CHAPTER 3 Synthesis and Characterization of CuS Thin Filmsshodhganga.inflibnet.ac.in/bitstream/10603/34594/9/09_chapter3.pdfsulphides/ selenides thin films. So the author deposited

Chapter 3 92

Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014

CHAPTER 3

Synthesis and Characterization of CuS Thin Films

Chapter 3 93


3.1 Introduction

Thin films is a system in which thin layer of materials are deposited on

substrates by various physical or chemical methods. Thin films helped in

miniaturization of devices or electronic components as well as tailoring properties of

the material as per need. Thin films play a vital role in optoelectronics systems such

as solar energy conversation, dye synthesized solar cell, Schottky barrier devices,

anti-reflecting coating on solar energy collectors, etc. Nowadays thin films made of

metal oxides, metal chalcogenides and transition metal dichalcogenides have

significant optoelectronic properties.

As a member of transition metal chalcogenides, covellite copper sulfide (CuS)

belonging to IB-VIA group has received much attention in recent time due to its

potential applications as solar radiation absorber and as a cathode material in lithium

rechargeable batteries [1-4]. The CuS in the form of thin films has found impending

attention in solar control coating [5], ammonia gas-sensor at room temperature [6],

dye-sensitized solar cell [7], etc. The CuS thin films have been deposited by various

methods such as, physical vapour deposition (PVC) [5], chemical vapour deposition

(CVD) [8], electro deposition (ED) [9], successive ionic layer adsorption and reaction

(SILAR) method [10], chemical bath deposition (CBD) [11], spray pyrolysis [12],

microwave assisted chemical bath deposition (MA-CBD) [13], liquid-liquid interface

reaction [14], chemical vapour reaction [15], atomic layer deposition (ALD) [16],

spray ion layer gas reaction [17], solution growth technique (SGT) [18],

photochemical deposition (PCD) [19], etc.

Literature shows CuS have been deposited by chemical bath deposition

(CBD), but no report of thin films deposition by dip coating technique. Till date dip

coating technique has been used to deposit oxide materials such as, ITO [21], TiO2

[22], ZnO [23], SnO2 [24], Cu1.5Mn1.5O4 [25], etc. but no report of deposition of metal

sulphides/ selenides thin films. So the author deposited CuS thin films by dip coating

technique and compared them with CBD deposited thin films. In this chapter, the

author describes the deposition of CuS thin films by these two low cost simple

techniques - chemical bath deposition (CBD) and dip coating. The entire process of

CuS thin films deposition by these two techniques were done at ambient condition,

thus making them promising, due to simplicity and the capability for cost effective

large area deposition, with no need of any sophisticated instrumentation.

Chapter 3 94


The chemical bath deposition and dip coating synthesized thin films were

comprehensively characterized for, stoichiometry, structure, microstructure, surface

morphology, optical absorption, electrical transport properties, etc. The achieved

results are deliberated in details in this chapter.

3.2 Deposition Methods of CuS Thin Films

Synthesis of copper sulfide (CuS) thin films on glass substrate were carried

out by chemical bath deposition (CBD) and dip coating techniques. In both the

techniques, the A.R. grade chemicals viz copper (II) chloride dihydrate (CuCl2•2H2O)

[Sd Fine-Chem Ltd., Mumbai, India], triethanolamine (TEA) (C6H15NO3) [SISCO

Chemical Pvt. Ltd, Mumbai], ammonia (NH4OH) solution [Chiti-Chem Corporation,

Vadodara, India], sodium hydroxide pellets (NaOH) [Loba Chemie Pvt. Ltd, Mumbai]

and thiourea (NH2CSNH2) [Chiti-Chem Corporation, Vadodara, India] were used

directly without further purification. Here copper (II) chloride dihydrate and thiourea

is the source of Cu+2

and S-2

ions, respectively. Triethanolamine and sodium

hydroxide works as a complexing agent, whereas ammonia solution was used for

adjusting pH of the bath solution to achieve the alkaline medium.

3.2.1 Substrate Preparation

The microscope glass slides of dimensions 76 mm × 26 mm × 1.35 mm [Blue

Ribbon, Microcil Manufacturers, Vyara, Gujarat, India] were used as substrates for

deposition of CuS thin films. The cleaning of the glass slide substrates were done by

boiling it in chromic acid for 1 hr and then kept overnight in the acid. This was

followed by wash with detergent and multiple distilled water wash. Finally the glass

slide was ultrasonically cleaned in methanol then dried in oven for 2 hr at 40 ᵒC before

using for deposition of thin film.

3.2.2 CuS Thin Films by Chemical Bath Deposition (CBD)

In the synthesis of CuS thin film by chemical bath deposition (CBD)

technique, firstly 5 ml of 1M copper (II) chloride solution was vigorously mixed with

4 ml of triethanolamine solution in a 100 ml glass beaker for 5 minutes. Then, 10 ml

of 25% aqueous ammonia solution (NH4OH) was added under continuos stirring for

10 minutes. After that 10 ml of 1M sodium hydroxide pellets was mixed into the

Chapter 3 95


solution in the beaker with continuos stirring for 5 minutes. Ultimately, 6 ml of 1M

thiourea solution was added and stirred for 5 minutes and 65 ml of double distilled

water was added to make the final solution 100 ml in volume (Figure1 (a)). The pH of

the bath solution was found to be 11.5. The glass slide substrate was immersed in the

bath solution and kept vertical in the beaker for deposition. The deposition was

carried out at room temperature. After 4 hrs, the glass slide was remove, rinsed with

double distilled water and allowed air drying (Figure 1 (b)).

(a) (b)

Figure 1(a) Bath Solution and (b) CuS thin film.

3.2.3 CuS Thin Films by Dip Coating Technique

Synthesis of CuS thin films by dip coating technique was done using the same

chemical solution as that of chemical bath deposition. In the dip coating technique

(Figure 2 (a)), the substrate was dipped into the prepared solution and then lifted

vertically from the solution for drying. Here the substrate dipping speed of 5 mmsec-1

and withdrawal speed of 8 mmsec-1

was set. Dip duration in the bath solution was 10

minute and between two successive cycles, the substrate was dried at room

temperature for 2 minute. This whole cycle was repeated for 4 hours having 20 dips.

A good quality and uniform greenish film of CuS was deposited on the glass substrate

shown in Figure 2 (b, c).

Chapter 3 96


(a)

(b) (c)

Figure 2 (a) Schematic diagram of dip coating mechanism (b) experimental unit and

(c) CuS thin film deposited by dip coating.

The CuS thin films deposition mechanism follows the chemical reaction:

CuCl2•2H2O + TEA ↔ Cu(TEA)2+

+ 2Cl- + 2H2O

Cu(TEA)2+

↔ Cu2+

+ TEA

NH4OH ↔ NH4+

+ OH-

(NH2)2SC + OH- ↔ CH2N2 + HS

- + H2O

HS- + OH

- → S

2- + H2O

Cu2+

+ S2-

→ CuS

Chapter 3 97


3.2.4 Gravimetric Method for Thickness Measurement

Thickness of the film is important factor in thin film study. Film thickness

measurement techniques are based on different principles such as from mass

difference, light absorption, interference effect, conductivity, capacitance, etc.

However, the microbalance (gravimetric method) method is easy way to determine

the film thickness. So this method was used for determination of film thickness of

deposited CuS thin films.

This method is based on the thickness determination from the mass of a film

which was deposited onto the substrate. Using a microbalance method a microscopic

glass slide substrate was weighed before and after the film was deposited on it. With

mass M1 for the substrate before deposition and M2 after deposition, the average

thickness t was calculated using the standard density (ρ) of the coated material by

using equation,

t =M

Aρ (3.1)

Where, t is the thickness of the deposited film on glass substrate, M is the difference

between masses M1 and M2, A is the area of the film surface and ρ is the density, here

density of CuS was taken as 4.76 gmcm-3

[26], same as that of the bulk. The obtained

thicknesses of the CuS thin films deposited by chemical bath deposition (CBD) and

dip coating technique is tabulated in Table 1.

Table 1 Thickness of the deposited films.

CuS Thin Films Chemical Bath Deposition Dip Coating

Thickness (μm) 0.20 0.18

3.3 Stoichiometric Composition

3.3.1 Energy Dispersive Analysis of X-rays (EDAX)

The stoichiometric composition of the chemical bath deposited (CBD) and dip

coating deposited CuS thin films were determined with the help of energy dispersive

ananlysis of X-rays (EDAX) using the JEOL 5610-LV at ERDA, Vadodara, Gujarat.

The typical EDAX spectrum of both chemical bath deposition and dip coating

deposited thin films are shown in Figure 3. The elemental analysis shows the presence

of copper (Cu), sulfur (S) along with some other elements. In both spectra Si(K) and

Chapter 3 98


Ca(K) peaks were observed which is due to the glass substrate. The relative average

atomic percentage ratio of Cu: S was about 1 for both the films, determined after

substraction of glass slide substrate elements indicating that the films are mostly

stoichiometric.

Figure 3 EDAX spectra of CuS Thin films.

Chapter 3 99


3.4 Structural Characterization

3.4.1 X-ray Diffraction (XRD)

The structural characterization of CuS thin films were done by X-ray

diffraction (XRD) employing Bruker D2 Phaser AXS, using CuKα radiation in the 2θ

range of 20ᵒ - 80

ᵒ at Charusat University,Changa, Gujarat. Figure 4 shows the XRD

pattern of as-deposited CuS thin films synthesized by chemical bath deposition and dip

coating techniques. The broad hump between 20ᵒ and 40

ᵒ is due to the amorphous glass

substrate. The analysis of X-ray diffractrograms by powder-X software shows that all

the diffraction peaks were indexed to be of CuS phase. Both the synthesized CuS thin

films possessed hexagonal structure with lattice parameters, a = b = 3.79 Å and c =

16.34 Å. These lattice parameters are in good agreement with the reported values of

the SGT deposited thin films; a = b = 3.7918 Å and c = 16.342 Å [6] and standard

JCPDS Card No. 06-0464; a = b = 3.792 Ǻ and c = 16.34 Ǻ. There is no presence of

diffractions corresponding to any other phase nor matches with other JCPDS files of

CuxS (1 x 2) except covellite CuS phase. This means, pure covellite CuS thin

films has been synthesized by using chemical bath deposition and dip coating

deposition techniques.

Figure 4 XRD patterns of CuS thin films.

Other than this, the XRD pattern showed large number of peaks in chemical

bath deposition synthesized thin films than dip coating synthesized thin films. This

Chapter 3 100


states, chemical bath deposition synthesized thin films structure possesses more

planes than dip coating synthesized thin films.

The crystallite sizes of the thin films were determined from the XRD pattern

employing the standard Scherrer’s relation [5, 27],

L = kλ/β2θcosθ (3.2)

Where L is the crystallite size, k is the shape factor which typically has value of unity

for the spherical shape particle; β2θ is the angular line width at half maximum

intensity (radians), λ is the wavelength of CuKα radiation (1.541 Å) and θ is the

Bragg’s angle in degree. The determined crystallite size of chemical bath deposition

and dip coating CuS synthesized thin films was 11 nm and 13 nm, respectively. The

observed crystallite size values are within the range of the reported crystallite size of

10-15 nm, for CuS thin films synthesized by chemical route [28].

3.4.2 Selected Area Electron Diffraction (SAED)

The selected area electron diffraction (SAED) patterns were recorded by

transmission electron microscope (TEM) at SICART, Vallabh Vidyanagar, India.

Both the pattern shows rings, Figure 6, clearly stating polycrystalline nature of the

samples. The d-values (distance between adjacent planes) and the indexing of

diffraction rings were done using the obtained SAED pattern. The obtained plane (1 0

4), (1 0 7), (1 0 8), (2 0 4), (0 0 12), (2 1 3) for CBD method and (0 0 6), (0 0 8), (0 0

9), (2 0 6) for dip coating method respectively matched with the XRD planes.

Figure 6 SAED patterns of CuS thin films.

Chapter 3 101


3.5 Surface Analysis

3.5.1 Optical Microscopy

(a) (b)

Figure 7 Surface microstructures as observed under optical microscope of

(a) chemical bath deposition and (b) dip coating synthesized CuS thin films.

The surface microstructures of the freshly synthesized thin films were

examined under Epignost’, Carl Zeiss Jena GmbH, West Germany optical microscope

at P. G. Department of Physics, Sardar Patel University, Vallabh Vidyangar, Gujarat.

The Figure 7 (a) and Figure 7 (b) shows the surface microstructures of the chemical

bath deposited and dip coating synthesized CuS thin films as seen under optical

microscope, respectively. In both the synthesis techniques it is seen that the substrates

are well covered by the thin films [29]. The dark spots all over the deposited film

surfaces may be the nucleation over growth.

3.5.2 Scanning Electron Microscope (SEM)

The surface morphology of thin films were studied using scanning electron

microscopy (SEM) employing JEOL JSM - 6380 LV at Department of Metallurgy, M.

S. University of Baroda, Vadodara, Gujarat. The SEM micrographs of the CuS thin

films synthesized by chemical bath deposition and dip coating techniques are shown

in Figure 8 and Figure 9 respectively. Various magnified image of the copper sulphide

(CuS) thin films by CBD and dip coating are shown in Figure 8 (a-c) and Figure 9 (a-

c) respectively. These films were uniform and cover the substrate very well. It can be

seen that the thin films were dense, smooth, and homogeneous without visible pores

Chapter 3 102


and spherical in shape. It is clearly seen that the range of the grains size varied from

few μm to nm in copper sulphide (CuS) films deposited by CBD method while in dip

coating, all the grains size were in nanometres (nm) range.

(a)

(b)

Chapter 3 103


(c) Figure 8 SEM images of CuS thin films deposited by CBD method having scale bar

of (a) 5μm (b) 2μm and (c) 1μm.

(a)

(b)

Chapter 3 104


(c)

Figure 9 SEM images of CuS thin films deposited by dip coating method having

scale bar of (a) 5μm (b) 2μm and (c) 1μm.

3.5.3 Atomic Force Microscopy (AFM)

The AFM surface analysis of both the deposited CuS thin films was done by

Nano Surf Easyscan-2 using non contact mode at Charusat University, Changa. The

obtained 2D and 3D images of the films are shown in Figure 10 and 11. AFM analysis

reveals that RMS values (Table 2) of dip coated thin film was small compare to

chemical bath deposited CuS thin film. Thus the AFM study concludes that the

surface quality obtained in dip coating thin films is better than CBD technique.

(a) (b)

Chapter 3 105


(c) (d)

Figure 10 AFM images (a, b) 2D and (c, d) 3D of Chemical Bath Deposited CuS thin

films.

(a) (b)

(c) (d)

Figure 11 AFM images of CuS (a, b) 2D and (c, d) 3D deposited by dip coating

technique.

Chapter 3 106


Table 2 Values of RMS and Peak-Valley height of the deposited thin films.

3.6 Optical Characterizations

3.6.1 UV–Vis–NIR Spectroscopy

Figure 12 Absorbance spectra of CuS thin films.

The optical and solid-state properties of CuS thin films synthesized by

chemical bath deposition and dip coating techniques were studied using the

absorbance spectra obtained employing UV-100-cyberlab Spectrophotometer at

Charusat University, Changa, Gujarat. The absorption spectra of the chemical bath

deposition and dip coating synthesized thin films are shown in Figure 12. In the

chemical bath deposition synthesized thin films, the strong absorption is in the

wavelength range of 325 nm to 600 nm. Whereas in the dip coating synthesized thin

films, the absorption is in the wavelength range of 300 nm to 550 nm. The optical

bandgap of the CuS thin films were determined from the absorption spectra using

Tauc’s relation [30],

Method Range RMS

nm

Peak-Valley height

nm

Chemical Bath Deposition 7.01μm2

14.04 206.19

2.93 μm2 9.32 86.45

Dip Coating 599 nm2

1.36 5.69

202 nm2

1.44 7.93

Chapter 3 107


(αhν)1/n

= A (hν – Eg) (3.3)

Figure 13 shows the plot of (αhν)2 versus hν for both, the chemical bath

deposition and dip coating deposited thin films, respectively. Ideally the graph should

be linear, the deviation from linearity at higher photon energies in case of thin films

synthesized by chemical bath deposition technique can be attributed to the presence of

structural irregularities and imperfections in the thin films.

Figure 13 Plot of (a) (αhν)2 versus hν of CuS thin films.

The direct optical bandgap values of chemical bath deposition and dip coating

synthesized CuS thin films are 2.2 eV and 2.5 eV, respectively. The obtained values

of direct and indirect bandgaps for both the thin films show that the values are in close

conformity to each other. The obtained direct bandgap values of 2.2 and 2.5 eV lies

within the reported direct bandgap range of 2.11 to 2.58 eV for CuS thin films

deposited by different methods viz SILAR, CBD, MA-CBD, SGT [3, 11,13, 18,31-

34].

The optical study of solids does not mean only the physical phenomena such

as refraction, reflection, transmission, absorption, polarization, interference of light

but also the interaction of photon energy with matter and consequent changes in the

electronics state. From reflection, transmission and absorption processes it is possible

to evaluate the optical constants such as refractive index (n), absorption index or

extinction coefficient (k) and absorption coefficient (α) and in turn also the complex

dielectric constant (ε) and optical conductivity (σ0) of a solid. Optical constants and

solid state properties of CuS thin films were studied employing the optical absorption

Chapter 3 108


theory [35]. According to conservation law of energy [36], the relation between

reflectance (R), transmittance (T) and absorbance (A) is,

A+T+R=1 (3.4)

Where, transmittance (T) was measured by T= 10(-A)

[37].

The optical constants, index of refraction (n) and the extinction coefficient (k)

was evaluated by the below relation [36],

n =1+𝑅

12

1−𝑅12

(3.5)

and

k = 𝛼𝜆

4П (3.6)

The variation of optical constants of CuS thin films, like refractive index (n) and

extinction coefficient (k) with incident photon energy (hν) is shown in Figure 14.

Figure 14 Plots of refractive index () and extinction coefficient (k) versus hν of CuS

thin films.

A dielectric constant gives the information regarding how light moves through

materials. A higher value of dielectric constant makes the distance inside the material

looks longer so that the light travels slowly. The complex dielectric constant is given

by [38],

ε = (n + ik )2

= ε r +ε i (3.7)

Where, real dielectric constant (εr) = n2 –k

2 which is the normal dielectric constant and

imaginary dielectric constant (εi) = 2nk represents the absorbance of radiation by free

Chapter 3 109


carrier. The variation of complex dielectric constant with incident photon energy is

shown in Figure 15.

Figure 15 Plots of complex dielectric constant (ε) versus hof CuS thin films.

The optical response of a transparent solid film is given by [36],

Optical conductivity σ0 =𝛼𝑛𝑐

4П (3.8)

where c is the velocity of light. Hence σo is the conductivity at the optical frequency

concerned and is not generally equal to the direct current (DC) or low frequency

conductivity. The optical response of CuS thin films is shown in Figure 16.

Figure 16 Optical conductivity (σo) versus hν of CuS thin films.

Chapter 3 110


3.7 Electrical Properties

3.7.1 Four probe d. c. Resistivity

The d. c. electrical resistivity variation with temperature in the range 301 K to

383 K was carried out on CuS thin films deposited by chemical bath deposition and

dip coating by four probe method. The resistance was measured from room

temperature to high temperature (301 to 383 K). From the measured resistance, the

resistivity at each temperature was evaluated by using the formula

ρ=2πsR (3.9)

Where s = the distance between two probes.

R = the resistance between two probes.

The resistivity data determined from the above equation are plotted as a function

of inverse of temperature and is shown in Figure 17 for deposited thin films. It

clearly shows that the d. c. resistivity values decreases with increase in temperature in

both the cases. This behaviour substantiates the semiconducting nature of the

synthesized CuS thin films.

The activation energy was determined from the plot, Figure 17, using equation,

Ea= 2.303 × kB × slope (eV) (3.10)

Where, kB = Boltzmann constant =8.617×10-5

eV/K.

The activation energies determined from the resistivity versus temperature

curve [39] were 0.0036 eV for chemical bath deposition and 0.0167 eV for dip coating

synthesized thin films. The activation energy in both the synthesized thin films may be

due to two dimensional surface conductivity, impurity and grain boundary scattering

[40]. The low values of activation energies in both thin films are due to uniform and

non porous nature of thin films leading to easy tunnelling of electrons from one island

to other. Gadave et al. [39] reported activation energy values of 0.6160 x 10-3

and 0.69

x 10-4

eV for their CBD CuxS thin films.

Chapter 3 111


Figure 17 Plot of log ρ versus 1/T.

The sheet resistance of the chemical bath deposition and dip coating deposited

thin films were determined by,

Rs=П𝑅

ln 2 (3.11)

Where R is the resistance obtained during four probe measurement.

Figure 18 Sheet Resistance (RS) versus T of CuS thin films.

Chapter 3 112


The sheet resistance (RS) of CuS thin films decreases with increase in

temperature for both the films as shown in Figure 18. The RS decrease with increase

in temperature for both the thin films, further corroborates the semiconducting

behaviour of the CuS deposited thin films. The absolute values of RS in case of dip

coating synthesized thin films are higher than chemical bath deposited thin films. This

may be due to continuous film formation on substrate in case of chemical bath

deposition which enhances the continuity of the films. While in case of dip coating

synthesized thin films, the film growth is layer by layer during each dip of the

substrate in the aqueous solution. Thus the dip coating synthesized thin films has less

conductivity, leading to increase in sheet resistance RS value.

3.7.2 Thermoelectric Power Measurement

The variation of thermoelectric power ‘S’ as a function of temperature in the

temperature range of 303 K to 473 K maintaining a temperature gradient of 5 K was

measured for chemical bath deposition and dip coating deposited CuS thin films. The

thermoelectric power measurement was done using setup Model-TPSS-200, Scientific

Solution, Mumbai, India. The thin film of CuS was kept on the top platform of two-

heaters and two pick-up copper probes were place on the film. The temperature

difference (∆T) between the two junctions could be controlled by a temperature

controller device. The Seebeck coefficient S variation as a function of the reciprocal of

temperature (T-1

) is shown in Figure 19. It clearly states that the absolute value of

Seebeck coefficient S increases with temperature in line with the semiconducting

behaviour. The p-type nature of conduction was confirmed by the positive sign of

absolute values of Seebeck coefficient S. The absolute values of Seebeck coefficient in

case of chemical bath deposited thin films are less than the dip coating deposited thin

films. This variation is due to high value of sheet resistance in case of dip coating

synthesized thin films than that of chemical bath deposited thin films.

Chapter 3 113


Figure 19 Plots of Seebeck coefficient ‘S’ versus 1000/T of CuS thin films.

The Fermi energy (EF) and scattering parameter(s) of CuS thin films were calculated

using standard expression [41] given below,

S = kB/e [A+EF/kB T] (3.12)

The values of Fermi Energy (EF), scattering parameter(s) and scattering

coefficient (A) were determined from the slope and the intercept respectively, for the

CuS thin films deposited by chemical bath deposition and dip coating technique. In the

above equation kB is Boltzmann constant, e is the electronic charge. The determined

values are tabulated in Table 3.

Table 3 Values of Fermi energy (EF), constant (A) and scattering parameter (s) of the

CuS thin films.

Method Fermi Energy

EF (eV)

Values of

constant

A

Scattering

parameter (s)

Chemical Bath

Deposition 0.0682 0.2213 2.2787

Dip Coating 0.4781 1.4401 1.0599

Chapter 3 114


3.7.3 Hall Effect Measurement

The Hall measurement was carried out on CuS thin films deposited by

chemical bath deposition and dip coating in order to evaluate the semiconductor type,

mobility and carrier concentration. The Hall effect measurement at room temperature

employing 1.5 kG electromagnet using van der Pauw technique was carried out on the

Hall set-up Model-DHE-22 using constant current source Model-DPS175 developed

by SES Instruments Pvt. Ltd., Roorkee, India.

The Hall mobility of the thin films were determined by measuring the

change in resistance (∆R), magnetic field (B) which was applied perpendicular to the

sample and thickness (t) of the sample. Hall mobility µ𝐻

is given by the relation,

Hall mobility

R

B

tH

(3.13)

Where, ρ is room temperature resistivity of the sample. The Hall coefficient RH and

the carrier concentration (p) were calculated using the formula,

Hall coefficient, HHR (3.14)

Carrier concentration,

HR

p1

(3.15)

The Hall coefficient RH, carrier concentration p, mobility μH and conductivity

type of the CuS thin films were evaluated from Hall Effect measurements, which are

tabulated in Table 4. The positive sign of the Hall coefficients clearly states the thin

films to be p-type in nature substantiating the Seebeck coefficient result of p-type

behaviour, well matched with other reports [3, 7, 11, 12, 16, 28, 30]. The obtained

values of the carrier concentration are in the range ~1017

cm-3

confirming the thin

films to be semiconductor. The obtained carrier concentration values are less in

comparison to the reported values of carrier (hole) concentrations, 1022

cm-3

and 1020

cm-3

for CuS thin films synthesized by reactive RF sputter [42] and spray pyrolysis

[12] processes, respectively. This departure in carrier concentration may be due to the

different synthesis techniques employed.

Chapter 3 115


Table 4 Values of Hall Coefficient (RH), Carrier Concentration (p), Hall Mobility (μH) and

Conductivity type of the CuS thin films.

Method Hall Coefficient

RH

(cm3.coul.

-1)

Carrier

Concentration

p (cm-3

)

Hall Mobility

μH

(cm2.V

-1.Sec

-1)

Conductivity

type

Chemical Bath

Deposition

11.0582 5.76 × 1017

0.3225 p

Dip Coating 31.4661 2.23× 1017

0.5826 p

3.8 Conclusions

The CuS thin films were synthesized on glass substrate by chemical bath

deposition and dip coating technique.

The EDAX analysis of the as-synthesized thin films showed that the films are

near stoichiometric.

The XRD analysis of the deposited thin films showed that the films are single

phase covellite CuS, possessing hexagonal structure with lattice parameters, a

= b = 3.79 Ǻ and c = 16.34 Ǻ in good agreement with the reported lattice

parameters [6] and JCPDS Card No. 06-0464. The values of the crystallite

sizes are in close proximity to the reported values [28].

The surface microstructures of chemical bath deposition and dip coating

synthesized CuS thin films by optical microscope showed that the substrates

are well covered. The SEM analysis showed that the as synthesized thin films

were uniform, homogeneous having no visible pores on the surface.

The AFM analysis showed that the surface quality of dip coating deposited

CuS thin films are better in comparison to the thin films deposited by chemical

bath deposition technique.

The optical absorbance spectra analysis of the chemical bath deposition and

dip coating deposited thin films showed that both the thin films possess direct

optical bandgap. The direct optical bandgap values determined from the

optical absorbance spectra of chemical bath deposition and dip coating

synthesized CuS thin films are 2.2 eV and 2.5 eV, respectively. The values of

direct bandgaps for both the deposited thin films show that the values are in

close agreement to each other and to the reported bandgap values [3, 5, 11, 13,

18, 31-34].

Chapter 3 116


The d. c. electrical resistivity variation with temperature showed that the

resistivity value decreases with temperature confirming the semiconducting

nature of the CuS thin films.

The Seebeck coefficient studies on both, chemical bath deposition and dip

coating synthesized thin films showed that the films are semiconducting and

p-type in nature.

The Hall measurement analysis showed that the Hall coefficients are positive

for both chemical bath deposition and dip coating synthesized thin films,

confirming the p-type nature of the thin films. The carrier concentrations

determined from the Hall measurements came out to be ~1017

cm-3

, stating the

chemical bath deposition and dip coating synthesized thin films to be

semiconducting in nature. These conclusions of Hall Effect analysis

corroborate the results of the d. c. electrical resistivity and Seebeck coefficient

measurements done on chemical bath deposition and dip coating synthesized

thin film.

Chapter 3 117


3.9 References

[1] J. S.Chung, H. J.Sohn, J. Power Sources 108 (2002) 226.

[2] Y. Lou, X. Chen, A. C. Samia, C. Burda, J. Phys. Chem. B107 (2003)

12431.

[3] S. D. Sartale, C. D. Lokhande, Mater. Chem. Phys. 65 (2000) 63.

[4] S. Bordiga, C. Paze, G. Berlier, D. Scarano, G. Spoto, A. Zecchina, C.

Lamberti, Catal. Today 70 (2001) 91.

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CHAPTER 3 Synthesis and Characterization of CuS Thin Filmsshodhganga.inflibnet.ac.in/bitstream/10603/34594/9/09_chapter3.pdfsulphides/ selenides thin films. So the author deposited