CHAPTER- 5
SYNTHESIS AND CHARECTERIZATION OF PURE
& Al-DOPED ZINC OXIDE NANOCRYSTALLINE
THIN FILMS PREPARED BY SPRAY PYROLYSIS
5.0 INTRODUCTION:
As already discussed in previous chapters 4 and 5 that Zinc oxide (ZnO), is one of
the most prominent and versatile metal oxide semiconductors with good electrical and
optical properties, thermal and chemical stability. It has distinct advantages over other
competitors, e.g., abundance in earth crust, non-toxicity, low material costs, chemical
stability, high transparency in the visible and near infrared spectral region, etc. [1, 3]. It is
a direct wide band gap semiconductor (Eg = 3.37 eV at 300 K) [2–7] with a similar
crystal structure to gallium nitride (GaN) wide gap semiconductor (Eg = 3.3 eV at 300 K)
[2,8]. For some of these, the interest in ZnO overlaps with that of GaN, which is widely
used for production of green, blue-ultraviolet, and white light-emitting devices and lasers
[7–9]. However, ZnO has some advantages over GaN, among which are the availability
of fairly high quality ZnO bulk single crystals [8] and a large exciton binding energy
(ZnO: Eex = 60 meV, GaN: Eex ~ 18–28 meV) [2]. On the other hand, ZnO has been
investigated intensively due to its unique characteristics that may enable its efficient
utilization in/as: varistors, piezoelectric devices such as surface-acoustic wave (SAW)
and piezoelectric sensors, transparent conducting oxide (e.g., electrode of solar cells),
antireflection coatings, field-effect transistors, phosphors (green phosphors in displays),
chemical and biological sensors, gas sensors in electronic noses, etc. [1, 5, 7, 9].
Furthermore, because of its transparency, its electro-optical and elasto-optical properties,
ZnO is attractive for integrated photonic devices [5, 9]. The performance and efficiency
160
of thin film-based devices are determined strongly by the structural, electrical and optical
properties of the component films. A study of these properties and their dependence on
the film characteristics is very important as it helps in optimizing film parameters for
better device applications. Further, the electrical and optical properties of the thin films
are determined by the process parameters during film growth such as flow-rate, substrate
temperature, concentration of the solution etc., as well as by the presence of impurities
and defects in the films. For transparent conducting oxide (TCO) thin films like ZnO, it is
always desired to improve the electrical conduction without affecting its excellent optical
properties. Also, on doping the properties of ZnO thin films drastically changes as we
observed in previous chapters. The most perspective materials for ZnO doping are Al
(ZnO:Al or AZO) and Ga (ZnO:Ga or GZO) because of the high solubility and behavior
during the growth, which allows obtaining the films of low resistivity (~ 10-4 Ohm cm)
and high transparency in the visible region of the spectrum [10]. In the present
investigation, Al is chosen as dopant material because of its easy and abundant
availability and also, because it enhances the electrical conductivity, optical transparency,
and even gas-sensing properties of the ZnO thin films, which has immensely important
for industrial and domestic applications. Figure 5.1 shows resistivity value through
various stages of ZnO film using chemical spray pyrolysis.
Figure 5.1.Resistivity values of ZnO thin films achieved so far.
161
5.1 BRIEF LITERATURE SURVEY ON PURE AND AL-DOPED
ZINC OXIDE NANOCRYSTALLINE THIN FILMS
Benny Joseph et al. studied the variation of structural, morphological, electrical
and optical properties of Al-doped ZnO film prepared using CSP technique [11]. The
films were highly transparent in visible region (97% at 550 nm) and electrically
conductive (resistivity of 2.45×10
-2
Ω cm]. XRD studies showed that the films were
polycrystalline in nature with (002) preferred orientation.
Aluminium-doped Zinc Oxide (AZO) thin films of 100 nm thickness were
prepared by systematically changing the number of “Diethyl Zinc(DEZ, Zn(C2H5)2)Distilled-water cycles” (at 150 °C) in ‘Atomic Layer Deposition’ (ALD) method [12].
ZnO films grown without any TMA- Distilled water cycle exhibited a polycrystalline
nature having (100), (002),(101), and (110) orientation. But AZO films crystallized
preferentially along the (100) direction as a result of doping with Al. The lowest
resistivity of 4.5×10 Ω cm) was observed for AZO films of ~100 nm thick and grown
with Al doping of 3.0 at. %.
Mingsong Wang et al. [13] reported that the Al-doped zinc oxide (AZO) thin
films have been prepared via a sol-gel process. Optical and photoluminescent properties
of the AZO films have been investigated. The UV absorption edge was blue shifted with
increasing Al doping concentration. Efficient green-yellow emission was obtained after
annealing at 850 0C. For the 850 0C -annealed samples, the green peak was red shifted
from 518 to 565 nm as the Al doping concentration increased from 0 to 2.0 at.%. In
addition, violet emission in the range of 400–450 nm was observed in the 850 0C annealed AZO films. The possible origins responsible for these emission bands have been
discussed.
The dependence of electrical and optical properties of the temperature of
substrate and target had been studies for Al- and Si- doped ZnO films, prepared using
R.F. sputtering [14]. The lowest resistivity was obtained for substrate temperature in the
range 180-220 °C, mainly due to the improvement in crystallanity.
It has also been reported in literature that several dopants have been used to
modify the properties of ZnO. For example; M. de la L. F. K. Shan et al. [15] has
reported Al doped ZnO thin films are fabricated on the glass substrates at different
162
temperatures by pulsed laser deposition (PLD) technique. The crystal structure, band gap
energy, refractive indices, and photoluminescence are found to depend on the growth
temperature. From X-ray diffraction measurements, we observe that the substrate
temperature of 300-400 oC is the best fabrication condition for the crystallization. From
the transmission, we find that the band gap energies decrease exponentially with the
substrate temperature. Spectroscopic ellipsometry (SE) is used to calculate the refractive
indices n of the thin films. It is found that n values of the thin films deposited at higher
temperatures are in the reasonable range. However, the n values of the thin films that
deposited at low temperatures deviate from the normal value. Photoluminescence
measurements (PL) are also used to characterize the thin films.
V. Musat et al. [16] has reported Transparent and conductive high preferential caxis oriented ZnO thin films doped with Al have been prepared by sol–gel method using
zinc acetate and aluminium chloride as cations source, 2-methoxiethanol as solvent and
monoethanolamine as sol stabilizer. Film deposition was performed by dip-coating
technique at a withdrawal rate of 1.5 cm min-1 on Corning (1737) glass substrate. The
effect of dopant concentration, heating treatment and annealing in reducing atmosphere
on the microstructure as well as on the electrical and optical properties of the thin films is
discussed. The optical transmittance spectra of the films showed a very good
transmittance, between 85 and 95%, within the visible wavelength region. The minimum
resistivity of 1.3x10-3 Ω cm was obtained for the film doped with 2 wt.% Al, preheated at
400 0C and post-heated at 600 0C, after annealing under a reduced atmosphere of forming
gas.
M. Karger et al. [17] Undoped and Al-doped ZnO films with dopant
concentrations of nominally 1% and 10% and a thickness of 100 nm have been grown on
SrTiO3 (001) by pulsed laser deposition at substrate temperatures between 650 °C and
820 °C. The epitaxial conditions were examined with high pressure in-situ reflection high
energy electron diffraction (RHEED) and ex-situ x-ray diffraction (XRD) measurements
in different geometries. The films are highly (1120)-oriented with a lattice mismatch
between the SrTiO3 [110] direction and the c-axis of about 3%. Atomic force microscopy
(AFM) revealed smooth surfaces with a roughness of drms ˂ 5 nm and different sized
islands.
163
H. Gómez-Pozos et al. [18] recently achieved the lowest resistivity of Al –
doped spray pyrolysed ZnO thin film (4.3×10−3 Ωcm). They achieved this by doping the
samples using ‘Aluminum 2,4 pentanedionate’followed by high temperature vacuum
annealing treatment.
M.K Jayaraj et al [19] has reported the conducting and transparent Al-doped
ZnO thin films by off-axis rf magnetron sputtering on amorphous silica substrates
without any post-deposition annealing. The electrical and optical properties of the films
deposited at various substrate temperatures and target to substrate distances were
investigated in detail. Optimized ZnO:Al films have conductivity of 2200 S cm–1,
mobility (18 cm-3) and average transmission in the visible range is higher than 85% show
very little temperature dependence.
Sumetha Suwanboon et al. [20] has reported Undoped, 10 wt% Al and 10 wt%
Mn doped ZnO thin films were deposited on a glass substrate by sol-gel dip coating. Al
ions played an important role in improvement of the c-axis orientation, while Mn ions
inhibited the growth along c-axis. The average grain size decreased when doping ZnO
with Al and Mn. The smallest average grain size was 25 nm, obtained with 10 wt% Mn
doping. The band gap values of prepared thin films varied in the range of 3.24 - 3.96 eV.
C. Guillen et al. [21] has reported Transparent and conductive Al-doped ZnO
(AZO) films have been grown with various thicknesses between 0.3 and 1.1 mm by
magnetron sputtering at room temperature onto soda lime glass substrates. After
deposition, the samples have been annealed at temperatures ranging from 150 to 450 oC
in air or vacuum. The visible transmittance ~ 85–90% and resistivity ~ 1.6–2.0 mΩ cm,
both parameters slightly decreasing when the film thickness increases. Heating in air or
vacuum produces further elongation of the crystalline lattice together with some increase
of the visible transmittance and a decrease of the electrical resistance that depends on the
heating temperature and atmosphere. The best characteristics has been obtained after
treatment in vacuum at ~ 350 oC, where the highest carrier concentrations are achieved,
giving visible transmittance 90–95% and resistivity ~ 0.8–0.9 mΩ cm for the AZO layers
with various thicknesses.
Sung-Hae Lee et al. [22] has reported the use of aluminum-doped zinc oxide
(AZO) layer as a transparent conducting oxide (TCO) layer in ZnO nanowire (NW)164
based dye-sensitized solar cells (DSSC’s). The well aligned, single crystalline ZnO NW
arrays that were grown on the AZO films exhibited a better DSSC performance (an
increased photocurrent density and fill factor) than those grown on the fluorine doped tin
oxide (FTO) films. The I-V characteristics and electrochemical impedance spectroscopy
measurements for the ZnO NW arrays on the AZO and FTO films clearly showed that the
superior DSSC performance was caused by the facilitated charge injection from the ZnO
NW to AZO, resulting from the formation of an ohmic contact. This study demonstrates
that the AZO films are more favorable for highly efficient ZnO NW-based photoenergy
conversion devices.
M. Ajili et al. [23] has reported the deposition of Aluminum doped zinc oxide
(AZO) thin films on glass substrate, by chemical spray pyrolysis. The structural, optical
and electrical properties of these films as a function of Al content were investigated (y=
([Al3+]/[Zn2+]) solution= 0, 1%, 2% and 3%, y is the concentration ratio in the spray
solutions). XRD results reveal the AZO films have a hexagonal wurtzite structure with
(002) preferred orientation. The average transmittance of all the films is higher than 80%
in a wide wavelength 375-2500 nm. The band gap energy (3.28 eV), calculated for all
samples with different Al dopant concentration. A minimum resistivity ρ of 1.328 Ω cm
is obtained for the film doped with 1 mol% Al. The carrier concentration of the films are
obviously increased until the Al3+ reaches its saturation content of about 2 at. % and the
Hall mobility μ decreased with increasing the aluminum dopant concentration. Doping
with Al3+ in the ZnO influences the volume composition of ZnO:Al thin films, makes the
film surface roughness increase until y= 2% (above 2% it decreases) and leads to the
decreasing of average lateral grain size.
Jungwoo Kang et al. [24] reported the deposition of thin films of gallium-doped
zinc oxide (GZO) (Ga2O3: 90 wt%, ZnO: 10 wt%) on glass substrates by sputtering and
aluminum-doped zinc oxide (AZO) (Al2O3: 3 wt%, ZnO: 97 wt%) targets simultaneously
using a direct-current (DC)/radio-frequency (RF) magnetron co-sputtering system. The
concentration of gallium (Ga) in the film was varied by using different RF powers for
sputtering the GZO target with the DC power for sputtering the AZO target fixed. A
minimum resistivity was obtained at an RF sputtering power of 200 W for the GZO target
when the DC sputtering power for the AZO target was fixed at 60 W. It was found that
165
the resistivity of AZO thin films (6.40 × 10−3 Ω-cm for 0 W) could be lowered by more
than one order by co-sputtering AZO and GZO targets to make AGZO thin films (2.14 ×
10−3 Ω-cm for 200 W) without lowering their transmittance at all.
S. Tewari et al. [25] has deposited the zinc oxide (ZnO) on cleaned glass
substrates by chemical spray pyrolysis technique using Zn(CH3COO)2 as precursor
solution. Also, aluminium-doped thin films of ZnO were prepared by using AlCl3 as
doping solution for aluminum. The dopant concentration [Al/Zn atomic percentage
(at%)] was varied from 0 to 1.5 at% in thin films of ZnO prepared in different
depositions. All the films were of zinc oxide having polycrystalline nature and possessing
typical hexagonal wurtzite structure with crystallite size varying between 100.7 and 268.6
nm. The electrical studies established that 1 at% of Al doping was the optimum for
enhancing electrical conduction in ZnO thin films and beyond that the distortion caused
in the lattice lowered the conductivity.
A.V. Shorokhova [26] has demonstrated the possibility of the high quality AZO
thin films fabrication by the PLD method using the second harmonic of the Q-switched
YAG:Nd3+ laser. The investigations of dependence of the AZO thin films properties on
the PLD conditions (concentration of Al in the target, buffer gas pressure) have been
conducted.
R. E. Treharne et al. [27] has reported a rapid screening methodology for the
development of transparent conducting oxides thin films. The methodology, based on a
combination of spectrophotometry, ellipsometry and 4-point probe measurements, was
used to map out the opto-electronic properties over a co-sputtered ZnO :Al2O3 film
deposited from separate ceramic targets of ZnO and Al2O3. Clear distributions for the
carrier density, ne, and mobility, μe, are determined as a function of wt%. Al2O3 content
within the film. A minimum resistivity value of 7.6 × 10−4 Ω cm was achieved for a
composition of 1.5 wt%. Al2O3.
Min-Chul Jun et al. [28] has reported the influences of aluminum and gallium
dopants (0 to 2.0 mol%) on zinc oxide (ZnO) thin films regarding crystallization and
electrical and optical properties for application in transparent conducting oxide devices.
Al- and Ga-doped ZnO thin films were deposited on glass substrates (corning 1737) by
sol–gel spin-coating process. A lowest sheet resistance of 3.3 × 103 Ωcm was obtained
166
for the GZO thin film doped with 1.5 mol% of Ga after post-annealing at 650°C for 60
min in air. All the films showed more than 85% transparency in the visible region.
Y. Okuhara et al. [29] has reported the preparation of aluminium-doped zinc
oxide (ZnO:Al) as heat reflective coatings by multi-target reactive sputtering using
metallic Zn and Al targets. To control the surface oxidation of each target during
sputtering, emission intensity of Zn plasma and current-voltage characteristics for the Al
target were monitored. The lowering oxygen content in addition to the optimizing Al
content achieved the carrier concentration Ne of 11.0×1020 cm-3 and the Hall mobility μ
of 25.0 cm2/Vs. The highest Ne provided the shortest plasma wavelength λp of 1375 nm,
which shifted the near-infrared reflectance spectrum closer to the visible region. The high
λp reduced the optical absorption and enhanced the reflectance. The local structure and
the carrier generation behaviour of the doped Al were evaluated by X-ray absorption fine
structure (XAFS) measurements.
Tao Wang et al. [30] have synthesised highly-transparent Al-doped ZnO porous
network thin films via solvothermal method for the first time. It is found that the
morphology of the nanostructures evolved from nanosheets to porous network structures
because of the different annealing temperature. The incorporation of Al and the contents
of NaOH play important roles in the morphology of porous network nanostructures.
A growth mechanism is proposed to explain these findings. Al is successfully doped into
the ZnO wurtzite lattice structure as revealed by X-ray diffraction (XRD) and verified by
the energy dispersive X-ray detector (EDX). The unique nanostructure thin films exhibit
a
typical
wurtzite
structure
and
strong
violet
emission.
Ultraviolet-visible
spectroscopy (UV-VIS) shows the films have a good transparency, and the absorption
edges are obviously blue shifted with a temperature increase. By optimizing the reaction
parameters, the morphology, and size of the network thin films could be tailored, which
will be very significant for fabricating novel and surface morphology controlled
photocatalysis, optoelectronics, solar energy and self-cleaning devices.
M.C. Jun et al. [31] reported that transparent conducting oxide (TCO) materials
with high transmittance and good electrical conductivity had been attracted much
attention due to the development of electronic display and devices such as organic light
emitting diodes (OLED’s), and dye-sensitized solar cells (DSSC.s). Aluminum doped
167
zinc oxide thin films (AZO) have been well known for their use as TCO materials due to
its stability, cost-effectiveness, good optical transmittance and electrical properties.
Especially, AZO thin film, which have low resistivity which is similar to that of ITO
films with wide band gap semiconductors. The AZO thin films were deposited on glass
substrates by sol-gel spin-coating process. As a starting material, zinc acetate dihydrate
(Zn(CH3COO)2 . 2H2O) and aluminum chloride hexahydrate (AlCl3 .6H2O) were used. 2methoxyethanol and monoethanolamine (MEA) were used as solvent and stabilizer,
respectively. After deposited, the films were preheated at 300 oC on a hotplate and postheated at 650 oC for 1.5 hrs in the furnace. We have studied the structural and optical
properties as a function of Al concentration (0-2.5 mol.%).
5.3 PRESENT INVESTIGATION
As already described and discussed in chapter 4, among the number of methods
used for the deposition of doped and un-doped ZnO thin films, the chemical spray
pyrolysis (CSP) represents the less expensive alternative, since it can produce large area,
high-quality and low cost thin films of easily doped with varying dopant concentrations,
by adding a suitable compound of the dopant to the spray solution. So in the present
work, the thin films of pure and Al-doped ZnO thin films have been synthesized by the
chemical spray pyrolysis method.
5.3.1 PREPARATION OF AZO THIN FILMS
In present investigation pure and Al-doped ZnO transparent conducting thin films
have been prepared by spray pyrolysis method onto the glass substrates at 4500C
temperature. 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 Aluminum was used as the source of dopant in the present
investigation, so 0.1 M Aluminum chloride hexahydrate [AlCl3•6H2O] (Sigma-Aldrich
≥99%) was 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
homogeneous solution. The dopant solution of Aluminum chloride hexahydrate
[AlCl3•6H2O] in varying doping concentration from 0 to 2.0 at% was added to starting
168
solution with continuous stirring until a homogeneous solution was obtained. Now, the
pure and Al-doped ZnO thin films were deposited by spraying the prepared solution in air
atmosphere on 1cm×1cm glass substrates preheated at 450 0C. Before deposition, the
glass substrates were ultrasonically cleaned in acetone solution and then rinsed by deionized water. After the successful deposition of doped and undoped films, the thickness
was measured by profilometery (Bruker’s Dektak XT). All the undoped and Al-doped
ZnO thin films of different doping concentrations were prepared separately under the
same parametric conditions, as given in table 5.1. After the successful deposition of films
from [Zn(CH3COO)2•2H2O] precursor by CSP, their structural, electrical and optical
properties have been investigated in details.
5.3.2 STRUCTURAL STUDIES
5.3.2 (a) Phase Identification
As already discussed in section 2.1 of chapter-2 that x-ray diffraction (XRD), a nondestructive 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 electronic and optical properties. Since the x-ray diffraction peaks of thin
films deposited on a substrate, contribution from substrate to the diffraction can
sometimes overshadow the contributions from thin film. Signals from the layer can be
maximized by carrying the measurements at fixed low glancing angles (α) of the primary
beam. The sample can then be characterized as a function of depth by varying the angle
of incidence between the critical angle of incidence (αc) for the total reflection of x-rays
and ~10. This system consists of x-ray source, divergence slit, stationary holder to hold
the sample (thin films or pellet), receiving slit, solar slit, anti- scatter slit, flat crystal
monochromator and proportional counter. The source was made up of Cu, which can
emit two radiationtions Kα and Kβ, and these come out of the beryllium window in the
line focus geometry. Cu-mirrors along with a divergence slit and solar-slit were used in
the incident beam side. The x-ray mirror converts the divergence of the beam by
diffracting it off a graded multi-layer. This process retains much more of the x-ray flux
and thereby an enormous gain in intensity can be obtained. The soller-slit permits only.
169
Table 5.1. The Spray growth parameters of pure and Al-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 5.2. Demonstrating the spray pyrolysis technique for depositing thin films.
170
axial rays to pass through it. The Cu-mirror filters out the Kβ component from the beam,
which initially consists to both Kα and Kβ rays. Finally pure Kα component comes out of
the mirror. These Kα rays are allowed to fall at a small glancing angle on the sample
placed above the IR (infra-red) controlled sample stage. The sample is kept stationary
during the measurement, whereas the detector alone is moves with an angle 2θ with
respect to the incident beam. Diffracted x-rays from the thin film sample pass through the
receiving soller slit and also through a monochromator (graphite flat crystal), and finally
fall on the detector, which is a scintillation counter. The incident angle is kept constant
during the measurement. The sample rotates in a vertical plane to take care of the
preferential orientation of the grains. The output from the detector goes to a computer,
which then records the spectrum. Figure 5.3 shows the representative x-ray diffraction
patterns of pure and Al-doped ZnO thin films with various Al doping concentration (0.5,
1.0, 1.5 & 2.0 at.%). 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 Al
or Zn are detected, indicating high purity of the product of Al-doped ZnO (abbreviated as
AZO). The lattice parameters of AZO have been calculated using high angle XRD peaks
corresponding to the reflection, such as, (002), (101) and (110) shown in Figure 5.3. All
the as deposited films have been found to have preferential orientation along (002) crystal
plane. The 2.0 at. % AZO thin film grows as single nanocrystalline because it has only
one reflection (002). It seems to be first of its kind in case of AZO thin films, to the best
of our knowledge. The variation of calculated lattice parameters of AZO with dopant
ratio of [Al]/[Zn] equal to 0, 0.5, 1.0, 1.5 and 2.0 by at.% are shown in Table 5.2. A
small decrease in the a-lattice parameter of the hexagonal unit cell has been observed
with increasing Al content. This may be possibly due to the strong covalent bonding in ab plane and difference in ionic radii of Zn+2(0.74 Å) and Al+3(0.54 Å) ions. However, the
c-lattice parameter increases due to the weaker covalent bonding along c-axis (since the
c-parameter (5.183Å) is larger than a-parameter (3.253Å)) in a-c plane.
171
Figure 5.3 The x-ray diffraction (XRD) patterns of nanocrystalline pure and Al-doped
ZnO Thin films prepared by spray pyrolysis method.
172
Figure 5.4 The variation of lattice strain with crystallites oriented at different direction
(hkl) for pure and Al-doped ZnO thin films, calculated from X-ray diffraction data.
173
(i)
Crystallite size
The crystallite size was calculated from x-ray diffraction data by using the Debye-
Scherrer formula;
𝐁 = 𝟎. 𝟗 ∗ 𝛌⁄𝐃 𝐜𝐨𝐬𝛉
𝐁
……………………(5.1)
Where B is 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 AZO as a function of Al doping concentrations is
shown in table 5.2. From the table it is reflected that the crystallite size increases with
increasing Al concentration in ZnO. The maximum crystallite size of 236.97 Å is found
for 2.0 at% Al-doped ZnO. This is due to the increasing the number of unit cells with
increasing the Al concentration of the ZnO 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;
𝐑𝟑
𝐧𝐕 = (𝟒⁄𝟑)𝛑 ( 𝐨⁄𝟖)
……………………………..(5.2)
𝟐
𝐕 = 𝟑√𝟑 𝐚 𝐜⁄𝟐 (for hexagonal structure)
Where
Then equation (5.2) becomes,
𝐧 = 𝐑𝟑𝐨 𝛑⁄
𝟗√𝟑𝐚𝟐 𝐜
…………………………. (5.3)
The calculated number of unit cells for different AZO films are listed in table 5.2. It is
evident from the table 5.2 that the number of unit cells in the crystallite/grain increases
with an increase in Al concentration in AZO thin films. The morphology of AZO thin
films as revealed by AFM (figure 5.6) also supports the enhancement of nanoparticles
size (~150–250 nm).
(iii)
Lattice strain
The lattice strain of the deposited thin films has been calculated by the formula given
below;
𝛆𝐡𝐤𝐥 =
(𝐝𝐡𝐤𝐥 − 𝐝𝐨 )
⁄𝐝
𝐨
………………………………..(5.4)
174
Where dhkl and do are the calculated and standard lattice spacing of the reflecting
planes respectively. The variation of calculated lattice strain of crystallites oriented at
different directions (hkl) for pure and Al doped ZnO thin films, are shown in figure 5.4
and their exact values are given in table 5.2. The crystallinity of AZO thin films has been
found to decrease with increasing Al doping since the observed XRD patterns show that
as Al concentration is increased, intensity of the XRD peak decreases monotonously. It
may probably be associated to the lattice disorder and strain induced (Table 5.2) in the
ZnO lattice due to the substitution of Zn2+ by comparatively lower ionic radius of Al3+
ions.
(iv) 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)]
…………………(5.5)
Where I(hkl) is the observed XRD peak intensities obtained from the AZO 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 0C substrate temperature, the variation of texture
coefficients of the AZO films with the different orientations (hkl) and doping
concentrations have been shown in figure 5.5 and 5.6 respectively. The value of texture
coefficient have been found in range of 1.237 and 7.493. These increment of texture
coefficient may be the possible reason for the enhancement of grain size of the AZO
films from 138 to 237 nm as the Al concentration in ZnO increased from 0 to 2.0 at. %.
The highest TC (hkl) has been found for grains textured along (002) plane for all AZO
films and this decrease with increasing Al concentration in AZO thin films.
5.3.2 (b) Surface Morphological studies
As compared to the SEM, the AFM studies are usually carried out to study the surface
morphology, detect any damages and cracks on the surface, surface homogeneity,
uniformity, nanostructure, nature of grains, and any other special features that are in
175
Figure 5.5 The variation of texture coefficient with crystallites oriented at different
direction (hkl) for pure and Al-doped ZnO thin films, calculated from X-ray
diffraction data.
176
Figure 5.6 The variation of texture coefficient with different Al concentration in ZnO for
crystallites oriented at different direction (h k l), calculated from X-ray diffraction data.
nanometer regime. The development of the AFM technique for surface analysis has led to
177
a detailed study of the surface of materials down to atomic scale because of its
exceptionally high vertical resolution. In this regards, the AFM study of nanostructure
substochiometric oxides can provide vital information about the high surface to-volume
ratio and the semiconducting behavior which are the combined properties prerequisite for
technological applications. In fact, AFM is the basic confirmation tool and it plays an
important role in nanotechnology. The AFM technique is based on the electron tunneling
between a sharp metal tip and the surface of the conducting/semiconducting solid surface.
In the present investigation the surface morphological examination of as prepared thin
films has been with atomic force microscopy (AFM).The results are shown in figures
5.7(a-e). The variation of average roughness of the films with varying Al doping content
are shown in figure 5.7. The AFM images (figure 5.7) demonstrate clearly that films have
a relatively smooth surface, consisting spherical AZO nanoparticles (NPs). 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 5.2). It is noteworthy to mention here
that, similar to XRD results, the grain/particle size, estimated from AFM data, was found
to increase and the average roughness of the films decreases (figure 5.8) with increasing
Al doping concentration. As the doping concentration increases, surface looks more
compact with relatively larger grains. Such overgrowth can be explained on the basis of
nucleation and coalescence process as shown in figure 5.9. Further 2.0 at. % aluminum
doping causes the formation of larger grains oriented along single (002) plane. 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 provide the
average mean crystallite size of grains/crystallites (single crystals) 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.
178
Figure 5.7 (a) Three dimensional Atomic Force Microscopy (AFM) images of as prepared
pure ZnO thin films at 4500C.
179
Figure 5.7 (b) Three dimensional Atomic Force Microscopy (AFM) images of as
prepared 0.5% Al-doped ZnO thin films at 4500C.
180
Figure 5.7 (c) Three dimensional Atomic Force Microscopy (AFM) images of as
prepared 1.0% Al-doped ZnO thin films at 4500C.
181
Figure 5.7 (d) Three dimensional Atomic Force Microscopy (AFM) images of as prepared
1.5% Al-doped ZnO thin films at 4500C.
182
Figure 5.7 (e) Three dimensional Atomic Force Microscopy (AFM) images of as
prepared 2.0% Al-doped ZnO thin films at 4500C.
183
Figure 5.8 The variation of average roughness with pure and Al-doped ZnO is calculated
from AFM.
Figure 5.9 The schematic of the effect of Aluminum doping concentration on surface
morphology of as grown of AZO thin films.
184
5.3.3 OPTICAL PROPERTIES OF AZO THIN FILMS
Since the optical properties of transparent conducting oxide (TCO) layer are one of
the major factors to contribute to the thin film’s applications. As the top layer of PV solar
cells (as discussed in section 1.3 of chapter-1), the TCO has to be a conductor and as
transparent as possible, because, in order to obtain a good performance cell, more light is
needed to reach the absorption layer to excite election-hole pairs (EHP). On the other
hand, different layers of solar cells connect to each other in series: a low resistive top
layer will reduce the voltage drop that can improve the output of the cell. Unfortunately,
high transmittance and low resistivity cannot easily be obtained at the same time.
Researchers are trying to find the optimal parameters to achieve the best match. This is
also the final purpose of the present thesis work. The optical absorption spectra of the
films shown in figure 5.10. This reveals that the as synthesized thin films have low
absorbance in the visible/near infrared region while the absorbance is high in the UV i.e.
ultraviolet region. Results are in good agreement with the results reports by other
investigators [32-34]. The absorption coefficient ‘α’ was found to follow the Tauc
relation [35, 36];
α = Ao(h- Eg)n / h,
…………………….. (5.7)
where 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, n
is a constant which is 1/2 for direct bandgap material and n is 2 for indirect bandgap
material. As our material was direct bandgap, So we put in equation (ii), n= ½;
α = Ao(h- Eg)1/2 / h,
…………………..(5.8)
The extrapolation of straight line to (αh)2 = 0 axis gives the value of energy of
bandgap. Plots of (αh)2 vs. the photon energy h for thin films of varying Al-doping
concentrations are shown in figure 5.12. Linearity of the plots indicates that the material
is of direct bandgap nature. The extrapolation of straight line to (αh)2 = 0 axis gives the
optical bandgap of about 3.76 eV of pure and Al doped ZnO thin films. The optical
transmission spectrum (T) was calculated from the optical absorption spectra data using
the well-known Beer’s Law [37];
(2-Absorbance)
10
= %T
185
.....................................(5.9)
The calculated optical transmission spectra (T) of the as synthesized AZO thin
films grown on glass substrates of varying dopant concentration (0 to 2.0 at %) as a
function of wavelengths are plotted in figure 5.11. The curves reveal clearly that the
average value of transmittance of thin films in the visible/near infrared range is about
92% - 96%. Thus, a nearly 4% increase in average transmission is observed with
increasing the Al-doping concentration in ZnO thin films and this is attributed due to the
decrease in the average roughness of thin films shown in figure 5.8.
5.3.4 ELECTRICAL PROPERTIES OF AZO THIN FILMS
Numerous investigations have been made on the electrical properties of transparent
conducting oxide films to understand the conduction phenomena [72]. Researchers have
made a systematic study on the effect of various parameters such as nature of substrate,
substrate temperature, film thickness, nature of dopants and its concentration etc [73, 74]
on the electrical properties of TCO films. The high conductivity of the TCO films results
mainly from non stoichiometry. The conduction electrons in these films are supplied
from donor sites associated with oxygen vacancies or excess metal ions. These donor
sites can be easily created by chemical reduction. Unintentional doping (which happens
mainly in the case of film deposition by spray pyrolysis), intentional doping and
contamination by alkali ions from the glass substrate can affect electrical conductivity.
One of the major factors governing the conductivity of TCO films is the carrier
mobility. The mobility of the carriers in the polycrystalline film is dependent on the
mechanism bywhich carriers are scattered by lattice imperfections. The various scattering
mechanisms involved in semiconducting thin films are acoustic deformation potential
scattering [75], piezoelectric scattering [76], optical phonon scattering [77] neutral
impurity scattering [78], ionized impurity scattering [79], electron-electron scattering
[80] and grain boundary scattering [81]. In the case of a polycrystalline film, the
conduction mechanism is dominated by the inherent inter-crystalline boundaries rather
than the intra-crystalline characteristics.
These boundaries generally contain fairly high densities of interface states that trap
free carriers by virtue of the inherent disorders and the presence of trapped charges. The
interface states results in a space charge region in the grain boundaries. Due to this space
charge region, band bending occurs, resulting in potential barriers to charge transport.
186
Figure 5.10 The Optical Absorbance spectra of pure and Al doped ZnO thin films.
Figure 5.11 The Optical Transmission spectra of pure and Al doped ZnO thin films.
187
188
Figure 5.12 Evolution of the (αhυ)2 vs. hυ curves of pure and Al-doped ZnO thin films
prepared from 0.1 M Zn(CH3COO)2•2H2O.
Table-5.3 Electrical and Hall Effect measurements as a function of different Al
concentration at substrate temperature of 4500C.
Aluminium
concentration in ZnO
Conductivity
Hall Cofficient
σ(Ω-cm)-1
(RH )
Carrier
Concentration
n(cm-3)
Hall
Mobility
μ(cm2/Vs)
Pure ZnO
2.321×102
0.4582
1.364×1019
106.3
0.5% Al-doped ZnO
3.221×102
0.263×10-1
2.375×1019
8.744
1.0% Al-doped ZnO
3.692×102
0.218×10-1
2.865×1020
8.052
1.5% Al-doped ZnO
4.336×103
0.156×10-1
4.001×1020
67.728
2.0% Al-doped ZnO
8.623×103
0.093×10-1
6.698×1020
80.366
189
The four-point probe method in the Van der Pauw configuration [82] is
widely used for the determination of the electrical resistivity (ρ), carrier concentration
(n), carrier mobility (μ), and Hall coefficient RH of a semiconductor layer. Details of the
method of measurement have already been discussed in section 2.3 of chapter-2.
The sheet resistivity of the Al-doped ZnO thin films from 0 -2.0 at.% were
found to be 89Ω/sq, 64Ω/sq, 33 Ω/sq, 17 Ω/sq and 9 Ω/sq deposited at substrate
temperatures Ts=450°C, respectively. As in the case of the optical transmittance, the
sheet resistivity of the film depends on the Al concentration. Here the deposition
temperature 450°C gave the optimum value of sheet resistivity and the optical
transmittance. This result may be related to the structure of the film such as its uniformity
and homogeneity. As seen from table 5.3, the resistivity of the doped films decreased as
the Al concentration increased from 0 at% to 2.0 at%. The maximum value for the
conductivity (σ) of 8.623×103 (Ω-cm)-1 was obtained for 2.0 at% Al-doped ZnO films.
This is due to the both carrier concentration and Hall mobility increased 6.698×1020 cm-3
and 80.366 cm2/Vs respectively at 450°C.
The conduction characteristics of the ZnO are primarily dominated by
electrons generated from O2- vacancies and Zn interstitial atoms. The electrical
conductivity in Al-doped ZnO film is higher than that in pure ZnO films due to the
contribution from Al3+ ions on substitutional sites of Zn2+ ions. When the films were
prepared by spray pyrolysis from the ceramic target, the decrease of resistivity with
increasing substrate temperature in the range 400-450°C was related to the improvement
of thin film crystal structure, more scattering centers had been formed for films deposited
at higher substrate temperature, thus concluding the ion impurity scattering as the
dominant factor for the decrease of conductivity by calculating the mean free path of
electron. It may be suggested here that, due to the rapid increase of Zn vapor pressure
with substrate temperature, re-evaporation of surplus Zn and dopants atoms should be
considered in determining the properties of the doped films. Finally it may also suggested
that the film deposited at higher substrate temperature of 450°C exhibited a decrease of
Zn content and increase of Aluminum dopant content and hence increasing the interstitial
defects.
190
5.3.5 CONCLUSION
The pure and Al doped zinc oxide thin films are prepared by 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 P63/mc. A small change in the lattice parameters (~ 0.35%) of the hexagonal
unit cell has been observed with increasing Al content. The 2.0 at.% Al-doped ZnO thin
film grows as single nanocrystalline because it has only one reflection (002). The
crystallite size & the number of unit cells in the particles of doped ZnO increases with an
increase in Al dopant concentration. The highest texture coefficient TC(hkl) has been
found for (002) plane for all the films and the TC(002) value decreases with increasing
Al concentration in ZnO thin films because the grains/crystallites size of doped ZnO
increases (138.14 to 236.97nm).
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
150 to 250 nm and roughness decreases with increasing the doping concentration of Al in
ZnO thin films from 0 to 2.0 at%.
The sheet resistivity of the Al-doped ZnO thin films from 0 -2.0 at.% were
found to be 89Ω/sq, 64Ω/sq, 33 Ω/sq, 17 Ω/sq and 9 Ω/sq deposited at substrate
temperatures Ts=450°C, respectively. Hall measurements studies showed that the as
grown films are n-type and degenerate semiconductors. The increase in Al concentration
resulted in the decrease of resistivity. However, the carrier concentration and Hall
mobility increased as the Al concentration increased from 0.5 at.% to 2.0 at.%, reaching
their maximum values of 6.698×1020cm-3
respectively.
191
& 80.366 cm2/Vs of AZO thin films ,
REFERENCES
1. K. Ellmer, A, Klein, Springer Series in Materials Science, 104, 1 (2008).
2. C. Bundesmann, R. Schmidt-Grund, M. Schubert, Springer Series in Materials
Science, 104, 79 (2008).
3. J. L. Chen, D, Chen, Z. H. Chen, Sci. China Ser. E Tech. Sci. 52, 88 (2009).
4. Y. K. Tseng, G. J. Gao, S. C. Chien, Thin Solid Films, 518, 6259 (2010).
5. O. Lupan, L. Chow, S. Shishiyanu, E. Monaico, T, Shishiyanu, V. Şontea, B. Roldan
Cuenya, A. Naitabdi, S. Park, A. Schulte, Mater. Res. Bull. 44, 63 (2009).
6. O. Lupan, S. Shishiyanu, V. Ursaki, H. Khallaf, L. Chow, T. Shishiyanu, V. Sontea,
E. Monaico, S. Railean, Sol. Energy. Mater. Sol. Cells 93, 1417 (2009).
7. H. Von Wenckstern, H. Schmidt, M. Brandt, A. Lajn, R. Pickenhain, M. Lorenz, M.
Grundmann, D. M. Hofmann, A. Polity, B. K. Meyer, Prog. Solid State Chem. 37,
153 (2009).
8. L. Znaidi, T. Touam, D. Vrel, N. Souded, S. B. Yahia, O. Brinza, A. Fischer, A.
Boudrioua, Coatings, 3, 126 (2013)
9. L. Schmidt-Mende, J. L. MacManus-Driscoll, Mater. Today, 10, 40 (2007).
10. Min-Chul Jun, Sang-Uk Park and Jung-Hyuk Koh, Jun et al. Nanoscale Research
Letters, 7, 639 (2012).
11. Benny Joseph , P.K. Manoj and V.K.Vaidyan,Ceramics International 32, 487 (2006).
12. Parag Banerjee,Won-Jae Lee, Ki-Ryeol Bae,Sang Bok Lee and G.W. Rubloff,
Journal of Applied Physics, 108, 043504 (2010).
13. Mingsong Wang, Ka Eun Lee, Sung Hong Hahn, , Eui Jung Kim,
Sunwook Kim, Jin Suk Chung, Eun Woo Shin, Chinho Park, Materials Letters, 100,
082019 (2008).
14. T. Minami, H. Sato, T. Sonoda, H. nanto and S. Takata, Thin Solid Films, 171, 307
(1989).
15. F. K. Shan, B. C. Shin, S. C. Kim and Y. S. Yu, Journal of the Korean Physical
Society, 42, S1374 (2003).
16. V. Musat, B. Teixeira, E. Fortunato, R.C.C. Monteiro, P. Vilarinho Surface and
Coatings Technology 180 , 659 (2004).
17. M. Karger and M. Schilling, Phys. Rev, B, 71, 075304 (2005).
192
18. H Gómez-Pozos, A Maldonado and M de la L. Olvera, Materials Letters, 61, 1460
(2007).
19. M. K. Jayaraj, A. Antony and M. Ramachandran, Bull. Mater. Sci., 25, 227 (2002).
20. Sumetha Suwanboon, Tanattha Ratan and Thanakorn Ratan, Walailak J Sci & Tech,
4, 111 (2007).
21. C. Guillen, J. Herrero Vacuum, 84, 924 (2010).
22. Sung-Hae Lee, J. Phys. Chem. C , 114, 7185 (2010).
23. M. Ajili, N. Jebbari, N. K. Turki and M. Castagne, International Renewable
Energy Congress, 5, (2010).
24. J. Kang, H. W. Kim and C. Lee, Journal of the Korean Physical Society, 56, 576
(2010).
25. S Tewari and A Bhattacharjee, Praman journal of physics,76, 153 (2011).
26. A.V. Shorokhova, D.A. Zuev, A.A. Lotin, O.A. Novodvorsky, O.D. Khramova,
Advanced Laser Technologies, 20, 2 (2012).
27. R E Treharne, K Hutchings, D A Lamb, S J C Irvine, D Lane and K Durose, J. Phys.
D: Appl. Phys ,45, 335102 (2012).
28. Min-Chul Jun, Sang-Uk Park and Jung-Hyuk Koh, Nanoscale research Letters, 7,
639 (2012).
29. Y. Okuhara, H. Matsubara, C. Numako and M. Takata, Journal of the Australian
Ceramic Society Volume, 49, 15 (2013).
30. Tao Wang, Liu Yanmei , Li Guang , Sun Zhaoqi , Lu Jianguo , Beibei Liu
Wu Mingzai , Cryst. Eng. Comm., 13, 2661 (2011).
31. M. C. Jun, J. H. Koh, J Nanosci Nanotechnol., 7, 3403 (2013).
32. Atul Gupta, H.S. Bhatti, D. Kumar, N.K. Verma, R.P. Tandon, Digest Journal
of Nanomaterials and Biostructures 1, 1 (2006).
33. S. Aydogu, O. Sendil, M.B. Coban, Chinese J. of Physics 50, 89 (2012).
34. N.F. Mott, R.W. Gurney, Electronic Processes in Ionic Crystals (Oxford University
Press,London: Oxford Univ. Press 1940.
35. J. Tauc, R. Grogorovici, A. Vancu, Phys. Stat Solidi 15, 627 (1966).
36. J.I. Pankove, Optical Processes in Semiconductors (Prentice-Hall, Englewood Cliffs,
N.J., 1971).
193
37. Suwanboon Sumetha, Chukamnerd Suchada, Anglong Utsanee, J. Sci. Technol. 29,
1563 (2007).
194