Investigation on the effect of Al concentration on the structural, optical and electrical properties of spin coated Al:ZnO thin films

Accepted Manuscript Investigation on the effect of Al concentration on the structural, optical and electrical properties of spin coated Al: ZnO thin films P. Raghu, N. Srinatha, C.S. Naveen, H.M. Mahesh, Basavaraj Angadi PII:

S0925-8388(16)33053-5

DOI:

10.1016/j.jallcom.2016.09.290

Reference:

JALCOM 39124

To appear in:

Journal of Alloys and Compounds

Received Date: 22 July 2016 Revised Date:

21 September 2016

Accepted Date: 27 September 2016

Please cite this article as: P. Raghu, N. Srinatha, C.S. Naveen, H.M. Mahesh, B. Angadi, Investigation on the effect of Al concentration on the structural, optical and electrical properties of spin coated Al: ZnO thin films, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.09.290. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Investigation on the effect of Al concentration on the structural, optical and electrical properties of spin coated Al : ZnO thin films Raghu P1, Srinatha N2,3, Naveen C S4, Mahesh H M1, $ and Basavaraj Angadi2,* 1

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Dept. of Electronic Science, Bangalore University, Bangalore, India 560056 2 Dept. of Physics, Bangalore University, Bangalore, India 560056 3 Department of Post Graduations studies in Physics, Vijaya College, RV Road, Bangalore, India 560004 4 Innovative Nano and Micro Technologies Pvt. Ltd., Bangalore, India 560056

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Abstract

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Al doped ZnO thin films were deposited using sol-gel spin coating method onto ITO coated glass substrate and their structural, optical and electrical properties were investigated through XRD, UV-Visible spectroscopy and two probe set up, respectively. Structural studies reveal that, undoped films are polycrystalline in nature where as Al doping show significant preferred orientation along c – axis. Optical studies reveal > 90% transparency in films and also an increase in the band gap energy with al doping, due to increase in the carrier concentration of the Al doped ZnO and the mechanism is well explained on the basis of Burstein – Moss effect. In addition, Urbach energy was estimated and found to increase with increase of Al content, indicating decrease in the defect density of the films, in supportive with the XRD results. Also, sheet resistance of Al doped ZnO films found to decrease with increase in Al concentration. The investigated results confirm that Al doped ZnO films are feasible and potential candidates for TCO applications.

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Keywords: AZO thin films; Spin coating; Atomic Force Microscope; Electrical properties; Band gap.

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*Corresponding Author: Dr. Basavaraj Angadi Department of Physics, Bangalore University, Bangalore – 560056, India Tel: +91-80-22961478 E-mail address: [email protected]

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1. Introduction

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In recent years, group III elements doped zinc oxide (ZnO) are receiving vast amount of attention as a potential candidates for many technological applications such as in optoelectronic devices [1], LED’s [2], flat panel display electrodes and also as n-type buffer material in the solar cells because of their optical band gap (3.3 eV) wide enough to transmit most solar radiation in the visible wavelength region [3-6]. In particular, Al doped ZnO (AZO) shows an excellent transparency over the entire visible spectrum and has better transport properties due to higher electron mobility [3], which is an important aspect for solar cell application. In order to utilize AZO thinfilms as a buffer layer for CdTe based solar cell applications, a strong c - axis orientation (002) perpendicular to the substrate is required. In addition to its maximum transparency in the visible wavelength region, optimum resistivity is also required in order to match with window layer in solar cells. It has been reported in the literature the maximum transparency can be achieved in ZnO by heavily doping with various dopants such as aluminum (Al) to obtain the required optical and electrical properties [7].

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2. Experimental

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Till date, many routes have been reported to deposit ZnO such as radio-frequency magnetron sputtering [8-9], magnetron sputtering [10], spray pyrolysis [11], metal-organic chemical vapor deposition (MOCVD) [12], sol–gel method [13], molecular beam epitaxy [14], atomic layer epitaxy [15], electron beam evaporation [16], pulsed laser deposition [17]. Among all, Sol–gel process received much attention because of its low cost and its simplicity. This process is capable of producing high-quality coatings even on both large- and small-sized substrates which can be employed for advanced applications [18]. In the present investigation, we have deposited pure and (1-3%) Al doped ZnO thin films by sol-gel spin coating method onto ITO coated glass substrates using 0.8 mol/L concentrated solution. We tried to enhance the optical properties such as transparency in the visible wavelength region with c-axis orientation and investigated the effect of Al doping on the structural, optical and electrical properties of ZnO towards solar cell applications as transparent and high resistive buffer layer.

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Undoped and 1-3% Al doped ZnO (AZO) thin films were prepared by the sol–gel spin coating method. Fig. 1 shows the flow chart of the procedure for synthesis of AZO thin films. Zinc acetate dihydrate (Zn(CH3COO)2. 2H2O), Aluminum Chloride Hydrate (AlCl3 xH2O), 2methoxyethanol and Di-ethanol-amine (DEA) were used as a source, dopant, solvent and stabilizer, respectively. The stoichiometric amounts of zinc acetate dihydrate, Aluminum Chloride Hydrate were dissolved in 2-methoxyethanol at room temperature to obtain source solution (A) and dopant solution (B), respectively. The mixture was stirred at 60 oC and diethanol-amine (DEA) was added drop wise to the solution A as a stabilizer until clear solution was achieved. Then solution B is added drop wise to the solution A in order to obtain the stoichiometric solutions of pure, 1, 2 and 3 mol% Al doped ZnO. The resulting precursor solutions were stirred at 60 oC for 2 h to form a clear and transparent homogeneous mixture. Then the solution (sol) was aged for 48 h at room temperature and then filtered and stored in a container. The concentration of the solution was 0.8 mol/L and molar ratio of DEA to zinc acetate was maintained at 1:1. Prior to the deposition, the ITO coated glass substrates (Sigma Aldrich) were cleaned thoroughly with soap water, tap water, deionized water and consequently

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cleaned with acetone and with DI water for 5 min each with the help of ultrasonic bath, then dried with air blower. Finally the substrates were baked at 350 oC for 10 min and cooled down to room temperature. Pure, 1, 2, 3 mol% Al doped ZnO thin films were obtained by spin coating the sol at a rotation speed of 3000 rpm for 60 sec and the wet films were heated at 350 oC for 10 min and cooled down to room temperature. The process of coating and subsequent drying at 350 oC was repeated for 10 times to get the desired thickness of the film. Finally, the as-prepared films were post annealed at 550 oC for 2 hr and cooled down to room temperature in a muffle furnace.

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The thickness of the Al doped ZnO films were estimated through gravimetric method using the formula. ( −  )
=

 Where, m2 is the mass of the film plus substrate and m1 is the mass of the substrate, respectively, after and before deposition. A is the area of the film, ρ is the bulk density of thin film material. The estimated thickness of Zn1-xAlxO (x = 0, 0.01, 0.02 and 0.03) films are 540 nm, 460 nm, 450 nm and 430 nm, respectively.

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Pure and Al substituted ZnO thin films were characterized through PXRD (Rigaku, ULTIMA, 40 kV, 30 mA) using Cu Kα radiation of wavelength 1.5418 Å. The surface topographic features and the morphology were examined though Atomic Force Microscope (AFM) images recorded on an AFM A100 instrument (APE Research) in non-contact mode. Optical properties were investigated through room temperature optical absorbance and transmittance spectra recorded in the 300 - 1000 nm wavelength range using UV-Visible spectrophotometer (Ocean Optics, USB 4000-XR). The sheet resistance of the films was measured by two probe set up using Keithley 2602A source measuring unit.

3. Results and Discussions

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3.1 Structural studies by XRD

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PXRD patterns of pure and AZO thin films on ITO substrates are depicted in Fig. 2. It is seen that, all the films exhibit multiple peaks indicating the polycrystalline nature of as deposited films. All the peaks are well matched with the JCPDS card no. 36-1451 belonging to the hexagonal wurtzite structure of ZnO with P63mc space group. Few other crystalline peaks (marked with *) along with the ZnO peaks are due to the ITO substrate. The PXRD pattern of annealed ITO substrate is also shown in Fig.2 as a reference. No impurities were observed in films even upto 3% Al substitution into ZnO host matrix, with in the detection limit of XRD. This indicates the substituion of Al at Zn in host matrix without leading to an impurity phase. Undoped ZnO exhibits polycrysalline in nature, whereas the Al doped films exhibits prefered orientation along (002) plane with increase in doping concetraion, which is evidenced form the increae in the intensity of (002) peak with Al concentration. This observation indicates that, even small percentage of Al substitution in ZnO films promotes all the crystallites to orient along c – axis / (002) plane. In addition, the intensity of the other peaks (100), (101) are suppresed with Al concentration. This c – axis orientation of AZO thin film is essential when used as a buffer layer in CdS/CdTe based solar cells [19]. Furthermore to see the effect of Al susbstitution, crystallite

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size and induced strain are calculated using following equations. The crystallite size was calculated for highest intensity peak, i.e. (002) peak, using Scherer’s formula shown in Eq. (1) =





…………………. (1)

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Where, k is Scherer’s constant, λ is the wavelength of X-rays used, β is the observed FWHM, and θ is the Bragg’s angle. Further, the lattice parameters ‘a = b’ and ‘c’ were determined by using the following equation (2) for hexagonal crystal system; 



     

= 





 +





……………. (2)

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Where, d is the inter-planar spacing determined using Bragg’s equation; ! = 2#$% &. The unit cell volume for hexagonal system was determined using the equation (3)

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' = 0.866 , - ……………. (3)

The dislocation density (δ), defined as the length of dislocation lines per unit volume, is estimated using the relation (4) . =



/

…………….. (4)

The strain (ε) present in thinfilms was estimated using the relation (5); 

………….. (5)

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0 =



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The estimated parameters such as, crystallite size (D), interplanar spacing (d), strain (ε), dislocation density (δ), lattice parameters and unit cell volume are tabulated in table 1. It is inferred from the table that, the estimated crystallite size is found to decrease for 1% Al doping and then increases for higher doping concentrations (Table 1) may be due to the decrease in the film thickness with increase in Al concentration. It may be due to the higher thermal energy / heat transfer rate (in-situ annealing) in case of lesser thickness films compared to those of higher thickness films during the post annealing treatment leading to the increase in crystallite size. Whereas an opposite trend in dislocation density, δ and strain, ε was observed with increase in Al concentration. The estimated lattice parameters (a = b & c), unit cell volume and inter-planar spacing (d) values are found to be in agreement with standard values of JCPDS card no. 36-1451 (Table 1). It is found that the unit cell volume found to increase with increase in Al content upto 2 wt% and then decreases. This is due to the relative difference between the ionic radii of Al3+ (0.535 Å) and Zn2+ (0.60 Å) in the tetrahedral symmetry. The substitution of Al3+ at Zn2+ site alters the concentration of defects (oxygen vacancies) in the film. As a result, the unit cell volume found to increase with increase in Al content up to 2% and then decrease. Similar non linear trend in the structural parameters are reported in literature [19]. In conclusion, Al substitution has less impact on the lattice parameters whereas lesser defects (dislocations) and strain was observed in the films with higher Al concentrations, indicating the homogeneity of the films.

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3.2 Atomic Force Microscopy (AFM)

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3.3 Optical studies by UV-Visible Spectroscopy

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Surface topography and morphology were investigated through Atomic Force Microscopy (AFM). The AFM micrographs of Zn1-xAlxO (where, x = 0, 0.01, 0.02, 0.03) thin films are depicted in Fig. 3. The scanning area was 5 µm × 5 µm. The values of average roughness (Ra), root-mean-square roughness (Rrms), Skewness (RSk) and Kurtosis (RKu) are listed in Table 2. From the figure, it can be clearly seen that undoped film is composed of many dense and less uniform grains of ~ 0.5 µm (500 nm) in size. With the increase in Al concentration, nucleation takes place and dense grains gradually become uniform columnar grains. Also, the grain size and surface roughness gradually decreases (Table 2). From the table, it can be seen that, all the AFM parameters found to decrease with increase in Al concentration, indicating the uniformity of the films. Hence, it can be concluded that, Al substitution makes films more uniform and homogeneous.

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To investigate the effects of Al doping into ZnO on the optical band gap, the optical absorption and the transmittance were recorded for the spin coated films using UV-Vis spectrophotometer and are depicted in Fig. 4 (a & b) respectively. From the absorbance spectra (Fig. 4(a)), it can be seen that, the absorption edge of Zn1-xAlxO is shifted towards lower wavelength region (blue shift) with increase in Al content. Also, the transmittance of the films has been increased with substitution of Al in ZnO (Fig. 4(b)). The transmittance of the bare ITO substrate is also included for the comparison with that of AZO films. In particular, the transmittance of > 90% in the visible region was achieved for 2% and 3% Al doped ZnO films with good crystallinity. This indicates Al doped films are potential candidates for the transparent conducting electrode applications.

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Detailed analysis was carried out using transmittance spectra (Fig. 4(b)) to estimate the refractive index (n) of the film by using transmittance maxima and minima, as a first approximation based on envelope method, as illustrated in Fig. 5. The refractive index was calculated using the equation (6), [20]

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= √[3 + 4(3  −



 )]

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Where,

3 = 62



∗

89 :8; 89 8;

<+

=>   

………………… (7)

The interference maxima and corresponding minima are represented by TM and Tm, respectively. ‘ns’ is the substrate refractive index, which is taken as 1.52, a standard value for glass. The thickness t was estimated by considering a maxima and subsequent minima of the envelope at corresponding wavelengths respectively, which is given by,

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 ∗

?
= @∗= (

………………..(8)

 :? )

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The estimated values of TMax, Tmin, n, and t according to the envelope method are tabulated in table 3. It is found that, the average refractive index of the film decreases with increase in Al concentration and is found to be in the range of 1.712 to 1.689. This decreasing trend with respect to thickness may be attributed to the increased absorption (decreased transmission) in the weak absorption wavelength region. A relative difference is observed in the estimated thickness of the films using envelope method and that of gravimetric method.

AℎC = D (ℎC − EF )=

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Further to investigate the effect of Al substitution in ZnO, the optical band gap energy (Eg), was calculated using Tauc’s equation (9), [21]. ......... (9)

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Where, B is a constant, ‘hν’ is the photon energy, ‘Eg’ is the optical energy band gap and ‘n’ is a number which characterizes the transition process. The exponent ‘n’ takes the values; 2, 3, 1/2 and 3/2 for indirect allowed, indirect forbidden, direct allowed and direct forbidden transitions, respectively. α(λ), is the optical absorption coefficient, which is determined using the following equation (10); H

A (!) = 2.303 I

......... (10)

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Where, A is the absorbance and t is the thickness of the film,

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The experimental optical absorption data was well fitted to the above equation (6) for n = 1/2 (direct allowed transition). The Tauc’s plots, (αhν)2 vs (hν), for Zn1-xAlxO (where, x = 0, 0.01, 0.02, 0.03) thin films are depicted in Fig. 6. The Eg values were determined by extrapolating the linear region of Tauc’s plots to meet at hν, i.e. for (αhν)2= 0. The estimated band gap of Zn1xAlxO (x=0, 0.01, 0.02 & 0.03) thin films are tabulated in table 4. It is observed that, the band gap energy of pure ZnO film (3.25 eV) is blue shifted as compared with the bulk ZnO (3.37 eV). This could be due to the smaller thickness and strain induced in the film. Further the Eg value increases with the increase in Al concentration due to the substitution of smaller ionic sized Al3+ at Zn2+. From the literature, the observed blue shift in Eg with Al content can be explained based on the concept of Burstein–Moss band-filling effect (BM shift) [22-24]. The optical band gap is defined as the minimum energy needed to excite an electron from the valence band to the conduction band therefore the Fermi level lies between the conduction and valence bands [24]. As the Al doping concentration is increased, the lowest states of the conduction band are occupied by free electrons contributed by Al dopants, which pushes the Fermi level towards higher in energy. As a result, the Fermi level moves into the conduction band and lies inside the conduction band. That is the donor electrons occupy states at the bottom of the conduction band, since Pauli principle prevents states from being doubly occupied, this means that the energy required to activate an electron from the valence band to the conduction band is more than the fundamental

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band gap [23- 24]. Hence the valence electrons require an extra energy to be excited to the higher energy states within the conduction band. Therefore, the optical band gap increases with increasing Al concentration. This type of blocking of low energy transitions is known as Burstein–Moss effect which indicates that the band gap becomes wider as the Al concentration increases, that is,

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Eg = Eg0 + ∆EgBM……………………….. (11)

Where, Eg0 is the intrinsic band gap and ∆EgBM is BM shift and is a positive quantity whose magnitude increases with the increasing carrier concentration.  ℏ  N







@ + @ ……………..(12) O

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∆EFKL =

Where kF is the Fermi wave vector and mh is the effective mass for holes in the valence band.

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The Urbach energy which is interpreted as the extent of band tails of localized states in forbidden energy gap is calculated using the following equation [25] A (C) = D 6

ℏQ

RSTUV

< ……………..(13)

Where, B is a constant and Etail is the width of the band tails. Using the above equation, Urbach

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energy is estimated from the reciprocal of the slopes of linear portion of the ln(α) versus hν curves (Fig. 7) in the lower photon energy region. The estimated Urbach energy values are tabulated in table 4. It is inferred that, the estimated values of Etail increases with Al concentration. This could be due to the shift in the optical absorption edge when doped with Al into ZnO. As a result of doping, the band structure get modified which indicates that, the dopant ratio is responsible for the width of localized states in the optical band of the films and causes an increase in the energy width of localized states thereby affecting the optical energy gap [25].

3.4 Electrical Properties

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I-V characteristics of the films were measured using Keithley two probe set up and are depicted in the Fig. 8. The sheet resistance of the films is estimated by determining the inverse slope of the linear fit of corresponding I-V plots. The estimated electrical resistivities of Zn1-xAlxO (x = 0, 0.01, 0.02 & 0.03) thinfilms are tabulated in table 4. It is observed that, the electrical resistivity decreases with increase in Al dopant concentration in ZnO films (Fig. 9). This may be due to the increase of free carrier concentration as a result of the donor electrons from Al dopants. The observed decrease in the resistivity is attributed to the substitutional replacement of Al3+ ions at Zn2+ cation sites. It is reported in the literature that [19, 26-27], the higher crystal orientation along c-axis can be thought to reduce resistivity, because of the shorter carrier path length in a caxis plane and the reduction in the scattering of the carriers at the grain boundaries. In addition to the effect of orientation, the increase in grain-packing density could partly contribute to the decrease in resistivity. Also, segregation of Al components at the grain boundaries would be possible at higher aluminum contents, and it could have a partial role in affecting the resistivity.

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But from the XRD results, no presence of any impurities was detected. This also corroborates with the XRD results, the c – axis orientation which actually alters the resistivity of the films as reported in the literature [19, 26-27]. Therefore the increased crystal orientation with Al substitution has the greatest effect on the increased grain packing density (reduced grain boundary) which results in the decrease of resistivity (higher conductivity of the films).

Conclusions

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Undoped and 1-3% Al doped ZnO thinfilms were prepared onto ITO coated glass substrate using sol-gel spin coating method and the effect of Al substitution on structural, optical and electrical properties were investigated. XRD results show the polycrystalline nature for undoped ZnO and with addition of Al into ZnO, preferred c – axis orientation was observed. No traces of impurities were observed confirming the substitution of Al at Zn site without formation of impurities. Further, decrease in the structural parameters such as lattice parameters and unit cell volume, dislocation density and strain were observed with increase in Al content. This observation was attributed to the substitution of smaller ionic radii of Al3+ (0.535 Å) compared to Zn2+ (0.60 Å) in the tetrahedral symmetry. The optical studies shows the blue shift in Eg as evidenced from the shift in the absorption band edge towards lower wavelength region, called BM effect and was discussed in detail as a function of carrier concentration. Transmittance spectra confirm that, Al substitution induces transparency in the host. In particular, transmittance of > 90% in the visible region was achieved for 2% and 3% Al doped ZnO thin films with good crystallinity, indicates Al doped films are potential candidates for the transparent conducting electrode applications. Also, the refractive index and the thickness of the films were estimated using transmission spectra by envelope method, which shows both thickness and the refractive index decreases with Al concentration. The estimated Urbach energy (Etail) found to increase with Al concentration due to the substitution of Al at Zn. This has affected the band structure of the ZnO thereby affecting the optical energy gap leads to increase in the width of localized states. Furthermore the sheet resistance of the films was estimated by determining the inverse slope of the linear fit of corresponding I-V plots. Estimated sheet resistance values were found to decrease with increase of Al concentration in ZnO. Hence, it is observed from the above investigated results that, Al doped ZnO films are suitable for TCO and solar cell applications as high resistive buffer layer.

Acknowledgements

Authors are thankful to the DST-FIST for providing XRD facilities at Department of Physics, Bangalore University, Bangalore. The authors are also thankful to APER, Bangalore for AFM measurements.

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Table 1: Estimated FWHM, interplanar distance, dislocation density, crystallite size, strain, Lattice parameters, volume of the unit cell (V) and optical band gap (Eg). d / (Å)

Dsch / (nm)

ε / (10-4)

δ / (1014 lines m2 )

2.5869 2.5863 2.5860 2.5864

21 20 22 23

16.97 17.12 15.94 15.42

22.66 25.00 20.66 18.90

Lattice Parameters a=b/ c/ (Å) (Å) 3.226 5.171 3.229 5.167 3.231 5.173 3.228 5.171

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=0 = 0.01 = 0.02 = 0.03

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x x x x

FWHM of (002) peak x (π/180) radians 0.4076 0.4112 0.3829 0.3705

V/ (Å3)

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Zn1-xAlxO

46.616 46.666 46.752 46.652

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Table 2 Summary of AFM analysis Rrms / (nm)

59.1 31.3 19.6 16.8

79.8 40.2 24.5 20.9

Skewness (RSk) 1.41 0.785 0.0675 -0.00583

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Kurtosis (RKu) 2.57 1.69 - 0.0869 - 0.148

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Ra / (nm)

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Zn1-xAlxO thinfilms x=0 x = 0.01 x = 0.02 x = 0.03

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Table 3: Estimated parameters (refractive index and thickness) of Al doped ZnO films using transmittance spectra Tmin

n

0.83 0.86 0.91 0.92 0.91 0.82 0.85 0.91 0.92 0.92

0.78 0.81 0.84 0.87 0.88 0.79 0.82 0.85 0.87 0.87

1.736 1.723 1.769 1.700 1.635 1.659 1.650 1.737 1.700 1.700

n (Average) 1.712

1.689

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x = 0.03

TMax

t(envelope) 560

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x = 0.02

λ/ (nm) 430 462 534 622 740 412 434 500 575 690

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Zn1-xAlxO

537

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Table 4: Estimated band gap energy, band tail width and sheet resistance of Al doped ZnO films. Etail / (eV)

3.25 3.28 3.30 3.32

0.132 0.160 0.187 0.195

Sheet resistance (Rs)/ (Ω) 282 283 279 250

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x=0 x = 0.01 x = 0.02 x = 0.03

Eg / (eV)

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Zn1-xAlxO

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Figure Captions

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Fig. 1 Flow chart of synthesis procedure for AZO thin films. Fig. 2 XRD patterns of Zn1-xAlxO (where, x = 0, 0.01, 0.02 & 0.03) thin films Fig. 3 Surface topography and morphology of Zn1-xAlxO thin films by AFM Fig. 4 UV-Visible (a) absorbance and (b) transmittance spectra of Zn1-xAlxO thin films Fig.5 Illustration of determining the refractive index and thickness using transmission spectra by envelope method Fig. 6 (AℎC) vs ℎC plots of Zn1-xAlxO (where, x = 0, 0.01, 0.02 & 0.03) thinfilms Fig. 7 ln(α) vs hν for Zn1-xAlxO thinfilms. Fig. 8 I-V characteristic plots of Zn1-xAlxO (where, x = 0, 0.01, 0.02 & 0.03) thinfilms. Fig. 9 Sheet resistance of AZO thinfilms for different Al dopant concentrations

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Raghu P et. al. Fig. 1

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o

on ITO; 550 C, 3k, 10 Spin

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*

x = 0.03

(112)

*

(103)

(110)

* ITO substrate peak (102)

(101)

*

(100)

Normalized Intensity / (a.u)

(002)

Zn1-xAlxO

x = 0.02

SC

x = 0.01

M AN U

x=0

ITO substrate

20

30

40

50

60

70

80

o

AC C

EP

TE D

2θ / ( )

Raghu P et. al. Fig. 2

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Raghu P et. al. Fig. 3

ACCEPTED MANUSCRIPT

Zn1-xAlxO

(a)

0.8

x=0 x = 0.01 x = 0.02 x = 0.03

0.6 0.4 0.2 0.0 300

ITO Substrace; 10 Spin, 60 sec

100 80

(b) Zn1-xAlxO

60

ITO - Substrate x=0 x = 0.01 x = 0.02 x = 0.03

40 20 0

400

500

600

700

800

300

400

RI PT

ITO Substrace; 10 Spin, 60 sec

Transmittance / (%)

Absorbance / (%)

1.0

500

600

700

800

900 1000

λ / (nm)

SC

λ / (nm)

AC C

EP

TE D

M AN U

Raghu P et. al. Fig. 4

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

Raghu P et. al. Fig. 5

ACCEPTED MANUSCRIPT

1.0

1.5

(a)

(αhν)2

0.5

(b)

1.0

0.5

3.28 eV

3.25 eV 2.5

3.0

3.5

0.0 2.0

4.0

1.5

/ (cm-2 eV2)

(αhν)2

0.5

3.30 eV 3.0

3.5

4.0

3.5

4.0

EP

TE D

hν / (eV)

x = 0.03

M AN U

(c)

1.0

AC C

/ (cm-2 eV2) (αhν)2

x = 0.02

2.5

3.0

hν / (eV)

hν / (eV) 1.5

0.0 2.0

2.5

SC

0.0 2.0

RI PT

/ (cm-2 eV2)

x = 0.01

(αhν)2

/ (cm-2 eV2)

x=0

(d)

1.0

0.5

0.0 2.0

2.5

3.32 eV 3.0

3.5

4.0

hν / (eV)

Raghu P et. al. Fig. 6

ACCEPTED MANUSCRIPT

19 x=0 x = 0.01 x = 0.02 x = 0.03

17

RI PT

ln α

18

16

14 1.5

2.0

2.5

3.0

SC

15

3.5

AC C

EP

TE D

M AN U

hν / (eV)

4.0

4.5

Raghu P et. al. Fig. 7

ACCEPTED MANUSCRIPT

0.010

0.0020 0.0015 0.0010 0.0005 0.0000

x = 0.01 Linear Fit of x = 0.01

0.008 0.006 0.004 0.002 0.000

0

1

2

3

4

5

0.0

0.5

Voltage / (V)

1.5

2.0

2.5

3.0

Current / (A)

0.006 0.004 0.002 0.000

0.008 0.006

SC

0.010 x = 0.02 Linear Fit of x = 0.02

x = 0.03 Linear Fit of x = 0.03

M AN U

Current / (A)

1.0

Voltage / (V)

0.010 0.008

RI PT

x=0 Linear Fit of x = 0

Current / (A)

Current / (A)

0.0025

0.004 0.002 0.000

0.0

0.5

1.0

1.5

2.0

3.0

AC C

EP

TE D

Voltage / (V)

2.5

0.0

0.5

1.0

1.5

2.0

2.5

Voltage / (V) Raghu P et. al. Fig. 8

ACCEPTED MANUSCRIPT

2000

RI PT

1500

1000

500

0 0.0

0.5

1.0

1.5

SC

Sheet resistance / (Ω)

2500

2.0

2.5

3.0

AC C

EP

TE D

M AN U

Al concentration / (%)

Raghu P et. al. Fig. 9

ACCEPTED MANUSCRIPT

Highlights

EP

TE D

M AN U

SC

RI PT

Al doped ZnO thinfilms were prepared by spin coating method. Effect of Al on Structural, optical and electronic properties were investigated. Uniform, homogeneous and transmittance (> 90%) films are obtained due to Al doping. Blue shift in Eg has been explained based on Moss-Bernstein effect.

AC C

• • • •