ugc progress report - new arts, science and …...arts, commerce and science college, ahmednagar and...
TRANSCRIPT
1
Acknowledgement
Report of UGC Research Project
Sanction By
UGC WRO, Pune
Year- 2013-2015
Title of the Project
“Synthesis and Characterization of Nano - Structured Zinc
Selenide Thin Films for Optoelectronic Applications.”
Submitted by
Dr. S. C. Karle Mr. N. T. Shelke
(Principal Investigator) (Co- Investigator)
P. G. Department of Physics
New Arts, Commerce & Science College
Ahmednagar-414001
2
Dr. S. C. Karle PG Dept of Physics New Arts Commerce & Science College Ahmednagar
31st March 2015 The Principal
New Arts Commerce & Science College Ahmednagar
Sub: Submission of documents for minor research project in the subject Physics.
Respected Sir,
I have been sanctioned a total grant of Rs 160000/- (One Lakh Sixty Thousand
only) for the minor research project entitled “Synthesis and Characterization of Nano -
Structured Zinc Selenide Thin Films for Optoelectronic Applications” by vide U.G.C.
letter no. 47-425/12 (WRO) dated 16th March 2013. Out of the sanction grand the sum
of Rs. 135000/- (Rs. One Lakh Thirty Five Thousand) has been received. The audited
consolidated utilization certificate, Audited consolidated statement of expenditure, final
report of the project, asset certificate, accession certificate.
I take this opportunity to submit the audited statement, utilization certificates and
the vouchers for Rs. 185237/- (One Lakh Eighty Five Thousand Two Hundred Thirty
Seven). Thus I have submitted all the vouchers, utilization certificate and audited
statement for the entire grants of Rs. 160000/- (One Lakh Sixty Thousand only). The
amount due is of Rs 25000/- (Rs Twenty Five Thousand only). Kindly accept all the
contents and do the needful.
Yours faithfully
Enclosure: (Dr. S. C. Karle) 1. Utilisation Certificate 2. Statement of Expenditure 3. PCR 4. Assets Certificate 5. Accession Certificate 6. Final project report
3
Acknowledgement
I would like to express my sincere thanks to University Grand Commission for the
award of the grants to undertake this minor research project. The novel scheme initiated
by university grand commission is unique and has helped to ignite the research talents in
the absence of funds among the teaching faculty.
I would like to appreciate the co-operation of my co-investigator, Mr. Nitin T.
Shelke who was constantly involved in the project with useful suggestions and truthful
discussions.
I also acknowledge the inspiration and facility provided by the management of
Ahmednagar Jilha Maratha Vidya Prasarak Samaj, Dr. B. H. Zaware, Principal, New
Arts, Commerce and Science College, Ahmednagar and Prof. V. K. Dhus, Head,
Department of Physics.
I also appreciate the sincere expertise provided by the entire technician in the
characterization process.
Place: Ahmednagar
Date: 14/07/2015 Dr. S. C. Karle
4
CONTENT
CHAPTER Page
I. INTRODUCTION
1.1 Introduction 5
References 8
II. LITERATURE SURVEY
2.1 Introduction 9
2.2 Thin Film Growth Process 11
2.3 Thin Film Growth Modes 14
2.4 Different Thin Film Deposition Methods 15
References 26
III. EXPERIMENTAL SET UP
3.1 Introduction 29
3.2 Influence Of Deposition Parameters On Thin Films Properties 31
3.3 Experimental Details 34
References 37
IV. CHARACTARIZATION
4.1 Introduction 39
4.2 Experimental Details 40
References 44
V. RESULT AND DISCUSSION
5
Chapter – I
INTRODUCTION
1.1 Introduction
Thin film Science has grown world-wide into a major research area. The
importance of coatings and the fabrication of new mixed / alloyed materials for industrial
applications have resulted in a tremendous increase of innovative thin film processing
technologies. Currently, this development goes hand-in-hand with explosion of scientific
and technological breakthroughs in microelectronics, optics and nanotechnology. A second
major field comprises process technologies for films with thickness ranging from 1micron
to several microns. These films are essential for a host of production areas e.g. thermal
coatings and wear protections, enhancing service life of tools and protect materials against
thermal and atmospheric influences. The rapidly changing needs for thin film materials and
devices are creating new opportunities for the development of new processes, materials
and technologies [1].
Nanoparticles and nano structured materials represent an emerging technology
that has an impact on an incredibly wide number of industries and markets. II – VI
polycrystalline semiconducting materials are under increased scrutiny because of their
world wide use in device cost reduction in the era of optoelectronics and photovoltaics. II –
VI semiconductor materials doped with transition metal ions have been widely investigated
recently for a number of electronic and optoelectronic applications [2, 3]. Binary / Ternary
/ quaternary forms of these materials provide a possibility of tailoring their properties and
hence project themselves as important semiconducting materials for future advancements
in view of device fabrication.
6
Semiconducting thin films have been found to be useful in the fabrication of a
number of solid state devices such as thin film transistors, solar and photovoltaic cells and
electroluminescent cells. Thin films can be prepared by various techniques including
vacuum evaporation, spray pyrolysis, sputtering, molecular beam epitaxy, chemical vapor
deposition, solution growth, screen printing and electrophoresis. A great deal of efforts has
been expended in the development of simple and low cost method for the deposition of
thin films suitable for device applications [4, 5]. These simple methods normally produce
polycrystalline films which are considered to be prime candidates for solar cell production.
Interest in developing higher efficiency and environmental friendly energy has
particularly lead to pursue extensive research on development and commercialization of
electrochemical energy storage and converter system such as solar cells. Solar cells will
provide efficient and low emission power generation and will solve traditional energy
supply issue for the next generation.
Solar cells (photovoltaic cells) are an electric device that converts light energy
directly into electrical energy. In this area, the Si solar cells are most popular. The dye
sensitive solar cells (DSSC) are high performance solar cells in the next generation. It is
simple to manufacture and low cost, pollution free. Generation of solar power is being
more efficient by the thin film technology. A thin film solar cell (TFSC) or thin film
photovoltaic cells (TFPV) is made by depositing one or more thin layers of photovoltaic
material on a substrate. Thin films can be produce by various physical and chemical
methods. The spray pyrolysis is the most familiar method. The deposition is done in air
atmosphere by the simple apparatus. The spray pyrolysis thin film deposition (SPTD)
method is a simple and versatile of making thin films and multilayered films. This
7
technique is used to prepare thin and thick films, ceramic coatings and powders. Also it
helps to dope any element in the ratio of required proportion through the solution medium.
During the last decade, a no. of authors has employed spray pyrolysis technique for
the preparation of nanoparticles. Wang et. al. [6] prepared Ni nanoparticles using low
pressure chemical spray pyrolysis technique (CSPT). Eslamian et. al. [7] developed a
mathematica model for the evaporation of the micro and nanosized solution droplets. The
model is used to predict whether the particles are fully filled or hollow. K. Okayamu et. al.
[8] studied the preparation of regulated nanoparticles by means of spray, a drying process
using a colloidal mixture as a precursor. In the CSP technique various parameters like air
pressure, deposition rate, substrate temperature, distance between substrate and nozzle,
cooling rate etc. affect the physical, electrical and optical properties of the thin films.
This work is motivation towards development of cost effect spray pyrolysis system
to study the effect of various parameters on the nanostructure and morphology of the thin
films. V. Senthilnathan et. al. [9] prepares the control system which automatically controls
the spray precursor as per user defined and having large mechanical arrangement. In our
present work, we fabricated an automatic control instrument without any mechanical
arrangement for the formation of their films by CSP technique [10-12].
Our present work reports the observations regarding the effect of substrate
temperature, precursor properties, distance between substrate and nozzle, air pressure, flow
rate of precursor solution. Using the CSP instrument, the ZnSe films have been prepared
and the results obtained have been reported.
8
References:
1. Thin films Science, T. wrangler, Max-Plank Institute Metallforsching, 70174
Stultgart, Germany.
2. A. Gillan, V. V. Fedrov et al, Appl. Phys. Lett. 86 (2005) 1.
3. U. HÖmmerich et al J. Crys. Growth 287(2006) 450.
4. G. Riveros et al, Sol. Energy Mater Sol. Cells 70 (2001) 255.
5. V. Kumar et al, Opt. Mater. 10 (1998) 253.
6. W. N. Wang et. al., Material science and engineering B, 111 (2004) 69.
7. M. Eslamian et. al., Nanotechnology, 17 (2006) 1674.
8. K. Okuyama et. al., Chemical Engineering Sciences, 58 (2003) 537.
9. V. Senthilnathan, S Ganeshan, S. Senthilnathan, M.V. Vasu, V. V. Gandhi, Journal
of Optoelectronics and Advanced, 12 (2010) 2145.
10. R.K. Nkum et al, Mater. Sci. Eng. B, 55 (1998) 102.
11. B. Su et al, Thin Solid Films 102 (2000) 361-362.
12. Mustafa Oztas et al, Materials Letters 61(2007) 343-346.
9
Chapter –II
LITERATURE SURVEY
2.1 Introduction
A thin film can be defined as a quasi two-dimensional material created by
condensing, atomic/molecular/ionic species of matter [1-5]. The fabrication of thin films
on a single crystal substrate is done by the deposition of individual atoms. On the other
hand thick films can be defined in a different way, as a low-dimensional material created
by thinning a three- dimensional material or assembling large clusters / aggregates / grains
of atomic /molecular / ionic species. For making different devices like, electronic devices,
instrument hard coatings, optical coatings, decorative parts etc. thin films have been widely
used for more than a half century. Although the thin film technology is a well-established
materials technology, the demand of twenty first century, for the development of new
materials such as nanostructured materials and/or a man made superlattices; it is still
evolving on a daily basis. Thin film technology is both an old and a current key material
technology. Thin film materials and deposition processes have been reviewed in several
publications and books [1-5].
There are several techniques available for thin films deposition on a single crystal
substrate like thermal evaporation, chemical decomposition, and the evaporation of source
materials by the irradiation of energetic species or photons. In general the growth process
of thin films exhibits the following features:
1. Thin films of all materials created by any deposition technique starts with a
random nucleation process followed by nucleation and growth stages.
10
2. Nucleation and growth stages are dependent upon various deposition conditions,
such as growth temperature, growth rate, the chemistry of the material and the
substrate and their structure.
3. The nucleation stage can be modified significantly by external agencies, such as
electron or ion bombardment.
4. Film microstructure, associated defect structure, and film stress depend on the
deposition conditions at the nucleation stage.
5. The crystal phase and the orientation of the films are governed by the deposition
conditions as well as by the structure of the substrate.
Film composition, crystal phase and orientation, film thickness, and microstructure,
are the basic properties of film, and can be controlled by the deposition conditions. Some
unique features like quantum size effects, impact of strain, consequence multilayer aspects
that cause variety of proximity effects are observed in thin films and cannot be realized in
bulk materials. Thin films have been extensively studied in relation to their applications for
making electronic devices in the latter part of fifties. Wiemer proposed thin film transistors
(TFTs) composed of cadmium sulfide (CdS) semiconducting films in the early sixties and
at the end of sixties the bulk Si-MOS (metal-oxide semiconductor) devices were
successfully developed [6]. In seventies, different kinds of novel thin-film devices were
proposed, including thin-film surface acoustic wave (SAW) devices [7], integrated thin-
film bulk acoustic wave (BAW) devices [8], and thin-film integrated optics [9] and several
other wide variety of thin-film devices were developed. Variety of multi-layered materials
including giant magnetoresistance (GMR) materials have been developed by using
sputtering technology, with its precise, controlled deposition. Prof. K. L. Chopra, one of
11
the pioneers of the thin film deposition technology once commented that, "The thin film
was in past considered as the fifth state of matter next to plasma, since the reliable
materials properties could not be obtained and thin films were considered to be different
from bulk materials", at present the thin films are considered as the first state of matter
[10].
2.2 Thin Film Growth Process
Three major steps that constitute a typical thin-film deposition process are, (i)
production of the appropriate atomic, molecular, or ionic species, (ii) transport of these
species to the substrate through a medium, and (iii) condensation on the substrate, either
directly or via a chemical and/or electrochemical reaction, to form a solid deposit. Based
on the various experimental and theoretical studies a general picture of the step-by-step
growth process is as follows:
• The unit species, on impacting the substrate, lose their velocity component normal
to the substrate (provided the incident energy is not too high) and are physically
adsorbed on the substrate surface.
• The adsorbed species are not in thermal equilibrium with the substrate initially and
move over the substrate surface. In this process they interact among themselves,
forming bigger clusters.
• The clusters or the nuclei, as they are called, are thermodynamically unstable and
may tend to desorb in time, depending on the deposition parameters. If the
deposition parameters are such that a cluster collides with other adsorbed species
before getting desorbed, it starts growing in size. After reaching a certain critical
size, the cluster becomes thermodynamically stable and the nucleation barrier is
12
said to have been overcome. This step involving the formation of stable,
chemisorbed, critical-sized nuclei is called the nucleation stage.
• The critical nuclei grow in number as well as in size until a saturation nucleation
density is reached. The nucleation density and the average nucleus size depend on a
number of parameters such as the energy of the impinging species, the rate of
impingement, the activation energies of adsorption, desorption, thermal diffusion,
and the temperature, topography, and chemical nature of the substrate. A nucleus
can grow both parallel to the substrate by surface diffusion of the adsorbed species,
and perpendicular to it by direct impingement of the incident species. In general,
however, the rate of lateral growth at this stage is much higher than the
perpendicular growth. The grown nuclei are called islands.
• The next stage in the process of film formation is the coalescence stage, in which
the small islands start coalescing with each other in an attempt to reduce the
substrate surface area. This tendency to form bigger islands is termed
agglomeration and is enhanced by increasing the surface mobility of the adsorbed
species, for example, by increasing the substrate temperature. In some cases,
formation of new nuclei may occur on areas freshly exposed as a consequence of
coalescence.
• Larger islands grow together, leaving channels and holes of uncovered substrate.
The structure of the films at this stage changes from discontinuous island type to
porous network type. Filling of the channels and holes results in the formation of a
completely continuous film.
Thus statistical process of nucleation, surface-diffusion controlled growth of the three-
dimensional nuclei, and formation of a network structure and its subsequent filling to give
13
a continuous film, these processes constitute the growth process. Growth stages and the
initial nucleation, depends on the thermodynamic parameters of the deposit and the
substrate surface, can be categorized as (a) island type, called Volmer-Weber (VW) type,
(b) layer type, called Frank-Van der Merwe (FV) type, and (c) mixed type, called Stranski-
Krastanov (SK) type. This is shown in Figure 2.1.
Figure 2.1: Three modes of thin film growth processes.
Island type is the most common growth process, available in almost all practical cases.
Except under special conditions, the crystallographic orientations and the topographical
details of different islands are randomly distributed, so that when they touch each other
during growth, grain boundaries and various point and line defects are incorporated into
the film due to mismatch of geometrical configurations and crystallographic orientations.
14
If the grains are randomly oriented, the films show a ring-type diffraction pattern and are
said to be polycrystalline. Even if the orientation of different islands is the same
throughout, as obtained under special deposition conditions, on suitable single crystal
substrates, a single-crystal film is not obtained. Instead, the film consists of single crystal
grains oriented parallel to each other and connected by low-angle grain boundaries. These
films show diffraction patterns similar to those of single crystals and are called epitaxial
single-crystal films.
2.3 Thin Film Growth Modes
Thin film growth modes in materials as reported in literature can be characterize in
three modes Volmer–Weber (VW) or island growth, Frank–Van der Merwe (FV) or layer-
by-layer growth, and Stranski–Krastanov (SK) or mixed type growth. These growth
mechanisms are shown in Figure 2.1 and described below one by one.
2.3.1 Volmer–Weber or island growth:
Shown in Figure 2.1 (a) occurs when the smallest stable clusters nucleate on the
substrate and grow into three-dimensional island features [11]. One simplistic explanation
for this growth behavior is that the atoms or molecules being deposited are more strongly
bonded to each other than to the substrate material. This is often the case when the film and
substrate are dissimilar materials. There are a few examples of such behavior in the growth
of oxide films on oxide substrates, but this growth mode is typically observed when metal
and semiconductor (i.e., Group IV, III–V, etc.) films are grown on oxide substrates.
2.3.2 Frank–Van der Merwe or layer-by-layer growth:
The opposite characteristics of Volmer–Weber or island growth, however, are
displayed in Frank–Van der Merwe or layer-by-layer growth (Figure 2.1 (b)) which occurs
15
when the extension of the smallest nucleus occurs in two dimensions resulting in the
formation of planar sheets [12]. In layer-by-layer growth the depositing atoms or
molecules are more strongly bonded to the substrate than each other and each layer is
progressively less strongly bonded than the previous layer. This effect extends
continuously until the bulk bonding strength is reach. A typical example of this is the
epitaxial growth of semiconductors and oxide materials. The field of oxide thin film
growth has developed around the ability to control materials through this and other similar
growth modes. Such capabilities have ushered in an era of unprecedented control of oxide
materials down to the single (or even half-) unit cell level.
2.3.3 Stranski–Krastanov mode:
This is the final growth mechanism shown in Figure 2.1 (c) which is a combination
of the layer-by-layer and island growth [13]. In this growth mode, after forming one or
more monolayers in a layer-by-layer fashion, continued layer-by-layer growth becomes
energetically unfavorable and islands begin to form. This sort of growth is fairly common
and has been observed in a number of metal-metal and metal–semiconductor systems.
These different growth modes can be described in more detail with simple thermodynamic
models for the nucleation and growth of film materials.
2.4 Different Thin Film Deposition Techniques
A layer of material ranging from fraction of nanometer to several micrometers in
thickness is known as thin film. The application of thin films in modern technology is
widespread. Semiconductor devices and optical coatings are the main application
benefiting from the construction of thin films. The methods employed for thin-film
deposition can be divided into two parts based on the nature of the deposition process viz.,
physical or chemical. The physical methods include physical vapour deposition (PVD),
laser ablation, molecular beam epitaxy,
gas-phase deposition methods and solution techniques. The gas
chemical vapour deposition (CVD) [1
spray pyrolysis [17], sol
precursor solutions. All possible deposition processes are shown in
Figure 2.2: Different physical and chemical thin film deposition processes.
Vapor deposition technique describes any process in which a solid immersed in a
vapor becomes larger in mass due to transference of material from the vapor onto the solid
surface. The deposition is normally carried out in a vacuum chamber to enable control of
the vapor composition. If the vapor is created by physical means without a chemical
reaction, the process is classified as physical vapor deposition (PVD), if the material
deposited is the product of a chemical reaction; the process is classified as CVD. Many
variations of these basic vapor deposition methods have been developed in efforts to
16
laser ablation, molecular beam epitaxy, and sputtering. The chemical methods comprise
phase deposition methods and solution techniques. The gas-phase methods are
deposition (CVD) [14, 15] and atomic layer epitaxy (ALE)
spray pyrolysis [17], sol-gel [18], spin- [19] and dip-coating [20] methods employ
All possible deposition processes are shown in Figure
Different physical and chemical thin film deposition processes.
Vapor deposition technique describes any process in which a solid immersed in a
vapor becomes larger in mass due to transference of material from the vapor onto the solid
ion is normally carried out in a vacuum chamber to enable control of
the vapor composition. If the vapor is created by physical means without a chemical
reaction, the process is classified as physical vapor deposition (PVD), if the material
e product of a chemical reaction; the process is classified as CVD. Many
variations of these basic vapor deposition methods have been developed in efforts to
and sputtering. The chemical methods comprise
phase methods are
(ALE) [16], whereas
] methods employ
Figure 2.2
Different physical and chemical thin film deposition processes.
Vapor deposition technique describes any process in which a solid immersed in a
vapor becomes larger in mass due to transference of material from the vapor onto the solid
ion is normally carried out in a vacuum chamber to enable control of
the vapor composition. If the vapor is created by physical means without a chemical
reaction, the process is classified as physical vapor deposition (PVD), if the material
e product of a chemical reaction; the process is classified as CVD. Many
variations of these basic vapor deposition methods have been developed in efforts to
17
balance advantages and disadvantages of various strategies based on the requirements of
film purity, structural quality, and the rate of growth, temperature constraints and other
factors.
PVD is a technique whereby physical processes, such as evaporation, sublimation
or ionic impingement on a target, facilitate the transfer of atoms from a solid or molten
source onto a substrate. Evaporation and sputtering are the two most widely used PVD
methods for depositing films. This section described briefly the typical physical and
chemical vapor deposition processes which are commonly used to grow epitaxial and
polycrystalline thin films of transition metal oxides.
2.4.1 Physical Vapour Deposition
2.4.1.1 Vacuum Evaporation:
The thermal evaporation process comprises evaporating source materials in a
vacuum chamber below 10-6 torr and condensing the evaporated particles on a substrate
[21]. In this process, thermal energy is supplied to a source from which atoms are
evaporated for deposition onto a substrate. Heating of the source material can be
accomplished by any of several methods. The simplest is resistance heating of a wire or
stripe of refractory metal to which the material to be evaporated is attached. Larger
volumes of source material can be heated in crucibles of refractory metals, oxides or
carbon by resistance heating, high frequency induction heating, or electron beam
evaporation. The evaporated atoms travel through reduced background pressure in the
evaporation chamber and condense on the growth surface. The deposition rate or flux is a
function of the travel distance from the source to the substrate, the angle of impingement
onto the substrate surface, the substrate temperature TS, and the base pressure.
18
2.4.1.2 Pulsed-Laser Ablation-Based Techniques:
Pulsed laser deposition (PLD) is an improved thermal process used for the
deposition of alloys and/or compounds with a controlled chemical composition. In laser
deposition, a high-power pulsed laser (1 J/shot) is irradiated onto the target of source
materials through a quartz window [22-24]. A quartz lens is used to increase the energy
density of the laser power on the target source. Atoms that are ablated or evaporated from
the surface are collected on nearby substrate surfaces to form thin films [25]. The target
material is locally heated to the melting point, melted, and vaporized in a vacuum. The
laser pulse may also provide photoemitted electrons from the target to make a plasma
plume and the evaporation mechanism may be complex since the process includes the
thermal process and the plasma process. By optimizing various parameters such as ablation
energy, base vacuum level, background oxygen pressure, distance between target and
substrate and the temperature of substrates, one can have desired deposition rate and
structural quality. Advantage of PLD technique is direct monitoring of cell-by-cell growth
by reflective high-energy electron diffraction (RHEED) pattern [26]. Major drawbacks of
this excellent technique are the limited area of uniform deposition and particle/particulate
ejection from the target [27]. To avoid the deposition of the microsized ejections, the
substrates are settled at an off-axis position.
2.4.1.3 Molecular Beam Epitaxy (MBE):
MBE is an example of an evaporative method. This growth technique can provide
film materials of extraordinarily good quality which are ideal for research purposes.
However, the rate of growth is very low compared to other methods, which makes it of
limited use for production of devices. In MBE, the deposition of a thin film can be
accurately controlled at the atomic level in an ultra-high vacuum (10−10 torr). A substrate
19
wafer is placed in the ultra-high vacuum chamber. It is sputtered briefly with a low energy
ion beam to remove surface contamination. This step is followed by a high temperature
anneal to relax any damage done to the growth surface during preparation. The substrate is
then cooled to the growth temperature, typically between 400 and 700 0C, and growth
commences by directing atomic beams of the film material, as well as a beam of dopant
material if necessary, toward the growth surface of the substrate. The beams are emitted
from crucibles of the growth materials which have been heated to temperatures well above
the substrate temperature to induce evaporation and condensation. The film composition
can be properly selected by accurate control of atomic ratio of each metallic electron beam
sources. O'Donnell et al. have used MBE to prepare manganite thin films and investigated
anisotropic magnetotransport [28].
2.4.2 Sputtering Process
In this section the details of sputtering phenomenon such as, the sputtering yield
and its dependence on various factors is provided. Then the types of sputtering
mechanisms have been discussed, and finally describing the complete sputtering system
used for the preparation of manganite thin film.
2.4.2.1 Sputtering Phenomenon:
When a solid surface is bombarded with energetic particles such as accelerated
ions, the surface atoms of the solid are scattered backward due to collisions between the
surface atoms and the energetic particles, as shown in Figure 2.4. This phenomenon is
called back-sputtering, or simply sputtering. When a thin foil is bombarded with energetic
particles, some of the scattered atoms transmit through the foil. The phenomenon is called
transmission sputtering.
20
Figure 2.4: The physical sputtering processes showing the collision process of
incident ions and the target atoms particles.
The word “spluttering” is synonymous with “sputtering.” Cathode sputtering,
cathode disintegration, and impact evaporation are also used in the same sense [29, 30].
Among the sputtering techniques, the simplest one is the DC sputtering. The DC sputtering
system is composed of a pair of planar electrodes. One of the electrodes is a cold cathode
and the other is the anode. The front surface of the cathode is covered with target material
to be deposited. The substrates are placed on the anode. The sputtering chamber is filled
with sputtering gas, typically argon gas at 5 Pa (4×10-2 torr). The glow discharge is
maintained under the application of DC voltage between the electrodes. The Ar+ ions
generated in the glow discharge are accelerated towards the cathode and hits the target
surface and results in the sputtering of the target. Before reaching the target surface the
Ar+ is converted to neutral Ar atom by capturing the stray electron on the surface of the
target or the surrounding. The sputtered atoms from the target then fall on the heated
substrate resulting in the deposition of the thin films. Sputtered atoms are generally
composed of neutral single atoms of the target material when the target is sputtered by
bombardment with ions having a few hundred electron volts. These sputtered atoms are
partially ionized (i.e., a few 1-5 % of the sputtered atoms are ionized) in the discharge
region. The removal of the target atoms by sputtering is initiated by the first collision
21
between incident ions and target surface atoms, followed by the second and third collisions
between the target surface atoms. The displacement of target surface atoms will eventually
be more isotropic due to successive collisions, and atoms may finally escape from the
surface. Depending on the incident ion energy which hit the target, different models has
been proposed for the collision mechanisms in sputtering. The first is the elastic-collision
theory in which the maximum possible energy is transferred in the first collision only. The
second theory is called linear cascade collision theory started in the 1960s by Sigmund,
Thomson, and Wehner on this theory [31-34]. Sigmund assumed that sputtering of the
target by energetic ions or recoil atoms results from cascades of atomic collisions [35].
Another model is probably due to the evaporation at local patches of the target
surface heated by the subsiding cascade similar to thermal evaporation. This process is
considered as a nonlinear cascade collision and/or thermal sputtering, in contrast to the
linear cascade collision model. Prior to the film deposition processes by magnetron
sputtering one needs a bulk form of thin film material generally called “target”. Target is
compactly-pressed sintered disk (as per the shape of the cathode) of metal, alloy or
compound oxides etc. of same chemical stoichiometric as required for the thin film.
Sputtering is an inefficient process, and most of the power input to the system appears
ultimately as target heating; thus these targets are usually mounted on a water-cooled
backing plate which also acts as cathode. The target is fixed on the backing plate by
mounting clips or other mechanical support
2.4.2.2 Magnetron Sputtering:
The basic concept of sputtering is ejection of surface atoms from the target surface
by momentum transfer by bombarding ions. The main advantage of sputtering is that it is a
non-thermal physical process. One of advantage of sputtering is to be utilized for etching
22
process as well as deposition depending on the ion energy. Several variants of the
sputtering process such as (i) DC, (ii) RF, (iii) DC / RF magnetron are used for thin films
fabrication. The schematic diagram of DC and RF sputtering is shown in Figure 2.3.
However, that there are important variants within each category (e.g., DC bias) and even
hybrids between categories (e.g., reactive RF). RF and DC magnetron sputtering
techniques are the popular methods for growing high quality manganite thin film [36].
Figure 2.3: The schematic diagram of DC and RF sputtering system.
Sputtering with transverse magnetic field has several advantages compared to other
sputtering depositions such as low heating of substrate and low radiation damage.
Therefore, magnetron sputtering techniques are suitable for temperature sensitive or
surface sensitive material deposition.
2.4.3 Chemical Deposition Process
2.4.3.1 Chemical Vapour Deposition:
Chemical vapor deposition (CVD) is the process of chemically reacting volatile
compound of a material to be deposited, with other gases, to produce a nonvolatile solid
that deposits atomistically on a suitably placed substrate. It has emerged one of the
powerful techniques of thin film growth. Among the reasons for the growing adoption of
23
CVD methods is the ability to produce a large variety of films and coatings of metals,
semiconductors, and compounds in a crystalline or vitreous form, possessing high purity
and desirable properties. Furthermore, the capability of controllably creating films of
widely varying stoichiometry makes CVD unique among deposition techniques. Other
advantages include relatively low cost of the equipment and operating expenses, suitability
for both batch and semicontinuous operation, and compatibility with other processing
steps. Hence, many variants of CVD processing have been researched and developed in
recent years, including low-pressure (LPCVD), plasma-enhanced (PECVD), metal-organic
(MOCVD) and laser-enhanced (LECVD) chemical vapor deposition. Hybrid processes
combining features of both physical and chemical vapor deposition have also emerged.
MOCVD has presently assumed considerable importance in the deposition of epitaxial
compound semiconductor films. However, the main obstacle of MOCVD for high
temperature superconductor (HTSC) and rare earth manganite oxides is the lack of
thermally stable precursors. MOCVD technique has been utilized to grow different
compositions of thin CMR manganite films [37].
2.4.3.2 Metal-Organic Chemical Vapor Deposition:
Metal-organic chemical vapor deposition (MOCVD) is of great importance for
large scale production of oxide thin films [38]. It is routinely used in the electronics
industry, has excellent film uniformity over large areas, is capable of conformal coating of
arbitrary geometries, can be done at relatively high partial pressures of oxygen, has easy
and reproducible control of film stoichiometry, has relatively high deposition rates, and
allows for multilayer growth, superlattices, and graded compositions [39,40]. MOCVD
works on the principle that one can create a complex organic molecule decorated with the
material desired for thin film growth. By passing an inert gas through a bubbler of a liquid
24
precursor, these molecules are transported to the reaction chamber and passed over a
substrate at high temperature. The heat helps to break the molecules and deposits the
desired material on the surface. One of the biggest challenges for MOCVD growth of
oxide materials is identification of the appropriate metal-organic precursors. Precursors for
materials with high atomic number typically have limited vapor pressure at room
temperature and thus it is essential to heat the bubblers and all the lines in the system to
avoid clogging. This requires careful attention so as to avoid hot spots where premature
deposition might occur as well as cool spots where condensation of the precursor can
occur. In the end, very high quality thin films of oxide materials can be created using this
technique [41].
2.4.4 Solution-Based Thin Film Deposition Techniques:
There are a variety of solution-based approaches for the creation of complex oxide
materials including sol–gel, chelate, and metal organic decomposition [42, 43]. Very
briefly, solution deposition usually involves four steps:
(1) Synthesis of the precursor solution,
(2) Deposition by spin-casting or dip-coating,
(3) Low-temperature heat treatment for drying and/or pyrolysis of organics, and formation
of amorphous films (typically 300–400°C),
(4) High temperature heat treatment for densification and crystallization (anywhere from
600 to 1100 ºC).
Such processes are highly scalable, cheap, and very quick. Great strides have been made in
utilizing such techniques to make high quality and highly oriented films for devices.
25
2.4.4.1 Low-Temperature Aqueous Solution Depositions:
In stark contrast to the previously reported growth techniques, there is a set of
aqueous solution-based deposition techniques that enable the creation of films at lower
temperatures (25–100°C). Processes such as chemical bath deposition (CBD), successive
ion layer adsorption and reaction (SILAR), liquid phase deposition (LPD), electroless
deposition (ED), as well as more modern variants such as photochemical deposition
(PCD), deposition assisted by applied fields, ferrite plating, liquid flow deposition, and
more can be used to create films of oxide materials at low temperatures[44].
2.4.4.2 Spray Pyrolysis Process
A wide variety of thin films has been deposited by applying the spray pyrolysis
technique. Various devices such as solar cells, sensors, and solid oxide fuel cells have been
prepared by using these films. Preparation conditions are mainly responsible for different
properties for such deposited thin films. Most critical parameter which influences the films
roughness, cracking, crystallinity, etc. is the substrate surface temperature. Atomization of
the precursor solution, aerosol transport, and decomposition of the precursor are the
processes mainly involved in spray pyrolysis technique.
For preparing dense and porous oxide films, ceramic coatings, and powders, spray
pyrolysis is the most suitable processing technique. Spray pyrolysis represents a very
simple and relatively cost-effective method, especially regarding equipment cost. In the
glass industry [44] and in solar cell production to deposit electrically conducting electrodes
[45] spray pyrolysis has been used for several decades. Typical spray pyrolysis equipment
consists of an atomizer, precursor solution, substrate heater, and temperature controller.
26
References
1. R.R. Chamberlin, J.S. Skarman, J. Electrochem. Soc. 113 (1966) 86.
2. Z.M. Jarzebski, J.P. Marton, J. Electrochem. Soc. 123 (1976) 199C,299C, 333C.
3. D.S. Albin, S.H. Risbud, Adv. Ceram. Mater. 2 (1987) 243.
4. J.S. Ryu, Y. Watanabe, M. Takata, J. Ceram. Soc. Jpn. 100 (1992)1165.
5. A.Ganguly , S.Chaudhuri, A.K.Pal, J. Phys.D: Appl. Phys. 34 (2001) 506.
6. W. Badawy, F. Decker, K. Doblhoffer, Sol. Energy Mater. 8 (1983)363.
7. J.C. Manifacier, J.P. Fillard, J.M. Bind, Thin Solid Films 77 (1981) 67.
8. B. Drevillon, K. Satytendra, P. Roca, I. Cabarrocas, J.M. Siffert, Appl. Phys. Lett.
54 (1989) 2088.
9. P. Grosse, F.J. Schmitte, G. Frank, H. Kostlin, Thin Solid Films 90 (1982) 309.
10. K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1.
11. S. Kulaszewicz, I. Lasolka, Cz. Michalski, Thin Solid Films 55 (1978) 283.
12. K. Sato, Y. Gotoh, Y. Hayashi, K. Adachi, H. Nishimura, Asahi Glass Co. Ltd.
Rpts. Res. Lab. 40 (1991) 233.
13. M.L. Olvera, A. Maldonado, R.A. Somoza, M. Konagal, M. Asomoza, Thin Solid
Films 229 (1993) 196.
14. C. Agashe, M.G. Takawale, B.R. Marathe, V.G. Bhide, Sol. Energy Mater.17
(1988) 99.
15. W. Badawy, F. Decker, K. Doblhoffer, Sol. Energy Mater. 8 (1983) 363.
16. W.A. Bryant, J. Mater. Sci., 12(7), 1285 (1977).
17. R.N. Ghoshtagore, J. Electrochem. Soc., 125(1), 110 (1978).
18. T. Suntola, Thin Solid Films, 216(1), 84 (1992).
27
19. C.C. Chen, M.M. Nasrallah, and H.U. Anderson, J. Electrochem.Soc., 140(12),
3555 (1993).
20. C.J. Brinker, A.J. Hurd, G.C. Frye, K.J. Ward, and C.S. Ashley,J. Non-Cryst.
Solids, 121(1–3) (1990) 294.
21. S. Armstrong, P.K. Datta, R.W. Miles, Thin Solid Films 126 (2002) 403.
22. C.J. Brinker, G.C. Frye, A.J. Hurd, and C.S. Ashley, Thin Solid Films, 201(1)
(1991) 97.
23. J.C. Manifacier, J.P. Fillard, J.M. Bind, Thin Solid Films 77 (1981) 67.
24. B. Drevillon, K. Satytendra, P. Roca, I. Cabarrocas, J.M. Siffert, Appl. Phys. Lett.
54 (1989) 2088.
25. P. Grosse, F.J. Schmitte, G. Frank, H. Kostlin, Thin Solid Films 90 (1982) 309.
26. S. Kulaszewicz, I. Lasolka, Cz. Michalski, Thin Solid Films 55 (1978) 283.
27. K. Sato, Y. Gotoh, Y. Hayashi, K. Adachi, H. Nishimura, Asahi Glass Co. Ltd.
Rpts. Res. Lab. 40 (1991) 233.
28. K. Belghit, M.A. Subhan, U. Rulhe, S. Duchemin, J. Bougnot, in :Proc. 10th EC
Photovoltaic Solar Energy Conf., Lissabon, Portugal,1991, p. 613.
29. H.R. Paes Jr., L.M.O.C. Pinho, W. Losch, in : Proc. 9th EC Photovoltaic Solar
Energy Conf., Freiburg, Germany, 1989, p. 63.
30. M.L. Olvera, A. Maldonado, R.A. Somoza, M. Konagal, M. Asomoza, Thin Solid
Films 229 (1993) 196.
31. J.F. Guillemoles, D. Lincot, P. Cowache, J. Vedel, Proc. 10th EC Photovoltaic
Solar Energy Conf., Lissabon, Portugal, 1991, p. 609.
32. C. Mazon, J. Muci, A.Sa-Neto, A. Ortiz-Conde, F.J. Garcia, in: Proc 22th IEEE
Photovoltaic Special Conf., 1991, p. 1156.
28
33. G.E. Rike, S.R. Kurtz, P.L. Philipp, L.M. Levinson, J. Appl. Phys. 27 (1985) 5512.
34. T. Minami, H. Sato, H. Nanto, S. Takata, Thin Solid Films 176 (1990) 277.
35. L. Ozawa, Cathodoluminescence, Kodansha Ltd., Tokyo and VCH Publishers, New
York, 1990, p. 255.
30. L.B. Wen, Y.W. Huang, S.B. Li, J. Appl. Phys. 62 (1987) 2295.
31. B. Li, S.R. Morrison, J. Phys. Chem. 89 (1985) 5442.
32. S. Kumar, B. Drevillon, J. Appl. Phys. 65 (1989) 3023.
33. P. Stanstny, R. Kuzel, V. Skacel, J. Less-Common Mater., 164±165 (1990) 464.
34. S.J. Golden, H. Isotalo, M. Lanham, J. Mayer, F.F. Lange, M.Ruhle, J. Mater. Res.
5 (1990) 1605.
35. H. Nakajima, S. Yamaguchi, K. Iwasaki, H. Morita, H. Fujimori, Appl. Phys. Lett.
53 (1988) 1437.
36. R. Singh, J. Narayan, Mater. Eng. B. 7 (1990) 195.
37. W. Kern, B. Tracy, RCA Rev. 41 (1980) 133.
38. M. Ottaviani, S. Panero, S. Morzilli, B. Scrosati, M. Lazzari, Solid State Ionics 20
(1986) 197.
39. K. Nagase, Y. Shimizu, N. Miura, Y. Yamazoe, Appl. Phys. Lett. 61 (1992) 243.
40. J. Zhang, K. Colbow, Appl. Phys. Lett. 58 (1991) 1013.
41. S.K. Deb, Phil. Mag. 27 (1973) 801.
42. I.F. Chang, B.L. Gilbert, T.L. Sun, J. Electrochem. Soc. 122 (1975) 955.
43. P.S. Patil / Materials Chemistry and Physics 59 (1999) 185.
44. D. Perednis & L. J. Gauckler, Journal of Electroceramics, 14, (2005)103
45. B.W. Faughnan, R.S. Crandall, P.M. Heyman, RCA Rev. 36 (1977) 177.
29
Chapter - III
EXPERIMENTAL SET UP
3.1 Introduction
Numerous materials have been prepared in the form of thin films from last decade
because of their potential technical value and scientific curiosity in their physical as well as
chemical properties. They have wide range of applications and extend from micrometer
dots to coatings of several square meters on window glasses. A large number of techniques
have been examined for the search of most reliable and cheapest method of producing the
thin films. These include oxidation of an evaporated metal film, reactive and nonreactive
sputtering techniques, chemical vapor deposition etc. and a number of methods involving
growth from chemical solutions, so-called chemical techniques. Owing to their scalability,
simplicity and inexpensiveness, chemical techniques have been mostly studied for the
sreparation of thin films. Moreover, these chemical techniques facilitate the materials to be
designed on a molecular level.
Spray pyrolysis is a versatile and effective technique to deposit metal oxide films.
It is an attractive method to prepare a wide variety of powders and thin film materials for
various industrial applications. Metal oxide, chalcogenide and even metal films have been
deposited using this technique. Spray pyrolysis opens up the possibility to control the film
morphology. The quality and properties of the films depend largely on the process
parameters. The most important parameter is the substrate surface temperature. The higher
the substrate temperature, the rougher and more porous are the films. If the temperatures
are too low the films are cracked. In between, dense smooth films can be obtained. The
deposition temperature also influences the crystallinity, texture and other physical
30
properties of the deposited films. The precursor solution is the other important spray
parameter which affects the morphology and the properties of the deposited films. In
addition, the film morphology and properties can be drastically changed by using various
additives in the precursor solution [1, 2].
Spray pyrolysis technique is a processing technique being considered in the
research to prepare thin and thick films, ceramic coatings, and powders. Unlike many other
film deposition techniques, spray pyrolysis represents a very simple and relatively cost-
effective processing method. It offers an extremely easy technique for preparing thin and
thick films of any composition. Spray pyrolysis does not require high-quality substrates or
chemicals and sophisticated instruments. The method has been employed for the
deposition of thin uniform films, porous films, and for powder production. Also
multilayered films can be easily prepared using this versatile technique. Spray pyrolysis
has been used for several decades in the glass industry [3-5] and in solar cell production [4-
7]. Typical spray pyrolysis equipment consists of a nozzle, precursor solution, substrate
heater, rate of spray, distance between nozzle and substrate and temperature controller.
Various reviews concerning spray pyrolysis techniques have been published. Mooney and
Radding have reviewed the spray pyrolysis method, properties of the deposited films in
relation to the conditions, specific films particularly CdS and device application [8, 9].
Tomar and Garcia have discussed the preparation and the properties of sprayed films as
well as their application in solar cells, anti-reflection coatings and gas sensors [10]. Albin
and Risbud presented a review of the equipment, processing parameters, and
optoelectronic materials deposited by the spray pyrolysis technique [11]. Pamplin has
published a review of spraying solar cell materials as well as a bibliography of references
on the spray pyrolysis technique [12].
31
3.2. Influence of Deposition Parameters on Thin Film Properties
Thin-film deposition, using the spray pyrolysis technique, involves spraying a
metal salt solution onto a heated substrate (Fig.1). Droplets impact on the substrate surface,
spread into a disk shaped structure, and undergo thermal decomposition. The shape and
size of the disk depends on the momentum and volume of the droplet, as well as the
substrate temperature. Consequently, the film is usually composed of overlapping disks of
metal salt being converted into oxides on the heated substrate.
Fig. 3.1. Schematic diagram of spray pyrolysis equipment.
This section deals with the influence of the main spray pyrolysis parameters on
structure and properties of the deposited films.
32
3.2.1. Influence of Temperature
Spray pyrolysis involves many processes occurring either simultaneously or
sequentially. The most important of these are aerosol generation and transport, solvent
evaporation, droplet impact with consecutive spreading, and precursor decomposition. The
deposition temperature is involved in all mentioned processes, except in the aerosol
generation. Consequently, the substrate surface temperature is the main parameter that
determines the film morphology and properties. It is generally observed that higher
substrate temperature results in the formation of better crystalline films, [13, 14]. By
increasing the temperature, the film morphology can change from a cracked to dense and
then porous and re-crystallization into larger grains is enhanced [15, 16]. In many studies
the deposition temperature was reported indeed as the most important spray pyrolysis
parameter.
3.2.2. Influence of Precursor Solution
The precursor solution is the second important process variable. Solvent, type of
salt, concentration of salt, and additives influence the physical and chemical properties of
the precursor solution. Therefore, structure and properties of a deposited film can be
tailored by changing composition of precursor solution. The structure of deposited TiO2
film was changing from a cracked to a crack-free reticular after the introduction of acetic
acid into the precursor solution. The change of morphology was attributed to the
modification of precursor solution chemistry. Films from chloride based precursors were
crystalline and highle photosensitive compared to those formed from nitrate based
precursors which were amorphous [17 - 19]. Concentration of spray solution also affects
the nature of the film formed. Usually it ranges from 0.001M to 0.1M and it is observed
33
that smooth films of columnar grains are obtained with low concentration and low spray
rates [20]. Chen et al. have shown that the morphology of the thin films can be changed
considerably by adding additives to the precursor solutions [21]. The change in
morphology was attributed to the chemical modification of precursor solution.
3.2.3. Flow Rate of Spray Solution
Another parameter influencing the properties of films formed is the spray rate of
the precursor solution. It has been reported that properties like crystallinity, surface
morphology, resistivity and thickness are affected by change in spray rate of the precursor
solution [22]. Generally it is observed and experimentally proved that smaller spray rate
favors formation of better crystalline films. Smaller spray rate requires higher deposition
time for obtaining films of the same thickness prepared at higher spray rate. Decrease in
crystallinity usually results in increased resistivity of the films. Higher spray rate results in
rough films. Also it is reported that films deposited at smaller rate are thinner due to higher
re-evaporation rate.
3.2.4. Distance between Nozzle and Substrate
During film formation by spray pyrolysis many process occur either sequentially or
simultaneously. This includes precursor solution atomization, droplet transport and
evaporation, spreading on the substrate, drying and decomposition of the precursor
solution. Deposition of thin films by spray pyrolysis can be divided into following steps
[23]:
1. The droplet splashes on the substrate, vaporizes, and leaves a dry precipitate in
which decomposition occurs.
34
2. The solvent evaporates before the droplet reaches the surface and the precipitate
impinges upon the surface where decomposition occurs.
3. The solvent vaporizes as the droplet approaches the substrate, then the solid melts
and vaporizes and the vapor diffuses to the substrate to undergo a heterogeneous
reaction.
3.3 Experimental details
The spray pyrolysis method is basically a chemical deposition technique in which
fine droplets of the desired materials are sprayed onto a preheated substrate. Continuous
films are formed on the hot substrate by thermal decomposition of the material droplets.
The chemical reactants are selected in such a way that the products other than the desired
compound will volatile at the temperature of deposition. Two necessary conditions for
good quality thin film preparation are to obtain a mist of the solution with small droplets
and their uniform distribution. To fulfill these conditions, a new (computer controlled)
spray and deposition chamber has been developed indigenously, which improves the
selectivity of the droplets that arrive close to the substrate. This spray system automates the
various fatigue and error creating processes when performed manually. Moreover,
parameters like spray rate of the solution and speed of spray head movement are controlled
precisely which are difficult to be controlled by manual process. A positive displacement
pump controlled by stepper motor and microprocessor is used to dispense the solution as
per requirement. The spray head movement is also controlled by stepper motor driven
linear stages in X direction. The temperature of the substrate heater plate is controlled
independently through a dedicated controller. In order to have better control on the
deposition conditions like spray rate, area of deposition and carrier gas pressure the stepper
35
motor controller has been connected to microcontroller. This arrangement is particularly
very useful for large area deposition of thin films with greater uniformity.
ZnSe thin films were deposited onto glass substrates by spray pyrolysis method at
400, 425, 450 and 475◦C substrate temperatures. 0.05M solutions of zinc chloride hydrate
(ZnCl2·H2O) and selenium dioxide or selenourea (H2NC(Se)NH2)were used as starting
materials. Glass microslides of the size 7.5cm×2.5cm were used as substrates. Prior to
deposition these substrates were washed with water, then boiled in concentrated (2M)
chromic acid and kept in double distilled water for 48 h. Finally the substrates were
ultrasonically cleaned for 10 min. The temperature of substrate was controlled by an iron–
constantan thermocouple. The spray rate employed was varied from 3 ml/min to 5ml/min
and kept constant throughout the experiment. Nitrogen gas was used as carrier gas. After
deposition, the films were allowed to cool at room temperature. The adhesion of the films
onto the substrate was quite good [24, 25].
3.3.1 Deposition of the ZnSe thin films
For the fabrication of ZnSe thin films, Zinc Chloride and Selenium dioxide or
selenourea were used as the source of materials for Zn2+, and Se2- ions, respectively. All
the precursors were of the analytical grade. The aqueous solutions of zinc chloride
dehydrate (ZnCl2·H2O) and Selenium dioxide (SeO2) or Selenourea (CH4N2Se) were
prepared by dissolving appropriate amounts of these salts (A.R.Grade) in double distilled
water. The equimolar solutions were mixed together in appropriate volumes to obtain the
Zn: Se ratio as 1:1 and then sprayed through a nozzle onto the preheated amorphous glass
substrates. The experiments were carried out in two sets: in the first set, the substrate
temperature was varied from 400◦C, at the intervals of 25◦C, to 475◦C by keeping all other
parameters constant, especially the concentration of spraying solution (0.05M) and in
36
second set the substrate temperature was held fixed at its optimized value of 450◦C and
concentrations of spraying solution were varied as 0.025M, 0.050 M, 0.075M and 0.1M.
The other parameters like quantity of spraying solution and spray rate are kept constant at
its optimized values 50cm3 and 4ml/min respectively [26, 27]. Various optimized
preparative parameters for deposition of ZnSe thin films summarized as follows:
Table 1: Various optimized preparative parameters for deposition of ZnSe thin films.
Sr. No. Parameter Optimized Value
1 Substrate temperature 4500C
2 Concentration of spraying solution 0.05 M
3 Spray rate 4 ml/min.
4 Carrier gas pressure 2 kg/cm2
5 Quantity of spraying solution 50 cm3
37
Reference:
1. P.S. Patil, Materials Chemistry and Physics 59 (1999) 185.
2. D. Perednis & L. J. Gauckler, Journal of Electroceramics, 14 (2005) 103.
3. J.M. Mochel, US Patent 2,564,707 (1951).
4. J.E. Hill and R.R. Chamberlin, US Patent 3,148,084 (1964).
5. A.R. Balkenende, A. Bogaerts, J.J. Scholtz, R.R.M. Tijburg, and H.X. Willems,
Philips Journal of Research, 50(3–4), 365 (1996).
6. S.P.S. Arya and H.E. Hintermann, Thin Solid Films, 193(1–2), 841 (1990).
7. C.H. Chen, E.M.Kelder, P.J.J.M. van der Put, and J. Schoonman, J. Mater. Chem.,
6 (5), 765 (1996).
8. J.B. Mooney and S.B. Radding, Annu. Rev. Mater. Sci., 12, 81 (1982).
9. S.C. Karle, Ph. D. Thesis, Savitribai Phule Pune University, Pune, India, (2008).
10. M.S. Tomar and F.J. Garcia, Progress in Crystal Growth and Characterization of
Materials, 4(3), 221 (1981).
11. D.S. Albin and S.H. Risbud, Advanced Ceramic Materials, 2(3A), 243 (1987).
12. B.R. Pamplin, Progress in Crystal Growth and Characterization of Materials, 1(4),
395 (1979).
13. H. H. Affify, S. A. Nasser, S.E. Demian, J. Mater. Sci.: Materials in Electronics,
2(3) 700 (1994).
14. A. Goswami, Thin Film Fundamentals, New Age International (P) Ltd, India
(1996).
15. K.L. Chopra, I. Kaur, Thin Film Devices and Applications, Plenum Press, Newyork
(1983).
38
16. N.H.J. Stezler, J. Schoonman, J. Materials Synthesis and Proceedings, 4(6) 429
(1996).
17. T.T. John, C. Sudha Kartha, K. P. Vijaykumar, T. Abe, Y. Kashiwaba, Appl. Surf.
Sci., 252 (2005) 1360.
18. R.P.M. Kumar, Ph. D. Thesis, Cochin University of Science and Technology, India
(2007).
19. J.B. Mooney, S.B. Radding, Annu. Rev. Mater. Sci. 12 (1982) 81.
20. T. T. John, Ph. D. Thesis, Cochin University of Science and Technology, India
(2004).
21. C.H. Chen, E.M. Kelder, and J. Schoonman, J. Eur. Ceram. Soc., 18 (1998)1439.
22. T. Sabestian, R. Jaykrishnan, C.S. Kartha, K.P. Vijaykumar, The Open Surface
Science Journal, 1 (2009) 1.
23. A.A. Yadav, M.A. Barote, P.M. Dongre, E.U. Masumdar, J. Alloys Compd. 493
(2010) 179.
24. A.A. Yadav, M.A. Barote, P.M. Dongre, E.U. Masumdar, Materials Chemistry and
Physics 121 (2010) 53.
25. A.A. Yadav, M.A. Barote, P.M. Dongre, E.U. Masumdar, Solar Energy 84 (2010)
763.
26. K.T. Ramakrishna Reddya, Y.V. Subbaiah, T.B.S. Reddy, D. Johnston, I. Forbes,
R.W. Miles, Thin Solid Films 431 –432 (2003) 340.
27. Mustafa Öztaş, Metin Bedir, Materials Letters 61 (2007) 343.
39
Chapter -IV
CHARACTERIZATION
4.1. Introduction
Recently, prime efforts are being given to trim down the cost of optoelectronic
consumer products to make them affordable, socially acceptable and easily viable. To
quench the quest of patrons, the only exclusive alternative is to exploit thin film structures
instead of single crystals. The II-VI, IV-VI and IIII-V chalcogenides and their derivatives
are attracting a treaty of anxiety in this respect. These tremendously imperative photonic
materials can be realized in practice by the congregation of substantial element deposition
techniques [1-11]. Chemical spray pyrolysis is a solitary inter alia, cheapest and
universally notorious technique for synthesizing these materials in thin film form [5-8, 10-
13]. The modus operandi in its form is simplest and requires no sophisticated
instrumentation and process control [5-8, 10-13]. The controlled and lethargic discharge of
reacting species facilitates better orientation of the crystallites with improved grain
structure [5, 10, 12]. Depending on the deposition conditions, film growth results into
nanosized yield product. Nanometric semiconductors exhibit unique narrative properties
due to their big and strong quantity of surface atoms in three-dimensional confinement of
electrons. Altering the size of a particle alters the degree of confinement of electrons and
electronic structure of the solid. In particular, there are band edges which are tunable with
particle size [2, 3, 5, 14]. ZnSe is one of such materials which has been industrially
exploited as photodetector, IR-emitter and solar energy converter including the
supplementary photonic devices and applications [2, 14-16]. The as-grown film materials
40
are proved to be similar with those produced by other sophisticated and exclusive methods.
Single crystals of binary ZnSe were curiously examined by Mustafa Öztaş et al [11]. In
this chapter, attempts have been made to study the structural properties of the ZnSe thin
films grown onto the glass substrates by a chemical spray pyrolysis process.
4.2. Experimental Details
4.2.1. Deposition of the ZnSe thin films
The method involved spraying aqueous solutions of ZnCl2, selenourea onto heated
glass substrates. The source to substrate distance was maintained at 25 cm and the
substrate temperature varied in the range, 350–450 0C. The solution concentration was
maintained at 0.1 M. The nitrogen gas was used as the carrier gas and the solution was
sprayed at a flow rate of 4 ml /min. The depositions were carried out in the dark in a closed
chamber to avoid the dissociation of selenourea into elemental selenium under
illumination. The various phases present and the structure of each phase were determined
using X-ray diffractometry.
The ZnSe thin films were prepared by spraying an aqueous solution of ZnCl2 and
SeO2 on a glass substrate kept at 450 °C. ZnSe thin films were spray deposited from an
aqueous solution containing 0.05 M ZnCl2 and 0.05 M SeO2. A 50 ml spraying solution
was prepared at the same ratio (Zn:Se=1:1), the spray flow rate was adjusted to about 3 ml
per min and the distance between the nozzle (head of the sprayed source) and the substrate
was kept at 20 cm.
4.2.2. The structural determinations
The X-ray diffraction (XRD) is an elegant technique for the determination of
crystal structure of the materials. The basic principle of X-ray diffraction is that, diffraction
generally occurs only when wavelength of incident radiation is of the order of the distance
41
between scattering centers. This state of diffraction is acknowledged as Bragg’s law and is
given as [17];
2����� = �λ …………………… (4.1)
Where, d is the interplanar spacing, q is the diffraction ang+le,
λ is the wavelength of X-rays and n is the order of diffraction.
For a cubic system (a = b = c and α = β = γ = 900), the interplanar spacing for any
set of planes is given by;
� =
√� �� �� ……………………….. (4.2)
Here, a, b, c are lattice parameters and h, k, l are Miller indices.
In early days, XRD was used only for the determination of crystal structure, but is now
applied to such diverse determinations as chemical analysis, stress measurements, phase
equilibrium and measurement of grain size, orientation of the crystallite, etc. For thin
films, the powder diffraction technique in conjunction with diffractometer is most
commonly used. In this technique, the diffracted radiations are detected by a counter tube,
which shifts along the angular range of reflections. The intensities are recorded on a
computer system. The dvalues are calculated using equation (4.1) for the known values of
q, λ and n. The X-ray diffraction statistical data thus obtained is printed in a tabular form
on a paper and is compared with Joint Committee Power Diffraction Standards (JCPDS) to
identify the mysterious material. X-ray powder pattern is a set of lines or peaks, each of
disparate intensity and position, on either a strip of photographic film or on a length of
chart paper. For a given substance, line positions are essentially fixed and are the
characteristics of that substance. The intensities may be depending on the technique of the
sample preparation and the instrumental stipulation.
42
a) Phase Identification
For the identification of a phase, a principle note is taken off the line positions
together with a semi-quantitative contemplation of intensities [17, 18]. Each crystalline
substance has its own characteristic diffraction pattern and this is used for its phase
identification. Mixtures of substances perhaps identified provided, of course, that
component phases are accessible for comparison. A line in powder form of phase of
interest is preferred and its intensity is compared with that of a suitable internal standard
line [17, 18]. The amount of phase present can be determined by interpolation from a
formerly constructed calibration graph of intensity against composition.The lattice
parameters of a solid solution series often show a small but detectable variation with
composition. This provides a productive approach of characterizing the solid solutions and
technically, the lattice parameters perhaps used as an indicator of the composition. If
composition reliance is linear then Vegard’s law is said to be obeyed. This is not actually a
law but rather is a generalization that applies to the solid solution formed by a random
substitution or sharing of ions. It assumes implicitly that the changes in unit cell
parameters with composition are governed purely by the relative sizes of the atoms or ions
that are active in the solid solution mechanism, e.g. the ions that swap each other in a
simple substitution mechanism. Exodus from Vegard’s law behaviour has been observed in
many solid solution series, particularly in metals. Other abrupt changes or discontinuities
may occur at certain compositions if either a change in symmetry of solid solutions or a
change in solid solution mechanism occurs [18].
b) Intensity consideration
Intensities of X-ray reflections are too significant for the two major reasons. Firstly,
the quantitative measurements of intensity are necessary in order to determine crystal
43
structures. Secondly, the quantitative or semi qualitative intensity data are needed in using
the powder finger print method to characterize the material. Intensities depend on several
factors as [17];
1. Structure factor: Structure factor depends on position of the atoms in the unit cell
and their scattering power.
2. Polarization factor: It is the angular dependence of intensity scattered by
electrons.
3. Lorentz factor: Lorentz factor is a geometric factor that depends on the particular
type of instrument used and varies with θ.
4. Multiplicities: Multiplicities is the number of reflections that contribute to an
observed powder lines, one from each set of planes that are superposed to give the
observed line. Multiplicities may be radically calculated if crystal symmetry is
known; the object is to find the maximum possible number of (hkl) combinations
which are equivalent.
5. Thermal vibrations: Thermal vibrations of atoms cause a decrease in intensities of
diffracted beams and increase in the background scatter.
6. Absorption factor: The absorption of X-rays by a sample depends on nature of the
sample and geometry of the instrument. Ideally, for single crystals, crystal should
be spherical so as to have same absorption factor in all directions.
A preferred orientation becomes known if the samples used in powder diffraction do not
have completely random arrangement of crystal orientations.
c) Determination of the grain size
When size of an individual crystal is less than about 100 nm, the term particle /
grain / crystallite size is usually used. The crystallite size was estimated
44
from the full width at half maximum (FWHM) of the most intense diffraction line
using Scherrer’s formula as follows [17, 18];
� =�.���
����� …………………………… (4.3)
Where, D is the crystallite size, λ is the wavelength of X-rays used, β is the Full
Width at Half Maxima of the peak (FWHM) in radians and �is the Bragg’s angle.
The X- ray diffraction data can also be used to determine the dimension of the unit cell.
This technique is not functional for the identification of individuals of multilayer or
percentage of the doping materials.
4.3 The XRD studies
The X-ray diffractograms were obtained for these composite structures in the 200 to
800 2θ-range with CuKα radiation (1.5406A°). These are shown in fig.4.1 (a, b) for two
samples. The diffractograms were further analyzed to compute the interplanar distances
(d), Miller indices, intensities of reflections (I/Imax) and lattice parameters for a series of
the films.
2 0 3 0 4 0 5 0 6 0 7 0 8 0
Zn
O
Zn
O
Zn
O
Zn
O
Zn
O
Zn
O
Zn
O
2 θθθθ
Inte
nsi
ty
Zn
O
45
2 0 3 0 4 0 50 6 0 7 0 8 0
Zn
O
Zn
O
Zn
O
Zn
O
Zn
OZ
nO
Zn
O
Inte
nsi
ty
2 θθθθ
Fig. 4.1 a) XRD diffractogram of ZnSe thin film by using Selenourea
b) XRD diffractogram of ZnSe thin film by using selenium dioxide
46
References:
1. K. L. Chopra, R.C. Kainthla, D. K. Pandya, A. P. Thakoor, Thin Film Solar Cells,
Plenum Press, N.Y, 1983.
2. N. C. Sharma, D. K. Pandya, H. K. Sehgal, K. L. Chopra, Thin Solid Films 32
(1977) 383.
3. M.T. Gutierrez, J. Ortega, Sol. Energy Mater. 20 (1990) 87.
4. J. Datta, C.Bhattacharya, S.Bandyopathyay, Appl.Surf.Sci.252(2006) 7493.
5. G. Hodes, Phys. Chem. Chem. Phys. 9 (2007) 2181.
6. K.T. Ramakrishna Reddy, Y.V. Subbaiah, T.B.S. Reddy, D. Johnston, I. Forbes,
R.W. Miles, Thin Solid Films 431 –432 (2003) 340.
7. E. Pentie, L.Pintilie, T.Botila, E.Ozbay, Optoelect. Adv. Mater. 3 (2001)525.
8. L. P. Deshmukh, R.V. Suraywanshi, E. U. Masumdar, M. Sharon, Solar Energy 86
(2012) 1910.
9. A.N.Chattarki, S.S.Kamble, L.P.Deshmukh, Materials Letters 67(2012)39.
10. A.N.Chattarki, L.P.Deshmukh, Rare Metal Mater. Engg.41 (2012) 28.
11. A. N. Chattarki, N. N. Maldar, L. P. Deshmukh, J. Alloys and Compds.597 (2014)
223.
12. Mustafa Öztaş, Metin Bedir, Materials Letters 61 (2007) 343.
13. B. A. Ezekoye, C. E. Okeke, Pacific J. Sci. 7 (2006) 108.
14. J. Zhu, S. Liu, O. Palchik, Y. Koltypin, A. Gedanken, Solid State Chem.153 (2000)
342.
15. A. Rogalski, Prog. in Quant. Elect. 27 (2003) 59.
16. S. Kumar, Z. H. Khan, M. A. Khan, M. Husain, Current Appl. Phy. 5 (2005)561.
47
17. B. D. Cullity, Elements of X-ray Diffraction, ed. Addison-Wesley, Massachusetts,
1956.
18. V. M. Raghavan, “Materials Science and Engineering”, in: V. M. Raghavan (ed),
Prentice-Hall of India, New Delhi, (1998).
48
Chapter – V
RESULT AND DISCUSSIONS
The chemical spray pyrolysis method was chosen as a low temperature processing
technique for deposition of thin films onto any kind of heating substrate. This technique is
also offers extremely easy way to prepare thin films with dopants, virtually any element in
any proportion by merely adding it in a spray solution. The effort was directed towards the
optimization of processing condition for the deposition of good quality nanostructured thin
films with variety of morphological study. The results of optimization studies on spray
parameters showed that the surface temperature is the most critical parameter affecting the
roughness and morphology, porosity, cracking and crystallinity of the thin film. Physical
and chemical properties of the thin films have shown to be extensively dependent on the
deposition temperature as well as solution flow rate and the type of precursor solution
among other parameters. The proposed system for deposition of thin films is easier to
maintenance, low cost, and has no sophisticated instruments. In this deposition process
heat is used to break molecules into elemental sources which are then deposited on
substrate. The coating is applied at elevated temperatures by spraying droplets of liquid
precursors onto hot substrates. It is used for depositing a wide variety of thin films which
are used in devices like solar cells, sensors, solid oxide fuel cells etc.
The deposition of ZnSe performed into two stages by changing the selenium
precursor. A) The method involved spraying aqueous solutions of zinc chloride (ZnCl2),
selenourea (CH4N2Se) onto heated glass substrates. The source to substrate distance was
maintained at 25 cm and the substrate temperature varied in the range, 350–450 0C. The
solution concentration was maintained at 0.1 M. The nitrogen gas was used as the carrier
49
gas and the solution was sprayed at a flow rate of 4 ml /min. The depositions were carried
out in the dark in a closed chamber to avoid the dissociation of selenourea into elemental
selenium under illumination. The various phases present and the structure of each phase
were determined using X-ray diffractometry.
B) The ZnSe thin films were prepared by spraying an aqueous solution of zinc
chloride (ZnCl2) and selenium dioxide (SeO2) on a glass substrate kept at 450 °C. ZnSe
thin films were spray deposited from an aqueous solution containing 0.05 M ZnCl2 and
0.05 M SeO2. A 50 ml spraying solution was prepared at the same ratio (Zn:Se=1:1), the
spray flow rate was adjusted to about 3 ml per min and the distance between the nozzle
(head of the sprayed source) and the substrate was kept at 20 cm.
Here the grown thin films were become uniform, pin-hole free and strongly
adherent to the substrate surface as the substrate temperature increases. The X-ray
diffraction data of the as deposited thin films at temperatures (4500C) by using different
selenium precursors indicated various peaks corresponding to ZnO. With an increase of
substrate temperature the intensity of the peaks corresponding to ZnO phase increased.
Thus the ZnSe thin film with chemical spray pyrolysis is difficult to deposit on glass
substrate. The XRD diffractgrams of as deposited thin films exactly matches with zinc
oxide. This might have been due to partial re-evaporation of Se from the film surface
(because of their high vapour pressures) so that the Zn-rich ZnSe reacted with atmospheric
oxygen to form ZnO.
The chemical spray pyrolysis technique is an important method for preparation of
variety of nanostructured oxide thin films. The films so prepared can be used in sensor
applications such as gas sensor, humidity sensor, phtosensor etc. which will serve for the
progress of human civilization.