CHAPTER 1

INTRODUCTION

1.1 Introduction

The definition of a thin film is one that considers the

contributing role of the surface region to the overall bulk properties

of the material (1). Thin films may be classified as (i) t < 100A,

ultrathin film (ii) t > 10,000A, thick film and (iii) 100A < t <

10,000A thin film. The scientific curiosity about the behaviour of two

dimensional solids has been responsible for the immense interest in

the study of the science and technology of thin films. Thin films

have made tremendous advances in the last decade, because of their

use in microelectronics, optical coatings, integrated optics, thin film

superconductivity, quantum engineering, communication and information

technology and solar cells. Using thin films, higher packing density,

higher speed performance and lower costs are obtained with decrease

in the size of the active electronic devices. In this thesis, a modest

attempt has been made to fabricate high quality thin films of ternary

chalcopyrite compounds viz. Copper Indium diselenide (CuInSe2, CIS),

Silver Indium diselenide (AgInSe2, AIS) and Copper Aluminium

diselenide (CuAlSe2, CAS) and their electrical, optical and structural

properties have been investigated.

1.2 Ternary chalcopyrite compounds

Many of the binary and ternary naturally occurring compounds

are known to exhibit semiconducting behaviour. Hahn et a1 (2) was

the first to synthesize the I-111-VI2 compounds in 1953. Goodman

and Douglas (3) discussed the possibility of semiconductivity in these

materials, a year later. CuInSez is one of the most promising

materials for thin film solar celis due to high absorption coefficient,

wide band gap range and easy conversion to n /p carrier type. The

efficiency of solar cells based on Cu(In, Ga)Sez has reached 17% as

reported by Tuttle et a1 (4). Laboratory-scale device efficiencies in

excess of 15% have been reported by several groups (5-7) for the

polycrystalline or amorphous CIS based cells. For integrated modules,

the maximum reported efficiencies are 11.2% for a 938 cm2 device

(8) and 10.4% for a 3883 cm2 array (9).

AgInSe2 is a material of special interest since it is a ternary

analog of CdSe which has been used in a number of electronic

devices. Although Cu-111-VI2 compounds have been studied extensively,

information on Ag-111-VI2 compounds is scarce. AgInSe2 is a direct

gap, n-type semiconductor with an energy gap of 1.19 eV (10) and

a melting point of 770°C. AgInSez is quite sensitive to optical and

thermal stresses (11).

CuAlSe2 is a wide gap member of I-III-VI2 type chalcopyrite

semiconductors and it is a promising material for blue light-emitting

devices. It has a direct band gap of 2.67 eV at room temperature

(12, 13). However, it is difficult to grow high quality Cu-A1-VI2

compounds using conventional methods.

Ternary chalcopyrite crystals are of technological importance,

since they show promise for application in the areas of visible and

infrared light emitting diodes, infrared detectors and solar cells.

1.3 Structure of chalcopyrites

The I-111-VIz (ABC2) chalcopyrite phase possesses the tetragonal

structure, with four formula units per cell (14). The prototype phase

is the mineral chalcopyrite, CuFeS2. Each A- and B- atom is

tetrahedrally coordinated to four C- atoms, while each C- atom is

tetrahedrally coordinated to two A and two B atoms. If the cations

A and B were distributed at random, the cubic zinc blend structure

would result. Thus the chalcopyrite structure is a superlattice of the

zinc blend structure with c/a ratio approximately equal to 2. The

tetrahedral coordination implies that the bonding is primarily covalent

with sp3 hybrid bonds dominating. I-111-VIz compounds can be

regarded as the ternary analog of 11-VI binary compounds. They

can be derived from the binary phases by ordered substitution of

other group atoms so as to maintain an electron-to-atom ratio of

4. The ordered arrangement of metal atoms (or cations) lead to

the formation of tetragonal superlattice. When the zinc blend cell

is redoubled and the atoms of the cation sublattice are replaced

by A and B atoms, the ordered chalcopyrite structure is obtained.

The presence of two cations with different chemical properties introduce

two kinds of structural distortions, one geometrical (tetragonal

distortion) and the other crystallographic (internal distortion) (15).

The quantity 2-c/a is a measure of the tetragonal distortion which

may occur as a result of ordering. Thus the chalcopyrite structure.

is a member of the tetrahedrally coordinated structure family derived

from the well known sphalerite structure by isoelectronic and ordered

substitution of the bivalent zinc atom as given in figure 1.1. Neglecting

the sublattice distortion, all atom positions and bonding directions

are conserved, but the symmetry is reduced from cubic to tetragonal

by doubling the elementary cell. Tablesl.1 and 1.2 list the lattice

parameters and melting points of CuInSe2, AgInSe2 and CuAlSez.

Table 1. I Lattice parameters of I-111-VIz compounds

Ref.

Tablel2. Melting points of I-III-VI2 compounds

Compound

CuInSez

AgInSe2

CuAlSez

Actually, the chalcopyrites are derived from the larger family

of compounds known as the adamantines. The adamantine family of

compounds (21) is made up of those compounds derived from one

or both of the tetrahedrally bonded diamond and lonsdaleite structures.

The four group IV elements C, Si, Ge and a - Sn possess the

structure, which consists of two interpenetrating fcc lattices arranged

so that each atom has four neighbours at the corners of a regular

tetrahedron. Formation of I-111-V12 compounds from tetrahedrally

co-ordinated group IV elements by substitution of elements so as to

maintain the electron-atom ratio equal to 4 is schematically shown

in figure 1.2. In order to explain the electronic structure of CuInSe2

and other compounds, one should take into account (i) the electronic

band structure (ii) the densities of states , (iii) the role played by

Melting point

O c

990

770

1200

Reference

19

19

20

Fig.l.1. a. Sphalerite structure b. Chalcopyrite structure

Group IV elements

Si, Ge

Group I1 - Group VI

ZnSe

Group I - Group I11 - Group VI

CuInSez Fig.l.2. Schematic diagram of formation of

I-111-VI2 compounds from group IV elements

the copper atom d orbitals and (iv) the energy band gap and t h e

valence band splittings. The s and p states give a maximum of 4

valence states for each value of n. When atoms form covalent bonds

in a crystal, we get a set of states directed from one atom towards

its nearest neighbours using hybridized orbitals. The energy in excess

required to form hybridized bonding orbitals is obtained by the

overall decrease in crystal energy. Actually, the number of orbitals

is conserved during hybridization and Ebond < Enon bond < Eorbital.

'ns' and 'np' valence electrons are the principal contributors to the

covalent bond. The presence of d electrons as in copper make the

valence levels mixed. As a result pd hybridization occurs and will

affect the electronic structure resulting in the band gap shrinkage

and minimization of spin-orbit valence band splitting as in CuInSez.

The band gap shrinkage in CuInSez can be assigned to non-ideal

anion position as detected by mismatch in classical atomic radii. The

calculation on electronic charge density for states in the upper valence

band show that for CuInSez, Cu-Se contact appears covalently bonded

with a significant ionic component, whereas the In-Se contact appears

to be non-bonding 1221. The In atom merely fills up the space

without forming a strong bond with the selenium atom. The weaker

In-Se bonding compared to the Cu-Se bond forms substitutions at

the In site rather than the copper site with relative ease. Thus the

valence stoichiometry is controlled by the copper vacancy formation

and subsequent substitution by indium.

1.4 Phase behaviour and defects in I-111-VI2 compounds

In I-111-VI2 compounds, defects are caused by vacancies (empty-

lattice sites), interstitials (atoms present on sites where they should

not be) and site interchanges by atoms (atom A on the site of B).

These defects break up the lattice periodicity which in turn diminish

the carrier mobility. Also the Fermi level is modulated by the

presence of these defects induced by variations in sublattice cation

ordering or due to non-stoichiometry. The ternary compounds show

two types of non-stoichiometry. One is due to the solid solution

and the other involves an excess or deficiency of electrons compared

to the number needed to make all bonds. The former can be reduced

by precipitation of minor phases (CuxSe, InySe) present in the

compound CuInSe2. Defects are formed by the vacancies, interstitials

and inter change of sites. The absence of room temperature mutual

solubility of 12-VI and I-111-V12 type compounds is caused by the

crystallo-chemical difference between the compounds. The 12-VI type

compounds have only n = 2.66 electron/atom, while I-111-VI2 type

compounds have n = 4 electron/atom. The VI anions have more

than 4 neighbours, resulting in a deficiency of electron to form

saturated homopolar sp3 bonds. In addition to the binary extremas

on the In2Se3-Cu2Se tie line, several copper poor phases of CuInSez

have been identified and studied. The compounds are Cu2InqSe7,

Cu3IngSe9, CuIn3Se5, CuIngSes, CusInSe4 (23, 24, 25, 26, 27). Any

phase along with the CuInSe2 phase will produce deviation from the

ideal chalcopyrite behaviour. It should also be noted that the presence

Tablel.3. Assignment of defect levels in CuInSez

V - vacancies i - interstitials

Sample

type

of oxygen plays an important role in the stability of Cu-Se and

In-Se binary secondary phases. The presence of these phases modulate

the activation energy and optical band gap of the chalcopyrite

Ionization energies (meV)

phase. A summary of the possible assignments of donor and acceptor

levels observed in CuInSez for different deviations from molecularity

and valence stoichiometry are given in Tablel.3 from an earlier report

* (28). Thus these defects play an important role in determining the

opto-electronic, microstructural ,and phase behaviour of I-111-VIz

compounds.

1.5. Earlier studies on I-111-VI2 compounds

In crystallography, the diamond structure is one of the simplest

and most symmetrical arrangement of atoms. Only silicon and

germanium crystallize in this structure. Grimm-Sommerfeld rule of

chemical bonding accounts for the semiconducting behaviour, change

in band gap and thermal conductivity of these elements. In ternary

compounds which are semiconducting, the drop in symmetry due to

superlattice formation opens the way for non-linear devices and

interesting optical properties. A detailed study on the chalcopyrite

structure, growth, luminescent studies, non-linear optical properties,

electrical-transport properties of I-111-VI2 and 11-IV-VIz compbunds

has been done by Shay et a1 (14). Zhuze et a1 (19) have given a

systematic investigation on the electrical properties of semiconducting

compounds with the general formula ABC2 and have studied the

general relationships between electrical properties and the chemical

bond nature. Temperature dependence of electrical conductivity of

CuInSe2, bulk carrier mobility, thermoelectric power and band gap

are studied. A detailed study on I-111-VI2 compounds grown under

minimum VI element pressure or annealed under maximum VI element

pressure has been given in an earlier paper (29). Room temperature

electrical properties of ten compounds of I-111-V12 family are discussed.

Band gap value, absorption edge, conductivity type, carrier mobility,

carrier concentration for the compounds are investigated in ,this paper.

In a short communication by Elliot et a1 (30), some electrical properties

of flash evaporated CuInSe2 thin films are reported. The composition,

morphology and crystallographic structure of chemical bath deposited

CuInSez thin films are studied by Padam (31). A detailed study on

the electrical and optical properties of flash evaporated p-CuInSez

thin films is done by Sridevi and Reddy (32). Temperature dependence

of conductivity and optical absorption corrected for background

absorption for CulnSez thin films are also reported here. Structural,

optical and electrical properties of spray pyrolitically deposited CuInSez

thin films are reported by Tembhurkar and Hirde (33). Plot of

conductivity versus temperature, dependence of absorption coefficient a

with photon energy hu and carrier mobility values are also given.

Electrical conductivity studies on both p- and n-type bulk CuInSez

have been done by Parkes et a1 (34). A detailed study on the

vacuum deposited p- and n-type CuInSez thin films has been done

in an earlier work (35). Varela et a1 (36) have made electrical

conductivity studies of polycrystalline thin films of CuInSez. Dependence

of electrical and optical properties of CuInSez thin films on the

composition has been done by Tuttle et a1 (37). The effect of substrate

temperature on these properties. is also investigated. Electrical and

luminescent properties of bulk CuInSez are studied by Migliorato et

a1 (38). The properties are studied on bulk CuInSez grown in various

selenium pressures. Electrical properties of some I-III-VIz chalcopyrite

compounds grown by solid state growth method are reported by

Ashida et a1 (39) in a recent work. In another paper (40), they

report the effect of annealing on the electrical and structural properties

of quasi flash evaporated CuInSe2 thin films. Recently, Schmidt et

a1 (41) report the electrical, optical and structural properties of

co-sputtered CuInSe2 thin film. Variation of activation energy and

band gap with compositions for solution grown non-stoichiometric

CuInSez thin films is investigated by Sharma and Garg (42). Dependence

of Hall parameters on temperature for CuInSez single crystals is

reported by Horig et al (43). Optical ban

CuInSe2 thin films is reported by Neu

work. A detailed study on the electri

properties of some I-III-VI2 compoun

12

vacuum evaporation has been done by Kazmerski et a1 (45). Temperature

dependence of the absorption edge for CuInSe2 has been investigated

by Horig et a1 (46). Structural and optical properties of spray

pyrolitically deposited CuInSe;! thin films are studied. by Agnihotri

et a1 (47). Influence of impurities and free carriers on the optical

properties of CuInSez thin films has been done by Sobotta eet a1

(48). Reflection and transmission spectra of laser deposited CuInSez

thin films are studied by Kindyak et a1 (49). Optical transitions

associated with the conduction band is studied in this paper. Infrared

optical characterization of thermally oxidized CuInSe;! thin films has

been done by Sobotta et a1 (50). The vibrational modes associated

with this compound are studied in detail. Varela et al (51) investigate

the dependence of refractive index and absorption coefficient on

the Cu-In percentage ratio.

Studies on the electrical and optical properties of AgInSe;!

single crystals have been done in detail by Lerner (17). The values

of thermal and optical band gap are given. Electrical and photoelectrical

properties of chalcopyrite semiconductor AgInSe2 are reported by

Navdeep Goyal (11). The optical properties of AgInSez thin films are

given by Abdelghany et a1 (52).

Preparation and properties of single crystal CuAlSe2 is reported

by Honeyrnan (18). Electrical conductivity, optical absorption and

photoconduction are also investigated in this paper. Raman studies

on CuAlSe2 are reported by Chichibu et al (53, 54). Photoluminescence

studies in CuAlSe2 epilayers grown by low- pressure MOCVD on

GaAs substrates are also investigated (55).

More detailed studies and reported values of various parameters

of chalcopyrite films deposited by various methods will be discussed

in the course of the thesis.

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