CHAPTER- 4
SYNTHESIS AND CHARECTERIZATION OF PURE
& Sn-DOPED ZINC OXIDE NANOCRYSTALLINE
THIN FILMS PREPARED BY SPRAY PYROLYSIS
4.0 INTRODUCTION:
One of the most important fields of current interest in materials science is
the study of fundamental aspects and applications of transparent conducting oxide thin
films (TCO). Transparent conducting films act, as a window for light to pass through the
active material beneath (where carrier generation occurs), as an ohmic contact for carrier
transport out of the photovoltaic but also as transparent carrier for electronics used
between laminated glass or light transmissive composites. Transparent conducting oxide
(TCO) thin films are semiconducting materials with a large bandgaps of energies
corresponding to wavelengths which are shorter than the visible range (380 nm to
750 nm).As such, photons with energies below the bandgap are not collected by these
materials and thus visible light passes through. Thus TCO have not only high optical
transmittance in the visible region but also have relatively high electrical conductivity
and high reflectance in the IR region. However, applications such as photovoltaics (PV)
may require an even broader bandgap to avoid unwanted absorption of the solar spectra.
This unique combination of physical properties i.e. transparency and electrical
conductivity, makes them suitable for a variety of applications in optoelectronic devices.
Consequently, various techniques for the growth of these TCO’s films have been recently
intensively investigated. The growth technique plays a significant role in determining the
properties of these films, because the same material deposited by two different techniques
usually may have different micro and macro properties. The simultaneous occurrence of
128
high optical transparency (>80%) in the visible spectrum and low electrical resistivity
(10-3Ωcm or less) is not possible in an intrinsic stoichiometric oxides, because of the
large optical band gap (>2.0 eV). Partial transparency and fairly good conductivity may
be obtained in very thin (<10 nm) films of metals. On the other hand, the only way to
have transparent conductors is to populate the conduction band with electrons in spite of
the wide band gap by controlled creation of non-stoichiometry or the introduction of
appropriate dopants. These conditions are very conveniently obtained in oxides of
cadmium (Cd), tin (Sn), indium (In), zinc (Zn) and their alloys in thin film form, prepared
by a number of different deposition techniques. The first report of a transparent
conducting oxide (TCO) was published in 1907, when Badeker [1] reported that thin
films of Cd metal deposited in glow discharge chamber could be oxidized to become
transparent while remaining electrically conducting. Since then, the commercial value of
these thin films has been recognized, and the transparent and electrically conducting
oxide films (TCO), e.g., In2O3:Sn (ITO), SnO2 (TO), ZnO and ZnO:Al (AZO), have been
extensively studied owing to their variety of applications in optoelectronic devices and as
gas sensors. The optoelectronic applications include the transparent electrodes for flat
panel displays (FPD’s), solar cells, light emitting diodes and transparent heating elements
for aircraft and automobile windows, heat reflecting mirrors for glass windows, and
antireflection coatings [2-6]. In FPD’s, the basic function of ITO is as transparent
electrodes. The volume of FPD’s produced, [and hence the volume of ITO coatings]
continues to grow rapidly. Specifically, for applications in the field of thin film solar
cells, the TCO can serve as an electrode and protection layer of the p-n junction, which is
the main part for the performance of solar cells. In terms of conductivity and
transmission, each TCO varies. Therefore, choosing the type of TCO is a major issue for
a new solar cell design. Beside the optoelectronic applications, transparent conductive
oxide (TCO) films have found a wide range of applications in electric equipments and
coatings where transparency is required. This material constitutes an important
commercial use in the manufacture of antifrost windshields (nesa glass) and in the
manufacture of thin film resistor.
At present, and likely well into the future, tin-doped indium oxide (ITO) material
offers the best available performance in terms of conductivity (~104 Ω-1cm-1) and
129
transmission (80-90%), combined with excellent environmental stability, reproducibility,
and good surface morphology. However, the enormously high cost of indium, toxicity
and the scarcity of this material create the difficulty in obtaining low cost TCO’s [7].
Hence search for other alternative TCO materials has been a topic of research for the last
few decades. It includes some binary materials like ZnO, SnO2, CdO and ternary
materials like Zn2SnO4, CdSb2O6:Y, ZnSO3, GaInO3 etc. Binary oxide, such as, ZnO is a
potential candidate for various applications in the field of optoelectronic devices in shortwavelength. However, the application of binary oxides e.g. ZnO and SnO2 thin films is
sometimes limited because these materials could become unstable in certain chemically
aggressive
and/or
elevated
temperature
environments.
The
introduction
of
multicomponent oxide materials resulted in the design of TCO’s films suitable for
specialized applications. This is mainly because one can control their electrical, optical,
chemical and physical properties by altering the chemical compositions. But the major
advantages of using binary materials are that their chemical compositions and depositions
conditions can be controlled easily. Even though CdO:In films have been prepared with a
-5
resistivity of the order of 10 ohm cm making them quite useful for flat panel displays
and solar cells, currently these are of not much importance because of the toxicity of Cd.
For a semiconductor to be useful, particularly in reference to optoelectronic
device, band gap engineering is a crucial step in device development. By allowing the
starting semiconductor with another material of different band gap, the band gap of
resultant alloy material can be fined tuned, thus affecting the wavelength of exciton
emissions. So the objective of the present chapter is to prepare pure and Sn-doped
nanocrystalline ZnO thin films on glass substrate using chemical spray pyrolysis and
optimize the substrate temperature, annealing temperature and thickness of the film by
investigating the structural, optical and electrical properties and to investigate the effect
of concentration of dopants such as Tin (Sn) on the structural, optical and electrical
properties of the nanocrystalline ZnO thin films. Thus the main effort is to decrease the
resistance and transparency of ZnO thin film deposited using chemical spray pyrolysis
(CSP) technique ( which is very simple and rather easy to vary the concentrations of Zinc
ion or dopant) and hence to vary / control the properties of the films through Sn-doping.
130
4.1 A REVIEW OF THE DOPING EFFECT ON ZnO THIN FILMS
It is often possible to impart some desirable properties to semiconductor by
introducing a controlled quantity of suitable impurity elements into it either during its
preparation or diffusing them afterwards through thermal or other treatments. This
process is called ‘doping’ and the impurities are known as ‘dopants’. The density of
impurities in semiconductors is one of the major parameter that can control position of
Fermi level. Doping of impurities in wide band gap semiconductors often induces
dramatic changes in electrical and optical properties.
ZnO is an n-type semiconductor; its physical properties like structural, optical,
electrical and magnetic properties can be modified by appropriate doping either by
cationic or anionic substitution. In order to meet the demands for several applications,
various dopants such as Al3+, F1-, Cu1+, Ag1+, Ga3+, ,In3+, Sn4+ and Sb5+ has been already
tried [8-17] to prepare high quality n- and p-type ZnO and efficiency of the dopant
depends on its electro-negativity and ionic radius. Doping of ZnO with the atoms of
elements of higher valency results in replacing Zn2+ atoms and hence improvement in the
electrical conductivity. When ZnO is doped with group III elements, [such as Al, Ga and
In], the dopants substitute Zn atoms while the group VII elements [such as Cl and F]
substitute O atoms; these two doping lead to n-type. However, p-type doping in ZnO may
be possible by substituting Zn site using group-I elements (Li, Na and K) or substituting
O using group-V elements (N, P and As).
In literature, it has been reported that several dopants have been used to modify
the properties of ZnO. For example Sun-Young Sohn, et al. [18] reported the preparation
of transparent conducting oxide films of indium-zinc-oxide (IZO) and Sn-doped IZO
(IZTO) on glass substrates at room temperature by rf-magnetron sputtering. The IZO
films have good electrical and chemical stability against the high-temperature-and-lowmoisture (HTLM) or the low-temperature-and-high-moisture (LTHM) stress for a short
time of 30 min, but their sheet resistances rapidly increase under an LTHM stress for a
long time of 240 hours. It has been found that the stability of IZO films is remarkably
improved if a suitable amount of (tin) Sn is doped. In particular, an IZTO film (with
In2O3:ZnO:SnO2 = 90:7: 3 wt.%) shows such an excellent electrical stability against
every moist-heat stress that its sheet resistances after stress are reduced by only a small
131
percentage of those before stress, as well as good optical properties including a wide
bandgap and high visible-light transparency, which is closely related with its smooth
surface morphology of an amorphous-like structure.
The report of S. Mondal, et al. [19] on the pure and manganese-doped zinc
oxide (Mn:ZnO) thin films deposited on quartz substrate following successive ion layer
adsorption and reaction (SILAR) technique showed that the film growth rate increased
linearly with number of dipping cycle. The X-ray diffraction revealed that polycrystalline
nature of the films increased with increasing manganese incorporation. Particle size
evaluated using X-ray line broadening analysis showed decreasing trend with increasing
manganese impurification. The average particle size for pure ZnO was 29·71nm and it
reduced to 23·76nm for 5%Mn-doped ZnO. The strong preferred c-axis orientation was
lost due to manganese (Mn) doping. The degree of polycrystallinity increases and the
average microstrain in the films decreased with increasing Mn incorporation.
Incorporation of Mn was confirmed from elemental analysis using EDX. As the Mn
doping concentration increases the optical bandgap of the films decreases for the range of
Mn doping reported here. The value of fundamental absorption edge was 3·22 eV for
pure ZnO and it decreased to 3·06 eV for 5%Mn:ZnO.
V. Gokulakrishnan et al. [20] reported ZnO and Mo-doped ZnO thin films
preparation by the spray pyrolysis technique and the XRD results showed that the films
were polycrystalline and belong to the ZnO hexagonal structure. The AFM studies
revealed that the RMS value of roughness decreases with increasing Mo concentration.
Average transmittance of ∼87% in the visible region (400–800 nm) was obtained for the
film doped with 1 at.%. The optical bandgap of the MZO films increases with increasing
Mo content of the samples. Annealing the ZnO and Mo doped ZnO films at 400 0C for 60
min improves the resistivity of the films. The minimum resistivity, maximum carrier
concentration and mobility of the Mo doped ZnO at Mo content of 1 at.% were 4.7×10 -2
Ωcm, 1.27×1019 cm-3 and 10 cm2V-1s-1 respectively. It can thus be concluded that highly
transparent MZO films prepared on Corning glass substrate by spray pyrolysis are
promising materials especially as transparent electrodes.
Nadir Fadhil Habubi, et al. [21] has reported Uniform and adherent Zn1-xMnxO
films have been deposited by using spray pyrolysis technique on glass substrates. The
132
optical properties and dispersion parameters of zinc oxide have been studied as a function
of doping concentration with Mn. Changes in direct optical energy band gap of cobalt
oxide films were confirmed after doping, The optical energy gap Eg increased from 3.13
eV for the undoped ZnO to 3.39 eV with increasing the doping concentration of Mn to 4
%. An increase in the doping concentration causes a decrease in the average oscillator
strength. The single-oscillator parameter has been reported.
Seval Aksoy, et al. [22] has reported the undoped and tin (Sn) doped ZnO films
were deposited by a spray pyrolysis method onto the glass substrates. 0.2 M solution of
zinc acetate in a mixture of ethanol and deionised water, in a volume proportion of 3:1,
was employed. Dopant source was tin chloride. The atomic percentage of dopant in
solution were Sn/Zn = 1%, 3% and 5%. The optical transmittance was about 76% in a
visible range for Sn-doped ZnO films. The absorption edge shifts to the lower
wavelengths with Sn dopant.
L.K. Munguti et al. [23] has reported the tin doped zinc oxide thin films
were deposited by reactive evaporation under various tin doping levels anging from 1%
to 8%. The deposition was done using Edwards Auto 306 coating unit at room
temperature (25°C) and 5.0 x 10-5 mbar of chamber pressure. The optical transmittance
spectra was obtained in the visible wavelength 380-750nm. The doped films showed high
transmittance >75% although slightly lower than that of undoped films. The band gap
ranged from 2.95-3.95eV with the lowest value been attained at 4% tin doping. For the
electrical characterization, sheet resistivity was carried using the four point probe at room
temperature (25°C). The sheet resistivity ranged from 24.3-26.7Ωcm although it
decreased with increase in doping concentration.
Sn-doped ZnO (SZO) films, prepared using SILAR technique, were used as
NO2 gas sensor and the results were reported [24]. Experimental results showed that the
sensitivity of ZnO films increased on Sn doping.
Y. Caglar et al. [25] prepared undoped and SZO thin film using spray
pyrolysis. Effect of Sn doping on structural and morphological properties of ZnO films
was investigated using XRD and SEM. XRD patterns confirmed that the film had
polycrystalline nature having (100),(002),(101),(102) and (110) reflections. While
133
pristine ZnO had the (101) preferred orientation, Sn-doped ZnO films were having (002)
orientation.
Nano structured pure and SZO synthesized for gas sensing application by thermal
evaporation technique [26]. SEM images indicated change in the growth pattern from
nanowire (for pure ZnO) to tetrapods [for SZO]. The response towards different gases for
pure and SZO were recorded. Pure ZnO nano-wires exhibit selective response towards
acetone vapour while for SZO, the response decreased. The deviation from stoichiometry
and the morphology of ZnO are probably responsible for such a difference in gas
response. Pure and Sn-doped ZnO showed nearly same crystallite size.
M.R Vaezi et al. [27] reported the preparation of SZO films from a Zinc complex
solution containing tin ions on to pyrex glass substrate using two-stage chemical
deposition process. Resistance of the undoped ZnO films is high and reduces to a value
4.2x10-2 ohm cm when 2.5% Sn is incorporated. All of the ZnO films have above 80%
transmittance in a range of 400-700 nm. The optical energy gap ( Eg) increases with the
increase of doping amount of Sn in the films. It varied from 3.05 to 3.18 eV depending on
the amount of Sn incorporated.
NO2 gas sensor was fabricated by successive ionic layer adsorption and
reaction (SILAR) technique and rapid photo thermal processing (RPP) of the SZO film
[28]. Influence of variation of Sn concentration in the chemical bath and the RPP
temperature on NO2 sensitivity of thin film sensor elements was investigated in this work.
Higher sensitivity was obtained at 5–10 at %, Sn concentration in the solution of ions and
RPP temperature of 550–6500C.
Another report was on preparation of Al or Sn doped ZnO films using spray
pyrolysis [29]. These films were deposited on either indium tin oxide (ITO) coated or
bare glass substrates. ZnCl2, AlCl3 and SnCl2 were used as precursors. The properties of
the films have been studied before and after annealing 4 h at 4000C in vacuum (10-3 Pa).
Here the lowest electrical resistivity achieved was due to doping with Sn (3x103 Ohmcm)
and this was further lowered by 2-3 order of magnitude after the vacuum annealing (0.9
Ohm cm).
Paraguay et al. [30] prepared sprayed ZnO film and doped with different
elements, Al, In, Cu, Fe and Sn. Sensitivity of the films were studied in two steps: first as
134
a function of their temperature (435-675 K) for a fixed ethanol concentration (40 ppm)
and the next case was as a function of ethanol concentration (4-100 ppm) for a fixed
temperature (675 K). A better sensitivity can be observed for Sn- and Al-doped films,
with a dopant/Zn ratio of 0.4 at.% and 1.8 at.%, respectively.
Bougrene et al. deposited ZnO:Sn films using spray pyrolysis technique on
glass substrates and physico-chemical properties of these films were studied [31].
Crystallinity and optical transmittance improved on doping. Lowest resistivity obtained
was 5x10-2 ohm cm.
Olvera and Maldonado [32] reported the deposition of high quality ZnO:Ga
films using CSP technique. Two different precursors were used for the deposition. lowest
resistivity obtained was 4.5x10-3 ohm cm and the average optical transmittance in the
visible region was about 85 %. Finally it is worthwhile to report some of the best results.
Different group tried to deposit ZnO films using different technique.
The best results have been reported by Hahn et al.[33]. They deposited highly
conductive polycrystalline ZnO films using MOCVD technique with resistivity of the
order of 3x10-4 ohm cm. Average optical transmittance was nearly 85%.
Minimum resistivity of 1.4x10-4 ohm cm was obtained with Al doping [34] by Jeong et
al. These films exhibited an average of 95% in the visible region.Chen et al [35] could get
lowest resistivity of 8.54x10-5 ohm cm in films deposited using PLD technique, which
showed an average optical transmittance of 88%. Chemical spray pyrolysis technique
generally resulted in resistive films compared to the samples from physical method like
sputtering and PLD. S.Major et al. [36] reported the lowest resistivity of 8-9×10-4 Ω cm
in Indium-doped spray pyrolysed ZnO thin films.
4.2 PREPARATION OF PURE AND DOPED ZINC OXIDE THIN
FILMS IN GENERAL:
Besides doping, thin film properties are strongly dependent on the method of
deposition and subsequent heat treatments, the substrate materials, the substrate
temperature, the rate of deposition, and the background pressure. Specific applications in
modern technology demand such film properties as high optical reflection/transmission,
hardness, adhesion, non-porosity, high mobility of charge carriers, chemical inertness
toward corrosive environments, and stability with respect to temperature. Somewhat less
135
required properties are stoichiometric composition and high orientation in single crystal
films. The need for new and improved optical and electronic devices stimulated, in
addition, the study of thin solid films of single elements, as well as binary and ternary
systems, with controlled composition and specific properties, and has consequently
accelerated efforts to develop different thin film deposition techniques. The thin film
deposition techniques can be classified according to the scheme shown in Figure 4.1.The
common techniques that have been used to grow TCO’s films include Chemical Vapor
Deposition (CVD), Spray Pyrolysis, Sputtering, Reactive and Plasma Assisted Reactive
Evaporation, Ion Beam Sputtering, Ion Plating and Filtered Vacuum Arc Deposition
(FVAD). Each of these techniques has its own advantages and disadvantages. For
example, spray techniques are very cheap, deposition parameters easily controllable but
the produced films are not so stable.
4.2.1 CHEMICAL VAPOR DEPOSITION (CVD)
Chemical Vapor Deposition (CVD) is a material synthesis method in which the
constituents in the vapor phase react to form a solid film on a substrate. Gas precursors
can be used directly, and liquid precursors can be used with a bubbler, in which a carrier
gas is passed through the liquid. The chemical reaction is an essential part of this
technique and should be well understood. Various types of chemical reactions are utilized
in CVD (Figure 4.2) for the formation of solids. In one type of reaction, a vapor precursor
that contains the material to be deposited is decomposed by reduction, e.g. using
hydrogen at an elevated temperature. Decomposition is accomplished by thermal
activation. Alternatively, plasma activation may be used to reduce or decompose the precursor at a lower temperature than with thermal activation [37]. CVD processes have
numerous other names, such as metal-organic CVD when plasma is used to induce or
enhance decomposition and reaction; low pressure CVD when the pressure is less than
ambient; and low-pressure plasma enhanced CVD PECVD when the pressure is low
enough that ions can be accelerated to appreciable energies from the plasma.
4.2.2 PULSED LASER DEPOSITON (PLD):Pulsed laser deposition (PLD) is an evaporation technique (Figure 4.3) in which
a laser pulse is used to ablate target material, producing a local plasma jet. The plasma
also contains energetic molecular clusters and macroparticles. The emission of these
136
Figure 4.1 A schematic diagram of thin film deposition techniques
Figure 4.2 A schematic drawing of the CVD technique [after 39].
137
macroparticles is a serious drawback. A solution to this problem is to use crossed laser
induced evaporation plumes to discriminate macroparticles ejected from the target. The
energy of the evaporated material depends on the laser pulse energy.
The energy spectrum of the plasma particles consists of a major relatively lowenergy component (1-100 eV) and a minor high-energy component (up to a few keV)
[38]. As this energetic impact of the evaporated material is kept responsible for a layer
growth with smooth surfaces, a choice of the proper laser pulse energy is required. Each
laser pulse evaporates a well-defined amount of material. Multilayer films can be very
accurately controlled by varying the number of laser pulses.
4.2.3 VACUUM EVAPORATION
Vacuum evaporation (including sublimation) is a physical vapor deposition
(PVD) process where material is thermally vaporized from a source and reaches the
substrate without collision with gas molecules in the space between the source and
substrate. The trajectory of the vaporized material is "line-of-sight." Typically, vacuum
evaporation is conducted in a gas pressure range of 10-5to 10-9Torr, depending on the
level of contamination that can be tolerated in the deposited film [39]. The basic system
and evaporator source configurations are shown in Figure 4.4. Deposition of thin films by
evaporation is very simple and convenient, and is the most widely used technique. One
merely has to produce a vacuum environment, and give a sufficient amount of heat to the
evaporant to attain the desired vapor pressure, and allow the evaporated material to
condense on a substrate kept at a suitable temperature [39]. The important process
parameters are the substrate material, source and substrate temperatures, source-substrate
distance, and background gas composition and pressure. Evaporants with an
extraordinary range of chemical reactivity and vapor pressures have been deposited. This
variety leads to a large diversity of source designs including resistance-heated filaments,
electron beams, crucibles heated by conduction, radiation, or rf-induction, arcs, exploding
wires, and lasers.
4.2.4 CHEMICAL SPRAY PYROLYSIS (CSP)
It is a process where a precursor solution, containing the constituent elements of the
compound, is pulverized in the form of tiny droplets onto the preheated substrate, where
upon the thermal decomposition of the precursor an adherent film of thermally more
138
Figure 4.3 Schematic diagram of the pulsed laser deposition system.
Figure 4.4 Conventional vacuum evaporation system and evaporator source
configurations [after 39].
139
stable compound forms. Spray pyrolysis involves several stages:
(1) generation of micro sized droplets of precursor solution,
(2) evaporation of solvent,
(3) condensation of solute,
(4) decomposition of the precursor or solute and
(5) sintering of the solid particles.
CSP is a convenient, simple and low-cost method for the deposition of large-area
thin films, and it has been used for a long time. Additionally it is a low cost method (the
device does not require high quality targets or vacuum) the composition and
microstructure can easily be controlled (facile way to dope material by merely adding
doping element to the spray solution) and the deposition takes place at moderate
temperatures of 100-500ºC. Furthermore it offers the possibility of mass production.
However as every other method, CSP has some disadvantages such as the possibility of
oxidation of sulfides when processed in air atmosphere, difficulties regarding the growth
temperature determination. Apart from that after a long processing time the spray nozzle
may become cluttered. Finally the films quality may depend on the droplet size and spray
nozzle. This method is useful for the deposition of oxides and is also a powerful method
to synthesize a wide variety of high purity, chemically homogeneous ceramic powders.
4.3 PRESENT INVESTIGATION
For improving the electrical conductivity and optical transmittance of ZnO thin
films, the various nanostructured doped ZnO thin films have been prepared by a variety
of physical deposition methods such as laser deposition [40], different sputtering methods
[41], atomic layer deposition [42] and chemical deposition methods such as chemical
vapor deposition [43] chemical spray pyrolysis [44], chemical bath deposition [45] and
the sol–gel process [46]. Among these methods the chemical spray pyrolysis method
represents the less expensive alternative, since it can produce large area, high-quality and
low cost thin films doped easily with varying dopant concentrations.
4.3.1 PREPRATION OF PURE AND TIN (Sn)-DOPED ZINC OXIDE (SZO)
FILMS BY CHEMICAL SPRAY PYROLYSIS (CSP)
The pure and Sn-doped ZnO transparent conducting thin films were prepared using
spray pyrolysis method onto the glass substrates at temperature of 4500C, which has been
140
optimized after several depositions. All the chemical reagents used in present
investigations were of analytical grade and used without any further purification. In a
typical procedure, the stoichiometric amount of 0.1 M solution of zinc acetate dehydrate
[Zn(CH3COO)2•2H2O] ( Merck ≥ 98 %) was first dissolved in methanol & double
distilled water in the volume ratio 3:1 respectively. Since the Tin was used as the source
of dopant in the present investigation, so 0.1 M Tin chloride pentahydrate [SnCl4•5H2O]
(Sigma-Aldrich ≥98%) was also dissolved in pure methanol only. The resulting solution
was stirred thoroughly with the help of a magnetic stirrer for 12 hours to yield a clear and
transparent homogeneous solution. The dopant solution of Tin chloride pentahydrate
[SnCl4•5H2O] in varying doping concentration from 0 to 2.0 at% was added to starting
solution with continuous stirring until a homogeneous solution was obtained. Now, the
undoped and Sn-doped ZnO (abbreviated as SZO) nanocrystalline (NC) thin films were
deposited by spraying the prepared solution in air atmosphere on 1cm×1cm glass
substrates preheated at 4500C in about 2 hours. Before deposition, the glass substrates
were ultrasonically cleaned in acetone solution and then rinsed by de-ionized water. After
the successful deposition of doped and undoped films, the thickness was measured by
profilometery (Bruker’s Dektak XT). All the SZO thin films of different doping
concentrations were prepared separately under the same parametric conditions, as given
in table 4.1. The flow chart for preparation thin film by the CSP is shown in figure 4.5.
4.3.2 STRUCTURAL CHARACTERIZATION
4.3.2 (a) Chemical Phase Identification
As already discussed in section 2.1 of chapter-2 that x-ray diffraction
(XRD), a non-destructive technique, is the most powerful technique used to uniquely
identify the crystalline phases present in materials and to measure their structural
properties (strain state, grain size, epitaxy, phase composition, preferred orientation, and
defect structure) of thin single or multi layer films, and the atomic arrangements (phase
of atoms), which affect their electrical/electronic and optical properties. So in the present
study, the gross/ average structural characterization (phase identification) of SZO thin
films was carried out by the Bruker D8 Advance X-ray diffractometer (XRD) with
rotating anode target, operating at UGC-DAE Consortium for Scientific Research (CSR),
Indore, India. All the XRD patterns of pure and SZO thin films were recorded in the 2θ
141
Table 4.1 The Spray growth parameters of pure and Sn-doped ZnO thin film samples.
Concentration of Zinc acetate solution
0.1 M
Nozzle-substrate distance
15 cm
Solution flow rate
0.2 ml/min
Substrate temperature
450 oC
Spraying time
10 min
Distance between nozzle and substrate
12 cm
Thickness of the all films
400 nm
Figure 4.5 Preparation procedure of a thin film by the spray pyrolysis process.
142
range from 200 to 900 with scanning speed and step of 2.0 0/min and 0.0295430
respectively. Figure 4.6 shows the representative powder x-ray diffraction patterns of
pure and Sn-doped ZnO with various Sn doping concentration (0.5, 1.0, 1.5 & 2.0 at.%).
The experimental peaks position and their Miller indices were compared with Joint
Committee on Powder Diffraction Standards (JCPDS) data. The presence of peaks with
considerable intensities, corresponding to the reflections such as (100), (002), (101) and
(110) in the x-ray diffraction patterns reveal that the as synthesised doped and undoped
ZnO thin films are pure crystalline hexagonal wurtzite phase of zinc oxide (JCPDS card
no. 89-1379) which belongs to the space group P63mc. No other reflection peaks from
impurities, such as other oxides of Sn or Zn are detected, indicating high purity of the
product of Sn-doped ZnO. The lattice parameters of SZO have been calculated using high
angle XRD peaks corresponding to the reflection, such as, (002), (101) and (110) shown
in Figure 4.6. All the as deposited films have been found to have preferential orientation
along (002) crystal plane. The variation of calculated lattice parameters of SZO with
dopant ratio of [Sn]/[Zn] equal to 0, 0.5, 1.0, 1.5 and 2.0 by at.% are shown in Table 5.2.
A small decrement of a and c -lattice parameter of the hexagonal unit cell has been
observed with increasing Sn content. This may be possibly due to the difference in ionic
radii of Zn+2(0.74 Å) and Sn+4(0.72 Å) ions.
(i) Crystallite size
The crystallite size was calculated from x-ray diffraction data by using the DebyeScherrer formula;
𝐁 = 𝟎. 𝟗 ∗ 𝛌⁄𝐃 𝐜𝐨𝐬𝛉
𝐁
...……………(4.1)
Whereas B the crystallite size, λ is the X-ray wavelength (1.5418 Å for CuKα), ϴ is the
Bragg angle and DB is the full width at half maximum (FWHM).
The calculated crystallite size of SZO as a function of Sn doping concentrations
is shown in figure 4.7. From the table it is reflected that the crystallite size decreases with
increasing Sn concentration in SZO. The minimum crystallite size of 94.6 nm is found
for 2.0 at% Sn-doped ZnO. This is due to the decreasing the lattice volume of unit cell
(table 4.2) because XRD peaks shifts at right side i.e. at high angle side, so that the lattice
143
Figure 4.6 The x-ray diffraction (XRD) patterns of nanocrystalline pure and Sn-doped
ZnO thin films prepared by spray pyrolysis method.
Table 4.2: Various parameters (Crystallite size, Lattice parameters, Volume of the cell and
Number of unit cells) calculated from x-ray diffraction patterns.
Sn dopant
Crystallite
concentratio
diameter/size
n in ZnO
Lattice parameters
a = b (Å)
c (Å)
Volume of
Number of
the cell (Å3) unit cells
(nm)
(n)
0
123.5
3.258
5.208
143.619
6863406
0.5
113.7
3.257
5.207
143.503
5360078
1.0
109.7
3.255
5.205
143.272
4821801
1.5
98.8
3.248
5.204
142.629
3538455
2.0
94.6
3.243
5.200
142.081
3118086
144
parameters a and c decreases hence number of unit cells decreases with increasing the Sn
concentration of in the SZO thin films, discus below in details.
(ii) Number of unit cells
To examine the average number of unit cells of crystallites, let us consider a crystallite P,
of diameter R0. The crystallite P, can be changed into n smaller identical unit cells of
edges, a & c, being the lattice constants, and V is the lattice volume then, the number of
unit cells will be;
𝐧𝐕 = (𝟒⁄𝟑)𝛑 (
Where
𝐑𝟑𝐨⁄
𝟖)
…………..……..(4.2)
𝟐
𝐕 = 𝟑√𝟑 𝐚 𝐜⁄𝟐 (for hexagonal structure)
Then equation (4.2) becomes,
𝐧 = 𝐑𝟑𝐨 𝛑⁄
𝟗√𝟑𝐚𝟐 𝐜
..........……………(4.3)
The calculated number of unit cells for different SZO films are listed in table 4.2. It is
evident from the table 4.2 that the number of unit cells in the crystallite/grain decreases
with an increase in Sn concentration in SZO thin films.
(iii) Texture coefficient
The preferential growth orientation has been determined using the well-known formula
for texture coefficient TC(hkl) given below;
TC(hkl)=
𝐈(𝐡𝐤𝐥)⁄𝐈𝐨 (𝐡𝐤𝐥)
⁄𝟏
(𝐧) ∑[𝐈(𝐡𝐤𝐥)⁄𝐈𝐨 (𝐡𝐤𝐥)]
.…………(4.4)
Where I(hkl) is the observed XRD peak intensities obtained from the SZO films, n is the
number of diffraction peaks considered, and Io (hkl) is the intensity of the most intense
peak of the textured films. With 450 °C substrate temperature, the variation of texture
coefficients of the SZO films with the different orientations (hkl) have been shown in
figure 4.8. The increment of texture coefficient in range of 1.560 and 1.916 may be the
possible reason for the enhancement of grain size of the SZO films decreases from 123.5
to 94.6 nm as the Sn concentration in ZnO increases from 0 to 2.0 at. %. A sample with
randomly oriented crystallite presents TC (hkl)=1, while the larger this value, the larger
abundance of crystallites oriented at the (hkl) direction. From the figure 4.8, it can be
145
Figure 4.7 Crystallite Size of pure and Sn-doped polycrystalline ZnO thin films.
Figure 4.8 The variation of texture coefficient with crystallites oriented at different
direction (hkl) for pure and Sn-doped ZnO thin films, calculated from X-ray
diffraction data.
146
seen that the highest (hkl) at (002) plane for SZO thin film i.e. the grains textured along
(002) plane for all SZO films and this increases with increasing Sn concentration in SZO
thin films. The morphology of SZO thin films as revealed by AFM (figure 4.9) also
supports the enhancement of nanoparticles size (~120–200 nm).
4.3.2 Surface Morphological studies
The surface morphological examination with atomic force microscopy (AFM)
of as prepared thin films are shown in figure 4.9 (a-e) and the variation of average
roughness of the films with varying Sn doping concentrations are shown in figure 4.10.
The AFM images (figure 4.9) demonstrate clearly that films have a relatively smooth
surface, consisting spherical and rod like SZO nanoparticles (NP’s). AFM images have
also been used to estimate the grain/particle size of the samples. The estimated values,
obtained using WSxM software with scan area 1x1 µm2, are in close agreement with
those obtained from the XRD data (see Table 4.2). It is noteworthy to mention here that,
similar to XRD results, the grain/particle size, estimated from AFM data, was found to
decrease and the average roughness of the films decreases (figure 4.10) with increasing
Sn doping concentration. It may also be noted that the particle sizes observed by AFM
are higher as compared with that calculated from the XRD data. This is due the fact that
the XRD technique provides the average/mean crystallite size of grains/crystallites while
AFM shows the particles which are agglomeration of many crystallites/grains. The XRD
and AFM outcome can be reconciled by the fact that smaller primary particles have a
large surface free energy and would, therefore, tend to agglomeration faster and grow
into larger grains.
4.3 OPTICAL PROPERTIES
The optical properties of SZO thin films have been explored by UV-VIS absorption
spectra recorded by Dual beam UV-VIS-NIR spectrometer (Perkin Elmer, Lambda 950)
in the wavelength ranging between 200 to 800 nm and the resistance has been measured
by four probe method at room temperature at UGC-DAE-CSR, Indore. The optical
transmission spectra (T) of the as synthesized SZO thin films grown on glass substrates
of varying dopant concentration (0 to 2.0 at %) as a function of wavelengths are plotted
in figure 4.11. The optical transmission spectrum (T) was calculated from the optical
147
Figure 4.9 (a) Two and three dimensional Atomic Force Microscopy (AFM) images of as
prepared pure ZnO thin films at 4500C.
Figure 4.9 (b) Two and three dimensional Atomic Force Microscopy (AFM) images of as
prepared 0.5% Sn-doped ZnO thin films at 4500C.
148
Figure 4.9 (c) Two and three dimensional Atomic Force Microscopy (AFM) images of as
prepared 1.0% Sn-doped ZnO thin films at 4500C.
Figure 4.9(d) Two and three dimensional Atomic Force Microscopy (AFM) images of as
prepared 1.5% Sn-doped ZnO thin films at 4500C.
149
Figure 4.9 (e) Two and three dimensional Atomic Force Microscopy (AFM) images of as
prepared 2.0% Sn-doped ZnO thin films at 4500C.
Figure 4.10 The variation of average roughness of pure and Sn-doped ZnO thin films.
150
absorption spectra data using the well-known Beer’s Law [47];
(2-Absorbance)
10
………………… (4.5)
= %T
The figure reveals clearly that the average value of transmittance of thin films in
the visible/near infrared range is about 88% - 96%. However, a nearly 8% increase in
average transmission is observed with increasing the Sn concentration in SZO thin films
and this is attributed due to the decrease in the average roughness of thin films shown in
figure 4.10 The optical absorption spectra of the films shown in figure 4.12 shows that
the as synthesized thin films have low absorbance in the visible/near infrared region
while the absorbance is high in the ultraviolet region. The results are in good agreement
with the reports by other investigators [48-50]. The variation of absorption coefficient (α)
with wavelengths was found to follow the Tauc relation [48-50];
α = Ao(h- Eg)1/2 / h,
…………………(4.6)
Here Ao is a constant which is related to the effective masses associated with the
bands and Eg is the bandgap energy, h is the photon energy, α is the absorption
coefficient. Finally the plots of (αh)2 vs. the photon energy h for various SZO thin
films of varying Sn-doping concentrations are shown in figure 4.13. Linearity of the
plots indicates that the as grown materials is of direct bandgap nature. The optical band
gap, estimated by the extrapolation of the straight line to (αh)2 = 0 axis gives the optical
bandgap is found to be increasing from 3.16 eV to 3.27 eV with Tin dopant
concentrations from 0 to 2.0 at.%. The change in band gap can be attributed due to the
Burstein-Moss band gap widening and band gap narrowing due to electron-electron and
electron-impurity scattering.
4.4 ELECTRICAL PROPERTIES
In order to know to study the effect of Sn-doping on the conductivity as well
as the conduction mechanism in the SZO thin films, the sheet resistance of all pure and
Sn- doped ZnO thin films were measured by collinear four probe method at room
temperature and graphically reported as a function of the dopant (Sn) concentration in
Figure 4.14. It is remarked that resistivity magnitude depends on Sn concentration in the
precursor solution. So, the sheet resistance decreases with the increase of Sn contents %,
151
Figure 4.11The Optical Transmission spectra of pure and Sn-doped ZnO thin films.
Figure 4.12 The Optical Absorbance spectra of pure and Sn-doped ZnO thin films.
152
Figure 4.13. Evolution of the (αhυ)2 vs. hυ curves of pure and Sn-doped ZnO thin films
prepared from 0.1 M Zn(CH3COO)2•2H2O.
153
reaching a minimum in the order of 3.12 x10-2 Ω/sq. at 2.0 at.% of Sn concentration. The
decrease in sheet resistance may be explained as follows: since the ionic radius of tin
(0.38 Å) is smaller than that of the zinc ion (0.64 Å), the tin atoms doped into a ZnO
lattice act as donors by supplying two free electrons when the Sn ions occupy Zn ion
sites. When a small amount of Sn is introduced in the precursor solution of ZnO, tin can
play the role of an effective donor in ZnO layers, which can be explained by the
substitution introduction of Sn4+ ions into the Zn2+ ions sites, generating free electrons.
Sn4+ ions substituted Zn2+ ions in the lattice induce positive charges in the material. In
order to maintain electrical neutrality, two negative electrons are induced to compensate
the excess positive charges. Hence the conductivity increases due to increasing the free
electrons in the film.
When comparing the resistivity of undoped ZnO thin films with Sn-doped ZnO thin
films, it is found that the resistivity decreases. The decrease in resistivity may be
explained as follows: since the ionic radius of tin (rSn4+ = 0.38 °A ) is smaller than that of
the zinc ion (rRZn2+ = 0.6 °A ), tin atoms doped into ZnO lattice act as donors by
supplying two free electrons when the Sn4+ ions occupy Zn2+ ion sites. This in turn
increases the free carrier concentration and hence, decreases the resistivity. The
mechanism of the conduction can be described by the following equation:
SnO2 (ZnO)
SnllZn + OxO + 1/2O2 + 2el
(4.7)
Sn4+ ions substituted Zn2+ ions in the lattice induce positive Snll Zn charges in the
material. In order to maintain electrical neutrality, two negative electrons are induced to
compensate the excess positive charges. Hence the resistivity decreases due to increasing
free electrons in the film.
4.5 CONCLUSION
The pure and Sn-doped zinc oxide thin films are prepared by chemical spray pyrolysis
technique from Zn(CH3COO)2•2H2O precursor. The analysis of x-ray diffraction patterns
revealed that as synthesised doped and undoped ZnO materials are pure nanocrystalline
(NC) thin films having hexagonal wurtzite phase of Zinc oxide, which belongs to the
space group P63mc. A small change in the lattice parameters of the hexagonal unit cell
has been observed with increasing Sn content. The crystallite size & the number of unit
154
Table 4.3 Electrical and Hall Effect measurements as a function of different Sn
concentration at substrate temperature of 450 0C.
Tin concentration in
Conductivity
Hall Cofficient
Carrier
Hall Mobility
ZnO
σ (Ω-cm)-1
(RH)
Concentration
μ(cm2/Vs)
n(cm-3)
Pure ZnO
5.871×102
- 0.6793
0.92 ×1019
398.8
0.5% Sn-doped ZnO
5.932×102
- 0.192×10-1
3.25 ×1020
11.3
1.0% Sn-doped ZnO
6.721×102
- 0.213×10-1
2.93 ×1020
14.3
1.5% Sn-doped ZnO
13.573×103
- 0.096×10-1
6.51 ×1020
130.3
2.0% Sn-doped ZnO
27.765×103
- 0.043×10-1
14.53 ×1020
119.3
Figure 4.14 The Sheet Resistance of the pure and Sn-doped ZnO nanocrystalline
materials, calculated by four probe method.
155
cells in the particles of doped ZnO decreases with an increase in Sn dopant
concentration. The highest texture coefficient TC (hkl) has been found for (002) plane for
all the films and the TC(002) value increases with increasing Sn concentration in ZnO
thin films because the grains/crystallites size of doped ZnO decreases (123.5 to 94.6
nm). Surface morphological examination with AFM in contact mode revealed the fact
that the ZnO nanoparticles (NPs) are spherical in shape with a diameter ranging between
120 to 200 nm and roughness decreases with increasing the doping concentration of Sn in
ZnO thin films from 0 to 2.0 at%. Nearly 8% increase in average transmission (88% 96%) is observed with increasing the Sn concentration in SZO thin films and this is
attributed due to the decrease in the average roughness of thin films and the energy
bandgap of Sn-doped ZnO thin films were obtained from optical absorption spectra by
UV-Vis absorption spectroscopy. Upon increasing the Sn dopant concentration the
optical bandgaps of the SZO films have also increases. The sheet resistance decreases
with the increase of Sn concentration, reaching a minimum in the order of 3.92 Ω/sq. at
2.0 at.% of Sn concentration.
156
REFERENCES
1. K.Badeker, Ann. Phys. (Leipzig). 22, 749 (1907).
2. D. Davazoglou, Thin Solid Films. 302, 204 (1997).
3. T. Minami, Semicond. Science Technol. 20, 35 (2005).
4. K. L.Chopra, S.Major, and D.K. Pandya, Thin Solid Films. 102, 1(1983).
5. B. G. Lewis and D. C. Paine, MRS Bull. 25, 22 (2000)
6. Ouerfelli, J. and et al., Materials Chemistry and Physic. 112, 198 (2008).
7. B.G. Lewis and D.C. Paine, MRS Bulletin, Aug. 25, 22 (2000).
8. Z.L. Wang, Z.C. Kang, Functional and Smart Materials—Structural Evolution and
Structure Analysis, Plenum Press, New York, (1998).
9. U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.
Cho Morkoc H, J. Appl. Phys., 98,, 0413014 (2005).
10. Pu Xian Gao, L. Wang Zhong, Journal of Applied Physics 97, 044304 (2005).
11. Zhong Lin Wang, Jinhui Song, Arrays Science, 312, 241 (2006).
12. C.S. Wei, Y.Y. Lin, Y.C. Hu, C.W. Wu, C.K. Shih, C.T. Huang C.H. Chang,
Sensors and Actuators 128, 18 (2006).
13. Italy Milan, N. Huby, Tallarida, G. Kutrzeba-Kotowska, Applied Physics Letters
94, 143501 (2009).
14. Long Wangcheng, Hu Jun, Liu Jun, He. Jinliang, Materials Letters, 64, 1081 (2010).
15. Aarthy Sivapunniyam, Niti Wiromrat, Myo Tay Zar Myint, Joydeep Dutta,
Sensors and Actuators B: Chemical, 157, 232 (2011).
16. T. Ates, C. Tatar, F. Yakuphanoglu, Sensors and Actuators A: Physical, 190, 153
(2013).
17. M. Willander, O. Nur, Q.X. Zhao, L.L. Yang, M. Lorenz, B.Q. Cao, J. Zúñiga Pérez,
C. Czekalla, G. Zimmermann, M. Grundmann, A. Bakin, A. Behrends, M. AlSuleiman, A. El-Shaer, A. Che Mofor, B. Postels, A. Waag, N. Boukos, A. Travlos,
H.S. Kwack , J. Guinard, D. Le Si Dang, Nanotechnol., 20, 332001 (2009).
18. Sun-Young Sohn, Hwa-Min Kim, Seoung-Hwan Park and Jong-Jae Kim, Journal of
the Korean Physical Society 45, S732 (2004).
19. S. Mondal, S. R. Bhattacharyya and P. Mitra, Bull. Mater. Sci. 36, 229 (2013).
20. V. Gokulakrishnan, S. Parthiban, K. Jeganathan & K. Ramamurthi, Ferroelectrics
157
423, 134 (2011).
21. Nadir Fadhil Habubi, Sami Salmann Chiad, Saad Farhan Oboudi, Ziad Abdulahad
Toma, International Letters of Chemistry, Physics and Astronomy 4, 1 (2013).
22. S. Aksoy, Y. Caglar, S. Ilican and M. Caglar, Optica Applicata XL, 1 (2010).
W.T. Seeber, M.O. Abou-Helal, S. Barth, D. Beil, T. HoÈ che, H.H. Aify and S.E.
23. Demian, Materials Science in Semiconductor Processing 2, 45 (1999).
24. S. T. Shishiyanu, T. S. Shishiyanu and O. I. Lupan, Sensors and Actuators 107, 379
(2005).
25. Yasemin Caglar , Seval Axsoy,Saliha Illican and Mujdat Caglar, Super lattice and
Microstructure 46, 469 (2009).
26. S.C. Navale and I.S. Mulla, Mater. Sci. Eng. C 29, 1317 (2009).
27. M.R.Vaezi and S.K. Sadrnezhaad, Materials Science and Engineering B 141, 23
(2007).
28. S.T. Shishiyanu, T.S. Shishiyanu and Oleg I. Lupan, Sensors and Actuators B 107,
379 (2005).
29. F. Kadi Allah , S. Y. Abe , C.M. Nunez , A. Khelil , L. Cattin, M. Morsli,J.C.
Bernede A. Bougrine , M.A. del Valle and F.R. Dıaz, Applied Surface Science 253,
9241 (2007).
30. F. Paraguay D. M. Miki-Yoshida , J. Morales, J. Solis and W. Estrada L. Thin Solid
Films 373, 137 (2000).
31. A. Bougrene, M. Addou, A. Kachouane, J. C. Bernede and M. Morsli, Mater. Chem.
Phys. 91, 247 (2005).
32. M. de la L. Olvera and A. Maldonado, Phys. Stat. Sol. (a) 196, 400 (2003).
33. B. Hahn, G. heindel, E. P. Schoberer and W. Gebhaardt, Semicond. Sci. Technol.13
788 (1998).
34. W.J. Jeong, S.K. Kim and G.C. Park, Thin Slid Films 506-507, 180 (2006).
35. M. Chen, Z. L. Pei, X. Wang, C. Sun and L. S Wen, J. Vac. Sci. Technol. A 19, 963
(2001).
36. S.Major, A.Banerji and K. L.Chopra , Thin Solid Films 108, 333 (1983).
37. H.L. Hartnagel, A.L. Dawar, A.K. Jain, C.Jagdish, Institude of Physics
Publishing. (1995).
158
38. A.A.Gorbunov, W. Pompe, A. Sewing, S.V.Gaponov , A.D.Akhsakhalyan, I.G.
Zabrodin, I.A.Kaskov, E.B.Klyenkov, A.P.Morozow, N.Salaschenko, R.Dietch,
H.Mai, S.Vollmar, Appl. Surf. Sci. 649, 96 (1996).
39. K.Seshan, Handbook of Thin Film Deposition Propcesses and Techniques, Noyes
Publications, William Andrew Publishing, Norwich, NewYork, U.S.A. (2002).
40. H. Kim, A. Pique, J. S. Horwitz, H. Murata, Z. H. Kafafi, C. M. Gilmore and D. B.
Chrisey, Thin Solid Films 377, 798 (2000).
41. K. Ellmer and R. Wendt, Surf. Coat. Technol. 93, 21 (1997).
42. J. Lim and C. Lee, Thin Solid Films 515, 3335 (2007).
43. J. L. Deschanvres, B. Bochu and J. C. Joubert, J. Phys. 4(3) , 485 (1993).
44. K. Krunks, O. Bijakina, V. Mikli, T. Varema and E. Mellikov, Physica Scripta T79 ,
209 (1999).
45. M. Kokotov, A. Biller and G. Hodes, Chem. Mater. 20(14), 4552 (2008).
46. T. Schuler and M. A. Aegerter, Thin Solid Films 351, 125 (1999).
47. Beer, Annalen der Physik und Chemie, 86, 78 (1852).
48. S. Aydogu, O. Sendil, M.B. Coban, Chinese J. of Physics 50, 89 (2012).
49. N.F. Mott, R.W. Gurney, London: Oxford Univ. Press (1940).
50.J. Tauc, R. Grogorovici, A. Vancu, Phys. Stat Solidi 15, 627 (1966).
159