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Review on Tin (II) Sulfide (SnS) Material: Synthesis,Properties, and ApplicationsN. Koteeswara Reddya, M. Devikab & E. S. R. Gopalca Center for Nanoscience and Engineering, Indian Institute of Science, Bangalore 560012,Indiab Department of Aerospace Engineering, Indian Institute of Science, Bangalore 560012, Indiac Department of Physics, Indian Institute of Science, Bangalore 560012, IndiaPublished online: 25 Aug 2015.

To cite this article: N. Koteeswara Reddy, M. Devika & E. S. R. Gopal (2015): Review on Tin (II) Sulfide (SnS)Material: Synthesis, Properties, and Applications, Critical Reviews in Solid State and Materials Sciences, DOI:10.1080/10408436.2015.1053601

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Review on Tin (II) Sulfide (SnS) Material: Synthesis, Properties,and Applications

N. Koteeswara Reddy*,1 M. Devika,2 and E. S. R. Gopal31Center for Nanoscience and Engineering, Indian Institute of Science, Bangalore 560012, India2Department of Aerospace Engineering, Indian Institute of Science, Bangalore 560012, India3Department of Physics, Indian Institute of Science, Bangalore 560012, India

Tin (II) sulphide (SnS), a direct band gap semiconductor compound, has recently receivedgreat attention due to its unique properties. Because of low cost, absence of toxicity, and goodabundance in nature, it is becoming a candidate for future multifunctional devicesparticularly for light conversion applications. Although the current efficiencies are low, thecost-per-Watt is becoming competitive. At room temperature, SnS exhibits stable low-symmetric, double-layered orthorhombic crystal structure, having a D 0.4329, b D 1.1192,and c D 0.3984 nm as lattice parameters. These layer-structured materials are of interest invarious device applications due to the arrangement of structural lattice with cations andanions. The layers of cations are separated only by van der Waals forces that provideintrinsically chemically inert surface without dangling bonds and surface density of states. Asa result, there is no Fermi level pinning at the surface of the semiconductor. This fact leads toconsiderably high chemical and environmental stability. Further, the electrical and opticalproperties of SnS can be easily tailored by modifying the growth conditions or doping withsuitable dopants without disturbing its crystal structure.In the last few decades, SnS has been synthesized and studied in the form of single-crystals andthin-films. Most of the SnS single-crystals have been synthesized by Bridgeman technique,whereas thin films have been developed using different physical as well as chemical depositiontechniques. The synthesis or development of SnS structures in different forms includingsingle-crystals and thin films, and their unique properties are reviewed here. The observedphysical and chemical properties of SnS emphasize that this material could has novelapplications in optoelectronics including solar cell devices, sensors, batteries, and also inbiomedical sciences. These aspects are also discussed.

Keywords SnS compound, IV–VI layered semiconductor, absorber materials, narrow band gapmaterial, structural and optical properties, optoelectronic devices

Table of Contents

1. INTRODUCTION........................................................................................................................................... 2

2. GENERAL PROPERTIES .............................................................................................................................. 3

3. CRYSTAL STRUCTURE OF SnS .................................................................................................................... 33.1. Structural Transition.................................................................................................................................. 4

3.2. Lattice Geometry ...................................................................................................................................... 4

3.3. Band Structure ......................................................................................................................................... 5

4. EXPERIMENTAL PROCEDURE.................................................................................................................... 6

4.1. Powders and Single Crystals ....................................................................................................................... 6

*E-mail: dr_nkreddy@rediffmail.comColor versions of one or more of the figures in this article can be found online at www.tandfonline.com/bsms.

1

Critical Reviews in Solid State and Materials Sciences, 0:1–40, 2015

Copyright � Taylor & Francis Group, LLC

ISSN: 1040-8436 print / 1547-6561 online

DOI: 10.1080/10408436.2015.1053601

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4.2. Thin Films ............................................................................................................................................... 7

4.2.1. Chemical Methods........................................................................................................................... 7

4.2.2. Electrochemical Deposition (ECD) .................................................................................................... 8

4.2.3. Chemical Vapor Deposition (CVD).................................................................................................... 9

4.2.4. Close-Space Vapor Transport (CSVT) ...............................................................................................10

4.2.5. Atomic Layer Deposition (ALD) ......................................................................................................10

4.2.6. Spray Pyrolysis ..............................................................................................................................11

4.2.7. Physical Deposition ........................................................................................................................11

5. RESULTS AND DISCUSSION........................................................................................................................14

5.1. Physical Properties of Single Crystals .........................................................................................................14

5.1.1. Structural Properties .......................................................................................................................14

5.1.2. Electrical Properties........................................................................................................................15

5.1.3. Optical Properties...........................................................................................................................16

5.2. Physical Properties of SnS Thin Films.........................................................................................................19

5.2.1. Temperature Effect.........................................................................................................................24

5.2.2. Thickness Effect ............................................................................................................................24

5.2.3. Substrates Surface Effect ................................................................................................................25

5.2.4. Annealing Effect ............................................................................................................................27

5.2.5. Doping Effect ................................................................................................................................29

5.2.6. Metallization of SnS .......................................................................................................................30

6. DEVICE APPLICATIONS OF SnS .................................................................................................................31

6.1. Photovoltaic Applications .........................................................................................................................32

6.2. Other Applications ...................................................................................................................................34

6.2.1. Photoelectrochemical Cells ..............................................................................................................34

6.2.2. Solid-State Batteries .......................................................................................................................35

7. SUMMARYWITH FUTURE ISSUES .............................................................................................................37

ACKNOWLEDGMENTS ......................................................................................................................................38

REFERENCES......................................................................................................................................................38

1. INTRODUCTION

The development of solid state devices has been a major

technology innovation, which has revolutionized electronics,

communication, computer hardware, and also general applica-

tions in entertainment industry and so on. It has pushed the

earlier thermionic devices to specialized pockets of applica-

tions. Semiconducting germanium (Ge), silicon (Si), gallium

arsenide (GaAs), and other materials have played a major role

in the new activities. Gallium nitride (GaN) has introduced

advances in optoelectronics. Among these materials, Si has a

special place of being applied widely in photovoltaic devices

also.1 However, the manufacturing cost of Si based devices

still remains high. From the last few decades, semiconducting

thin film technology has received much attention because of

the wider applications in various fields of science and technol-

ogy. The utilization of thin films in the fabrication of devices

drastically reduces the materials cost. Further, various deposi-

tion and characterization techniques have also been developed

for the realization of various thin film materials. Development

of solution growth techniques has further reduced the cost of

production since these techniques are cheaper, scalable, and

simple. Nevertheless, the cost of Si-based photovoltaic sys-

tems appears to have reached a plateau and further reduction

appears to be difficult.

Over past few decades the usage of renewable energy is

gradually increasing to meet the increase of energy demand.

Among the renewable energy sources, the solar energy seems

to be the most attractive and promising alternative energy

source due to its availability, non-pollutant, and universal

nature. Cadmium telluride (CdTe), copper indium selenide

(CIS), copper indium gallium selenide (CIGS), and amorphous

silicon (a-Si) are four important thin-film technologies often

used in photovoltaic (PV) solar cells.2,3 No doubt, the develop-

ment of binary and ternary thin film materials has created path-

ways compared to the Si-dependent technology, due to the

favorable properties and simplicity in their synthesis.

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However, in real practice these materials failed to achieve

expected goals due to their toxicity, high cost, and lack of

reproducibility. Efforts are still continuing at various laborato-

ries to identify and develop new class-materials for PV device

applications where the constituent materials are not toxic and

are readily available in a low cost manner. The factors that

should be considered in developing new semiconductor mate-

rials include (1) a suitable energy band gap, (2) the ability to

deposit the material using a low-cost deposition technique,

and (3) abundance of the elements.

Dittrich et al. surveyed many materials in view of their pos-

sible PV applications and suggested that the sulfosalts are

more promising candidates for future solar cell devices.4,5

Among these sulfosalts, tin mono sulfide (SnS) is one of the

best compounds suitable for various applications particularly

for fabrication of solar cell devices due to its unique physical

properties. Contrary to the lack of abundancy of indium and

gallium elements for CIS and CIGS devices, the constituent

elements of SnS are abundant in the Earth’s crust. As com-

pared to the perceived toxicity problems (Cd, Se) associated

with the usage of CdTe, CIS, and CIGS, Sn and S are safe in

view of environment as well as lives of human-beings. As a

result, SnS has evolved as a safe and sustainable semiconduct-

ing material. In particular, SnS has attracted some attention

because its band gap lies in between those of Si (1.12eV) and

GaAs (1.43 eV). Today, the cadmium selenide (CdSe) is one

of the most commonly known photoconductors due to its enor-

mous photoconductivity to dark conductivity ratio. The toxic-

ity and hazardous nature of Cd and Se elements is shadowing

its applications in the field of optoelectronics. In this view, the

SnS can fulfill the above requirements as an alternative to

CdSe for photo-conducting applications. The growth process-

ing of SnS is almost pollution-free and moreover, the cost,

availability, toxicity, and stability of SnS are seen to be highly

appropriate not only for solar cell applications but also as coat-

ing material for heating mirrors.6

2. GENERAL PROPERTIES

In this section we summarize the physical properties (struc-

tural, optical, and electrical properties), structural transitions

(room temperature phase to high temperature and high pres-

sure phases), crystal structure, and band structures (of all the

phases) of SnS.

SnS is one of the binary compound belongs to IV–VI group.

It is a layer-structured compound like germanium sulfide

(GeS), germanium selenide (GeSe) and tin selenide (SnSe),

etc. Owing to their layered structures, SnS also exhibits strong

anisotropic vibrational properties7,8 and therefore these struc-

tures show significant differences in their physical properties

when measurements are made along their crystallographic

axes. Further, the layers in SnS compound are coupled with

weak van der Waals forces. The presence of week forces in

SnS provides intrinsically a chemical inert surface without

dangling bonds and surface density states.9,10 As a result, the

surface of SnS becomes free from Fermi level pinning. This

fact makes SnS to be chemically and environmentally inert.11

At normal temperature and pressure (NTP), the SnS compound

usually possesses orthorhombic disorder crystal-structure with

a space group of Pbmn .D162h/

12 having a D 0.432, b D 1.12 and

c D 0.398 nm (JCPDS card No: 39-0354) as lattice parame-

ters. Further, SnS exhibits chemical stability in acidic solu-

tions,13 high melting and boiling points of 880 and 1230�C. Atroom temperature (RT), the work function and dissociation

energies of SnS are »4.214 and 4.5 eV,15 respectively.

The crystal structure of SnS is stable over a range of

compositions for which Sn : S ffi 1 : 1C x. Generally, the

excess of non-metallic atoms induces the proportionate Sn

vacancy sites (VSn), and thus, every anion would introduce

two positive holes in SnS lattice and thereby the material

becomes a positive-type (p-type) conductor.15 Upon increas-

ing the temperature or pressure, the SnS undergoes a struc-

tural transition from disorder to order structure. The pressure

coefficient of energy gap (dE/dP) of SnS is observed as

¡1.3 § 0.1£10¡4eV/MPa. This value is almost twice larger

than that of the other layered-structure compounds.16 Under

NTP conditions, SnS has dual band gaps: indirect and direct.

Depending on the growth technique and stoichiometry, the

indirect band gap of SnS varies between 1.07 and 1.25 eV

and direct band gap located at slightly higher energies, varies

between 1.30 and 1.39 eV with high fundamental absorption

coefficient larger than 104 cm¡1, which is greatly higher than

that of presently existing materials like CdTe, CIS or CIGS.

Further the refractive index of SnS single crystals increases

from 3.3 to 3.7 with the increase of incident photon energy

from 0.5 to 2.0 eV.17

The electrical and optical properties of SnS material can be

tailored by doping with suitable dopants like Ag, Sb, Cl, and

N.18 It is also known that depending on the content of tin, the

SnS can change its conductivity type from p-type to n-type.

Apart from these properties, over the last few decades SnS is

attracting the world-wide scientists as low-toxicity and cost-

effective material in the field of semiconductor science and

technology hese unique properties make SnS as an appropriate

candidate for various applications including photovoltaic devi-

ces, solid-state lubricants, near-infrared detectors, lithium ion

batteries, and sensors, which will be described in later sec-

tions. In addition, SnS is also expected as an excellent holo-

graphic register system in view of its remarkable optical

properites.

3. CRYSTAL STRUCTURE OF SnS

In 1935, Hoffman invented the crystal structure of SnS as

orthorhombic and assigned a D 0.398, b D 0.433, and c D1.118 nm as lattice parameters of unit cell.12 In virtual view,

structure of SnS is slightly disordered NaCl-type structure

since the highly electronegative S atoms draw electron pair

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from Sn and becomes [Ne] 3s2 3p6 and [Kr] 4d10 5S2 5p0. Fur-

ther, the nonbonding 5s lone pair electrons of the Sn strongly

distort the lattice from a rock-salt structure to distorted ortho-

rhombic layered structure. In these disorder-layered structures

each “Sn” atom is coordinated by six “S” atoms with three

short Sn–S bonds (»0.266 nm) within the layer, i.e., intralayer

and three long Sn-S bonds (at distances »0.338 nm) in the

next layer, i.e., interlayer (Figure 1). The lines connecting Sn

with intralayer S-atoms are approximately mutually perpen-

dicular and the same is true for the inter-layer S-atoms. It

implies that these layered-structures are connected along c-

axis with weak van der Waal’s forces. As a result, the layers

of SnS compound can be easily cleaved perpendicular to its c-

axis.

3.1. Structural Transition

At NTP, the SnS exhibits a stable low-symmetric phase,

i.e., a-SnS phase. Upon increasing temperature, it under-

goes λ-type phase transition to high-symmetric phase, i.e.,

b-SnS. In crystallographic view, orthorhombic SnS changes

to tetragonal one.19 The lattice parameters of the tetragonal

phase are a D 0.423 and c D 1.151 nm. The low tempera-

ture phase crystallizes in germanium sulfide (GeS) type

structure (B16) with the space group Pbnm .D162h/, whereas

the higher temperature phase crystallizes in thallium iodide

(TlI) type structure (B33) with the space group Cmcm .D172h/.

Here, the Pbnm .D162h/ is a subgroup of Cmcm .D17

2h/ of index

2,20 i.e., Pbnm retains half of symmetry elements of Cmcm.

The a!b transition in SnS is a second-order transition as per

the usual classification.21 Further, upon transition of a-phase

to b-phase, two stronger bonds in a-phase changes to four

rather weak bands in the b-phase. At the phase transition

temperature (Tc D 878 K), the axial ratio of lattice-parameter

“a” and “c” decreases continuously from a/c >1 to a/c <1.22

The crystal and band structures of these SnS phases are

described below based on the earlier reports.14,19,23

3.2. Lattice Geometry

The structure of low-temperature phase (<878 K) consists

of slabs with two atoms width (Figure 2a). The intra-layer

Sn-S bond length perpendicular to the slabs is small

(»0.263 nm). However, two of the interlayer Sn-S bonds are

shorter and stronger than those of intra-layer bonds and other

two bonds are much weaker. Hence, the coordination number

of Sn-S atoms is 3 (2 C 1), which results the a-phase SnS

obtaining a highly distorted octahedral crystal structure.23 The

FIG. 1. Schematic of diagram of double-layered structured

SnS. (� Cambridge University Press. Reprinted with permis-

sion from Cambridge University Press.30 Permission to reuse

must be obtained from the rightsholder.)

FIG. 3. Dispersion of the energy bands in a-SnS. (� Ameri-

can Physical Society. Reprinted with permission from Ameri-

can Physical Society.14 Permission to reuse must be obtained

from the rightsholder.)

FIG. 2. The crystal structure of (a) b-SnS and (b) a-SnS

(Strong Sn-S bonds are indicated by lines). (� American Phys-

ical Society. Reprinted with permission from American Physi-

cal Society.14 Permission to reuse must be obtained from the

rightsholder.)

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crystal structures of b-phase SnS is shown in Figure 2b and it

appears like NaCl-type slabs.14. In this case, the Sn atoms are

slightly pushed out of the slabs and the interlayer Sn-S bond

lengths in the plane of the slabs are equal to 0.296 nm and the

intra-layer Sn-S bond length perpendicular to the plane of the

slabs is 0.263 nm. These structures have the coordination

number of the atoms as 4 C 1.

3.3. Band Structure

The energy bands distribution along selected directions in

the Brillouin zone (BZ) of a-SnS is shown in Figure 3.14 It

clearly reveals that the S 3s energy states are residing far from

the other energy states of the valence band (VB) with a gap of

4 eV and a large part of Sn 5s states occupies the bottom of

VB. The middle of VB mainly contains S 3p and Sn 5p states,

which are fairly oriented in slabs between Y-T-Z planes. It

indicates that these energy states are probably not contributing

in the bonding of Sn and S atoms along X-direction but, these

are responsible for the building of Sn-S bonds along the slabs

of Y–Z plane. In the upper part of the VB there are a large

number of Sn 5p orbitals, which strongly mixed with S 3p

orbitals. All these states contribute in the formation of strong

covalent bonds along the X-direction (perpendicular to the

slabs). On the other hand, the conduction band (CB) states

along the Z–U line have nearly the same energy. This figure

clearly represents that the CB minimum occurs at G, whereasthe maximum of the VB is not at G.24 Thus, there is an indirectgap between the VB and CB of 1.6 eV (0.79 eV24) and a direct

gap is 1.8 eV (1.77 eV24). The nature of energy band over the

G! X line clearly indicates that at the lowest energies of the

CB the charge carrier mobility is drastically increased,

whereas, it is decreased at highest energies of the VB. These

are probably due to the presence of strong anisotropy of the

system along G! X direction.25 Along G plane, the spreading

and crossing of bands along the mostly hybridized directions

are increased.

The dispersion of the energy bands along selected direc-

tions of b¡SnS is shown in Figure 4. The lowest bands

belonging to S 3s orbitals are well separated from the other

bands with the energy gap of »3.5 eV. The S 3s bands have

nearly the same energy in large part of the BZ and are sepa-

rated at G with the separation energy of 1.6 eV. Above this,

there are two energy bands at the bottom of VB, which belong

to Sn 5s orbitals and partial contribution of S 3s orbitals. In

large part of the BZ, these bands are nearly degenerate and are

strongly separated at points X and Y. Above this, there are six

FIG. 4. Dispersion of the energy bands in b-SnS. (� Ameri-

can Physical Society. Reprinted with permission from Ameri-

can Physical Society.14 Permission to reuse must be obtained

from the rightsholder.)

FIG. 5. (a) Valence band density of states calculated for the band structures of SnS along the symmetry lines D, S, and L, andelectron density distribution for localized states in the forbidden band on the (010) plane (� John Wiley & Sons, Inc. Reprinted

with permission from John Wiley & Sons, Inc.26 Permission to reuse must be obtained from the rightsholder.); (b) VSn and (c)

VS. (� John Wiley & Sons, Inc. Reprinted with permission from John Wiley & Sons, Inc.27 Permission to reuse must be obtained

from the rightsholder.)

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bands related to S 3p states hybridized strongly with Sn 5p

states, which are responsible for the formation of covalent Sn-

S bonds. Noticeably the energy band pairs in most parts of the

BZ are degenerate in special directions. Between the VB and

the CB there is an indirect gap of 0.3 § 0.1 eV, and a direct

gap of 1.4 § 0.1 eV. Theoretically evaluated density of states

in VB of SnS (Figure 5a) reveals that SnS is probably contain-

ing six distinguishable valance bands.26 Three of them belong

to half-filled states and other three belongs to fully occupied

states. Based on the hierarchy of orbital energies, the evaluated

peaks can be assigned as: first three peaks to S (3pz), Sn-S

(hybridized 5py and 3py), Sn (5px) and other three peaks to S

(pair 3px), Sn (pair 5s), and S (pair 3s). The relative separation

between these peaks is 0.5 (S 3s - Sn 5s), 1.8 (S 3s - S 3px),

3.2 (S 3s - Sn px), 4.5 (S 3s - Sn py), and 6.5 (S 3s - S pz) eV.

In terms of electronic configuration, the SnS contains 40

valence electrons per unit cell since each unit cell consists of

four Sn and four S atoms, as shown in Figure 1 and every Sn

(5S2 5p2) atom shares four valence electrons and S (3s2 3p4)

atom contributes six valence electrons to the lattice. From the

Green-function analysis of SnS energy spectrum it can be

understood that the cationic vacancy (VSn) contains two local-

ized levels: one is in the gap between energy spectrum

branches of the main crystal at E D ¡12.6 eV and the other

one is in the fundamental forbidden band at E D C0.31 eV.27

Usually, the anionic vacancy (VS) creates a deeper energy

level in the forbidden band at E D C0.37 eV. The localized

states in the fundamental forbidden band are unoccupied for

either VSn or VS. This analysis also shows that the defect

potential of VS is slightly higher and deeply localized than that

of VSn. However, the electron density distributions for the cat-

ionic and anionic vacancies are similar (Figures 5b and 5c)

due to stronger defect potential of VS. Therefore, the creation

of a vacancy in the ideal crystal drastically increases or

decreases the density of states as compared to an unideal

crystal.

4. EXPERIMENTAL PROCEDURE

The growth of SnS materials in single crystal, polycrystal-

line, thin films and other forms has been carried out by various

well known chemical, mechanical and physical methods. A

brief description about the synthetic methods and optimized

growth/deposition conditions of SnS single crystals and thin

films is given in the following sections.

4.1. Powders and Single Crystals

The growth of SnS crystals is mainly carried out by four

methods: wet chemical method, mechanochemical (top-

down), melt growth, and melt-mixing. Among these techni-

ques, melt growth using Bridgman technique and melt-mixing

methods are well established.

A. Wet Chemical Method. Compound semiconductor

materials are synthesized by wet chemical method

using appropriate quantities of salts. Stoichiometric

polycrystalline SnS powders have been synthesized by

passing hydrogen sulphide (H2S) through an acidic

solution of stannous chloride (SnCl2) dissolved in

aqueous solution having the pH »1 (Figure 6).13.

Although this is a simple and low-cost route for the

synthesis of SnS crystals, the handling of sulfur precur-

sor, i.e., H2S, is difficult and also toxic.

B. Mechanochemical Synthesis (Ball Milling). It is one of

the simplest ways to prepare crystals of various materials. In

this method, hardened steel-balls are kept in a container

(Figure 7a) along with required materials and filled with

inert gas. The fine particles are produced by rotating the con-

tainer around its own axis with high speed. Here, the particle

FIG. 6. Schematic representation of wet-chemical method.

FIG. 7. Schematic representation of (a) the ball-milling appa-

ratus and (b) horizontal Bridgman apparatus.

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size of the final product depends on the balls size. If the size

of the balls is large then the outcome of the product contains

smaller grains. Stoichiometric SnS compounds have been

synthesized in a planetary activator by milling of homo-

geneously mixed individual elements (i.e., Sn and S) in

methanol with the weight ratio of 1:3.7. It was carried out

under argon atmosphere by maintaining thimble axes at a

rotational speed of 70 rpm with 1000 mps2 centrifugal

acceleration.28 Using the same methodology, small SnS

nanoparticles have been produced by continuous mixing of

powders with the combination of 100 steel-balls having

5 mm diameter in the duration of 60 min.

C. Bridgman Method (Melt Growth). The Bridgman tech-

nique, a well-known technique, basically consists of a

two-zone furnace (Figure 7b). Here, one zone will be

maintained at low temperatures in order to allow sufficient

overpressure of chalcogenide(s) within the sealed system.

The other zone of the furnace, contains the polycrystalline

compound materials, will be at slightly higher tempera-

tures, i.e., just above the melting point of the compound.

Upon moving the furnace to higher temperatures, initially

the source compound melts and upon going from hot to

cold side, the melt cools and solidifies. By placing a seed

crystal at the left-hand side of the melt, single crystal of

the compound can be obtained with specific orientation.

Single crystals of SnS have been prepared by vapor con-

densation method. Pure and stoichiometric SnS compound

was taken in a silica boat and kept in evacuated quartz

container at a temperature of 400�C. Then, it was moved

through a high temperature zone of 900�C with slow-rate

of movement (»1 cm h¡1).29

D. High Temperature Melt-Mixing. It consists of a high

temperature furnace (»1200� C) and two ceramic tubes,

as shown in Figure 8. The inner tube is connected to a syn-

chronous DC (direct current) motor and placed horizon-

tally on two supporting legs. The furnace is connected to

high power source through a temperature controller. The

required homogeneous compound can be obtained at

higher temperatures by keeping the vacuum sealed quartz

ampoule charged with appropriate materials at the center

of the inner tube under regular and continuous rotation.

The temperature of the furnace has to be increased at slow

rates up to higher temperatures slightly above the melting

point of the expecting compound. SnS compounds have

been synthesized by high-temperature melt-mixing

method under the laboratory conditions using high pure

“Sn” and “S”.30 Appropriate amounts of Sn and S were

weighed (»2 g) and transferred into a cleaned quartz

ampoule (70 mm in length and 9 mm diameter). Ampoule

was flame sealed under a high vacuum of 10¡5 Torr and

loaded into a home-built horizontal rotary furnace. It was

heated slowly up to 950�C in steps of 100�C per h and

kept at 950�C for two days under constant and continuous

horizontal rotation. Finally, it was allowed to cool slowly

to RT in the span of 24 h and the SnS compound was

recovered by breaking the ampoule.

4.2. Thin Films

SnS thin films have been prepared by using various chemi-

cal and physical methods. In general, the deposition of thin

films in chemical methods mainly depends on chemical reac-

tion, whereas in physical methods, the deposition takes place

through evaporation or ejection of materials from source. In

this section we briefly describe the growth/deposition methods

adopted for the preparation of SnS films and also highlight the

optimal growth/deposition conditions for the development of

stoichiometric SnS films.

4.2.1. Chemical Methods

This section deals with various wet-chemical methods

used for the growth of SnS thin films. It is well known that

the chemical methods are quite simple and economical.

They require less instrumentation and low-temperature.

Major assets of these methods are possible in-situ doping

and excellent control over the deposition conditions.

Depending on the nature of chemical reaction and process,

the deposition methods are categorized as thermal deposi-

tion and electro deposition. Although these methods are

simple and economic it is very difficult to achieve device

grade films and also, the control over chemical composition

of the deposited films is very poor.

A. Chemical Precipitation or Bath Deposition. It is a sim-

ple method used for the deposition of thin films of various

materials and their compounds. SnS films have been pre-

pared on conducting and non-conducting substrates from a

bath solution containing 0.1–1.0 M tin chloride (SnCl2)

and sulfur dissolved in deionized water (DIW) and pro-

pionic acid (PA), respectively, at a bath temperature

»100�C.31 Here, initiation of nucleation on the surface of

solids through ion-by-ion condensation leads to formation

of compact and adherent film. Smooth, adherent and thick

(»1 mm) films have been obtained within a short time of

30 min. By adding potassium gluconate (PG) or tartaric

acid (TC) the growth rate of SnS films have been con-

trolled well since they serve as Sn2C complexing agents.

FIG. 8. Schematic diagram of high temperature melt-mixing

method.

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This process works well with the powder source of Sn, and

very smooth films can be obtained by using anhydrous

sources.

SnS films with a maximum thickness of »0.12 mm

have been prepared at RT using chemical precipitation

method.32 The films deposited on glass substrates from the

bath solution consists of 5 ml (0.1 M) thioacetamide, 5 ml

(0.1 M) Sn-precursor prepared by dissolving SnCl2 in gla-

cial acetic acid, 15 ml of (7.4 M) tri-ethanolamine (TEA)

and 8 ml of (14 M) ammonia. The growth rate of SnS

films has been accelerated by maintaining the bath temper-

atures at around 75�.33 Further, SnS films have been

deposited on glass substrates at the bath temperature of

30�C by using a mixed solution, which contains 30 ml of

0.15 M SnCl2 2H2O, 30 ml of 0.7 M NH4F, 5 ml of 2 M

Na2S2O3 5H2O, and 6 ml of 0.25% NH4OH in a reaction

time of 24 h.34

B. Immersion or Dipping. This is another simple and eco-

nomic technique35 applied for the growth of various

thin films. A major process involved in this technique is

sequential dipping of substrates in cold and hot solu-

tions in regular intervals. SnS thin films have been pre-

pared by immersion or dipping of substrate(s) in a cold

solution of 25 mM sodium sulfide or ammonium sulfide

[Na2S or NH4ð Þ2S] and then in a hot (80–90�C) 25 mM

solution of SnCl2. In this method, upon immersion of

substrate in the hot solution a chemical reaction takes

place on its surface and final precipitation form as a SnS

thin film. The films adhesion to the substrate is very poor

and can be rubbed easily when the pH of the solution is

less than 3. By maintaining pH of the solution between 3

and 7 we can obtain adhesive and uniform SnS films

even on glass substrates. Further increase of pH upto 12,

except 7, enhances the growth rate and crystalline quality

of the SnS films.

Alternatively, SnS films have also been obtained by

dipping the substrates in a mixed solution prepared

with stannous chloride dihydrate (2.22 M) and thiourea

(1.31 M) [SnCl2 2H2O and SC(NH2)2] at the pH of 3,

followed by backing at 300�C in a furnace for the

duration of 5 min.36 In this technique, the thickness of

the films can be easily controlled by varying the num-

ber of dips. Further, the SnS films have been obtained

even at RT by successive ionic layer adsorption and

reaction (SILAR) method. In this approach the sub-

strates has been immersed in 0.1 M SnSO4 solution

dissolved in dilute H2SO4 (pH D 1.8) for 25 s and 10 s

in 0.1 M Na2S (pH D 12.5) solution followed by rins-

ing with deionized water at each steps for 15 s.37

4.2.2. Electrochemical Deposition (ECD)

Electrochemical deposition is a method used for the depo-

sition of metals as well as compound materials. In this

technique the deposition of substance occurs only upon pass-

ing the electric current through the conducting medium even

at low temperatures. In general, the deposition is carried out

in an electrochemical cell (ECC). ECC usually have three

electrodes namely working electrode (WE), where the depo-

sition takes place, counter electrode (CE) electrode for clos-

ing the circuit and reference electrode (RE), for referring the

voltage between electrolyte and WE. A schematic diagram is

shown in Figure 9. Based on nature of process the ECD

method can be broadly categorized into two types: anodic

ECD (AECD) and cathodic CED (CECD). Apart from these

methods, brush plating method also belongs to ECD method-

ology. All of these methods are simple, low-temperature,

scalable, and low-cost techniques. Also, these methods have

good control over the deposition rates as well as chemical

composition of the compound material since the deposition

occurs under electrically driven reactions. However, as com-

pared to CECD method, AECD method has major drawback

as its self-limiting growth rate makes it difficult to achieve

thicker films.

A. Cathodic Electrochemical Deposition (CECD) (Poten-

tiostatic). Stoichiometric SnS thin films have been

obtained at RT using the CECD method from the 0.01 M

aqueous solution consisting of tin chloride (SnCl2) and thi-

osulphate (Na2S2O3 (TS)) at the pH of 1.5.38 The deposi-

tion has been carried out onto titanium or ITO-coated

glass substrates by applying ¡0.7 V constant potential

with respect to RE. Further, the SnS thin films have also

been obtained by just replacing SnCl2 with 7 mM stan-

nous sulfate (SnSO4) solution at a potential of ¡1V.39 The

thickness of SnS films has been controlled by varying

either Sn2C concentration or WE potential. By adding eth-

ylene-diamine-tetra-acetic acid (EDTA) to the aqueous

solution of SnCl2 and thiosulphate, excellent improvement

FIG. 9. Schematic diagram of three-electrode electrochemical

cell.

8 N. KOTEESWARA REDDY ET AL.

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in the adhesion of films with substrate has been achieved

along with the improvements in their thickness, uniformity

and photoactivity.10 Here, EDTA additive play a great role

as a chelating agent and minimizes the direct reaction of

Sn2C and S, and thereby improves the longevity of the

deposition.

B. Constant-Current ECD (CCECD) (Galvanostatic).

Implementation point view, it is a simpler technique than

the potentiostatic modes since CCECD method does not

require any reference electrode and also the thickness of

the deposited films is simply related to the deposition cur-

rent density. This technique works only with two electro-

des: WE and CE under a constant current. Stoichiometric

SnS thin films were obtained on ITO substrates by

CCECD method using the a bath solution consisting of

20 mM of SnSO4 and 100 mM of Na2S2O3 at pH of 2.7

and Sn2C/S2O2¡3 ratio of 0.2 under a current density of

3.0 mA cm¡2 with a reaction time of 1.5 h.40

C. Pulsed Electrochemical Deposition (PECD). Pulsed

ECD is a unique technique, which effectively improves

the morphology as well as optical properties of as-depos-

ited films.41 By applying potential with shorter pulses in

shorter intervals as: “On” potential at ¡1V for 1 s and

“Off” potential at 0 V for 1 s to WE nearly stoichiometric

SnS films have been obtained even at RT.

D. Brush Plating Method. Brush plating is a cold electro-

lytic method usually applied to deposit a thin layer of

material(s) on a conductive surface without dipping the

object in a bath of electrolyte. This method provides good

adhesion and is used in the manufacturing industry for var-

ious applications. This is a simplest, cost-effective, and

scalable method. The brush plating equipment includes

power packs, solution(s), plating tools, anode covers, and

auxiliary equipments. The schematic diagram of brush

plating system is shown in Figure 11b. The axial brush is

typically a stainless steel or conducting rod, wrapped with

a cloth material. The brush has to be connected to a posi-

tive terminal of DC voltage and the negative terminal is

connected to substrate. The wrapped cloth holds the plat-

ing solution and also prevents direct contact with sub-

strate. Initially, the brush has to dip in the plating solution

and then by moving the brush continuously on the

substrate with a slow rate of movement the desired film

can be obtained. SnS films were deposited by brush plat-

ting method42 using the mixture solution 5 mM SnCl2 and

2.5 mM Na2S2O3 at the pH of 1.5 by applying constant

current density of 80 mA cm¡1 for 5 min.

4.2.3. Chemical Vapor Deposition (CVD)

Chemical vapor deposition (CVD) is a versatile technique

used for the deposition of thin films as well as nanostructures.

It has relatively simple instrumentation, ease of processing,

possibility of depositing different type of materials and eco-

nomic viability. It is also widely used in the semiconductor

industry. A simple CVD system consists of a reaction cum

growth chamber with temperature controller, precursors cum

carrier gas regulators, and vacuum pump. In a typical CVD

process, one or more volatile precursors purged into the CVD

chamber through regulators react and/or decompose on the

surface of the substrate kept at higher temperatures and form

desired compound as film or nanostructure. The volatile

byproducts produced in these processes are exhausted with the

combination of carrier gas through the vacuum pump. The

flow rate, gas composition cum concentration, growth temper-

ature, partial pressure in the reaction chamber, and chamber

geometry are the typical processing variables for the growth of

desired structures. Depending on the growth environment, the

CVD techniques are broadly classified as metalloorganic CVD

(MOCVD), atomic layer epitaxy (ALE), vapor phase epitaxy

(VPE), plasma enhanced CVD (PECVD), atmospheric pres-

sure CVD (APCVD), rapid thermal CVD (RTCVD), and low

pressure CVD (LPCVD) techniques.

A. Plasma-Enhanced Chemical Vapor Deposition

(PECVD). In recent years there has been an increasing

interest in PECVD technology to meet the demand for

novel and better materials in science and technology. In

this technique, the plasma produced by radio-frequency

(RF), direct-current (DC), or microwave field plays a

major role as a reaction initiator by promoting the chemi-

cal reaction through ionization and dissociation of precur-

sors, and leads the formation of a variety of thin film

materials on temperature sensitive substrates. Since the

plasma produces energetic ions/particles, there is usually

FIG. 10. Schematic diagram of (a) PECVD system and (b) APCVD system.

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good adhesion of the film to the substrate. The schematic

diagram of PECVD setup used for the preparation of SnS

thin films is presented in Figure 10a. Good quality SnS

films have been achieved by PECVD technique at the sub-

strate temperatures higher than 200�C using H2S (2 sccm,

standard cubic centimeter per min) and SnCl4 (2 sccm)

gases as sources of Sn and S and hydrogen gas (15 sccm)

as carrier gas.43 The other deposition conditions including

deposition pressure, total glass flow rate, 13.56 MHz RF-

plasma power density, and inter-electrode distance are

kept as constant at 150 mTorr, »20 sccm, 0.023 Wcm¡2

and 3 cm, respectively. The thickness of the SnS films has

been well controlled by varying RF-power and/or growth

temperature.

B. Atmospheric Pressure CVD (APCVD). APCVD works

at normal pressures and is mainly used in the glass produc-

tion at large scales. The advantage of APCVD is that the

films can be deposited in shorter times. In this technique,

the precursors of materials combine and react at atmo-

spheric pressure and deposit on the substrates kept at high

temperature. Further, it undergoes nucleation followed by

coalition and finally forms thin film of the desired mate-

rial. The growth rate of the films is quite high at about

»2 mm min¡1. A simple APCVD system used for the

development of SnS films is schematically presented in

Figure 10b. Good quality SnS films were obtained using

APCVD technique by purging of SnCl4 (1.5 lmin¡1) and

H2S (0.3 l min¡1) gases mixed with high-pure nitrogen

gas onto heated substrates at»550 § 5�C.44 Alternatively,the SnS films have also been obtained with the help of tri-

n-butyl-tin trifluoroacetate .Bun3SnO2CCF3/ organic pre-

cursor along with H2S gas at slightly low-growth tempera-

tures, »450�C.45 The flow rates of the precursors and

nitrogen carrier gas are maintained at 0.4, 0.6 and

11.6 sccm, respectively. Thick films (»750 nm) were

obtained within the time of 15 min.

Further, Hibbert et al.46 developed a novel and volatile

precursor of (fluoroalkythiolatin) tin (IV) (CF3CH2S)4Sn

for the deposition of tin sulfide thin films. By using this

precursor (boiling point is around »35�C), nearly stoichio-

metric SnS films (Sn:SD1:0.96) have been achieved in

APCVD technique at the substrate temperatures varied

between 525 and 600�C. Similarly, two asymmetric tin

dithiocarbamate precursors namely SnMe3(S2CNMeBu)

and SnBu(S2CNMeBu)3 have been developed by Kana

et al.47 and also applied successfully for the growth of SnS

films in APCVD. Parkin et al.48 also developed a single

source precursor of Sn(SCH2CH2S)2 and obtained single-

phase SnS films using aerosol-assisted CVD (AACVD)

technique at the growth temperatures above 500�C in

absence of H2S. By following the same approach, Bade

et al.49 obtained stoichiometric SnS films at the growth

temperature of 475�C using a new single volatile precur-

sor, tribenzyltin(IV)chloride thiosemicarbazones.

4.2.4. Close-Space Vapor Transport (CSVT)

The close-space vapor transport (CSVT) method is similar

to thermal evaporation method. However, it consists of a verti-

cal reactor made with a quartz tube (Figure 11a) with the

diameter of 2 cm and length of 10–20 cm. As shown in the

figure, the source material and substrate are separated with

quartz spacers of thickness 0.3—a few mm. A solid iodine

slab is placed at the upper part of the reactor and sealed under

vacuum. Finally, the complete system is placed on a SiC heat-

ing element that is operated by a voltage regulator. The ther-

mal gradient between the source and substrate can be adjusted

by using a movable heating-coil, which also allows for pre-

heating of the substrate. SnS films have been deposited using

CSVT technique50 onto glass substrates at a substrate tempera-

ture of 500�C by evaporating pure SnS compound for duration

of 10 min.

4.2.5. Atomic Layer Deposition (ALD)

ALD is a unique technique used for the deposition of vari-

ous thin films and nanostructures by a sequential spraying of

FIG. 11. Schematic diagram of (a) the CSVT system and (b) brush plating system.

10 N. KOTEESWARA REDDY ET AL.

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precursor materials. Basically, the sprayed precursors react

with the surface of the substrate one at a time in sequential

steps and form continuous thin films. Due to its self-limiting

and sequential feeding of precursors, the depositions are con-

formal and can also be controlled at atomic scales. Using ALD

method the films can be finely controlled for thickness and

deposited in a large area with excellent reproducibility. Also,

lower deposition temperatures can be used in order to deposit

the films on temperature sensitive substrates. A simple ALD

system consists of three major parts such as control unit of pre-

cursors, deposition chamber and vacuum pump, as shown in

Figure 12a. Stoichiometric SnS films have been deposited on

glass, sapphire and Si substrates by ALD technique through

the sequential purging of 2,4-pentanedionate (Sn-precursor),

nitrogen (carrier gas) and hydrogen sulfide (S-precursor) with

the combination of nitrogen in the intervals of 1-30-1-30 s.

The depositions have been carried out at the chamber base

pressure of 1 Torr and temperature of 175�C.51 Here the nitro-gen gas flow rate was maintained at 100 sccm and the partial

pressures of Sn- and S- precursors were maintained at 10 and

150 mTorr.

4.2.6. Spray Pyrolysis

The spray pyrolysis technique involves the spraying the

solution, which contains the solvable salts of the constituent

elements of the desired compound, onto a heated substrate.

The sprayed droplet upon reaching the surface of hot-substrate

undergoes pyrolytic decomposition and forms a single crystal-

lite or a cluster of crystallites of the product. The volatile

byproducts and the excess solvents escape in the vapor phase.

Here, the heated substrate provides the thermal energy neces-

sary for the decomposition and subsequent recombination of

the constituent species. This is followed by sintering and re-

crystallization of the clusters of crystallites giving rise to con-

tinuous film. A simple spray system (Figure 12b) consists of a

spray head connected to two channels and substrate heater

cum controller. The purified compressed air/nitrogen and solu-

tion are fed to the spray nozzle from opposite sides and are

then sprayed onto hot-substrates kept at below the spray noz-

zle. The substrate temperature is controlled using a tempera-

ture controller.

SnS films have been obtained using spray pyrolysis tech-

nique by spraying the equimolar solution prepared SnCl2 and

n, n-dimethyl-thiourea ((CH3NH)2CS) in a mixture DIW and

isopropyl alcohol (1:3) with a concentration of 0.1 M.52 Single

crystalline and stoichiometric SnS films have been obtained by

depositing them at a substrate temperature of »380 § 10�C by

keeping other deposition conditions including solution flow

rate (»10 cm3 min¡1), gas flow rates (»10 l min¡1), nozzle to

substrate distance (30 cm), and spraying time (10 min) as con-

stant. Further, Reddy et al. synthesized SnS films by using the

same technique and optimized the deposition conditions

including substrate temperature (300–375�C) and solution

concentration (0.09–0.13 M) for the growth of single crystal-

line and stoichiometric device grade films.53,54 Recently, this

method has been also applied by Sajeesh et al.55 for the

growth of SnS thin films and optimized the growth tempera-

ture for single phase films as 375 § 25�C with the precursor

equimolar concentration of 0.15 M.

4.2.7. Physical Deposition

The deposition of thin films in the physical deposition

methods occurs mainly through evaporation or ejection of

material from the source and thereby condensation onto the

substrates under high vacuum. The films deposited using

these techniques are qualitatively better than those from

chemical methods due to the presence of high-vacuum.

Based on the method applied for material evaporation, there

are a variety of physical vapor deposition techniques. Resis-

tive heating, flash evaporation, electron beam heating, laser

heating, arc evaporation, sputtering, etc. have been devel-

oped and applied for the deposition of various materials.

The methods, which were used for the deposition of SnS

films, are briefly described below.

4.2.7.1. Thermal Evaporation. Thermal evaporation tech-

niques (resistive-, electron beam-, and flash- evaporation are

FIG. 12. Schematic diagram of (a) the ALD system and (b)

spray pyrolysis system.

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the simplest techniques as compared to all other physical

vapor deposition (PVD) techniques. This technique basically

involves on the transformation of source material into vapor,

transport of this vapor onto the substrate surface and finally

condensation of the vapor on the substrate to form the thin

film.

A. Resistive Evaporation, A schematic diagram of simple

vacuum coating system used for the deposition of SnS

films is shown in Figure 13. It consists of a stainless-steel/

glass bell jar mounted on the top flange of the unit. A three

stage diffusion pump backed by a two stage rotary pump is

used to create a vacuum of the order of 10¡5 Torr. Here, a

liquid nitrogen trap connected between the chamber and

diffusion pump to avoid the back streaming of oil into the

chamber. Further, two Pirani gauges and one Penning

gauge are used to measure the fore-vacuum and high-vac-

uums, respectively, and a radiant heater generally used to

heat the substrates. The thickness of the film is monitored

using a quartz crystal thickness monitor. High quality stoi-

chiometric SnS films have been obtained by resistive

evaporation of high-pure SnS powder (»4N) onto glass,

ITO, and Al-substrates at the substrate temperature of

»300 § 50 �C under a vacuum of 10¡6 Torr.56

Alternatively, single-phase SnS films have been achieved

by co-deposition method at the substrate temperature of

300�C with a rate of 2 nm s¡1 under a pressure of 10¡5

Torr.57 The depositions have been done through the evap-

oration of high pure Sn and S from two different boats.

Here, the source-to-substrate distance has been maintained

as 15 cm and substrates were placed exactly at center and

normal to both the sources as shown in Figure 14a. Fur-

ther, Devika et al.58 developed polycrystalline SnS thin

films on glass substrates by resistive evaporation of high-

pure SnS compound with the substrate temperatures varied

between 275–325�C under the vacuum of 10¡6 Torr. Here,

the other deposition conditions such as the source to sub-

strate distance, film thickness, and the rate of deposition

were maintained constant as 14 cm, 0.5 mm, and 1 nm

s¡1, respectively.

B. Electron-Beam (e-Beam) Evaporation, The deposition

process in e-beam evaporation is similar to resistive

evaporation except nature of source. Here, the evapora-

tion of the material(s) is achieved by high-intensity

e-beam source instead of thermal heating. In e-beam

evaporation, an electron beam is aimed at the source

material causing local heating and evaporation. A sche-

matic diagram of e-beam evaporation is shown in

Figure 14b. High quality stoichiometric SnS films have

been obtained using e-beam evaporation of SnS powder

on to glass substrates at the substrates temperature of

300�C under a vacuum of 10¡6 Torr.59 Here, the dis-

tance between source and substrate is about 24 cm and

the rate of deposition is maintained at 3 nm s¡1.

4.2.7.2. Molecular Beam Epitaxy (MBE). Epitaxy or epi-

taxial growth is a method applied for the deposition of thin

layers of single crystalline material(s) over the lattice-

matched substrates through molecular vapor deposition. If

FIG. 13. Schematic diagram of thermal evaporation

system.

FIG. 14. Schematic diagram of (a) the co-evaporation appara-

tus and (b) e-beam evaporation apparatus.

12 N. KOTEESWARA REDDY ET AL.

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the substrate is made with the same material of deposition

then the process can be treated as homo-epitaxy, or simply,

epitaxy. However, when the deposited film is different

material from substrate, it is called as hetero-epitaxy. MBE

is an ultra-high-vacuum (UHV)-based system used to pro-

duce high-quality epitaxial films of variety of materials

including metals, insulators, and superconductors. In prin-

ciple, the MBE consists of atoms or clusters of atoms pro-

duced by heating up a solid source of material

(Figure 15a). They then travel in an UHV environment and

deposit on a hot substrate surface and form a film. Major

control over the vacuum environment as well as on the

quality of the source materials allows the development of

high quality crystalline films compared to other non-UHV

techniques. Epitaxial grown SnS films have been obtained

by MBE technique on slightly lattice matched magnesium

oxide substrates at substrate temperatures varied from 100–

335�C with a thickness of 135 nm.60

4.2.7.3. Sputtering. Sputtering is a technique belonging

to physical deposition methods. It usually consists of a deposi-

tion chamber, radio-frequency (RF)-source and vacuum pump.

The substrates placed in a vacuum chamber and the source

material, i.e. target, placed above the substrates as shown in

Figure 15b. Inert gas, generally argon, is introduced at lower

pressures. Plasma is created using an RF power source, and so

the gas becomes ionized. The ionized-ions are accelerated

toward the surface of the target and release atoms from target

due to their bombardment. The released atoms condense on

the surface of the substrates. Highly crystalline and amorphous

SnS films have been obtained on glass substrates by RF-sput-

tering (RF, 13.65 MHz) method at substrate temperature of

300 § 50�C, under the argon gas pressure of 5 £ 10¡2 Torr.61

Here, the other deposition parameters such as RF power, anode

voltage, cathode voltage, and target to substrate distance have

been maintained at 45 W, 1 kV, 40 mA and 8 cm, respec-

tively. Further, Hartman et al. successfully deposited SnS

films by RF-sputtering on glass substrates by keeping them at

16.5 cm away from target using sulfur-rich SnS target.62 The

depositions were carried under argon plasma (»150 W) with

the base pressure of 10¡7 Torr by varying argon pressure from

5–60 mTorr.

4.2.7.4. Sulfurization. The sulfurization setup (Figure 16)

basically consists of a two-zone furnace, quartz tube (length-

»1 m and diameter » 0.1 m) and a vacuum pump. One end of

the tube is connected to a vacuum system and the other end is

connected to a gas-line, i.e., carrier gas. In general, the high

vapor pressured sulfur is placed at the low temperature zone

and the sample which has to be sulfurized is placed at the

higher temperature zone in the flow-direction of carrier gas.

Single-phase SnS films have been obtained through the sulfuri-

zation of sputtered tin metallic layers on glass substrates at

temperatures varied between 300 and 350�C under a vacuum

of 10¡2 Torr for 20 min.63 Minemura et al.64 developed SnS

thin films by sulfurization of Sn-sheet in two-zone vertical fur-

nace in a reaction time of 76–103 h. Here, the sulfur source

and Sn-sheet were kept at 275�C and 200�C, respectively.Alternatively, the SnS films were obtained through the sulfuri-

zation of Sn-films on glass as well as Mo-coated glass sub-

strates with the thickness of 600 nm in a vacuum chamber at

200�C for 60 min.

4.2.7.5. Hot-Wall Deposition. Hot-wall deposition

method is frequently used for the deposition of single crystals

and thin films of various materials. It mainly consists of three

parts: a quartz-tube (hot-wall system), a vacuum chamber, and

a vacuum pump. One side enclosed hot-wall tube directs the

vapor from the source to the substrate placed at the open end

of the tube (Figure 17). The quartz-tube is heated by Kanthal

wire wounded closely along its length. The evaporated mole-

cules from the source, placed at the bottom of the tube, deposit

on the substrate by migration through the column of tube. The

whole arrangement is placed in a vacuum chamber and gener-

ally the deposition are carried out at pressures >10¡5 Torr.

SnS films were developed by hot-wall deposition on glass

FIG. 15. Schematic diagram of (a) the molecular beam epi-

taxy apparatus and (b) rf-sputtering apparatus.

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substrates kept at 10 cm away from open-end of the tube.65

The wall-temperature was kept at 550�C and the films were

deposited at different growth temperatures varied between 210

and 300�C under a pressure of 10¡6 Torr.

5. RESULTS AND DISCUSSION

The physical properties like crystal structure, chemical

composition, morphology, electrical, and optical properties of

SnS grown in single crystals and thin films are reviewed in the

following sections and discussed.

5.1. Physical Properties of Single Crystals

The Bridgeman technique has been frequently used for the

synthesis of SnS single crystals. The as-synthesized SnS crys-

tals appear like shiny flakes, which can be easily cleaved in a

direction perpendicular to the c- axis similar to tin mono sele-

nides.66 Thin platelets upto the thinness of 1 mm can be

obtained by adhesive tape pulling technique. The SnS crystals

obtained from a high-temperature melt-mixing method have

an excellent chemical stoichiometry with the Sn/S atomic

percent ratio of »1 and uniformly appended slices like surface

morphology (Figure 18).30 These SnS crystals are preferen-

tially oriented along the [010] direction and have orthorhom-

bic structure.

5.1.1. Structural Properties

Single crystals of SnS compound exhibit a phase transition

from a-phase to b-phase at a critical temperature (Tc) of

878 K22 due to the displacements of Sn and S atoms along the

[100] direction (Figure 19). The displacement of Sn and S

atoms positional parameters occurs from the distorted values

of orthorhombic phase to those of ordered values of the high

temperature b-phase. While increasing temperature, the lattice

parameters “a” and “c” vary continuously and approach each

other at Tc, whereas “b” increases continuously, as shown in

Figure 20. At around Tc, the axial ratio (a/c) also abruptly

changes from a/c>1 to a/c <1. On the other hand, while

decreasing temperature from 600 to 100 K, the crystal struc-

ture of SnS crystals remains stable as orthorhombic.30 The

FWHM (full width at half maximum) value of (040) peak

increases with the decrease of temperature (Figure 21) and,

however, the overall changes in FWHM value is found to be

0.029�. As compared to GaAs,67, the impact of temperature

fluctuations on SnS material is low since the volume expan-

sion coefficient of the SnS is 3 £ 10¡6 K¡1 whereas for GaAs

it is about 6 £ 10¡6 K¡1.67

The lattice parameters of orthorhombic (a-phase) as well

as tetragonal (b-phase) SnS crystals are slightly temperature

dependent.19 Both the phases exhibit different thermalFIG. 17. Schematic diagram of hot-wall deposition setup.

FIG. 16. Schematic diagram of two-zone sulfurization

apparatus.

FIG. 18. FESEM image of SnS compound synthesized by

high-temperature melt-mixing method. (� Cambridge Univer-

sity Press. Reprinted with permission from Cambridge Univer-

sity Press.30 Permission to reuse must be obtained from the

rightsholder.)

14 N. KOTEESWARA REDDY ET AL.

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expansion coefficients along their crystallographic axes. The

thermal expansion coefficients of orthorhombic phase along

three principal crystallographic axes are d[100]D¡89.1£10¡6,

d[010]D 80.1£10¡6 and d[001]D 35.6£10¡6 K¡1 and for tetra-

gonal phase, d[100]D¡26£10¡6 and d[001]D 51.8£10¡6 K¡1.

Therefore, the expansion or compression in both the phases is

clearly anisotropic.68 On the other hand, while increasing sur-

rounding pressure of SnS, orthorhombic (a-SnS) under goes a

phase transition to monoclinic (g-SnS) at around the pressure

of 17.5 § 1 GPa.24 The linear compressibility of this layered

compound along its crystallographic axes is 0.008, 0.0122,

and 0.0036 GPa¡1, which is attributed to decrease of S–S gap

and thereby increase of repulsive forces. Noticeably, the linear

compressibility parallel to the staking direction of the layers is

1.5 times higher than along “a” direction and 3.4 times higher

than parallel to the “b” direction.68

SnS crystals exhibit excellent structural stability upon dop-

ing of rare-earth (RE) elements. Such stability in the crystal-

line structure of SnS has been observed by Nasirov et al. for

the first time.19 RE elements (like Nd, Sm, and Gd) doped SnS

crystals exhibit temperature independent structural properties.

For examples, the RE elements doped SnS crystals with differ-

ent concentrations varied between 0.001 and 0.002 wt.%

showed an excellent crystalline stability upto the elevated tem-

peratures of »600�C19 and the observed lattice parameters

from Nd, Sm, and Gd doped SnS crystals for two typical con-

centrations are given in Table 1.

5.1.2. Electrical Properties

Usually SnS single crystals exhibit p-type conductivity.

At RT, the conductivity of these crystals is of the order of

10¡1–10¡4 V¡1 cm¡1.69,70 These SnS crystals show a

hole-concentration of the order of 1017 cm¡3 at RT and it

FIG. 19. Variation of the positional parameters (x and y) of Sn

and S atoms of SnS as a function of temperature. (� Elsevier.

Reprinted with permission from Elsevier.22 Permission to

reuse must be obtained from the rightsholder.)

FIG. 20. Variation of the lattice constants of SnS compound

with temperature. (� Elsevier. Reprinted with permission

from Elsevier.22 Permission to reuse must be obtained from

the rightsholder.)

FIG. 21. Variation of FWHM of SnS compound as a function

of temperature. (� Cambridge University Press. Reprinted

with permission from Cambridge University Press.30 Permis-

sion to reuse must be obtained from the rightsholder.)

REVIEW ON TIN SULFIDE MATERIAL 15

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is quite temperature independent between 100 and 500 K.

The Hall-mobility .mH / of holes perpendicular to c-axis of

SnS crystal, increases with increasing temperature from 100–

175 K (i.e., mH 1T3/2) as shown in Figure 22,29 which is

probably attributed to the ionic impurity scattering. Above

this temperature (>175 K), the hole-mobility is proportional

to T¡5/2, which indicates that at higher temperatures the lat-

tice scattering of carriers dominate the hole-mobility.71 At

RT, the mobility of holes along the c-axis is approximately

six times smaller than the mobility perpendicular to the c-

axis,29,72 which is probably attributed to the differences in

lattice constants of orthorhombic SnS.73 Besides this, the car-

rier density of p-type SnS crystals is increased upto 1019

cm¡3 by doping of silver (Ag) and the conductivity of SnS

changed from p-type to n-type by doping of antimony (Sb).72

The Sb doped SnS crystals show low carrier concentration of

the order of 10¡14 cm¡3.

Upon increasing temperature from 140 and 523 K, the

p-type SnS single crystals exhibit two distinguishable con-

duction regions namely extrinsic and intrinsic regions

(Figure 23).70 These regions are well separated by a broad

region treated as transition region, which occurs in between

60 and 140�C. The variation of electrical conductivity in

the extrinsic region is strongly attributed to the concentra-

tion of ionized acceptors, whereas in the intrinsic region

the conductivity is strongly related to the excitation of car-

riers from VB to CB. Upon treating the SnS crystals with

hydrogen at relatively low temperatures (»200�C), their

conductivity greatly decreases from 10¡4 to 10¡8 V¡1

cm¡1.69 This abrupt decrease in the conductivity of SnS

crystals is due to the drop in excess of sulfur atoms with

hydrogen treatment that results in a decrease in the positive

holes concentration and thereby its conductivity gradually

reduces to lower values.

5.1.3. Optical Properties

The optical properties of SnS crystals have been studied by

using various techniques. Before 1970s, the scientists identi-

fied SnS as an indirect band gap material. However, in 1977

Chamberlain et al. observed a direct band gap in SnS crystals

TABLE 1

Lattice parameters of orthorhombic (SnS)1-x(LnS)x crystals

(� Springer Science and Business Media. Reproduced with

permission from Springer Science and Business Media.19 Per-

mission to reuse must be obtained from the rightsholder)

Ln x a (nm) b (nm) c (nm)

Nd 0.001 0.3978 0.4322 1.1193

0.002 0.3978 0.4339 1.1193

Sm 0.001 0.3962 0.4527 1.1176

0.002 0.3978 0.4323 1.1193

Cd 0.001 0.3984 0.4326 1.1189

0.002 0.3969 0.4328 1.1247

FIG. 22. Hole mobility (mp) perpendicular to the c-axis as

a function of temperature for three different SnS crystals.

(� Elsevier. Reprinted with permission from Elsevier.29

Permission to reuse must be obtained from the rightsholder.)

FIG. 23. Temperature dependence of the electrical conductiv-

ity of SnS compound. (� Elsevier. Reprinted with permission

from Elsevier.70 Permission to reuse must be obtained from

the rightsholder.)

16 N. KOTEESWARA REDDY ET AL.

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along with indirect gap.74 As a continuation of this, Lukes

et al. noticed a strong direct interband transition in SnS crys-

tals while studying their electroreflectance measurements.75

Further, Valiukonis et al. also suggested the optical transition

in SnS compound as a direct allowed transition.16

Temperature-dependent infrared photoconductivity meas-

urements on SnS crystals reveal that while decreasing temper-

ature from RT to 35 K the indirect band gap (Eg) of

the crystals increases from 1.13 § 0.02 to 1.22 § 0.02 eV,

whereas their direct band gap remains stable at »1.43§0.02 eV.74 Below its indirect band, it exhibits three distin-

guishable peaks at 0.55 (E1), 0.66 (E2) and 0.83 (E3) eV (Fig-

ure 24). The peak E1 originates from the transition of carriers

between the doubly-ionized acceptors to the CB, and E2 origi-

nates from transitions between the VB and the acceptor states.

The temperature dependence of SnS indirect optical band gap

can be expressed by a linear equation:76

Eg D 1:21¡ 4:8£T£ 10.¡ 4/.eV ; T in K/;

and their refractive index “n” can also be formulized by the

following equations:77

na Eð ÞD 3:523C 0:692E2 {Ekanb Eð ÞD 3:523C 0:467E2 {Ekb

Under polarized light (E), SnS exhibits a direct transition at

1.3 § 0.02 eV when Ejja. However, these crystals exhibit a

strong direct transition at around 1.59 eV when Ejjb along

with a weak transition at previous position (Figure 25).75 It

clearly demonstrates the existence of different forbidden gaps

in SnS crystals along its crystallographic axes. Contrary to

this, Valiukonis et al. observed the lowest direct energy gap

(Eb) of 1.3 eV for SnS when Ejjb, i.e., 1L4¡1L4 allowed for

optical transitions and the next direct energy gap (Ea) of

1.6 eV when Ejja, i.e., 1L1¡1L1 is allowed for transitions. At

RT, the SnS single crystals are strong absorbents for the wave-

lengths below 1 mm. These crystals have similar absorption

coefficients along it’s “a” and “c” axes (Figure 26).

Further, the SnS crystals exhibit a refractive index no(λD 0)

of 3.6 and dielectric constant of 19.5. The effective mass of

holes in SnS crystals along its three principle axes is

m�a Dm�

b D 0:2mo;m�c Dmo; where mo is the rest mass of the

electron; and the effective charge on the atoms is e� D 0:7eo,where eo is the electron charge.29,72 It is also realized that the

values of no and e* are isotropic since the total oscillatory

strength of three mutually perpendicular Sn-S vibrations is

one ({cos2u1 C cos2u2 C cos2u3 D 1, where u is the angle

between Sn-S vibration and direction of light polarization)

that makes the oscillator strength independent of the direction

of polarization. However, the polaron effect at lower frequen-

cies is small in SnS because of its week hole-phonon coupling

constant (»0.2).78

SnS crystals exhibit Raman peaks at 40, 49, 70, 85, 95, 160,

164, 192, 208, 218, and 290 cm¡1 (see Figure 27) and infra-

red peaks at 69, 99, 145, 178, 188, 220, and 222 cm¡1.8 Due

to the structural anisotropy, SnS exhibits a low static dielectric

constant .eo/ of 32 when the electrical vector .E¡ / is parallel

to a-axis .Ejj a / or c-axis .Ejj c / than for the b-axis .Ejj b /,(eo D 42). On the other hand, a high frequency dielectric

FIG. 24. Photoconductivity curves for SnS in the spectral

region below the band edge. (� IOP. Reproduced with permis-

sion from IOP.19 Permission to reuse must be obtained from

the rightsholder.)

FIG. 25. Electroreflectance spectrum of SnS single crystal for

the Ejja and Ejjb polarizations. (� Elsevier. Reprinted with

permission from Elsevier.75 Permission to reuse must be

obtained from the rightsholder.)

REVIEW ON TIN SULFIDE MATERIAL 17

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constant .e1 / value of SnS crystals obtained along b-axis

.Ejj b / or c-axis .Ejj c / is higher (»16) than that the value

(»14) obtained along a-axis .Ejj a /. Using the Raman and

infrared data of SnS (see Tables 2 and 3) the splitting factor

(D) raised by interlayer coupling is calculated by using the

equation for the split-off modes .v§ /.79,80

v§ D v2o §D2� �1/2

;

where no is the frequency of the corresponding intra-layer

mode. The splitting between Raman and infrared peaks, the

calculated values of no, D and (no/D)2 are given in Table 2.

Here, the values of intra-layer and interlayer force constants

(k/q (no/D)2, where k and q are the intralayer and interlayer

force constants, respectively) are very close and are in between

5 and 7. This implies that SnS is a layered compound. How-

ever, by comparing this value with similar compounds like

GeS (k/q D 15) it is concluded that SnS is a weak layer-like

compound than the GeS, which can be considered as an inter-

mediate compound between layer-like and three-dimensional

FIG. 26. Absorption spectra of SnS at room temperature for

three different polarizations. (� John Wiley & Sons, Inc.

Reprinted with permission from John Wiley & Sons, Inc.16

Permission to reuse must be obtained from the rightsholder.)

FIG. 27. Raman spectra of SnS recorded at room temperature.

(� American Physical Society. Reprinted with permission

from American Physical Society.8 Permission to reuse must be

obtained from the rightsholder.)

TABLE 2

Frequencies of the six pairs of Raman and infrared active

phonons together with the corresponding values of no, D and

(no/D)2. The frequencies are given in cm¡1 (� IOP. Repro-

duced with permission from IOP.79 Permission to reuse must

be obtained from the rightsholder)

Splitting nC n¡ no D (no/D)2

Ag-B1u 111 97 104 37.5 7.7

Ag-B1u 216 183 199.5 80.7 6.1

Ag-B1u 264 220 242 100.8 5.7

B3g-B2u 160 190 175 70.8 6.1

B2g-B3u 170 200 185 76 6

B2g-B3u 194 229 211 84 6.4

TABLE 3

A comparison of Raman and infrared frequencies which were

split in accordance with the given equation (� IOP. Repro-

duced with permission from IOP.79 Permission to reuse must

be obtained from the rightsholder)

Ag B1u B2g B3u B3g B2u

264 220 194 229 160 190

216 183 170 200

111 97 78

50 47 58

18 N. KOTEESWARA REDDY ET AL.

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crystals. Further, the SnS crystals grown with a hole-concen-

tration of the order of 1017 cm¡3 exhibit a plasma frequency

(vp) of 100 cm¡1, which clearly emphasizes the presence of

complex plasmon-phonon coupling effect.81

5.2. Physical Properties of SnS Thin Films

The SnxS films grown by chemical bath deposition (CBD)

method exhibited polycrystalline orthorhombic crystal struc-

ture and their elemental composition ratio (Sn/S) varied

between 1.5 and 1.31 While introducing 10% H2O in the aque-

ous solution, the resulting films contained mainly single-phase

SnS and exhibited average indirect optical band gap of 1.15 §0.15 eV and activation energy of around 0.56§0.05 eV. On

the other hand, the films grown in absence of H2O showed

Sn2S3 phase as predominant and they showed indirect band

gap of 1.7 § 0.2 eV. Further, the films grown in the presence

of PG and TA exhibited SnS2 phase as dominant phase. These

films are thicker than the above films and also have smooth

and uniform surface, which exhibited the optical band gap of

»2.2 eV. The SnS films grown at RT are amorphous in nature

and consist of n-type conductivity. The optical indirect band

gap of these amorphous films is found to be »1.51 eV.32 How-

ever, the SnS films developed at bath temperature of 75�Cexhibited good crystalline properties along with photoconduc-

tivity as compared to the films deposited at RT.33 The SnS

films grown at 30�C by CBD on glass substrates with a thick-

ness (t) of 290 nm exhibited slightly sulfur-rich chemical com-

position and consist of p-type conductivity.34 These films also

have smooth surface morphology (Figure 28a) and orthorhom-

bic polycrystals, which are preferentially oriented along the

(040) planes. The band gap of these films is found to be direct

at around 1.31 eV along with high absorption coefficient.

The SnS films grown by dipping methods exhibited

mainly polycrystalline orthorhombic crystal structures and

their degree of preferred orientation increased with increas-

ing pH of the hot solution.35 The conductivity of these

films is drastically increased with the increase of alkalinity

of SnCl2 solution. These films also showed an acceptor

activation energy of »0.3 eV. The SnS films developed by

dipping followed by baking are smooth, shiny and strongly

adherent to the substrate.36 The thickness of these films

strongly depended on the number of dippings of substrate

in solution. For example, the films grown by 5 dippings in

a mixed solution of 2.22 M SnCl2 2H2O and 1.31 M SC

(NH2)2 at pH of 3 followed by baking at 300�C for 5 min

have a thickness of around 500 nm. These films are nearly

single-crystalline and grown exclusively along the <040>

directions. Further, these SnS films exhibited a photocon-

duction band edge at »1.4 eV. However, the SnS films

grown at RT on glass as well as ITO substrates by SILAR

method with the thickness of 200 nm exhibited a polycrys-

talline orthorhombic crystal structure.37 These as-grown

films are slightly tin-rich in chemical composition and

show direct optical band gap of 1.43 eV. The PL analysis

of these films revealed that the as-grown SnS films have

tin-vacancies that are located at deeper levels since these

films exhibit a broad band centered at 625 nm (1.82 eV).

SnS thin films grown by cathodic electrochemical deposi-

tion (CECD) on conducting substrates (ITO and titanium)

using aqueous solutions containing 0.01 M SnCl2 and 0.01 M

thiosulphate ions under the electro-potential of ¡0.7 V are

slightly tin-rich and have smooth and uniform surfaces.38

These tin-rich films exhibited polycrystalline orthorhombic

crystal structure, and contrary to above results, showed p-type

conductivity. These films also showed indirect band gap of

around »1 eV. Similar results have also been observed by

Subramanian et al.82 from the SnS films grown under the

same conditions. However, the SnS films grown in the pres-

ence of EDTA exhibited improved crystallinity and good sur-

face coverage (Figure 28b)10 than the SnS films grown

without EDTA. These SnS films have densely grown rocksalt

like crystal morphologies (Figure 28c), and exhibits slightly

higher indirect band gap (»1.1 eV). These results indicate that

the addition of EDTA to the aqueous solution of Sn and S

probably control the formation of Sn and S ions and thereby

SnS clusters. This could lead to grow a highly-quality SnS

films with a slow-rate of nucleation.

Ichimura et al. obtained stoichiometric SnS films by ECD

techniques using SnSO4 and Na2S2O3 as precursors of Sn and

S.39 The films deposited on In2O3 coated glass substrates at

higher Sn2C concentrations (>5 mM) with a constant electro-

potential of ¡1 V are thicker (2.5 mm) and uniform than the

films grown at lower Sn2C concentrations (<1 mM). However,

upon increasing of Sn2C concentration by keeping other depo-

sition conditions as constant, the elemental ratio of SnS films

remained as nearly constant, whereas this ratio linearly

decreased with the increase of pH as well as electro-potential.

The SnS films grown in the duration of 1 hour are slightly tin-

rich in nature and consists of polycrystalline orthorhombic

crystals. These tin-rich films also exhibited direct and indirect

band gap of around 1.3 and 1.0 eV, respectively. Further,

good-quality SnS films have been obtained on ITO substrates

by cathodic ECD method using an aqueous solution 5 mM

SnSO4 and 25 mM Na2S2O3 at a pH of 2–3 under a constant

deposition potential of ¡0.73 V (vs. SCE) and bath tempera-

ture (Tb) of 40�C.83. The as-deposited films are highly stoi-

chiometric and consist of irregularly aligned nanocubes like

surface morphology (Figure 28d). However, upon increasing

bath temperature from 30–50�C, the tin-rich SnS films gradu-

ally become sulfur-rich in nature, and their direct band gap

gradually decrease from 1.48 to 1.24 eV, but the absorption

coefficient remains stable (>104 cm¡1). Using the same

approach, Kamel et al.84 achieved SnS thin films on stainless

steel substrates using the bath solution made with 25 mM

SnSO4, 250 mM potassium thiocyanate (KSCN), and 250 mM

Na2SO4. The films grown at the lower current densities

(»2.2 mA cm¡2) exhibited orthorhombic crystal structures

REVIEW ON TIN SULFIDE MATERIAL 19

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having (040) plane as a preferential orientation. While increas-

ing the current density, the (101) plane becomes more domi-

nant than the (010) plane.

SnS films have been prepared by galvanostatic ECD

method on FTO glass substrates from nitrogen purged warm

(50–90�C) acidic aqueous solution (100 ml) at a pH of 2.5.

The solution is prepared with SnCl2 (50 mM) and Na2S2O3

(150 mM) precursors along with tartaric acid added as an

additive.85 Here, the electro-potential is varied between

¡1and 0.4 V with a rate of 150 mV s¡1 at a current density of

¡3 mA cm¡2. The as-deposited films have a thickness of

around 650 nm, exhibited stoichiometric chemical composi-

tion and orthorhombic crystal structure with (111) as preferred

orientation. These films also showed direct optical band gap

of around 1.05 eV. Thus, these studies reveal that the presence

of organic acids in the precursor solution promotes the unifor-

mity and adherence of the as-deposited films. The SnS thin

films grown by CCECD method on ITO substrates using

a bath solution of 20 mM of SnSO4 and 100 mM of Na2S2O3

under the deposition conditions of pH D 2.7, Sn2C/S2O2¡3 D

0.2 and current densityD3.0 mA cm¡2 in the reaction time of

1.5 h are exhibited nearly stoichiometry between its constitu-

ents.40 The SnS films grown at lower pH, Sn2C/S2O2¡3 and

current densities are more sulfur-rich in nature, whereas at

higher values, the films have clear Sn-rich chemical composi-

tion. SnS films grown under optimized conditions have rods

like morphology, Figure 29a,b and their adhesiveness to the

substrate is also better. These films also exhibited p-type

conductivity and their direct optical band gap and electrical

resistivity have varied between 1.21 and 1.42 eV, and 7.5 and

20 Vcm, respectively.

SnS films have been grown by pulsed ECD in three-

electrode ECC under ‘On’ potential of ¡1V for 1 s and

“Off” potential of 0 V for 1 s (vs. SCE) using the bath

solution prepared with 30 mM SnSO4 and100 mM

Na2S2O3 at a pH of 2.7. The as-grown SnS films are nearly

sulfur-rich in nature.41 Upon increasing the pulse intervals

from 1 to 10 s, the sulfur content in the films increased.

Besides this, the films grown under pulsed ECD method

with large intervals are more uniform, thicker and have

large direct band gap (1.67 eV) as compared to the films

grown by normal ECD (Figure 29c,d). Further, highly stoi-

chiometric SnS films have been deposited by pulsed ECD

method using the mixed solution consisting of »20 mM of

SnSO4 and »100 mM of Na2S2O3 under constant pH of 2–

3 and “On” potential of ¡0.75 by varying the “off” poten-

tial between ¡0.1 and 0.5 V.86 The SnS films grown under

these conditions have polycrystalline orthorhombic crystal

structure and are grown preferential along the <111>

direction. However, the lattice constants of these SnS films

slightly varied with the increase of “off” potential, as pre-

sented in Table 4. While increasing “off” potential from

¡0.1 to 0.5 V the sulfur-content of SnS films increased

and their grain size also increased along with the surface

roughness (Figure 30). All these films showed p-type con-

ductivity and upon increasing “off” potential their

FIG. 28. (a) Backscattering SEM image of SnS films grown

on glass substrate (� IOP. Reproduced with permission from

IOP.34 Permission to reuse must be obtained from the right-

sholder.), (b) low and (c) high magnification SEM images of

SnS films deposited on titanium substrate by ECD under the

guidance of EDTA (� Elsevier. Reprinted with permission

from Elsevier.10 Permission to reuse must be obtained from

the rightsholder.), and (d) SEM image of SnS films grown at a

bath temperature of 40�C. (� Elsevier. Reprinted with permis-

sion from Elsevier.83 Permission to reuse must be obtained

from the rightsholder.)

FIG. 29. (a) Low and (b) high magnification SEM images of

the as-grown SnS films on ITO substrates at Tb D RT, pH D2.7, Sn2C/S2O2¡

3 D 1/5, J D 3.0 mA cm¡2 and t D 1.5 h

(� Elsevier. Reprinted with permission from Elsevier.40 Per-

mission to reuse must be obtained from the rightsholder.); and

SEM images of SnS films deposited with a thickness of

2.5 mm by (c) normal ECD and (d) pulsed ECD with 10 s

interval condition. (� Elsevier. Reprinted with permission

from Elsevier.41 Permission to reuse must be obtained from

the rightsholder.)

20 N. KOTEESWARA REDDY ET AL.

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electrical resistivity and optical band gap are varied

between 17 and 43 Vcm and 1.23 and 1.34 eV, respec-

tively. Stoichiometric SnS films have been realized on ITO

substrates by three-electrode ECD method using the aque-

ous solution made by the procedure described above86 in

the presence of EDTA at Sn2C:EDTA:S2O2¡3 D1:1:4 and

different electrode potentials varied between ¡0.95 to

¡1.0 V for the duration of 1.5 h.87 The obtained films are

smooth and also have orthorhombic crystal structure.

SnS films grown by PECVD technique at the growth tem-

peratures above 200�C have purely single SnS phase and

exhibit (111) plane as preferred orientation, whereas the films

grown at below this temperature contain mixed phases.43 The

thickness of SnS films grown by PECVD method increased

with the increase of either the growth temperature or plasma

power. These single phase SnS films have p-type conductivity

and show indirect band gap of 1.16 V along with the phonon

energy of 0.18 eV. The conductivity of these films is found

to be of the order of 10¡6 V¡1 cm¡1 with deep acceptors

activation energy of 0.3 eV.

The SnS films grown by APCVD method at higher temper-

atures (>540�C) are gray in color and show excellent

stoichiometry between its constituent elements. The films

grown at below this temperature are yellowish in color and

sulfur-rich in nature probably due to the presence of SnS2 and

Sn2S3 phases.44,88 Structural analysis of these films showed

that at low temperatures (»300�C) Sn-S deposits as SnS2phase, and upon increasing temperature from 300 to higher

temperatures, this phase gradually undergoes decomposition

and forms Sn2S3 phase as an intermediate phase and at higher

temperatures it forms SnS phase (»550�C). This is in accor-

dance with the observations made by Piacente et al. who

investigated the sublimation properties of tin sulfides.89 These

single phase SnS films showed the Sn 3d5/2 binding energy

peak at 485.7 eV and S 2p3/2 peak at 161 eV and a few Raman

peaks at 96,163, 189, 220, and 288 cm¡1.

Further, good quality SnS films have been deposited at the

substrate temperature of 450�C using Bun3SnO2CCF3 and H2S

as precursors in APCVD method. The as-grown SnS films are

chemically stoichiometric and have polycrystalline ortho-

rhombic crystal structure.45 The surface of these films has

cuboids-like crystal morphology and also exhibited optical

band gap of 1.5 eV. SnS films have been grown by APCVD

method using (CF3CH2S)4Sn precursor as single-precursor at

above the growth temperatures of 525�C. These SnS films also

exhibited nearly stoichiometric chemical composition and

orthorhombic crystal structure.46 Noticeably, the films grown

at below 400�C clearly consist of SnS2 phase, whereas the

films grown at medium temperatures (between 400 and

525�C) contain mainly Sn2S3 along with small traces of SnS2phase. The morphology of these phases also significantly

changed from plates to cubes via wires and rods with the

increase of substrate temperature from 400–600�C, shown in

Figure 31.

The SnS films (Figure 32a) grown by CSVT technique at a

growth temperature of about 500�C for 10 min duration

showed slightly Sn rich in chemical composition along with p-

type conductivity.50 The crystallites of these films have ortho-

rhombic structure that are preferentially oriented along the

<111> direction. The films grown by CSVT showed a low

electrical resistivity and Hall mobility of 14.5 Vcm and

3.73 cm2 V¡1s¡1 along with a higher carrier density of 1017

cm¡3. These films also exhibited direct band gap of 1.32 eV

with a high absorption coefficient (104 cm¡1). SnS films have

TABLE 4

The composition, lattice parameters and grain size of the SnS films deposited at different Voff (� Elsevier. Reprinted with per-

mission from Elsevier.86 Permission to reuse must be obtained from the rightsholder)

Voff (V) Sn/S (at.%) a (nm) b (nm) c (nm) a/c Grain size (nm)

0.5 48.68/51.32 0.4431 1.1128 0.3970 1.116 26.93

0.3 49.26/50.74 0.4426 1.1125 0.3971 1.114 21.54

0.1 50.41/49.59 0.4426 1.1124 0.3973 1.114 23.94

¡0.1 51.5/48.5 0.4429 1.1134 0.3969 1.116 21.54

FIG. 30. SEM images of SnS films grown by pulsed ECD at

different ‘off’ potentials of (a) 0.1, (b) 0.3, (c) 0.5, and (d)

¡0.1 V, respectively. (� Elsevier. Reprinted with permission

from Elsevier.86 Permission to reuse must be obtained from

the rightsholder.)

REVIEW ON TIN SULFIDE MATERIAL 21

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been grown by ALD technique on glass as well as silicon sub-

strates by sequential feeding of 2, 4-pentanedionate (Sn-pre-

cursor), nitrogen (carrier gas) and hydrogen sulfide (S-

precursor) with nitrogen gas in the intervals of 1-30-1-30 s. at

the growth temperature of 175�C under a base pressure of 1

Torr. The ALD grown SnS films have clear stoichiometry

between the constituents and exhibits a direct optical band gap

of 1.87 eV.51

The SnS films deposited by spray pyrolysis method at the

substrate temperatures between 370–390�C have purely SnS

phase, which is oriented along the (111) plane.52 The films

grown below 370�C contained mixed phases such as SnS and

Sn2S3, whereas the films grown at higher temperatures

(>390�C) showed SnO2 as dominant phase. The crystallinity

of the sprayed SnS films increased with the increase of sub-

strate temperature. The single phase SnS films exhibit p-type

conductivity and their indirect optical band gap and activation

energy are found to be 1.27 and 0.54 eV, respectively. Using

the same method, Thangaraju et al. developed SnS films at the

substrate temperatures between 345 and 355�C on glass sub-

strates.90 Contrary to the above results, these SnS films are

amorphous and exhibit n-type conductivity. Besides this, these

amorphous SnS films show dark and photo-conductivity in the

order of 10¡3 and 10¡2 V¡1 cm¡1, respectively and an indirect

band gap of 1 eV.

Thin films of tin sulphide (SnxSy) have been deposited on

antimony-doped tin oxide-coated glass (ITO:Sb) and bare

glass substrates by using spray pyrolysis method.53,54 The

depositions have been made using equimolar solutions of

tin chloride and thiourea at different substrate temperatures

from 100–450�C and concentrations between 10–200 mM.

The films deposited in the temperatures between 300

and 375�C and concentrations between 90 and 130 mM

exhibited nearly stoichiometric chemical composition

(Sn/S»1.03), single phase (SnS) and showed a strong (111)

preferred orientation with an average grain size of 370 nm.

The surface of these films also had uniform morphology, as

shown in Figure 32b. These single-phase films exhibited

p-type conductivity with an average electrical resistivity of

30 Vcm and a net carrier concentration of 2£1015 cm¡3.

These layers also showed a direct energy band gap of

»1.32 eV with an absorption coefficient, >105 cm¡2 at

FIG. 31. SEM images of the SnS films deposited at different

temperatures of (a) 400, (b) 450, and (c) 550 and (d) 600�C.(� The Royal Society of Chemistry. Reproduced with permis-

sion from The Royal Society of Chemistry.46 Permission to

reuse must be obtained from the rightsholder.)

FIG. 32. SEM image of SnS films grown by (a) CSVT tech-

nique at a growth temperature of 500�C in the duration of

10 min (� Springer Science and Business Media. Reproduced

with permission from Springer Science and Business Media.50

Permission to reuse must be obtained from the rightsholder.)

(b) spray pyrolysis method at a temperature of 350�C(� Elsevier. Reprinted with permission from Elsevier.54 Per-

mission to reuse must be obtained from the rightsholder.), and

(c) XRD profile for SnS films deposited by thermal evaproa-

tion method with a thickness of (top) 18 and (bottom) 135 nm.

(� IOP. Reproduced with permission from IOP.91 Permission

to reuse must be obtained from the rightsholder.)

22 N. KOTEESWARA REDDY ET AL.

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above the fundamental absorption edge. The films deposited

at other conditions deviated from chemical stoichiometry

and also exhibited additional phases like SnS2, Sn2S3, and

SnO2 along with SnS phase.

Vacuum-evaporated SnS films at a substrate temperature of

300§50�C exhibit orthorhombic crystalline structures with a

preferential orientation of (040) and higher grain sizes,

>300 nm.56 These films are slightly S-rich in composition and

show high absorption coefficient, >104 cm¡1 with a direct

optical band gap of 1.48 eV. These SnS films also exhibit p-

type conductivity with a carrier density, Hall-mobility, and

resistivity of »1015 cm¡3, »400–500 cm2/Vs, and »13–

20 Vcm, respectively. The activation energy of these films is

found to be around, 0.28–0.34 eV due to the presence of deep

acceptor levels. However, the films deposited at below 200�Cexhibited (111) orientation as a dominant plane. As contrary

to this, Johnson et al.91 achieved single-phase SnS films by

vacuum-evaporation of SnS compound even at RT. These SnS

crystallites are oriented along the (111) plane (Figure 32c) and

have a direct band gap of 1.3 eV and photoconductivity of

10¡3 V¡1 cm¡1.

SnS films have been deposited at different substrate temper-

atures (30–300�C) by a thermal co-evaporation technique.57

The films grown at 300�C exhibited single-phase SnS by hav-

ing (040) peak as preferred orientation of orthorhombic crystal

structure (Figure 33a). The surface of these films contains

flakes-like crystalline morphology (inset Figure 33b). These

single-crystalline films showed the electrical resistivity of

6.1 Vcm with the activation energy of 0.26 eV. Further, the

SnS films exhibited an optical band gap of 1.37 eV with a

high optical absorption coefficient, >104 cm¡1. On the other

hand, the SnS films had been developed by resistive evapora-

tion of high-pure SnS compound at different substrate temper-

atures (20–325�C).58 As usual, the films grown at higher

substrate temperatures, >275�C, have nearly good-stoichiom-

etry and polycrystalline nature (Figure 34a). The surface mor-

phology of these films appears like monotonically dispersed

beaks-like crystallites as shown in Figure 34b. These films

also exhibited low electrical resistive and have a direct optical

band gap of 1.35 eV with absorption coefficient, »105 cm¡1.

The SnS films deposited by e-beam evaporation at the sub-

strate temperature of 300�C with a thickness of 810 nm

showed nearly single-crystalline nature with the crystallites

preferentially oriented along the (111) planes of orthorhombic

unit cell.59 These films also showed indirect and direct band

gaps of 1.24 and 1.38 eV and have a high absorption coeffi-

cient, >104 cm¡1.

Single crystalline SnS films deposited by RF-sputtering

method have better chemical stoichiometry between its

constituents. Here, the SnS films were deposited with dif-

ferent Sn and S ratios by simply changing the anode volt-

age.60 While increasing the Sn/S ratio from 0.1 to 1 the

color of the films changed from light-yellowish to heavy

gray. Stoichiometric SnS films exhibited a direct optical

band gap in between 1.43 and 1.46 eV and p-type conduc-

tivity along with low electrical-resistivity of »20 Vcm.

Further, while increasing the argon partial pressure from 5

to 60 mTorr under the power of 150 W, the thickness of

the films deposited in a fixed durations (60 min) decreased

from 1.58 to 0.23 mm and the surface morphology also

changed (Figure 35a–d).62 The SnS films grown at low

pressures have (002) plane as preferential orientation; how-

ever with increasing pressure, (111) plane becomes

FIG. 33. (a) XRD profile and (b) SEM image of SnS films

deposited by co-evaporation method at a substrate temperature

of 300�C. (� Springer Science and Business Media. Repro-

duced with permission from Springer Science and Business

Media.57 Permission to reuse must be obtained from the

rightsholder.)

FIG. 34. (a) XRD profile and (b) SEM image of SnS films

deposited by thermal evaproation method at a substrate tem-

perature of 275�C. (� AIP. Reproduced with permission from

AIP.58 Permission to reuse must be obtained from the

rightsholder.)

REVIEW ON TIN SULFIDE MATERIAL 23

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dominant (Figure 35e) and Sn/S ratio also increases. The

resistivity of the films increased from 1.1 to 33 kV cm

with pressure and their indirect band gap decreased from

1.07 to 1.02 eV.

Single-phase SnS films grown by two-step process or sulfu-

rization method consist of 200 nm thickness and orthorhombic

crystal structure. The as-grown crystallites are preferentially

oriented along the [111] plane.63 These sulfurized films have

slightly tin-rich chemical composition, and regular shaped

nanoparticle morphology with narrow porosity, as shown in

Figure 36. These films also exhibited p-type conductivity and

have an electrical resistivity of 102 Vcm with the activation

energy of 0.65 eV. Moreover, these films show a high absorp-

tion coefficient (>104 cm¡1) along with a direct optical band

gap of 1.35 eV. Alternatively, SnS films have been grown by

sulfurization of Sn-sheet in two-zone furnace for 76–103 h as

well as 600 nm thick Sn-film coated glass substrates in vac-

uum chamber for 60 min at a temperature of 200�C.64 The as-grown films in both the routes showed SnS phase along with a

few other phases like S and Sn. The final thickness of SnS

films grown in vacuum chamber is found to be 4.5 mm. These

films have polycrystalline crystals and exhibit a direct optical

band gap of 1.3 eV.

The SnS films grown by hot-wall deposition method at dif-

ferent substrate temperatures between 210 and 300�C showed

nearly stoichiometry (Sn/S D 1.07) with min sulfur defi-

ciency.65 These films exhibited (040) plane as preferred orien-

tation and had p-type conductivity. While increasing substrate

temperature, the direct band gap of these films decreased from

1.27 to 1.07 eV due to the decrease of film thickness.

5.2.1. Temperature Effect

Reddy et al. studied the structural and optical properties of

microcrystalline SnS films at different temperatures under a vac-

uum of 10¡6 Torr.92 Here, the SnS films were deposited by resis-

tive thermal evaporation method on glass substrates at a

substrate temperature of 300�C. At RT, the SnS films showed

orthorhombic crystal structure with lattice parameters of a D0.424, b D 1.107, and c D 0.397 nm. These films also showed

(measured under vacuum) an optical band gap of 1.47 eV with a

high absorption coefficient, 105 cm¡1. X-ray diffraction studies

at different temperatures (100–598 K) showed that the structure

of SnS films remains stable and their unit cell volume increased

with the increase of temperature. On the other hand, the band

gap of SnS films slightly decreased (Figure 37) with the increase

of temperature from 4–300 K. The overall change is about

0.03 eV. As compared to similar compound GaAs, the change in

optical band gap of SnS films is much less since the change is

0.1 eV for GaAs. Thus the change in optical band gap of SnS

films with the chance of temperature is marginal.

5.2.2. Thickness Effect

It is well known that the physical properties of thin film mate-

rials depend on deposition parameters as well as on the film

FIG. 35. SEM images of SnS films grown on glass substrate

by RF-sputtering method at different argon partial pressure of

(a) 5, (b) 10, (c) 30, and (d) 60 mTorr; and (e) XRD profiles of

SnS films grown on glass substrate by RF-sputtering method

at different argon partial pressure of (a) 5, (b) 10, (c) 30, and

(d) 60 mTorr. (� Elsevier. Reprinted with permission from

Elsevier.62 Permission to reuse must be obtained from the

rightsholder.)

FIG. 36. SEM image of SnS films grown by sulfurization

method. (� Elsevier. Reprinted with permission from Elsev-

ier.63 Permission to reuse must be obtained from the

rightsholder.)

24 N. KOTEESWARA REDDY ET AL.

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thickness.93–95 Thus, the understanding of film thickness on the

physical properties of SnS films is important. There have been a

few reports on this issue, which are described below.

Tanusevski et al. deposited SnS films by e-beam evapo-

ration method with different thicknesses from 410–810 nm

by keeping other deposition parameters as constant.59 With

increasing film thickness, their surface roughness increased

from 22 to 51 nm (Figure 38) and all the films showed p-

type conductivity. The SnS films were grown at RT by

chemical bath deposition using the aqueous solution pre-

pared with 0.8 g of NH4F, 1.125 g of SnCl2. 2H2O, 6 ml

of 0.25% NH4OH, and 5 ml of 2M Na2S2O3 in 100 ml of

distilled water at a pH of 7.96 By increasing the deposition

time from 4 to 18 h the SnS films were deposited with

ifferent thicknesses (Figure 39). The thickness of SnS films

increases in the beginning and then gets saturated, probably

due to the ionic product of the solution becoming equal to

solubility product.

Devika et al. investigated the impact of film thickness

on the physical properties of SnS films.97 Here, the SnS

films were deposited on glass substrates by thermal resis-

tive evaporation at a substrate temperature of 300�C with

different thicknesses between 0.1 and 1.5 mm. While

increasing the film thickness, the SnS films becomes more

stoichiometric and their average crystallite size gradually

increased from 30 to 120 nm. However, the SnS films

deposited at �0.5 mm thickness showed (111) orientation

as dominant, whereas, at higher thickness (040) peak

becomes more dominant (Figure 40a). These SnS crystalli-

tes exhibited different electrical and optical properties

depending on their preferential orientations. For example,

SnS films deposited with 0.75 mm have an optical band

gap of about 1.5 eV, whereas films deposited with 0.5 mm

thickness showed the band gap of 1.35 eV. Further, the

SnS films exclusively oriented along the <111> direction

exhibited a low electrical resistivity and activation energy

(39 Vcm and 0.28 eV) than the films oriented along the

<040> directions (65 Vcm and 0.3 eV) (Figure 40b).

5.2.3. Substrates Surface Effect

It is well known that the decrease of lattice mismatch with

substrate reduces the defect state density, and electrical resis-

tivity of the thin films with considerable improvement in their

grain sizes. This could significantly enhance the efficiency of

optoelectronic devices. In particular, the films to be adopted in

the fabrication of optoelectronic devices should possess two

basic requirements: (1) high absorbance (or transmittance) and

FIG. 37. Variation of energy band gap of SnS films as a func-

tion of temperature. (� AIP. Reprinted with permission from

AIP.92 Permission to reuse must be obtained from the

rightsholder.)

FIG. 38. AFM images of SnS films deposited by e-beam evaporation with a thickness of (a) 410 and (b) 810 nm. (� Elsevier.

Reprinted with permission from Elsevier.59 Permission to reuse must be obtained from the rightsholder.)

REVIEW ON TIN SULFIDE MATERIAL 25

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(2) low electrical resistivity. There have been several methods

developed to obtain low-resistive metal-chalcogenide films

including by reducing the lattice-mismatch between film and

substrate, and creation of excess metal atoms in the host

films.98,99 In this view, a few groups have been tried to obtain

low-resistive SnS films by selecting nearly lattice-matched

substrates, annealing and doping of foreign elements. In gen-

eral, thin films have very little strength and cannot be made

self-supporting. They must be manufactured onto suitable sup-

porting substrates. Here, the selection of substrate is a critical

issue since it must be compatible with the film material in

every way; chemically, structurally, with respect to tempera-

ture and stress stability. In particular, how different substrates

affect the physical properties of SnS films is briefly described

below.

Nozaki et al. prepared the epitaxial SnS films by deposit-

ing them on MgO(001) substrates using molecular beam

epitaxy.60 The as-grown SnS films on at a substrate temper-

ature of 335�C have smooth surface morphology (Figure 41)

and orthorhombic crystal structures. The epitaxial relations

between SnS film and substrate are found to be: (010)SnSjj(001)MgO and [100]SnSjj[100]MgO or [001]SnSjj[100]MgO. These films showed slightly low unit cell volume

(0.193 nm3) than that of its bulk counterpart (0.194 nm3).

Upon increasing the substrate temperature from RT to

600�C or decreasing “a/c” ratio, the lattice parameters “b”

and “c” of epitaxial SnS films increased, whereas the lattice

parameter “a” decreased. From these investigations the

authors realized that the interfacial interaction between the

SnS film and substrate has strong impact on the lattice

dimensions of the films, particularly along the weak bond

direction, not only near the substrate surface, but over the

whole thickness of the film. While decreasing the a/c ratio

the indirect energy band gap of the SnS films has increased.

However, the indirect band band-gap measured along direc-

tions perpendicular to the b-axis is larger than that along

the b-axis.

The substrate surface effect on the physical properties of

SnS thin films has been investigated by Devika et al.100 Here,

the SnS films were deposited on glass, ITO-coated glass, Si

(100) single crystal wafer and Ag-coated glass substrates using

the resistive thermal evaporation method. All the as-deposited

films exhibited nearly stoichiometric chemical composition

(Sn/S atomic% ratio of »1.05) and different surface morphol-

ogies (Figure 42). These films have orthorhombic structured

crystallites and all of them, except the films grown on Si sub-

strates, showed (111) as a dominant peak. The films deposited

on Si substrates exhibited (0 4 0) as a dominant peak. As com-

pared to other structures, the SnS films deposited on ITO

FIG. 39. The dependence of thickness of SnS films on the

deposition time in the bath solution. (� IOP. Reproduced with

permission from IOP.96 Permission to reuse must be obtained

from the rightsholder.)

FIG. 40. (a) XRD patterns of SnS films deposited at two dif-

ferent thickness of 500 and 750 nm and (b) electrical resistiv-

ity as a function of film thickness (inset shows magnified

view of the curved portion between 500 and 1000 nm).

(� The Electrochemical Society. Reprinted with permission

from The Electrochemical Society.97 Permission to reuse must

be obtained from the rightsholder.)

26 N. KOTEESWARA REDDY ET AL.

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substrates have high rms surface roughness (»14.9 nm) and

average grain size (»225 nm). These films also showed very

low electrical resistivity of 8.9 £ 10¡3 Vcm. The physical

properties of the SnS films deposited on different substrates

are presented Table 5.

Moreover, this group has developed SnS films on nearly

lattice matched substrates and investigated their physical

properties.101 A single crystalline aluminum (Al(100))

sheet has been chosen as lattice matched substrate due to

lower mismatch between Al and SnS (7.14 and 1.41%

along a and c lattice parameters, respectively). The SnS

films grown on these substrates are highly crystalline and

epitaxially oriented along the <101> direction. For com-

parative understanding, the XRD spectra of SnS films

grown on amorphous and lattice-matched substrates are

shown in Figure 43. The rms surface roughness of these

films is found to be »2.5 nm. Raman spectroscopic analy-

sis shows that these epitaxial SnS films have a good crystal

quality and phase purity. The PL analysis reveals that these

films are probably consisting of three types of localized

defect-states namely interstitial Sn, and vacancies of Sn

and S atoms.

5.2.4. Annealing Effect

In general, the resistivity of the films strongly depends on

preparation conditions and techniques. As stated above,

creation of excessive metal atoms in the SnS films leads

their resistivity to lower values, which would be achieved

through various heat treatments. A thermal treatment of as-

grown films also improves their structural quality as well as

stability.102–104 Also, it relieves accumulated strain energy,

diminishes defects, and enlarges their grain size since the dis-

locations and other structural defects will move towards grain

boundaries and adsorb/decompose with the surface.

The SnS films grown by dipping method under optimized

conditions have been annealed in vacuum at 285�C for 4

and 24 h and open air at 300–400�C for 30 min.35 Upon

annealing the SnS films in open air for a short time, the con-

ductivity type of the films changed from p- to n-type, and

the electrical conductivity increased by one order of magni-

tude. The films annealed in vacuum for a long time showed

a drastic change in their chemical composition and also

exhibited a new SnS2 phase by partial release of tin. In case

of open air annealing for a long time, SnS films get oxidized

and formed SnO2 phase. As per these observations, the

release of tin atoms with annealing of SnS films probably

leads their conductivity to n-type and have a donor activa-

tion energy of »0.2 eV. Upon annealing the chemically

deposited SnS films at above 300�C in air, a phase conver-

sion from SnS to SnO2 through an intermediate phase of

SnO2-x has been observed by Nair et al.105 While increasing

annealing time from 0 to 10 min at higher temperatures

(�400�C), the electrical resistance of SnS films decreased

from 1010 to 104 Vcm¡2 and further prolongation of anneal-

ing time, it drastically increased due to the formation of

SnO2 from SnO2-x phase. These investigations also reveal

that low-resistivity SnS films can be obtained by annealing

them at below 350�C in air.

Johnson et al. studies the impact of annealing on the

physical properties of stoichiometric SnS films deposited by

FIG. 41. SEM image of SnS film grown on MgO substrate at a

substrate temperature of 335�C. (� Elsevier. Reprinted with

permission from Elsevier.60 Permission to reuse must be

obtained from the rightsholder.)

FIG. 42. SEM images of SnS films grown on different sub-

strates. (a) glass, (b) ITO/glass, (c) Si, and (d) Ag/glass.

(� IOP. Reproduced with permission from IOP.100 Permission

to reuse must be obtained from the rightsholder.)

REVIEW ON TIN SULFIDE MATERIAL 27

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vacuum evaporation method. Here, the annealing tempera-

ture was varied from RT to 300�C and executed under air

and argon (Ar) atmosphere for 5 min.91 The films annealed

in air showed good improvement in their conductivity with

the increase of annealing temperature upto 50�C, and above

this temperature, it decreased. In case of Ar atmosphere,

the conductivity increased upto 100�C and above this, it

gradually decreased with annealing temperatures. After

annealing, the SnS films grown by brush plating method

exhibited an average thickness of about 1.5 mm.42 These

annealed films showed slightly tin-rich chemical composi-

tion and polycrystalline orthorhombic crystal structure.

These films have an average grain size of about 320 nm

and exhibit p-type conductivity. The electrical resistivity of

annealed films varied between 10 and 15 V cm. These films

have a high absorption coefficient of »104 cm¡1 and an

indirect band gap of 1.1 eV. The SnS films, grown by

cathodic electrodeposition on ITO substrate at a bath tem-

perature of 50 § 20�C under the conditions reported else-

where,101 were annealed under vacuum at 250�C for the

duration of 30 min.83 Upon annealing, the SnS films

showed an excellent improvement in their crystallinity and

surface-smoothness. Further, the average grain size of the

films is also increased from 580 to 760 nm. These films

exhibited p-type conductivity and high transmittance. How-

ever, the as-deposited as well as annealed SnS films showed

an indirect optical band gap of 1.15 eV and electrical resis-

tivity of 20 Vcm with deep-acceptor activation energy

between 0.34 and 0.45 eV.

The SnS films deposited by thermal evaporation method at

RT are amorphous in nature.106 Upon annealing the SnS films

between 150 and 300�C for 1 h under the vacuum of 10¡6

Torr, the films become polycrystalline and their crystallites

are preferentially oriented along the <111> direction. How-

ever, the films annealed at 200�C exhibit better quality than of

the films annealed at other temperatures. At all wavelengths,

the annealed SnS films exhibit lower refractive index and

absorption coefficient as compared to the as-grown films. Fur-

ther, the amorphous SnS films show a higher indirect (lower

direct) band gap of 1.4 eV (2.18 eV) than that of crystalline

films 1.38 eV (2.33 eV). The SnS films grown by chemical

bath technique exhibited stable structural properties upto the

annealing temperature of 250�C in argon environment for 1

and 8 h.96 However, the films annealed at 300�C for 1 h

showed slight improvement in the intensity of preferential ori-

entation, i.e., (111). The transmittance of these films increased

with the increase of annealing temperature as well as time.

While increasing annealing temperature and time, the direct

band gap of the films remained constant at 1.38 eV, whereas

the indirect band gap increased between 0.96 and 1.14 eV.

Further, the SnS films annealed at 250�C even at longer tim-

ings showed lower photoconductivity as compared to as-

grown films, and it is high for the films annealed at 300�C for

1 h, which attributed to the crystallinity of the films.

Devika et al. studied the impact of normal annealing on

the physical properties of SnS films.107 The films were

deposited on glass substrates and annealed at various

TABLE 5

The physical properties of SnS films deposited on different substrates by thermal resistive evaporation method (� IOP. Repro-

duced with permission from IOP.100 Permission to reuse must be obtained from the rightsholder)

Sub.

Lattice parameters (nm)Preferred

orientation (PO)

L (nm)

DPO rms roughness (nm) r (Vcm) Eg(eV)a b c Avg. Along PO

Glass 0.433 1.121 0.399 (111) 168.3 154 16.5 7.83 38.8 1.35

ITO 0.468 1.133 0.373 (111) 220.1 405 4.76 14.93 8.9£10¡3 1.62

Si 0.428 1.114 0.404 (040) 180.0 80.0 — 8.36 4.67 1.55

Ag 0.433 1.121 0.398 (111) 171.6 164 3.48 10.83 0.26 1.86

FIG. 43. XRD spectra of SnS films grown on glass and Al

substrates. (� John Wiley & Sons, Inc. Reprinted with permis-

sion from John Wiley & Sons, Inc.101 Permission to reuse

must be obtained from the rightsholder.)

28 N. KOTEESWARA REDDY ET AL.

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temperatures from 100–400�C in a vacuum of 10¡5 Torr

for 1 h. Here, the films annealed at 400�C remained on

glass substrate as patches due to partial re-evaporation of

SnS. While increasing the annealing temperature, the SnS

films become tin-rich in nature and at higher temperatures,

the Sn/S ratio of the films was found to be 1.1. The crystal

structure of annealed SnS films, however, remained stable

as orthorhombic and the change in their lattice parameters

was also marginal. The grain size and rms surface rough-

ness of the films decreased with the increase of annealing

temperature (Figure 44). While increasing annealing tem-

perature, the resistivity of SnS films decreased from 37 to

9 Vcm. However, the films annealed at 100�C exhibited

slightly lower optical band gap (1.35 eV) and activation

energy (0.24 eV) than the other films. Yue et al. studied

the impact of annealing on the physical properties of SnS

films, which were grown by three-electrode ECD

method.108 The growth was carried out onto ITO coated

glass substrates using a bath solution of 30 mM SnCl2 and

100 mM Na2S2O3 with a pH of 1.8 at a growth tempera-

ture of 30�C in the reaction time of 30 min. Then, the

films were annealed at different temperatures between 100

and 250�C in air for 1hr. Upon annealing, the films exhib-

ited drastic change in their surface morphology (Figure 45)

and chemical composition. The crystallinity as well as

grain size of the films increased with increasing annealing

temperature upto 150�C and above this temperature, the

SnS films got oxidized and formed SnO2 phase. Further,

the SnS films exhibited increase in absorption while

increasing annealing temperature (upto 150�C) and their

optical band gap also increased from 1.31 to 1.39 eV.

Devika et al. observed a few interesting physical properties

from the annealed SnS films by rapid thermal annealing

(RTA) process.109 Here, also, the SnS films were deposited by

thermal evaporation technique on glass substrates and treated

by RTA for a short time of 1 min at different temperatures

from 300–550�C under N2 atmosphere. As usual, the Sn/S

ratio and crystallinity of the films increased with the increase

of annealing temperature. The resistivity of the annealed films

decreased upto the annealing temperature of 400�C and above

this it slightly increased (Figure 46). The SnS films annealed

at 400�C exhibited a low electrical resistivity of 5 Vcm

(as compared to normal annealing films) with considerably

high Hall mobility and carrier density of 99 cm2 V¡1 s¡1 and

1.3 £ 1016 cm¡3, respectively. Another noticeable point here

is that the SnS films survived significantly upto 500�C anneal-

ing temperature upto and as usual, the films exhibited sublima-

tion at higher temperatures (»550�C).

5.2.5. Doping Effect

Alternative to annealing, the low-resistive SnS films can be

obtained by the creation of excess of metal atoms through

incorporation of suitable dopants. In this direction, Albers

et al.72 investigated the doping effect of antimony (Sb) and sil-

ver (Ag) on SnS single crystals and observed n-type conduc-

tivity with a carrier concentration of 10£19 cm¡3 in Sb-doped

SnS crystals and p-type conductivity with a concentration

10£18 cm¡3 in Ag-doped SnS crystals.

Doping of antimony (Sb) into sputtered SnS films fol-

lowed by annealing between 350 and 450�C in hydrogen

atmosphere also changed the conductivity of SnS films from

p-type to n-type and their resistivity fell down to 2 Vcm.60

The impact of Ag doping on the physical properties of SnS

films has been investigated by Devika et al.110 Here, the Ag

doping was achieved by an instant evaporation of SnS and

Ag from a single boat and deposited on glass substrates

with a thickness of 0.5 mm at a substrate temperature of

FIG. 44. SEM images of (a) as deposited and annealed SnS

films at (b) 100 and (c) 400�C. (� IOP. Reproduced with per-

mission from IOP.107 Permission to reuse must be obtained

from the rightsholder.)

REVIEW ON TIN SULFIDE MATERIAL 29

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275�C. All the films exhibited crystal structure as ortho-

rhombic and (111) plane as preferred orientation. The grain

size and crystallinity of the films were considerably

improved with increasing Ag concentration. However, the

electrical resistivity (Figure 47a) and activation energy of

the doped SnS films decreased upto 15 at.% and above this

concentration, they increased. The optical band gap of the

films also decreased with increasing concentration (Fig-

ure 47b). Here, the formation of new Sn-S–Ag ternary phase

is identified as a probable reason for the deviation in electri-

cal and optical properties of Ag doped SnS films.

In-doped SnS films with a thickness of 400 nm have been

obtained on glass substrate using CBD technique at a growth

temperature of 45�C.111 The in-situ doping of In at different

concentrations was obtained by the addition of various

amounts of InCl3 4H2O to the bath solution consisting of 1 g

SnCl2 2H2O dissolved in 5 ml of acetone, 10 ml 50% trietha-

nolamine, 4 ml 1M thioacetamide, 5 ml 25% NH3 H2O, and

distilled water. At all doping concentrations, the SnS films

exhibited orthorhombic crystal structure. While increasing the

In-doping concentration from 0 to 5 wt. % the grain size

and surface roughness of SnS films gradually decreased

(Figure 48) and above this, the Sn content of the films

decreased with increasing In-concentration. However, the SnS

films showed slight decrease in optical direct band gap

(1.39 eV) and electrical resistivity (in M Vcm) up to the dop-

ing of 1.5 wt. %, and above this, both the parameters

increased. Recently, Sn-doped SnS films have been obtained

by ex-situ diffusion of Sn by the annealing of thermally evapo-

rated Sn layer on sprayed SnS films55,112 under a vacuum of

10¡5 Torr at 100�C for 30 min. Although the crystal structure

of SnS films remained unaltered with the increase of Sn-dop-

ing concentration, they exhibited a new Sn2S3 binary phase at

higher concentration. Upon increasing the doping concentra-

tion of Sn, the resistivity of the SnS films decreased upto 6 mg

of Sn-diffusion and above this, the resistivity slightly

increased. The direct band gap of the Sn doped SnS films also

remained as constant at 1.33 eV.

5.2.6. Metallization of SnS

Despite the progress on SnS and its devices, there are sev-

eral technological challenges in the development of high-qual-

ity SnS films and fabrication of efficient devices. Among

them, selection and development of compatible, low-resistive,

and thermally stable Ohmic contacts to SnS films is one of the

crucial and challenging tasks. To identify such contacts, it is

desirable to use elements which should easily form inter-

metallic compounds with high melting points after formation

of Ohmic contact. The selected Ohmic contact should also

have good adhesion, smooth surface, and low sheet resistance.

The existing reports about the metallization of SnS structures

are described below.

The contact behavior or metallization of highly sulfur-rich

SnS films grown on ITO substrate by pulsed ECD method has

been studied with different metals including Al, Ag, Au, and

In.113 Here, prior to the deposition of metal contacts, the films

had been annealed at 200�C under nitrogen atmosphere. All

these contacts gave excellent Ohmic electrical contacts with

SnS films between ¡1 and C1 V. However, as compared to

Al contacts, the other contacts have excellent Ohmic behavior

FIG. 45. The FESEM images of (a) as-grown and annealed

SnS films at (b) 100, (c) 150 and (d) 250 �C. (� Elsevier.

Reprinted with permission from Elsevier.108 Permission to

reuse must be obtained from the rightsholder.)

FIG. 46. Variation of resistivity and mobility of SnS films as a

function of RTA temperature. (� Springer Science and Busi-

ness Media. Reproduced with permission from Springer Sci-

ence and Business Media.109 Permission to reuse must be

obtained from the rightsholder.)

30 N. KOTEESWARA REDDY ET AL.

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and also allow high current in the order of mA. Upon anneal-

ing (100–300�C), the resistivity of the films decreased and the

activation energy of SnS films with all contacts is found to be

»0.05 eV. Metallization of SnS thin films grown on glass sub-

strates with different metals (M D Ag, Al, In, and Sn) has

been studied by Devika et al.114 Further, the stability of the

M/SnS structures was examined by RTA annealing at different

temperatures, 300–500�C. The as-deposited SnS films have

slightly tin-rich chemical composition (Sn/S»1.1) and prefer-

entially oriented orthorhombic crystals along the <010>

direction. All M/SnS structures, except Ag/SnS, have excellent

Ohmic contact behavior between §10 V, which indicates that

these M/SnS structures follow field emission (FE) or tunneling

transport mechanism. However, the Ag/SnS contacts exhibited

Ohmic behavior only between §6 V (Figure 49a) and beyond

these voltages, it showed nonlinear trend due to the presence

of bias-dependent interface charges or states present between

Ag and SnS film, which probably control the barrier height

and thereby the current flow across the junction. Therefore,

Ag/SnS structures follow two types of transport mechanisms

depending on applied bias-voltage. At lower voltages (<6 V),

the structures follow FE mechanism, whereas at higher vol-

tages, follow thermionic emission (TE) mechanism, i.e.,

Schottky or rectifying behavior. The resistivity and activation

energy of M/SnS structures are given in Table 6. Here, the In/

SnS structures have shown lower electrical resistivity than

other M/SnS structures.

On the other hand, the M/SnS structures annealed at

various temperatures showed considerably better Ohmic

behavior. However, while increasing annealing temperature

(AT), In/SnS and Sn/SnS structures exhibited degradation

in their electrical properties due to re-evaporation of metal

layers. The Ag/SnS structures showed an excellent

improvement in their Ohmic trend with the increase of AT

(Figure 49b), which is probably due to the diffusion of Ag

atoms into SnS crystal lattice. Thus, the RTA treated Ag

metal contacts at higher temperatures could be also useful

for SnS based devices since they exhibited better and sta-

ble properties as compared to other M/SnS structures.

More recently, Mathews has studied the electrical proper-

ties of SnS films using Al contacts.115 Here, the SnS films

were deposited on florin doped tin oxide substrate by

pulsed ECD method with a thickness of 410 nm. The as-

deposited SnS films exhibited two types of carrier transport

mechanisms such as diffusion and recombination with Al

contacts and also had two-shallow traps activation energies

of 0.14 and 0.27 eV.

6. DEVICE APPLICATIONS OF SnS

SnS semiconductor is a potentially useful material for a

variety of applications including photovoltaics, optoelectron-

ics, chemical sensors, soli-state batteries, holographic, etc.,

due to its unique properties like direct band gap, high absorb-

ability, tunable electrical properties, thermal and chemical sta-

bility, and layered structure. Even though the basic concepts

of SnS are well understood at this time, some practical con-

straint including a lack of proper methodology to produce

highly conductive structures have hampered its potentials and

advantages. In this section, we review photovoltaic and other

applications.

FIG. 47. Variation of (a) resistivity and (b) energy band gap

of SnS films as a function of Ag-concentration. (� The Elec-

trochemical Society. Reprinted with permission from The

Electrochemical Society.110 Permission to reuse must be

obtained from the rightsholder.)

FIG. 48. SEM images of (a) undoped and (b) 0.015 g doped

SnS films. (� Shanghai University. Reprinted with permission

from Shanghai University.111 Permission to reuse must be

obtained from the rightsholder.)

REVIEW ON TIN SULFIDE MATERIAL 31

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6.1. Photovoltaic Applications

SnS came into limelight as a candidate mainly for the

development of solar cell devices due to its narrow band gap

and high absorption. In this direction, variety photovoltaic

devices have been realized by using p-SnS films as absorber

layer and other n-type materials as window layers, which are

discussed below.

Noguchi et al. prepared SnS films-based photovoltaic (PV)

devices with the configuration of n-CdS/p-SnS on indium

doped tin oxide (ITO) substrate with Ag as Ohmic contacts.

All the layers were developed using thermal evaporation

method, and the schematic diagram of the device is shown in

Figure 50a.56 Under dark, the as-deposited devices exhibit a

high diode quality factor of 3.5 and saturation current density

of 2.83 £ 10¡4 A cm¡2. These devices showed good photovol-

taic performance (Figure 50b) under the illumination of

100 mW cm¡2 light and exhibited a short-circuit current (Isc)

of 7 mA cm¡2, open-circuit voltage (Voc) of 0.12 V with a fill

factor of 0.35. The observed photoconversion efficiency is of

about 0.29%. Ristov et al. realized SnS based solar cells by

using two different materials as window layers and Schottky

contact namely cadmium oxide (CdO), cadmium tin oxide

(Cd2SnO4), and SnO2:F with the configuration of CdO/SnS/

Ag, Cd2SnO4/SnS/Ag, and SnO2:F/SnS/Ag. The device photo-

voltaic characteristics were studied under the illumination of

day light, 50 and 100 mW cm¡2 as shown in Figure 51a–b.116

The open circuit voltage of the devices developed with CdO

and Cd2SnO4 window layers is higher than the other devices,

probably due to the possible formation of very thin interlayer

of cadmium sulfide (CdS). As compared to other devices, the

devices developed with SnO2:F electrodes showed good pho-

toresponse in ultraviolet region, and exhibited a maximum

sensitivity at 520 nm (Eg D 2.4eV).

Sanchez-Juarez et al. have fabricated SnS2/SnS solar cell

devices and estimated their photovoltaic conversion efficiency

under the illumination of 70 mW cm¡2 light.116 Here, thin

films of SnS2 and SnS have been deposited onto the transpar-

ent conductive oxide (TCO) substrate by plasma enhanced

chemical vapor deposition (PECVD) method with a thickness

of 0.15 and 0.35 mm and then, the aluminum contacts were

deposited by thermal evaporation with diameter and thickness

of 0.2 cm and 0.8 mm. The final configuration of the device is

glass/TCO/n-type SnS2/p-type SnS/Al, shown in Figure 52a.

Under dark, these devices exhibited good rectification behav-

ior and the ratio between forward and reverse bias currents

within the range of applied voltages of §1.0 V is found to be

>300. These devices also showed a reverse saturation current

of about 3.7 £ 10¡7 A and diode factor of 2.7. Under illumina-

tion, a typical device exhibited photovoltaic effect (Fig-

ure 52b) with Voc D 0.35 V and Isc D 1.5 mA cm¡2.

Miyawaki et al. developed ZnS/SnS solar cell devices by

depositing ZnS films on 0.6 mm thick p-type SnS thin films

using photochemical and electrochemical deposition methods,

respectively.117 The as-deposited device showed (Figure 52c)

good rectifying behavior and under 100 mW cm¡2 light these

structures exhibited Isc and Voc as 0.95 mA cm¡2 and

135 mV, respectively. The same group have also developed

SnS based solar cell devices by using CdS and Cd1-xZnxS films

as window layers and studied their PV performance under the

illumination of 100 mW cm¡2 light.118 The PV conversion

FIG. 49. Current vs. voltage plots of M/SnS (M D Ag, Al, In,

Sn) (a) as-grown structures measured at room temperature,

and (b) Ag/SnS structures measured at different temperatures.

(� AIP. Reprinted with permission from AIP.114 Permission

to reuse must be obtained from the rightsholder.)

TABLE 6

The evaluated electrical resistivity and activation energy of M/

SnS structures (� AIP. Reprinted with permission from

AIP.114 Permission to reuse must be obtained from the

rightsholder)

Metal Resistivity (Vcm) Activation energy (eV)

Ag 25.71 0.31

Al 16.78 0.17

In 4.81 0.22

Sn 8.5 0.16

32 N. KOTEESWARA REDDY ET AL.

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efficiency of CdS/SnS cells grown under optimized conditions

showed as 0.22%, whereas the cells developed with a configu-

ration of Cd0.87Zn0.13S/SnS exhibited 0.71%, which can be

seen from Figure 53.

SnS-based solar cell devices have been developed by

Avellaneda et al. using CdS as window layer and also pre-

pared cells by using CuS and Cu2SnS3 as buffer layers.119

The performance of these devices has been measured under

the illumination of 85 mW cm¡2 light, and the obtained

results are shown in Figure 54. Here, the SnS, CdS, CuS,

and Cu2SnS3 films were deposited with a thickness of

400–500, 100, 50–100, and 100 nm, respectively by

chemical bath deposition method. The configurations of all

the cells and their photovoltaic responses are given in

Figure 54. From the simplest CdS/SnS device they

observed Voc as 380 mV and Jsc as 0.05 mA cm¡2. The

cells developed with an additional absorber layers i.e. CuS,

which was heated at 473 K in air for 10 min in order to

reduce the CuS thin film resistance, showed a Voc D310 mV and Jsc D 1 mA cm¡2. Finally, Cu2SnS3 layer

grown CdS/SnS device, which was developed by annealing

the 100 nm CuS layer coated CdS/SnS structures in a

nitrogen atmosphere at 315�C for 1 h, showed photovoltaic

characteristics of VocD 340 mV and Jsc D 6 mA cm¡2.

Here, the performance of CdS/SnS/ Cu2SnS3 device is

comparatively better than that of other cells. Ghosh et al.

developed solar cell devices by evaporating CdS and SnS

materials onto ITO substrate with the thickness of 0.3 and

1 mm, respectively.120 Here, the device properties were

investigated with post chemical treatment of the window

material by CdCl2 and their I–V characteristics were mea-

sured under dark and illumination of 100 mW cm¡2 light,

which are shown in Figure 55a–c. The efficiency of these

devices with and without window layer chemical treatment

is found to be 0.08% and 0.05%, respectively. Further, as

compared to untreated layers, the devices treated with

CdCl2 exhibited a high absorptivity particularly in visible

wavelength range, 400–700 nm. This group also developed

SnS/ZnO cells using thermal evaporation and electrodepo-

sition methods, respectively, and observed good photovol-

taic conversion efficiency of 1.29%.121

Nanocrystalline tin sulfide (SnS) based solar cell device has

been developed by Wang et al. using TiO2 as window layer,

and their photovoltaic behavior were studied.122. The devices

showed an open-circuit voltage (Voc) of 471 mV, a short-cir-

cuit current density (Jsc) of 0.3 mA cm¡2 and the conversion

efficiency (h) of 0.1% under 1 sun illumination, as shown in

Figure 55d. Stavrinadis et al. developed solar cell devices by

using 140 nm thick SnS and lead sulfide (PbS) NP films with

the configuration of ITO/SnS/PbS/Al (Au).123 A device made

with Au contacts shows an open circuit voltage Voc D 0.2 V

and short circuit current Isc D 4.2 mA cm¡2 with an overall

FIG. 51. Current-voltage characteristics of (a) CdO/SnS, and (b) Cd2SnO4/SnS PV cells under different illuminations. (� Elsev-

ier. Reprinted with permission from Elsevier.116 Permission to reuse must be obtained from the rightsholder.)

FIG. 50. (a) Schematic diagram of SnS based solar cell and

(b) current-voltage characteristics of SnS/CdS device under

illumination. (� Elsevier. Reprinted with permission from

Elsevier.56 Permission to reuse must be obtained from the

rightsholder.)

REVIEW ON TIN SULFIDE MATERIAL 33

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power conversion efficiency of 0.27%. However, the devices

fabricated with Al contacts showed considerably a high Voc of

0.44 V and low Isc of 1.2 mA cm¡2 with a conversion effi-

ciency of 0.17%.

6.2. Other Applications

Apart from the photovoltaic applications, SnS has also

received good attention in different fields including photoelec-

trochemical cells, lithium-ion batteries, infra-red detectors,

etc. Based on its recent advancements a few of them are briefly

described below.

6.2.1. Photoelectrochemical Cells

Polycrystalline tin monosulphide samples were prepared by

passing H2S through an acidic solution of stannous chloride

and their photoelectrochemical behavior were studied in Fe3C/Fe2C, Ce4C/Ce3C, and I2/I¡ redox couples.13 As compared to

other electrolytes (Table 7), the SnS samples showed a high

photocurrent with Ce4C/Ce3C electrolyte, probably due to the

formation of a maximum band bending with this redox system.

From the cyclic voltammetric studies it is revealed that the

SnS remains stable against photocorrosion in the Ce4C/Ce3C

redox couple over a period of 60 h. Further, photoelectro-

chemical (PEC) device was fabricated with the configuration

of SnS/Ce4C, Ce3C/Pt and studied under the light intensity of

12 mW cm¡2. I–V characteristic plot of the device is shown in

Figure 56a and its photoconversion efficiency is found to be

0.63%.

Subramanian et al. also fabricated PEC solar cell devi-

ces using cathodic electrodeposited SnS films with the con-

figuration of p-SnSjFe3C, Fe2CjPt and found a short-circuit

current density of 0.65 mA cm¡2 and open-circuit voltage

of 320 mV under 100 mW cm¡2 illumination.82 The over-

all light conversion efficiency for this device is found to

be 0.54%. The same group has examined the performance

of PEC by fabricating the cells with as-grown SnS films

by brush plating method and vacuum annealed films.11 The

photocurrent versus photovoltage curves of the cells under

illumination of 100 mW cm¡2 light are shown in Fig-

ure 56b. The conversion efficiency of the PEC cell fabri-

cated with as-deposited SnS film is very low, 0.21%,

which is probably due to the high series resistance (Rs) of

200 V and low shunt resistance (Rsh). Upon using annealed

SnS films, the PEC conversion efficiency of the device is

increased to 0.51%. This improved efficiency is attributed

to the decrease in Rs and substantial increase in Rsh value.

Further, a considerable improvement in the conversion effi-

ciency (»0.63%) of the device has been obtained by

FIG. 52. (a) Schematic diagram of SnS2/SnS solar cell, (b) current vs. voltage plot of the device under dark (inset shows its I vs.

V characteristics under illumination) (� Elsevier. Reprinted with permission from Elsevier.117 Permission to reuse must be

obtained from the rightsholder.), and (c) current vs. voltage characteristics of ZnS/SnS heterojunction cell. (� Elsevier.

Reprinted with permission from Elsevier.118 Permission to reuse must be obtained from the rightsholder.)

FIG. 53. Current density vs. voltage characteristics under AM

1.5 for the Cd0.87Zn0.13S/SnS cell (inset shows the schematic

representation of the fabricated solar cell structure). (� Elsev-

ier. Reprinted with permission from Elsevier.119 Permission to

reuse must be obtained from the rightsholder.)

34 N. KOTEESWARA REDDY ET AL.

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photo-etching of vacuum annealed p-SnS films in 0.1 M

FeCl3–FeCl2, 0.05M H2SO4 solution for 30 s. This increase

is attributed mainly to the removal of unwanted material

over the film surface as well as in the grain boundaries by

the etching process.

6.2.2. Solid-State Batteries

Recently, solid-state batteries with a solid electrolyte have

received great concern due to their high reliability and safety.

For example, In/LiCoO2 solid-state cells with Li2S–P2S5 solid

electrolyte exhibited excellent rechargeable performance with

high capacity and excellent cyclability at RT.124 In this regard,

novel tin-sulfide based compounds have considered as an

appropriate negative electrode with a suitable sulfide solid

electrolytes. Among other tin-sulfide compounds, SnS

becomes more prominent due to its excessive electron

exchangeability. As result, there have been a few reports on

the development of SnS-based solid-state lithium batteries,

which are briefly reviewed below.

Hayashi et al. developed SnS-based amorphous electro-

des by mixing 80 mole% of SnS and 20 mole% of P2S5by mechanical milling technique.125 Then, the solid-state

cells with SnS, SnS-P2S5 have been fabricated by using

80Li2S.20P2S5 as electrolyte. Here, P2S5 sulfide used as a

continuous network former between both the materials.

The charge-discharge curves of Li-In/80SnS¢20P2S5 cells

have higher potential than those of Li-In/SnS based cells,

see Figure 57a. The charge and discharge capacities for

first cell are about 1070 and 590 mA h g¡1, whereas for

second cell these are about 720 and 310 mA h g¡1. On

the other hand, the discharge capacity of 80SnS¢20P2S5based cells is 1.5 times higher than that of conventional

graphite anode materials used in commercialized lithium

ion secondary batteries. Thus, the addition of P2S5 to SnS

strongly enhances the reversible capacity of the cells. The

cycling performance of measured discharge capacity for

two typical cells is shown in Figure 57b. The discharge

capacity of both the cells gradually decreased with increas-

ing of cycling number. However, even after 50 cycles

these cells showed considerably high capacity of about

400 and 270 mA h g¡1. The same group also fabricated

Li-In/67SnS¢33P2S5 and studied its performance by using

80Li2S.20P2S5 as electrolyte.126 The charge capacity of

the cell with the 67SnS.33P2S5 glass is found to be

1070 mA h g¡1. The better performance of the

67SnS¢33P2S5-based cell is probably attributed to the

establishment of continuous LiC conduction paths by the

self-formed high LiC ion conductive Li2S–P2S5 matrix

around Sn nanoparticles.

Aso et al. developed SnS nanostructures-based solid-

state cells with the configuration of Li-In/80Li2S3.20P2S5/

SnS (different nanostructures) and their charge-discharge

FIG. 54. Current vs. voltage characteristics of three different

photovoltaic structures of (a) CdS/SnS, (b) CdS/SnS/CuS, and

(c) CdS/SnS/Cu2SnS2. (� Elsevier. Reprinted with permission

from Elsevier.120 Permission to reuse must be obtained from

the rightsholder.)

TABLE 7

Photocurrent observed with different electrolytes for n-SnS (� Elsevier. Reprinted with permission from Elsevier.13 Permission

to reuse must be obtained from the rightsholder)

Electrolyte Photocurrent (mA cm¡2) Eredox (V vs. NHE*)

Ce4C/Ce3C(0.1 M in 0.5 M H2SO4) 312 C1.42

Fe3C/Fe2C(0.1 M in 0.5 M H2SO4) 143.8 C0.71

I2/I¡(0.1 M aqueous) 60.7 C0.31

*NHE-Normal hydrogen electrode.

REVIEW ON TIN SULFIDE MATERIAL 35

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curves under the current density of 0.13 mA cm¡2 at 25�Care shown in Figure 58.127 Here, all the cells were

discharged and then charged in the voltage range from

0–2.0 V (vs. Li). The cells using SnS particles with short

needle, plate, and long needle, respectively, showed the

initial discharge capacities of 1090, 880, and 1000 mA h

g¡1, these are slightly low compared to theoretical capacity

is ca. 1140 mA h g¡1. All the cells exhibited the higher

initial discharge capacity than the cell with mechanically

milled SnS particles with micrometer-size, 720 mA h g¡1.

The increase of the initial discharge capacity with the all-

solid-state cells using prepared SnS particles is attributable

to decreasing the particle size. The initial discharge capac-

ity of the all-solid-state cell using needlelike SnS particles

as an active material was larger than that of the cell with

plate like SnS particles. In addition, the cell using SnS par-

ticles with short needle retained the discharge capacity of

620 mA h g¡1 after 15 cycles under the current density of

0.13 mA cm¡2 at 25�C. This result suggests that the use of

needlelike particles as an active material improves the

solid-solid interface in composite electrodes and forms a

continuous lithium ion conducting paths in composite elec-

trodes. Thus, the one-dimensional structure of needlelike

particles as an active material is probably favorable in all-

solid-state cells.

FIG. 55. Current vs. voltage characteristics of SnS/CdS junction (a) before and (b) after CdCl2 treatment and (c) spectral

response of the treated heterojunction (� Elsevier. Reprinted with permission from Elsevier.121 Permission to reuse must be

obtained from the rightsholder.); (d) Current density-voltage (J–V) plot of the SnS/TiO2 cell in the dark and under 100 mW

cm¡2 illumination (the inset by SEM shows that cross-sectional image of the structure based on SnS on TiO2). (� Elsevier.

Reprinted with permission from Elsevier.123 Permission to reuse must be obtained from the rightsholder.)

FIG. 56. (a) Output power curve of the n-SnS electrode under

the incident light intensity of 12 mW (� Elsevier. Reprinted

with permission from Elsevier.13 Permission to reuse must be

obtained from the rightsholder.) and (b) plots of power output

characteristics for a typical p-SnS/Fe3C, Fe2C/Pt PEC solar

cell. (� Elsevier. Reprinted with permission from Elsevier.11

Permission to reuse must be obtained from the rightsholder.)

36 N. KOTEESWARA REDDY ET AL.

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7. SUMMARYWITH FUTURE ISSUES

In this article, we have reported a brief review on the

synthesis of SnS material in the form of bulk, thin films,

and nanostructures using different synthetic techniques

(chemical and physical vapor deposition methods) and their

physical properties. A brief description about all these syn-

thetic methods and optimized deposition conditions for the

growth of stiochiometric and single phase SnS structures is

presented. The variation of physical properties of SnS struc-

tures including structure, composition, electrical and optical

properties with the change of growth or deposition parame-

ters are also reported. Finally, we have also highlighted the

recent achievements in SnS structures based device applica-

tions including photovoltaics, photoelectrochemical cells,

and solid-state lithium batteries.

The basic properties of SnS compound reveal that the

crystal structure of SnS is considerably stable over a large

temperature range (4–873 K) as a low symmetric disor-

dered orthorhombic structure. However, with the increase

of surrounding temperature, its structure gradually changes

towards high symmetric-ordered tetragonal structure. The

SnS structures possess both indirect and direct band gaps

but the band gaps of disordered structures are slightly

higher than the ordered ones. These properties strongly

emphasize that SnS structures have become one of the

alternative candidates in the field of science and technol-

ogy as active component for various device applications.

The above discussions show that SnS can be synthesized in

any form including single crystals and thin films (amorphous or

polycrystalline) by using all most all well-established techniques

starting from low-cost and low temperature methods to higher

cost capital intensive techniques. The quality of SnS structures

can also be easily tuned by changing the growth or deposition

conditions of the associated technique. Moreover, without dis-

turbing the crystal structure, the optical and electrical properties

of SnS can be tailored through various external post-treatments

including thermal, chemical and plasma ions routes.

Even though there have been good progress in the synthesis

and characterization SnS structures, several key challenges are

still remain as crucial issues that have to be overcome for the

FIG. 57. (a) Charge–discharge curves of all-solid-state cells

Li–In/80SnS¢20P2S5 and Li–In/SnS at the 1st cycle with

80Li2S¢20P2S5 glass-ceramic solid electrolyte and (b) cycling

performance on discharge capacity for the all-solid-state cells

using the 80SnS¢20P2S5 and SnS electrodes. (� Elsevier.

Reprinted with permission from Elsevier.126 Permission to

reuse must be obtained from the rightsholder.)

FIG. 58. Initial charge-discharge curves of the all-solid-state

cells Li-In/80Li2S.20P2S5/SnS with different morphologies.

(� American Chemical Society. Reprinted with permission

from American Chemical Society.128 Permission to reuse must

be obtained from the rightsholder.)

REVIEW ON TIN SULFIDE MATERIAL 37

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realization of its potential applications in devices. A few

important key challenges about SnS thin films and devices are

given below.

A. The as-deposited SnS films exhibited stable structural

properties along with good chemically stoichiometry and

optical properties. Surprisingly the SnS films developed

by various methods suffer a lot with high resistance, which

is in the order of kilo to mega Ohms. Thus, it is very cru-

cial to realize good quality and low-resistive films for the

development of efficient devices.

B. Although there have been a few reports on the develop-

ment of low-resistive SnS films exclusively with thermal

treatment, still the realized lowest resistance for SnS films

is in the order of kilo Ohms. Therefore, systematic investi-

gations on the realization of low-resistive films have been

taken up by post treatment of SnS films either with heat,

chemical, or plasma treatment.

C. It is well known that by using appropriate substrate and/or

suitable layer deposited substrate, high-quality SnS films

can be developed. Thus, it could be more appropriate to

grow or deposit SnS films on an appropriate buffer layer

coated substrate.

D. An alternative to these methods, low-resistive SnS films

can also be realized by doping of suitable p-type dopants.

E. SnS films crystallized predominantly in two orientations

such as (111) and (010) depending on growth conditions

and have been exhibited different surface morphologies.

Thus, it has to be addressed since the physical properties

of SnS films strongly depend on morphology and crystal

structure, all of which require significant further research.

F. Regards to photovoltaic applications, though it received

considerable attention, still the device performance is very

poor. Thus, further studies are required to improve the cur-

rent density and efficiency. This could be achieved by

adopting good quality and highly conductive SnS films.

FUNDING

Dr. N. K. Reddy wishes to acknowledge CSIR for the sanc-

tion of Senior Research Associateship under the scheme of

Scientist’s pool (No. 13(8525-A) 2011-Pool). Dr. M. Devika

wishes to acknowledge UGC for the sanction of Dr. D.S.

Kothari Postdoctoral fellowship (No. F.4-2/2006(BSR)/13-

703/2012(BSR)). Prof. Gopal wishes to thank Indian National

Science Academy (INSA) for honorary scientific position.

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