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Chapter-2 Thin Film Preparation

Methods for Deposition of SnSe Films

2.1 Introduction

The art and science of the fabrication of thin solid films have an accelerated

growth because of its impact in the electronic devices. The development of number of

thin film devices in which, semiconductor thin films are mainly used, has

revolutionized various fields. The investigations of different parameters governing the

properties of semiconductor thin films lead to manipulation of these properties for the

progress of humankind. The development of thin film technology has led to its

application in diverse fields from microelectronics to optics, space science to aircrafts,

and superconductivity to photovoltaic and to spintronics. Thus, the thin film studies

have directly or indirectly caused the advancement of many new areas of research in

solid-state physics and chemistry and will continue to play increasingly important

roles in the study of a variety of problems of basic and technological importance.

Initially use of thin films was centered on the preparation of anti-reflecting

coating for lenses, multilayer interference filters, automobile headlights and

decorative coatings. Industrialization of thin film technology has been started in the

beginning of the 20th century.

In recent years, thin film science has grown world-wide into a major research

area. The importance of coatings and the synthesis of new materials for industry have

resulted in a tremendous increase of innovative thin film processing technologies.

Currently, this development goes hand-in-hand with the advancements of scientific

and technological breakthroughs in microelectronics, optics and nanotechnology [1,

2]. A second major field comprises process technologies for films with thicknesses

ranging from one to several microns. These films are essential for a multitude of

production areas, such as thermal barrier coatings and wear protections, enhancing

service life of tools and to protect materials against thermal and atmospheric

influences [3, 4]. Presently, rapidly changing needs for thin film materials and devices

25

are creating new opportunities for the development of new processes, materials and

technologies. Therefore, basic research activities will be necessary in the future, to

increase knowledge, understanding, and to develop predictive capabilities for relating

fundamental physical and chemical properties to the microstructure and performance

of thin films in various applications. In basic research, special model systems are

needed for quantitative investigations of the relevant and fundamental processes in

thin film materials science. In particular, these model systems enable the investigation

of nucleation and growth processes, solid state reactions, the thermal and mechanical

stability of thin film systems and phase boundaries. Results of combined experimental

and theoretical investigations are a prerequisite for the development of new thin film

systems and the tailoring of their microstructure and performance.

2.2 Brief out Line of Thin Film Preparation Methods

Thin film technology has become extremely important in recent years due to a

dramatic increase in the industrial usage of thin films with its applications in many

diverse areas and the need for new and improved optical and electronic devices. Thus,

in present day technological development, thin films play an important role. Thin film

deposition techniques have a key role in the fabrication of sophisticated solid state

microelectronic devices. A large number of techniques have been successfully utilized

to prepare solid thin films. However, the methods of preparation of thin films may be

divided essentially into four groups, as given bellow.

Physical vapour deposition(PVD)

Sputtering

Chemical vapuor deposition(CVD)

Chemical deposition

2.2.1 Physical vapour deposition

Physical vapour deposition is the most common technique used for deposition

of metal, alloy and many compound films. The primary requirement for the method is

to achieve a vacuum of 10-5 torr or better. In this method the material evaporates or

sublimates in vacuum due to thermal energy (resistive heating) and then the vapor

stream of atoms or molecules condenses on a cooler substrate so as to form a

26

continuous and adherent deposits of desired thickness [5]. Physical vapour deposition

can be achieved by a variety of methods like thermal evaporation, electron beam

evaporation, radio frequency induction heating, laser beam evaporation, electron gun

heating etc. [6-8]. Multi-component alloys or compounds which tend to distill

fractionally may be evaporated by flash evaporation [9-11]. Some of these methods

are discussed bellow.

2.2.1.1 Resistive heating (thermal evaporation)

Different methods are used for evaporation of materials to be deposited. The

most commonly used method is thermal evaporation technique. Thermal evaporation

of the material can be achieved by a variety of physical methods. The oldest process is

evaporation from a boat or a wire helix sources, which consists of a refractory metal

(tungsten, tantalum, or molybdenum) having high melting point and very low vapor

pressure, are heated by an electric current. Vapor sources of different designs are used

for evaporation depending on the evaporant material have described by many authors

[12-14]. Even though new and modified techniques have been developed, electrical

resistive sources are still used extensively to prepare thin films of elements [15-21],

oxides [22-26], dielectrics [27] and semiconductor compounds [28-34].

2.2.1.2 Electron beam evaporation

The electrons emitted from a tungsten cathode are accelerated by high

voltages are focused on the substance to be evaporated which is kept in a water-

cooled crucible. The kinetic energy of the electrons is transformed into thermal

energy, so that the material in the crucible melts and evaporates into the vacuum.

High purity films can be prepared by this method because crucible materials or

reaction products are practically excluded from evaporation. We can evaporate any

material using this method and rate of evaporation is higher than that of resistive

method.

2.2.1.3 RF heating

Radio frequency can be used to heat the evaporant. By suitable arrangement of

RF coils, levitation and evaporation can be achieved, thereby eliminating the

possibility of contamination of the film by the support crucible.

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2.2.1.4 Laser evaporation

High power, pulsed laser beams can be used for ejecting particles into the

vacuum. The laser source, kept outside the vacuum system, is focused on to the target

material, and the ejected material is deposited on the substrate, placed in front of the

target material inside the vacuum. An advantage of laser evaporation is that because

of its high energy, alloys can be deposited without a change of composition as in flash

evaporation techniques.

2.2.1.5 Flash evaporation

This method is useful in the preparation of multi-component alloys or

compounds that tend to fractionate in the evaporation process. In this method, the

selected material is prepared in powder form with as small a grain size as possible,

and its small quantity is dropped on to a boat that is hot enough to ensure that the

material is evaporated instantaneously. The temperature of boat should be high

enough to evaporate the less volatile material fast. Various methods (ultrasonic,

mechanical, electromagnetic etc.) are used to drop the powder on the boat.

A drawback of this technique is the difficulty in preoutgassing the evaporant

powder. Degassing the powder can be accomplished to some extent by vacuum

storage for 24 - 36 hours prior to deposition.

2.2.2 Sputtering

Sputtering is a process of ejection of atoms from the surface of a material by

bombardment with energetic particles. In this process the substance to be deposited

(target) is kept in the form of an electrode whose surface atoms or molecules are

ejected by momentum transfer from the bombarding ions. The ejected or sputtered

atoms are allowed to condense on a substrate to from a thin film. If the ejection is due

to bombardment by positive ion, the process is known as cathodic sputtering. The ions

required for bombardment is usually obtained by maintaining a glow discharge due to

an applied electric field within the vacuum chamber. Over the years various sputtering

techniques have been developed. The techniques are glow discharge sputtering, triode

sputtering, ion plating, magnetron (direct and reactive) sputtering, ion beam sputtering

reactive sputtering, RF sputtering, etc [35-37].

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2.2.2.1 Glow discharge sputtering

The simplest sputtering arrangement is the glow discharge sputtering system.

In this method an electric field is applied between two electrodes in an inert gas at low

pressure. The plate of the material to be deposited is used as the cathode and the

substrate is mounted on the anode facing the target. When the electric field is applied,

a glow discharge is formed. By the impact of positive gas ions on the target plate, the

atoms of the cathode material will be ejected, which eventually condense on the

substrate as a thin film.

2.2.2.2 Triode sputtering

In this case extra electrons are injected in the discharge by thermionic

emission from an additional hot cathode. The total ionization and the ionization

efficiency are increased by accelerating these electrons. This method is often

combined with a magnetic field.

2.2.2.3 Ion-beam sputtering

In this method ions of the inert gas are produced in a discharge and then

extracted in a separate vacuum chamber and accelerated on to the target. By this

method it is possible to sputter at a pressure even below 10-3 torr, and the

incorporation of the inert gas atoms into the growing film can be reduced.

2.2.2.4 Magnetron sputtering

Magnetron sputtering technology has made significant progress since its

development, for the high rate of the deposition of metal, semiconductor and

dielectric films. In comparison to conventional diode sputtering, magnetron

sputtering, apart from obtaining high deposition rates at lower operating pressures,

makes it possible to obtain high quality films at low substrate temperatures. For a

simple planar magnetic system, a planar cathode is backed by permanent magnets that

provide a toroidal field with field lines forming a closed path over the target surface

(cathode). The secondary electrons generated are trapped in cycloidal on it near the

target and present self-heating of the substrate. The confinement of the plasma and the

resultant intense plasma allow magnetron-sputtering systems to operate at much lower

29

pressures and lower target voltages than are possible for r.f. diode sputtering. Here the

deposition rates are higher and cover large deposition areas.

2.2.2.5 RF Sputtering

This can be used to sputter insulator films directly and also possible to

sputter at low pressures. In this case an r.f. potential is applied to the metal

electrode placed behind the dielectric plate target.

2.2.2.6 Reactive sputtering

Films of oxides, nitrides etc. of metals can be prepared by introducing the

reactive gas along with the inert sputtering gas in to the sputtering system. The main

advantage of this method is the variation of the composition from the pure metal to

the highest reaction product caused only by variation of the pressure of reactive gas.

2.2.3 Chemical vapour deposition (CVD)

The deposition of thin film from gaseous phases by thermal decomposition or

chemical reactions on substrates at high temperature is known as the CVD process.

Sometimes a carrier gas is also introduced either to control the rate of reaction or to

prevent undesired reactions at the prevailing elevated temperature.

It is an important and popular technique for the preparation of thin films of a

wide variety of materials, on various substrates. Films of high purity and quality, with

required composition and doping levels, can be prepared by this method. In this

method, solid films are formed on the substrate, maintained at a suitable temperature,

by the reaction of the constituents in their vapor phase. The chemical reaction itself is

an important characteristic of all CVD processes. Several types of chemical reactions,

such as chemical transfer, thermal decomposition (pyrolysis), reduction, oxidation,

nitride and carbide formation etc. are available to carry out the CVD processes.

2.2.3.1 Photochemical vapour deposition

Photo CVD is a recently developed, low temperature deposition technique for

the preparation of high quality films for many technological applications. In this

process, the photochemical deposition takes place when high energy photons

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selectively excite states in the surface-absorbed or gas phase molecules leading to

bond rupture and the production of free chemical species to form the films or to react

to form compound films on the adjacent substrate. Ultra violet lamps or lasers are the

sources used for the photochemical deposition.

2.2.3.2 Plasma-enhanced chemical vapour deposition

It is a versatile technique for depositing a wide variety of film materials for

microelectronic, photovoltaic, and many other applications. In this process, the

plasma produced by r.f., dc or microwave fields are used to promote the chemical

reactions.

2.2.4 Chemical methods

In chemical deposition method, thin films are deposited on the substrate from

aqueous solution either by passing a current or by chemical reaction under appropriate

conditions. Large or small and even or uneven surfaces of all types – conducting or

insulating can be coated with relative ease by this cost effective method. The electro-

deposition, spray pyrolysis, closed space sublimation (CSS), anodization, solution

growth, etc. are different chemical deposition techniques normally used for the

deposition of thin films [38-40]. These methods used simpler equipments and are

more economical than physical methods. Chemical methods of thin film deposition

include the following.

2.2.4.1 Electrolytic deposition

This is an electrochemical process in which the anode and cathode are mersed

in a suitable electrolyte and the passage of electric current serves to deposit the

material on the cathode. By this method, it is possible to deposit only on metallic

substrates and the film may be contaminated by the electrolyte.

2.2.4.2 Electroless deposition

Electroless deposition is simple, and is possible for large-area deposition from

a solution, in which no electrode potential is applied, unlike electrodeposition. The

rate of deposition depends on the temperature of the bath and the chemical reduction

in some cases needs to be stimulated by a catalyst.

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2.2.4.3 Anodization

This is another electrochemical process, used to deposit oxide films over

certain metals. Here when the electric current is passed, the anode reacts with

negative ions from the electrolyte and forms an oxide coating.

2.3 Choice of Deposition Process

The deposition technique can be chosen depending upon the type of material

and its intended use for various applications which is actually decided by various

parameters characterizing the material. The ideal deposition process of thin film

should meet following functional criteria:

capability of deposition of any material, metal, alloy, simple or complex chemical compounds,

ability to form crystalline or amorphous deposits,

retention of the stoichiometry in compounds,

good growing power,

proper adhesion to different substrates,

low residual stress and concentration of defects (line, point, area or volume)

ability to deposit over a wide range of deposition parameters,

ability to control the structural parameters like residual stress, dislocation density, and

ability to control deposition parameters such as deposition temperature, deposition rate, thickness etc.

All the deposition methods have their inherent advantages and limitations.

Hence the choice of certain deposition technique for is to be made very carefully.

Vacuum evaporation is one of the oldest techniques for thin film deposition.

However, it is still widely used both in laboratory and in industry. This technique

possesses three basic stages-

Generation of vapor from the condensed phase, solid or liquid.

Transfer of the vapor from the source to the substrate in presence of low pressures.

Condensation of the vapor molecules on the surface of the substrate.

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Generally, 10-5 torr pressure is used for normal evaporation. When

evaporation is made in vacuum, the evaporation temperature is considerably lowered

and the incorporation of oxides and other impurities is reduced strongly. Among

vacuum evaporation techniques, the most of the above criteria can be met in the

method of thermal evaporation technique. The simplicity and versatility of vacuum

evaporation to obtain materials of desired qualities particularly for the fabrication of

devices on insulating substrates are some advantageous attributes of thermal

evaporation method. The thermal evaporation technique has been most extensively

used for the preparation of thin layers of photoconductors.

It is well known that the structural and optical parameters of the deposited thin

films are very much dependent on method of deposition. Thermal evaporation method

is the most common method in producing thin films because of its suitability,

reproducibility and high deposition rate. Various deposition parameters are easier to

maintain and control in case of thermal evaporation technique in comparison with

other techniques. This simplicity makes this technique more common and attractive

for thin film deposition.

In this work it was decided to study different properties of SnSe thin films and

their use in Schottky barrier devices. Many thin film preparation techniques have been

reported for the deposition of SnSe thin films by early researchers such as electro-

deposition [41], chemical bath deposition [42], laser ablation [43], hot wall deposition

[44] etc.. N. Kumar et al [45] and R. Indirajith et al [46] have reported thermal

evaporation technique for the deposition of SnSe thin films. Hence, in present

investigations, efforts have been made to deposit thin films of SnSe using thermal

evaporation technique.

2.4 Processes of Formation of Thin Film

The important processes involved in the formation of thin film are

condensation, nucleation and growth on the substrate. Transformation of a substance

from its vapour phase to the liquid or solid phase is called condensation, which in this

case is initiated by the formation of small clusters or nuclei through combination of

several incident atoms adsorbed on the substrate to form finally a coherent film is

termed as growth.

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2.4.1 Condensation

The partial vapour pressure in the gaseous phase should be equal to or greater

than its vapour pressure in the condensed phase at the same temperature for

condensation to occur on a substrate. If the substrate material is different from the

vapour material, the impinging vapour atoms are first adsorbed on the surface of the

substrate and condensation is initiated by the formation of small clusters of the

adsorbed atoms. These small clusters then act as the initial centers of condensation.

In this process the impinging vapour atoms on approaching the substrate

surface within a few atomic diameters come under the influence of the field of force

of the surface atoms of the substrate and may follow several interactions like:

an impinging atom may be reflected almost instantly retaining most of its kinetic energy.

there may be physical adsorption of vapour atoms under influence of Van der Waals force of the substrate atoms.

there may be chemical adsorption due to force of the type forming chemical bond between a vapour atom and a substrate atom.

impinging atom may form immediate association with other atom or atoms already adsorbed after finite stay time, or it may strike to the surface of the substrate and then migrate literally to join a cluster of adsorbed atoms.

If the kinetic energy of an incident atom is less than the energy of desorption,

the thermal accommodation takes place by giving up its excess kinetic energy through

lattice vibration of the substrate and the atom is adsorbed. Under appropriate

conditions, when the rate of adsorption becomes greater than the rate of desorption,

the small clusters (consisting of monomers) begin to enlarge, leading to stable

nucleation and growth. Several workers [47-49] have studied condensation

phenomena experimentally.

2.4.2 Nucleation and Growth

In the process of depositing thin film on a substrate the first stage is the

formation of adsorbed monomers of one or more atoms which in the second phase,

under the action force similar to surface tension, or capillarity combine to form small

clusters. These clusters like the monomers are not stable and likely to be desorbed

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unless they attain a certain critical size. On reaching a critical size, they continue to

grow forming stable condensates. Several theories are proposed to explain the

formation of critical clusters leading to the growth of stable condensates. Out of all

these, the two principal theories are the capillarity model and the atomistic model

[50].

Capillarity model [51] predicts that the free energy of a cluster passes through

a maximum as it increases in size, and this maximum of free energy corresponds to

the critical size of the cluster, which becomes stable above this critical size, but may

re-evaporate below this critical size. This is similar to the process of formation of

liquid droplets from vapours as explained by classical capillarity theory.

Figure 2.1 A Schematic diagram of growth process from vapour phase by different steps.

In the latter process, rate at which the critical nuclei grow is given by the

initial number of nuclei per unit area of the substrate and the rate at which the

adsorbed monomers diffuse to join them. Capillarity model of the theories of

nucleation enables one to evaluate free energy of formation of cluster of critical size

and shows as to how the critical size and nucleation rate depend on the substrate

temperature, impingement flux and the nature of the substrate. The atomistic or small

cluster model of the theories of nucleation [52] is most identical with the capillarity

model. The important prediction to be noted in this theory is that if the size of the

35

critical nucleus changes by even one atom with super saturation, a discontinuity will

occur in the nucleation rate versus temperature curve, and this may be verified

experimentally. Lewis [53], comparing the results of nucleation process derived from

capillarity and small cluster models, shows that the fundamental concepts underlying

the two models are identical. The essential difference between them is that the small

cluster model uses discrete arrangement of atoms where as the capillarity model uses

simple geometrical shapes of the clusters. Experimental tests as to the validity of the

nucleation theories have been made by several workers [54] by different techniques.

The best among them were the direct observations of the cluster formation and their

growth in the electron microscope [55, 56]. The prediction of the nucleation theories,

in general have been found to be true even through relatively crude experimentations.

However, the observations have revealed distinct four stages of film growth

leading to a continuous film as given bellow.

Nucleation and island formation

Coalescence of islands

Channel formation and

Formation of continuous film.

The existence of barriers for nucleation as predicted by nucleation theories

favours the formation of islands in the initial stage of film growth. When the

nucleation barrier is large, a few large islands are formed initially, which persist up to

relatively high average film thickness, resulting in the formation of a coarse grained

film. Under the small nucleation barrier, large number of islands is formed, which

becomes continuous at relatively low average film thickness and finer grained film is

obtained. As the deposition is continued, the islands grow in their sizes and come to

each other. The large islands appear to grow by coalescence of smaller islands and the

island density decreases monotonically. Surface diffusion is predominant for mass

transfer between the islands during coalescence. Shape and area of an island, which

has just been formed by coalescence, change during and after coalescence. According

to Pashley et al. [57] the driving force for the coalescence process is provided by the

change in the surface energy. As the islands continue to grow by coalescence, change

of shape still occurs particularly in the vicinity of the junction of islands and

consequently islands become elongated and flatten to increase surface coverage. They

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join to form a network in which the deposited material is separated by long, irregular

and narrow channels. Occasionally, the channels are uniformly wide, homogeneously

distributed and possess approximately matching curvature over small area. The

channels are the uncovered area of substrate. The final stage of the growth is to fill

these empty channels by considerable deposition and as such this is a slow process. If

the uncovered surface area of the channels is large, secondary nuclei are formed in the

channels with the progress of deposition and they grow coalescence and fill in as

before and the film becomes continuous. The growth sequence discussed above was

first deduced by Andrade [58] and was found in remarkable agreement with electron

microscopic observations [59]. In the case of polycrystalline layers, the coalescing

pairs of islands have randomly distributed relative orientations.

2.5 Factors Affecting the Growth, Structure and Film Properties

Various factors that affect the growth, structure and properties of a deposited

film are nature of the substrate, substrate temperature, source temperature, presence of

gas inside the chamber, annealing, contamination by impurities and presence of

defects on the substrate surface etc. [60]. The temperature of the substrate and

thickness of the film are two very important parameters affecting its properties. The

mobility of the just deposited ad atoms on the substrate surface can be enhanced by

increasing the temperature of the substrates during the deposition process. Higher

mobility of the ad atoms give the ad atoms more chance to grow together in ordered

manner leading to formation of thin films with large crystallites. Low mobility on the

other hand leads to the formation of a film of amorphous nature. If the substrate is the

face of a single crystal, periodic forces of cohesion induce an oriented growth called

the epitaxial growth. Thus, a thin film can be deposited in crystalline, polycrystalline

and amorphous form.

2.6 Requirements of the Thermal Evaporation Technique The complete deposition process of thin films by thermal evaporation

technique can be divided into three major stages as mentioned bellow.

Transformation of solid material, to be deposited, into gaseous form.

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Transport of the created material, to the substrate, from the evaporation source, in the form of vapor stream and

Deposition of the material on the substrate and growth of the film.

In reference to these steps, the requirement of the thermal evaporation

technique can be discussed. In the first stage, the transformation of the material takes

place from solid to gaseous state. For example, if this is to be accomplished by

resistive heating, then it is always essential that the material of the evaporation source

(i.e. boat) should withstand very high temperatures (which are always greater than the

melting/boiling point of the material to be evaporated). In addition, the power supply

used for such resistive heating should be capable of delivering enough current to the

boat so that it reaches these high temperatures conveniently.

In the second stage, the transport of the gaseous form of the material from the

evaporation source to the substrate takes place. This process can be better

accomplished only if the mean free path of the evaporated gas molecules is greater

than the distance between the evaporation source and the substrate. If this condition is

not fulfilled, then the vapor molecules will be diverted from their normal path due to

collisions with the ambient gas molecules or among themselves. Therefore, it is

always essential that the pressure in the chamber should be reduced substantially.

(The normal desired pressure is always less than 10-5 torr.)

In the third stage, the nucleation and condensation of the vapor molecules on

the substrates takes place to form a continuous film. This process can be optimized if

the rate of incidence of the vapor molecules at the substrate is kept less than the rate

of nucleation. Besides, it is always essential that the rate of impingement of the

ambient gas molecules should be very small compared to that of the evaporated

molecules in order to minimize the incorporation of impurities in the thin film.

2.7 Vacuum Coating Unit

The most important factor in thermal evaporation technique is the high

vacuum, which can minimize the interaction between the residual gases and the

sensitive surface of growing films. For preparation of thin films the vacuum pumps

capable of creating pressures around 10-6 torr are required to be connected to the

system. For this purpose, vacuum coating unit model: 12A4D (Make: Hind High

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Vacuum Co. Ltd. Bangalore) was used. The schematic diagram and photograph of the

deposition system is shown in figure 2.2. The system comprises of conventional

diffusion pump backed by a direct drive rotary pump of pumping speed 300 lit/sec.

The diffusion pump is of diffstack type with liquid nitrogen trap, water cooled

chevron and has a pumping speed of 500 lit/sec. The ultimate vacuum achievable by

the system is ~ 10-7 torr, which is measured at various stages by Pirani and Bayard-

Alpert gauges. The main gadgetries in the work chamber consist of thermal

evaporation sources and flash evaporator assembly with high tension (HT) ion

cleaning facility. It also consists of rotary substrate holder for obtaining uniform

thickness over large substrate area. Operating roughing, backing and baffle valves and

pouring liquid nitrogen in the trap in a proper sequence is required for attaining

ultimate vacuum in minimum time. The vacuum chamber after loading the substrates

to be coated may be allowed to follow ion-cleaning process. For this, the required

level of vacuum is about 10-2 to 10-3 torr. It is desirable to heat the system up to

around 250°C for about half an hour to remove the impurities adsorbed on to the walls

of the chamber and then flushed by an inert gas before going to high vacuum, for

which a high purity argon gas cylinder can be connected to the main chamber and the

flow can be controlled by a needle valve.

This is because of the fact that contamination free and chemically non-

interacting processes are necessary to be pursued particularly in case of

semiconductor devices, the vacuum system after achieving the ultimate vacuum is

normally isolated from the pumping system and then the thin film deposition process

may be initiated. The vacuum level can be monitored by Pirani gauge and ultimate

pressure is measured with the help of Bayard Alpert ionization gauge for better

accuracy.

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Figure 2.2 Schematic and Photographs of vacuum coating unit (Model: 12A4D).

The thickness of the deposited films can be measured using thin film

deposition controller (INFICON’s SQC-310). The supply of power to the evaporation

source and the rate of deposition may be controlled automatically by the deposition

controller. Here as an evaporation source, rectangular tungsten/tantalum boats were

used because its melting point (3410 °C / 3020 °C) is very high compared to melting

point (861°C) of the material, i.e. tin selenide, to be evaporated in present

investigations. Bringing an externally accessible shutter between the source and the

substrate may control thickness at any stage of deposition. The complete unit is fitted

with a number of various safety interlocks for electrical connections, water flow and

proper pressure levels.

2.8 Substrate Selection

Suitable supporting material (substrate) is required for the deposition of thin

films. The surface of the substrate plays a major role in the nucleation and growth

process of the film and thus influences the thin film properties considerably. Idealy

substrate should have the following requirements [61, 62].

The surface should be flat and smooth.

High mechanical strength

High resistivity

High thermal conductivity.

40

To minimize thermal stress nearly same coefficient of thermal expansion with that of the film and

Low cost.

Glass is most widely used as substrate material for deposition of

polycrystalline films. In the present study microscopic non conducting glass slides of

thickness 1.35 mm (Sunbeam Glass Works, Vyara-India) were used as substrate

material due to their fulfillment of most of the requirements as good substrates.

2.9 Substrate Cleaning

For proper adhesion of the films and for producing reproducible film

properties, the substrates should be highly cleaned and uncontaminated. Usually the

fine dust particles due to packaging, fingerprints and sticking of different impurity

atoms are the common contaminants. The removal of these contaminants by different

cleaning techniques depends upon the nature of the substrate and the type of

contaminants. The chemical reagents, such as acids, alcohol or alkalis with proper

concentrations remove the contaminants by breaking the bonds between the

contaminant molecules as well as between the contaminants and substrate. Acid

converts oxide layer, if any, and greases into water-soluble compounds. The

ultrasonic cleaning is another recommended process for removing gross contaminants

such as greasy particles and fingerprints. This procedure enhances the dissolution of

residues sticking on the substrate by the intense local stirring action by the shock

waves created in the solvent. Here the glass substrates were first cleaned by ultrasonic

vibrations followed by through washing with detergent and acetone. The drying of

cleaned wet substrates is also critical because of probable recontamination due to

adsorption of gaseous particles and dust. So, necessary precautions were taken in

drying the substrate in an atmosphere free from any air borne contaminants. For that,

the cleaned substrates were kept in a clean pettry dish and then pettry dish containing

substrates was put inside a cleaned closed stainless still oven for drying for an hour at

373 K.

2.10 Substrate Heating The radiant heater fitted above the substrate holder assembly was used to raise

the temperature of the substrate during deposition to any desired value up to 500◦C

41

(called substrate temperature). The power to the radiation heater was supplied and

controlled from outside through an autotransformer. The temperature was measured

with the help of a copper-constantan thermocouple which was connected to digital-

temperature controller (Model: DTC-07 DP, ASABA). By this method the substrate

temperature could be raised up to 573 K.

2.11 Thickness Measurement

For in-situ measurement of thickness of the films, it is required to know the

rate of deposition of the material, which can be known by using crystal oscillator. The

thickness of the film can also be measured even after removing the film from the

vacuum coating unit by mainly two methods. In the first method, the mass of the

deposited material is measured, which, along with the known density of the material,

is used to estimate the thickness while in the second method, the multiple beam

interference pattern is used for exact estimation of the thickness.

The method of in-situ measurement of thickness allows both monitoring and

controlling the deposition rate and film thickness. This is one of the most appropriate

methods which have been used in most of the vacuum system. In present

investigations, the control and estimation of the thickness has been achieved by using

the thin film deposition controller (Model: INFICON’s SQC-310 C) connected with a

quartz crystal sensor fitted in the vacuum system using a flexible feed through.

The use of quartz crystal oscillator to determine small quantities of deposited

material was first explored by Sauerbrey in 1957. This is based on the measurement

of changes in the resonant frequency of the quartz crystal oscillator with loading by

evaporated film when operated in a particular mode of vibration.

Thin film deposition controller INFICON’s SQC-310 used in this investigation

is shown in figure 2.3. INFICON’s SQC-310 series instruments are multi-channel

QCM based deposition controllers. They provide a unique combination of accuracy

and powerful features in a compact, low cost instrument.

The standard SQC-310 can control two evaporation sources using two quartz

crystal sensors simultaneously. Eight process control relays, and eight digital inputs

42

are included to support a broad range of external devices. The number of sensors,

outputs, and digital I/O can be doubled with an optional expansion card.

Figure 2.3 Photograph of SQC-310 deposition controller.

2.12 Preparation of SnSe Thin Films

First of all, the gadgets of the vacuum chamber were cleaned by acetone and

another cleaning process mentioned earlier. After cleaning, the rectangular tungsten/

tantalum boat is fixed between the filament holders from where the high current is

applied to the boat in order to evaporate the material. Then enough quantity of

analytical grade high purity reacted tin selenide (SnSe) powder (99.999%, Alfa Aesar)

is kept in this rectangular boat. High quality, microscopic glass slides are used as

substrates. Substrates were cleaned ultrasonically followed by washing with detergent

and acetone. The cleaned substrates were fixed in the vicinity of quartz crystal sensor

on the substrate holder (with heater) having nearly the same tooling factor for in situ

thickness measurement. Cleaned substrates have been covered by specially designed

masks in order to prepare the thin film sample for different characterizations, for

example whole glass slide was coated for optical characterization where as glass

slides coating with 1 × 1 cm2 area were used for electrical measurements and for the

fabrication of diode. In addition to this, pure NaCl crystals as substrates have also

been placed close to these glass substrates in order to prepare thin film samples for

electron microscopic studies.

43

Then the rough vacuum, around 10-3 torr, in the chamber was achieved with the

help of a rotary pump. During this process, only the roughing valve was opened. Once

this was accomplished, the diffusion pump was switched on. The roughing valve was

closed and the backing valve was opened. After sometime (40 - 45 minutes) the baffle

valve was opened. Before deposition, to have a better adhesion, substrates were heated

at 150◦C for one hour by radiant heater as discussed earlier. When 10-6 torr pressure was

attained in vacuum chamber, the heater connected to the evaporation source was

switched on which in turn slowly heated the source of SnSe to temperatures greater than

the melting point. This allowed the vaporization of SnSe material. The quartz crystal

thickness monitor was switched on before starting the heating process. The thickness of

the deposited films and the rate of evaporation were noted from thickness monitor.

After the whole process was over, the baffle valve was closed and the

diffusion pump was also switched off. When diffusion oil (heater) cooled down to

nearly room temperature, the backing valve was closed. The rotary pump was

switched off and immediately the air inlet valve connected to the rotary pump was

opened to restrict the rotary oil from entering into the system. The chamber was

opened and the deposited films on the glass substrates were taken out with great care

so that they do not get damaged.

Thus, thin films of SnSe at room temperature with 8, 10, 12 and 15 kÅ

thicknesses and thin films of thickness 10 kÅ at different substrate temperatures (308,

338, 368, and 398 K) were deposited on glass substrates using thermal evaporation

technique as mentioned above. Similarly thin films of different stoichiometry i.e.

Sn0.2Se0.8, Sn0.3Se0.7 and Sn0.8Se0.2 were also deposited. These all the films were

studied for their chemical composition and structural, optical as well as electrical

properties which are discussed in subsequent chapters.

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