CHAPTER- 5

SYNTHESIS AND CHARECTERIZATION OF PURE & Al-DOPED ZINC OXIDE NANOCRYSTALLINE THIN FILMS PREPARED BY SPRAY PYROLYSIS5.0 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

of thin film-based devices are determined strongly by the structural, electrical and optical

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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.

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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

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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 c-

axis 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.

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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)-

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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

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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 for

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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

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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. 2-

methoxyethanol and monoethanolamine (MEA) were used as solvent and stabilizer,

respectively. After deposited, the films were preheated at 300 oC on a hotplate and post-

heated 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

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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 de-

ionized 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 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 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.

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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

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Figure 5.2. Demonstrating the spray pyrolysis technique for depositing thin films.

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

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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 a-

b 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.

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Figure 5.3 The x-ray diffraction (XRD) patterns of nanocrystalline pure and Al-doped

ZnO Thin films prepared by spray pyrolysis method.

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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.

(i) Crystallite size

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The crystallite size was calculated from x-ray diffraction data by using the Debye-

Scherrer formula;

B= 0 .9∗λDB cosθ ……………………(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;

nV =( 43 )π ( Ro

3

8 ) ……………………………..(5.2)

Where V=3√3 a2 c2

(for hexagonal structure)

Then equation (5.2) becomes,

n=Ro3 π

9√3 a2 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;

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ε hkl=(dhkl−do )

do ………………………………..(5.4)

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 )/ I o ( hkl )

( 1n )∑¿¿¿

…………………(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 I o(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

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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

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

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diffraction data.

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.

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nanometer regime. The development of the AFM technique for surface analysis has led to

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

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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.

Figure 5.7 (a) Three dimensional Atomic Force Microscopy (AFM) images of as prepared

pure ZnO thin films at 4500C.

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Figure 5.7 (b) Three dimensional Atomic Force Microscopy (AFM) images of as

prepared 0.5% Al-doped ZnO thin films at 4500C.

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Figure 5.7 (c) Three dimensional Atomic Force Microscopy (AFM) images of as

prepared 1.0% Al-doped ZnO thin films at 4500C.

182

Figure 5.7 (d) Three dimensional Atomic Force Microscopy (AFM) images of as prepared

1.5% Al-doped ZnO thin films at 4500C.

183

Figure 5.7 (e) Three dimensional Atomic Force Microscopy (AFM) images of as

prepared 2.0% Al-doped ZnO thin films at 4500C.

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Figure 5.8 The variation of average roughness with pure and Al-doped ZnO is calculated

from AFM.

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Figure 5.9 The schematic of the effect of Aluminum doping concentration on surface morphology of as grown of AZO thin films.

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.

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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];

10(2-Absorbance)

= %T .....................................(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

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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.

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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.

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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

σ(Ω-cm)-1

Hall Cofficient

(RH )

Carrier

Concentration

n(cm-3)

Hall

Mobility

μ(cm2/Vs)

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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

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

191

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.

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

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their maximum values of 6.698×1020cm-3 & 80.366 cm2/Vs of AZO thin films ,

respectively.

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