Farooqui et al., J Adv Sci Res, 2016, 7(1): 01-08
Journal of Advanced Scientific Research
Available online through http://www.sciensage.info/jasr
1
ISSN
0976-9595
Review Article
METAL OXIDE THIN FILMS: A MINI REVIEW
Khadher AL-Rashdi1, Mazahar Farooqui1, 2, Mohammad Mohsin1, Gulam Rabbani1*
1
Post Graduate and research center, Maulana Azad College, Aurangabad, India 431001
2
Dr. RafiqZakaria College for women, Aurangabad
*Corresponding author: mazahar_64@rediffmail.com
ABSTRACT
The main objective of this work is to provide glimpses of the research which is based on the preparation of thin films of metal oxides
using the sol-gel method, and research work of those studies on the structure, physical and chemical characteristics and other
properties, it also recognize on the equipment and means by which samples, films were examined which produced in this research
and explain the idea of Sol-gel process and advantages of this process makes an important and preferable to the rest of the other
methods, as well as displays their flaws, and applications of films produced.
Keywords: Metal oxide, thin films, Sol-gel method, nanostructure, optical properties, Electrical properties.
1. INTRODUCTION
The field of material science and engineering
community’s ability to conceive the novel materials with
extraordinary combination of chemical, physical and
mechanical properties has changed the modern society. There
is an increasing technological progress. Modem technology
requires thin films for different applications [1]. The advent of
controlled thin film growth about seven decades ago
revolutionized many areas of science and technology [2]. Thin
film studies have directly or indirectly advanced many new
areas of research in solid state physics and chemistry which are
based on phenomena uniquely characteristic of the thickness,
geometry, and structure of the film, when we consider a very
thin film of some substance [3], Ultra-thin layers of material
deposited on another maternal are of immense importance.
The fabrication of integrated circuits consists of deposition and
selective removal of series of thin films. Thin film
microelectronics and optoelectronics are among the strongest
technological drivers of our economy, a fact manifested by the
explosive growth in communications and information
processing, storage and display applications. Technologies
have fertilized expanding thin film uses in diverse areas e.g.
coating of all kinds, bio-technology and the generation and
conservation of energy. Thin film applications are issues
rooted in material science and engineering. Involvement with
thin films dates to the metal ages of antiquity [4].
The processing of materials into thin films allows easy
integration into various types of devices. The properties of
material significantly differwhen they are analysed in the form
of thin films. Most of the functional materials are rather
applied in thin film form due to their specific electrical,
magnetic, optical properties or wear resistance. Thin film
technologies make use of the fact that the properties can
particularly be controlled by the thickness parameter. Thin
films are formed mostly by deposition, either physical or
chemical methods. Thin film are used in microelectronic
devices, magnetic thin films in recording devices, magnetic
sensors, gas sensor, A. R. coating, photoconductors, IR
detectors, interference filters, solar cells, polarizer’s,
temperature controller in satellite, superconducting films,
anticorrosive and decorative coatings [5].
1.1. History of thin film development
Thin-film technology is simultaneously one of the oldest
arts and one of the newest sciences. Involvement with thin
films dates to the metal ages of antiquity. Consider the ancient
craft of gold beating, which has been practiced continuously
for at least four millennia. Gold’s great malleability enables it
to be hammered into leaf of extraordinary thinness while its
beauty and resistance to chemical degradation have earmarked
its use for durable ornamentation and protection purposes [6].
Nanotechnology, as a concept, was started by Richard
Feynman [7] at an American Physical Society meeting on
1959. The term “nanotechnology” was first used and defined
by Norio Taniguchi at Tokyo University of Science in a paper
from 1974. He stated that “Nanotechnology’ mainly consists
of the processing of, separation, consolidation, and
deformation of materials by one atom or by one molecule.”
Eric Drexler stated that “Nanotechnology’ mainly consists of
the processing of, separation, consolidation, and deformation
of materials by one atom or by one molecule”. In the 1980s
Eric Drexler emphasized the technological significance of
nanoscale phenomena and materials [8, 9]. In the same time
new experimental techniques were invented, which gave
nanotechnology a great boost [10]. The invention of the
scanning tunnelling microscope (STM) in 1981 by Gerd Binnig
and Heinrich Rohrer [11] awarded with Nobel Prize in Physics
in 1986, and atomic force microscopy (AFM) in 1986 by
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Gerard Binning, Calvin F. Quate and Christopher Gerber had
very big impact in the development of nanotechnology and
nanoscience. In general, these techniques allowed not only
observation of nano objects, yet also precise manipulations of
single atoms and molecules [12]. Now-a-days transition
electron microscopy (TEM) and scanning electron microscopy
(SEM) are used for observation of the nanomaterials. Nobel
Prize in Physics was awarded to Ernst Ruska in 1986, more
than 50 years after he constructed first electron microscope in
1928 and first with better resolution comparing to optical
microscope in 1933. For years the “top-down” approach has
been the mainstream of nanotechnology [13]. Fullerenes were
prepared in 1985 by Harry Kroto, Richard Smalley and Robert
Curl, who together won the 1996 Nobel Prize in Chemistry.
Smalley’s research is related with formation of inorganic and
semiconductor clusters using pulsed molecular beams and time
of flight mass spectrometry. As a consequence of this
expertise, Curl introduced him to Kroto in order to
investigate a question about the constituents of astronomical
dust. These are carbon rich grains expelled by old stars such as
R Corona Borealis. The result of this collaboration was the
discovery of C60 and the fullerenes as the third allotropic form
of carbon. Subsequent discoveries included the fullerenes and
the larger family of fullerenes the following year [14, 15].
2. TERMS INVOLVED
2.1. Thin Film
A thin film is a layer of material ranging from one of a
nanometer (monolayer) to several micrometers in thickness.
Layer of material with one dimension was much smaller than
the other two. A familiar application of thin films is the
household mirror, which typically has a thin metal coating on
the back of a sheet of glass to form a reflective interface. The
process of silvering was once commonly used to produce
mirrors. A very-thin-film coating (less than about 50
nanometers thick) is used to produce two-way mirrors [16].
2.2. Nanocrystal
A nanocrystal is a crystalline particle with at least one
dimension measuring less than 1000 nanometers (nm), where
1 nm is defined as 1 thousand-millionth of a meter (10-9 m)
[17].
Fig. 1: (a) The Wurtzite Structure Model of Zn–O; (b) The
Tetrahedral Coordination of Zn–O
2
2.3. Gel
A gel consists of a three dimensional continuous
network, which encloses a liquid phase. In a colloidal gel, the
network is built from agglomeration on colloidal particles
[18]. A gel is a solid, jelly-like material that can have
properties ranging from soft and weak to hard and tough. Gels
are defined as a substantially dilute cross-linked system, which
exhibits no flow when in the steady-state. By weight, gels are
mostly liquid, yet they behave like solids due to a threedimensional cross-linked network within the liquid. It is the
cross linking within the fluid that gives a gel its structure
(hardness) and contributes to the adhesive stick (tack). In this
way gels are a dispersion of molecules of a liquid within a solid
in which the solid is the continuous phase and the liquid is the
discontinuous phase [19].
2.4. Sol
A sol is a dispersion of the solid particles in a liquid
where only the Brownian motions suspend the particles. A gel
is a state where both liquid and solid are dispersed in each
other, which presents a solid network containing liquid
components [20]. Examples include blood, pigmented ink, cell
fluids and paint artificial sols may be prepared by dispersion or
condensation. Dispersion techniques include grinding solids to
colloidal dimensions by ball milling and Bredig's arc method.
The stability of sols may be maintained by using dispersing
agents sols are commonly used in preparing sol-gel [21].
Fig.2: Shows both the sol and gel
2.5. Colloid
A colloid is a type of mixture in which one substance is
dispersed evenly throughout another. A colloidal system
consists of two separate phases: a dispersed phase (or internal
phase) a continuous phase (or dispersion medium). Generally,
the gravity of the dispersion is negligible and the interaction
between particles is always a short-range interaction [18, 20].
3. GENERAL METHODS OF PREPARATIONS
Although research into oxide thin films has continued since
the 1960s, it was the discovery of high temperature
superconductivity [22] in 1986 which provided a major
impetus to the research in the field of multi-component
complex oxide thin films. These oxides are the subject of
scientific studies because they represent immense promise for
the 21st century solid state devices. Although in the past these
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materials have been used as bulk materials for many
applications, it is the thin film form of these oxides which
makes them more attractive for various applications.
Significant progress has been made towards studying the
growth mechanisms of the epitaxial thin films by various
techniques such as RHEED, LEED, and STM [23]. Over the
years various processes have been developed for the deposition
of metal oxide thin films. Almost all of these deposition
techniques can be broadly divided into two categories, namely
physical vapour deposition processes (PVD) and chemical
processes. PVD processes include laser ablation, sputtering,
evaporation while chemical processes are chemical vapour
deposition techniques (CVD), liquid phase epitaxy, sol-gel and
metal organic deposition (MOD) and spin coating. There have
been many extensive reviews on the deposition of epitaxial
oxide films [24, 25]. We can Show methods of preparing thin
films as follows.
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systems used for materials synthesis, the interactions are of a
covalent nature and the gel process is irreversible. The
gelation process may be reversible if other interactions are
involved. Phase is negligible, and the interaction between
particles is always a short-range interaction [18, 20, 24, 28].
4.1. Method of sol-gel
4.1.1. Theory Methods
A sol-gel synthesis usually consists of two main steps.
The first step is to create a colloidal solution, with a solid
phase dispersed in a continuous liquid phase. The second step
is to convert the solution into a gel. This is done by hydrolysis,
by adding a gelling agent or by hydrothermal treatment. In this
step, the sol is chemically transformed into a gel which means
that the dispersed and continuous phases shift places. In other
words, a gel consists of liquid droplets dispersed in a
continuous solid matrix. In spin coating, the more or less
gelatinous solution is placed upon the substrate, which then
rotates at a given speed and duration, leaving only a thin layer
of solution on the substrate. This is a comparably low-cost and
easy chemical technique usually done in ambient atmosphere.
The procedure can, however, also be done in an artificial (e.g.
inert) atmosphere [29].
Scheme 1: General methods for preparation of thin films
4. Sol-Get Synthesis
The sol-gel process is a wet-chemical technique widely
employed recently in the fields of materials science and
ceramic engineering. Such methods are utilized primarily for
the fabrication of materials (typically a metal oxide) starting
from a chemical solution which acts as the precursor for an
integrated network (or gel) of either discrete particles or
network polymers [26]. In this chemical procedure, the
solution gradually converts into a gel like diphasic system,
which contains both liquid phase and solid phase. The
morphologies of these two phases range from discrete particles
to continue polymer networks. In this method the chemical
composition of the product can be controlled. In this method
the solution can be doped with organic dyes and rare earth
metals, Processing and producing thin films of metal oxides.
Nano materials derived by this method have wide applications
in electronics, medicine, and optics [27]. In a polymer gel the
particles have a polymeric sub-structure made by aggregates of
sub-colloidal particles. Generally, the sol particles may
interact by van der Waals forces or hydrogen bonds. A gel may
also be formed from linking polymer chaos. In most gel
Fig.3. Schematic representation of the different stages and
routes of the sol-gel technology
4.1.2. Experimental method
In this chemical procedure, the sol (or solution)
gradually evolves towards the formation of a gel-like biphasic
system containing both a liquid phase and solid phase whose
morphologies range from discrete particles to continuous
polymer networks. In case of the colloid the volume fraction
of particles (or particle density) maybe so low that a significant
amount of fluid may need to be removed initially for the gellike properties to be recognized. This can be accomplished in
any number of ways. The simplest method is to allow time for
sedimentation to occur, and then pour off the remaining
liquid. Centrifugation can also be used to accelerate the
process of phase separation. Removal of the remaining liquid
(solvent) phase requires a drying process, which is typically
accompanied by a significant amount of shrinkage and
densification. The rate at which the solvent can be removed is
ultimately determined by the distribution of porosity in the
gel. The ultimate microstructure of the final component will
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be strongly influenced by changes imposed upon the structural
template during this phase of processing.Afterwards, a thermal
treatment, or firing process, is often necessary in order to
favour further Polycondensation and enhance mechanical
properties and structural stability via final sintering.
densification and grain growth. One of the distinct advantages
of using this methodology as opposed to the more traditional
processing techniques is that densification is often achieved at a
much lower temperature. The precursor sol can be either
deposited on a substrate to form a film, or for any pot with a
suitable form, or used to synthesize powders [30].
4.2. Advantages of the sol-gel process
Technology Sol-Gel has developed a growing interest in
recent years for the installation of various metal oxides
prepared by this technique because of the advantages of this
technique [31-34], these advantages are as follows:
Better homogeneity from raw materials, Better purity from
materials, New no crystalline solids outside the range of
normal glass formation, Lower temperature of preparation,
Saving energy, Minimizing evaporation losses, Minimizing air
pollution, No reactions with containers, thus purity, Bypassing
crystallization, Good mixing for multi-component systems,
Effective control of particle size, shape, and properties, The
creation of special products such as films, Better products
from the special properties of the gel, The creation of new
non-crystalline solids outside the range of normal glass
formation, The fine tuning of chromatographic selectivity via
the possibility of creating hybrid organic-inorganic materials,
The possibility of designing the material structure and
property through the proper selection of sol-gel precursor and
other building blocks, The possibility of achieving enhanced
stationary phase stability and performance in chromatographic
separations [26,35-37].
4.3. Limitations of Sol-Gel
Despite its advantages, sol-gel technique never arrives at
its full industrial potential due to some limitations, e.g. weak
bonding, low wear-resistance, high permeability, and difficult
controlling of porosity. In particular the limit of the maximum
coating thickness is 0.5 µm then the crack-free property is an
indispensable requirement. The trapped organics with the
thick coating often result in failure during thermal process.
The present sol-gel technique is very substrate-dependent, and
the thermal expansion mismatch limits the wide application of
sol-gel technique [37, 38]. And disadvantages of sol-gel
process can be summed as follows:
Large shrinkage during processing, Residual fine pores,
Residual hydroxyl, Residual carbon, Health hazards of organic
solution, Long process time.
5. THIN FILMS DEVELOPED BY SOL-GEL SYNTHESIS
In recent years, the thin films metal of oxides has become
interesting because of its unique pmpertied [39, 40], Now-adays tremendous amount of interest has been generated in the
4
study of metal oxide materials because they lead to grow a
new generation of optics, sensors, photocatalysis, electro
catalysis, anticonosion, electronic and magnetic applications
The oxides of transition metals are like Zn, Cu, Ni, Ti, Sn, Fe,
Co and W etc. [41]. The intriguing physical characteristics
observed in nanoscale metals and semiconductors are related
to the changes in electronic structure as well as the large
surface-to-volume ratios, which make them vastly different
from corresponding bulk solids. These unique properties have
led to the development of synthetic methods that permit
exquisite control over materials size, shape, and disparity in
the nanometer length scale [42-44]. However, most of the
applications envisioned, and the devices that are likely to be
constructed, will not be solution based or single particle based
but will be formed from nanoparticle assemblies in the solid
state. Hence, developing methods for the assembly of
nanoparticles into solid state structures, while retaining their
characteristic physical properties, is an important challenge
[24].
5.1. ZnO Thin film
ZnO is one of the candidate materials for many
applications due to its significant physical and chemical
properties [45]. On substrates of glass, using the sol-gel
method, deposition of thin films of ZnO was examined at
different rates. The thin films with semiconductor II-VI is
prepared using this method. The effect of deposition
parameters on the optical and structural properties of thin
films of ZnO is also studied zinc acetate dehydrate,
methoxyethanol and monoethanolamine (MEA) are used as
raw material. Film deposition was performed by dip-coating
technique at a withdrawal rate of 1.5 cm min-l on Corning
1737 glass substrate [46], by sol-gel method using zinc acetate
and aluminium chloride as cations source, methoxyethanol as
solvent and monoethanolamine as sol stabilizer, The film
obtained doped with 2 wt.% Al, it was observed that.The
optical transmittance spectra of the films showed very good
transmittance, between 85 and 95%, within the visible
wavelength region, and the minimum resistivity of 1.3=10-3Ω
cm [46].Epitaxial films can be synthesized by using ZnO buffer
layers on sapphire (001) substrates. Chemical vapour
deposition preceded the sol-gel process epitaxial provides
these buffer layers of 50 am ZnO and significantly improved
the crystallinity of the sol-gel ZnO (12 By sol-gel process).
The uses of zinc acetate dehydrate, ethanol and ethanol
Secretary precursors, solvents and stability. In the case of Aldoped ZnO, aluminium nitrate monohydrate is added to the
precursor solution with an atomic percentage equal [47] to 1
or 2 % Al. Zinc oxide (ZnO) thin films were deposited on
microscope glass substrates by sol-gel spin coating method
[48]. The film is prepared from zinc acetate, alcohol isopropyl
and dual ethanol Secretary in starting materials, solvents and
stability respectively. The effect of different annealing
temperature on structural, morphological and optical
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properties of ZnO thin films has been reported [49].
Nanostructure ZnO thin films with different concentrations of
Ni+2 doping on aluminium foil substrates is also prepared the
ZnO films are annealed at different temperatures (100 ºC,
300ºC and 500ºC) [50]. Exhibited preferred orientation along
(002) plane when increasing PVP concentration into solution
[51], ZnO film is also prepared pH values of 4, 6, 8 and 10
[52], then deposited on the (ff0- glass, Silicon wafer and glass)
substrates by spin- coating (1000, 2000 and 3000) rpm for
Nano-thin film (67, 91, 182) nm and then thermally treated in
a furnace heat [53]. The ZnO nanowires were
electrochemically grown onto a ZnO sol-gel spin-coated seed
layer. Thin films and samples were deposited onto crystalline
quartz substrates covered by an Au electrode, able to be used
in a quartz crystal microbalance. A quartz crystal microbalance
placed in a vacuum chamber was used to quantify the amount
and kinetics of water adsorption onto the samples. Nanowire
samples, which have higher surface areas than the thin films,
adsorb significantly more water [54]. A sol-gel deposition
method applied to ZnO thin film as a semiconductor layer for
transistors (TFT). Experimental results indicate that ZnO TFT
and flexible ZnO TFT are better or at least comparable to a
conventional transistor, and the flexible ZnO TFT exhibits
superior electrical performance [55, 56]. Aluminium doped
zinc oxide polycrystalline thin films (ZAO) were prepared by
sol-gel dip- coating process. The sol was prepared from an
alcoholic solution of zinc acetate using DEA (Di Ethyl Amine)
as a stabilizer. It was prepared on glass substrates by spincoating method [57], the film can be used as solid white
lightsources for various potential applications [58]. The film on
conducting glass support (Sn02: F over layer) shows
Microstructure which changes on varying film preparation
conditions, especially the sintering temperature. The films
prepared at sintering temperature ≈ 600ºC, yield maximum
photocurrent and are more efficient for photo splitting of
water [59].
6. CHARACTERIZATION OF THIN FILMS
The role played by thin films was largely a utilitarian one,
necessitating measurement of film thickness and optical
properties. However, with the explosive growth of thin-film
utilization in microelectronics, there was an important need to
understand the intrinsic nature of films with the increasingly
interdisciplinary nature of applications, new demands for film
characterization and other property measurements arose. It
was this necessity that drove the creativity and inventiveness
that culminated in the development of an impressive array of
commercial analytical instruments. These are now ubiquitous
in the thin-film, coating, and broader scientific communities.
There are modem techniques used in the characterization of
thin electronic materials and devices. Among their
characteristics are the unprecedented structural resolution and
chemical analysis capabilities over small lateral and depth
dimensions. Some techniques provide information on the first
5
few- atom layers of the surface only. Others probe more
deeply, but in no case are depths much beyond a few microns
accessible for analysis. Virtually all of these techniques require
a high or ultrahigh vacuum ambient. Some are nondestructive. In common, they all utilize incident electron ion,
or photon beams. These interact with the surface and excite it
in such a way that some combination of secondary beams of
electrons, ions, or photons are emitted, carrying off valuable
structural and chemical information in the process. A rich
collection of acronyms has emerged to differentiate the various
techniques. These abbreviations are now widely employed in
the thin-film and surface science literature. General testing
and analysis of thin films is carried out with equipment and
instruments which are wonderfully diverse in character [6].
Structural characterization on thin films of a sample of metal
oxides (ZnO) conducted a number of different types of
analysis, Scanning Electron Microscopy (SEM), X-ray
diffraction (XRD), Transmission Electron Microscopy (TEM)
and X-ray, and atomic force microscope (AFM) and according
to the growth of movies on Shearer formula size, and the
optical properties were measured by UV-Visible
spectrophotometer.
By X-ray diffraction models (XRD), growth was calculated
using the size of the films Shearer equation. Using dual- beam
spectrum and field built absorbance measurements and visual
permeability recorded in over the wavelength 190-900 nm.
Studies have revealed that the optical absorption is within the
scope of the energy gap directly. In thin films of ZnO in the
dark and under UV optical thin films shows sufficient optical
band gap of which were obtained from ZnO can be used as a
Photoelectric material [39]. The optical transmittance spectra
of the films showed very good transmittance, between 85 and
95%, within the visible wavelength region. The minimum
resistivity of 1.3x10-3Ω cm [46], the observation of six-fold
symmetry in X-ray diffraction scan spectra confirmed the
epitaxial nature of the sol-gel ZnO films. Exciton
recombination were dominant in the photoluminescence,
while defect-related emissions around 3.31 to 3.32 eV, which
are regularly observable in polycrystalline sol-gel films, were
not observed in the sol-gel ZnO with ZnO buffer layers [63].
Wave guiding properties of the thin films have been also
studied using m-lines spectroscopy. The results indicate that
our films are monomodes at 632.8 nm with propagation
optical loss estimated [47] around 1.6 dB/cm, XRD pattern of
ZnO thin films shown polycrystalline wurtzite with a
preferential (0 0 2) orientation. The annealed ZnO thin films
are highly transparent in the visible region (400-700 nm) and
have a sharp absorption edge in the ultraviolet region. The
absorption edge analysis showed that the optical band gap
energy, Eq. for the ZnO thin films were between 3.26 eV and
3.28 eV. SEM analysis has shown completely different surface
morphology behaviour for annealed ZnO thin films. These
results suggest that the nan crystalline ZnO thin films prepared
by this sol-gel process have good axis orientation and optical
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properties [48]. The prepared films were annealed at different
temperature from 350 ºC to 550 ºC. The crystal structure and
orientation of the films were investigated by using X-ray
diffraction (XRD). The Scherer formula was used to calculate
the grain size of the films. The grain size of the crystallites was
found to be in the range of 13-26 nm. From UV spectra the
optical energy band gap was evaluated (3.27 eV-3.3leV). The
surface morphology of the film was analysed by Scanning
Electron Microscope (SEM). The surface morphology of the
film is homogeneous and continuous. The films showed high
transparency (>90%) in the visible region. When the
annealing temperature was increased above 450 ºC,
transmittance was decreased. This may be due to diffusion of
impurity ions from the soda lime glass [49], and characterized
by means of SEM and XRD). The crystallite size of prepared
ZnO films increases with increasing of the film annealing
temperature as well. The crystalline structure is hexagonal.
The addition of 1 to 15 wt % of Ni+2 to the start solution
modifies the morphology of films the ganglia-like hills and the
wrinkles become smaller [50]. The structural properties were
investigated by x-ray diffraction (XRD) method and atomic
force microscope (AFM). The optical properties were
measured by UV-Vis spectrophotometer. The XRD patterns
showed that the films formed preferred orientation along caxis which increased as a function of polyvinyl pyrrolidone
(PVP) concentration. The films gave the crystallite size of 1518 nm calculated by Scherrer’s formula and grain size of 48-70
nm measured by AFM at different PVP concentrations. The
direct optical band gap of the films [51] was in the range of
3.80-4.08 eV. X-ray diffraction analysis reveals that ZnO is in
hexagonal structure. Scanning electron microscopic images
show that the grain size increases with increase in pH. The
surface morphology improves with increase of pH values.
TEM analysis reveals formation of ZnO nanocrystalline with
an average grain size of 44 nm. Optical band studies show that
the films are highly transparent and exhibit a direct band gap.
The increase in pH values of range 4,6,8 and 10 decreases the
band gap energy from 3.32 to 3.14 eV [52], The crystal
structure of ZnO films was investigated through X-ray
diffraction (XRD). The X-ray diffraction spectra of ZnO film
annealed at 450°C. Then the samples were examined by XRay, as well as UV showing that the best efficiency of the solar
cell is obtained when the thickness of the polymer 67 nm and
91 nm where the polymer was 2.1%. The thickness improves
the structure of the solar cell and also helps to increase the
current density and gives the best efficiency [53]. X-ray
diffraction measurements reveal in ZnO nanowire and
nanocrystal a typical diffraction pattern of ZnO wurtzite
structure. Scanning electron microscopic images of nanowires
samples show the presence of nanowires with hexagonal
sections, with diameters ranging from 30 to 90 nm.
nanocrystallites with sizes between 10 and 33 nm. SEM
morphological studies revealed nanowires with hexagonal
sections, the optical properties of the nanowire array confirm
6
the presence of a direct semiconductor absorption edge, with
band gap energy at 3.29 cV, close to accepted value for ZnO.
The optical properties of ZnO thin films (with band gap
energy at 3.35 eV) are influenced by quantum size effects due
to the small dimensions of their primary nanocrystalline
structure, while the optical properties of ZnO nanowires are
not affected due to their relative larger nanometric diameter.
QCM measurements quantified the amount of adsorbed water
and adsorption kinetics for ZnO thin films and nanowires.
Occupying the same macroscopic area, one-dimensional
nanowires adsorbed larger amounts of water because they have
a higher surface-to-volume ratio than thin films. ZnO
nanowires are more sensitive to low water vapour pressures
(i.e., low % RH) than ZnO thin films, likely due to a higher
number of defects on their walls [54], were studied by
measuring structural and optical properties by X-ray
diffractrometry and UV-Vis-NIR spectroscopy. SEM images of
the films showed a smooth grain size and the microstructures
of films became denser. The optical transmittance was
between 91% and 98% in the visible and near IR regions. An
increase of Eq. values from 3.16 eV to 3.32 eV was obtained
when the ratio of Al changes from 0% to 4% [57]. ZnO thin
films on PS. Shoes a higher and a narrower (002) diffraction
peak was observed from the ZnO thin films on PS, and their
residual stress was relaxed, which indicates that the crystal
quality of the ZnO thin films was enhanced due to the skeleton
on the PS. The crystallinity of the ZnO thin films on PS was
further increased and the residual stress was further relaxed by
the post-annealing. A white light luminescence with blue,
green, and red emission peaks was observed from the ZnO
thin films on PS, and the intensities of the emission peaks
became uniform with the post-annealing [60] & measured by
employing a UV-Vis scanning spectrophotometer suggest that
the films are efficient UV absorber and moderate-weak
absorber of visible light. Thus, their use for PEC splitting of
water is possible. However, use of appropriate dye-sensitizers
would be helpful to expand absorption to higher wavelengths;
this can be attributed to the better optical absorption and
decreased electrical resistivity of the samples, observed
absorption spectra and the optical band gap of the films [58].
These deposited films can be characterized by various methods
such as X-ray diffraction (XRD), surface profilometer,
ellipsometry and ultraviolet visible spectroscopy [60], the
structural, optical, and electrical properties of ZnO thin films
have been investigated and it is found to be influenced by the
thickness of the film. The surface morphology ZnO thin films
that are porous and also formed aggregates of ZnO
nanoparticles compare to composite ZnO: The optical band
gap found to be increased with the increasing of the thin films
thickness for ZnO thin films. The conductivity of ZnO found
to be decreased as the thickness increase and conductivity for
composite ZnO [61, 62].
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7. Thin metal films applications
ZnO is a very promising material for semiconductor device
applications [64, 65]. For example, ZnO has been given much
attention as a host material for transparent conducting film
because impurity-doped ZnO films show high transparency
transmittance, between 85 and 95%, in a visible region and
low resistivity around 1.3 x l0-3 cm [46]. ZnO is a widely
used functional material with wide and direct band gap, large
exaction binding energy, and excellent chemical and thermal
stability [47, 55, 63]. ZnO is inexpensive, relatively abundant,
chemically stable, easy to prepare, nontoxic and most of the
doping materials that are used with it are also readily available
[47]. ZnO thin film is applied to the transparent conductive
film and the solar cell window because of the high optical
transmittance in the visible region. Studies on the application
of ZnO thin film to the surface acoustic wave (SAW) device
and film bulk acoustic resonator (FBAR) filter are being made,
because of its excellent piezoelectric properties, ZnO thin
films can be used as a window material in photovoltaic
applications [39, 47-50, 59, 63]. Zinc oxide has been regarded
as an excellent semiconductor material for the solar cell due to
its high electron mobility as well as the high chemical and
thermal stability [50]. ZnO thin film presents investigating
optical, acoustical and electrical properties which meet extent
applications in the fields of electronics, optoelectronics and
sensors [48, 51]. ZnO wide band gap opens the possibility of
creating ultraviolet (UV) light emission diodes (LEDs) and
white REDs with superior, Due to its large excited binding
energy of about 60 meV they can be used as transparent
electrodes in displays and metal oxide semiconductor in
optoelectronic devices [46, 50, 51].
Overall, the thin metal films applications can be
summarized the following:
The ultra-pure materials serve as electrodes, adhesion, or
diffusion layer in coatings used for TFT-LCDs used in GPS
systems, cell phones, computer monitors and TV screens,
Electronics and data storage. The sputtering target materials
can be used for the manufacture of integrated circuits and
optical data storage devices such as CDs and DVDs. Electrodes
molybdenum is chosen for its resistivity and good ohmic
contact property. Photovoltaic (PV) solar energy industryRotary and planar sputtering target materials made of high
purity molybdenum can be applied for CdTe and CIGS- based
solar cells while rotary NiV targets are used for Si thin film
solar cells. Diffusion barriers: Tantalum, molybdenum, and
tungsten thin film layers can prevent inter diffusion between
two materials in electronic devices. Optical coatings:
Reactively sputtered tantalum and niobium films have low
absorption and high refractive index. The purity and
uniformity of H.C. Starck rotary target materials enables
coaters to achieve excellent control of each separate deposition
process.
7
8. CONCLUSIONS
The review article describes the sol-gel method for the
synthesis of method oxide thin film particularly of ZnO. It also
gives advantages and disadvantage of sol-gel method, along
with theoretical aspect of metal oxide and the application of
thin films, and it also cover morphological studies and
description with various spectral technique.
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