Material characterizations of Al:ZnO thin films grown by aerosol assisted chemical vapour deposition

Accepted Manuscript Material characterizations of Al:ZnO thin films grown by aerosol assisted chemical vapour deposition Vipin K. Kaushik, C. Mukherjee, Tapas Ganguli, P.K. Sen PII:

S0925-8388(16)32389-1

DOI:

10.1016/j.jallcom.2016.08.022

Reference:

JALCOM 38523

To appear in:

Journal of Alloys and Compounds

Received Date: 12 April 2016 Revised Date:

11 July 2016

Accepted Date: 3 August 2016

Please cite this article as: V.K. Kaushik, C. Mukherjee, T. Ganguli, P.K. Sen, Material characterizations of Al:ZnO thin films grown by aerosol assisted chemical vapour deposition, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.08.022. 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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Material Characterizations of Al:ZnO Thin Films Grown by Aerosol Assisted Chemical Vapour Deposition Vipin K. Kaushik

C. Mukherjee

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Dept. Appl. Phy., Shri G. S. Institute of Technology and Science, Indore-452 003, India E-mail: [email protected]; [email protected] Tel: +91-0731-2582441; Fax: +91-0731-2432540

Tapas Ganguli

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Mechanical and Optical Support Section, RRCAT, Indore-452 013, India E-mail: [email protected]

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Material Research Laboratory, Indus Synchrotron Utilization Division , RRCAT, Indore-452 013, India E-mail: [email protected]

P. K. Sen

Dept. Appl. Phy., Shri G. S. Institute of Technology and Science, Indore-452 003, India E-mail: [email protected]

Abstract

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In the present work, optical, electrical, structural and morphological characterization of polycrystalline thin films of ZnO and Al:ZnO have been reported. Aerosol assisted chemical vapour deposition technique has been adopted for the growth of the films on the glass substrate at 480 o C. Al-fraction in Al:ZnO thin films was determined by using energy dispersive x-ray analysis. In the visible to near-infrared regions, all the films were found to exhibit high average transmittance ≥ 80%. Blue shift of energy bandgap, from 3.20 to 3.50 eV, has been observed with increasing Al-fraction in the Al:ZnO thin films. For highly doped Al:ZnO thin films, both theoretical and experimentally measured values of carrier concentration (≈ 1020 cm−3 ) were greater than Mott’s critical value (≈ 1019 cm−3 ) and supports the blue shift of the bandgap. The figures of merit of the films are found to be around 10−2 Ω−1 suggesting the applicability of Al:ZnO thin films as a transparent electrode. The observed variation of carrier effective mass with increasing Al-doping agrees quantitatively well with the theoretical calculations. At high Al-fraction (≥ 12 at.%), the compressive strain generated in ZnO lattice is due to the presence of amorphous Al2 O3 or AlOx at the grain boundaries. X-ray diffraction suggests that the films were single phase and polycrystalline in nature. Atomic force microscopy of Al:ZnO films reveals a systematic change in surface morphology with increasing Al-fraction in the films. The results demonstrate the potentiality of producing thin films of transparent conducting oxides with good electrical, optical, structural and morphological properties via a low-cost deposition technique. Keywords: Polycrystalline thin films; Aerosol assisted chemical vapor deposition; X-ray diffraction; Atomic force microscopy; Figure of Merit, Electrical, optical and structural Preprint submitted to Thin Solid Films

August 4, 2016

ACCEPTED MANUSCRIPT properties. 1. Introduction:

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In modern optoelectronic among many of the II-VI semiconducting materials, zinc oxide (ZnO) is of particular interest because of its applications in the general area of light emitting diodes, ultra-thin transistors and in amplification of UV emission [1, 2, 3]. In its intrinsic form, ZnO is an n-type semiconductor having hexagonal (wurtzite) structure in which each atom of zinc is surrounded by four oxygen atoms in tetrahedral coordination [4]. In an ideal wurtzite crystal, the axial ratio (c/a) and a so-called u parameter that measures the displacement of each atom with respect to the next along the c-axis bears the relationship uc/a =(3/8)1/2 where c/a = (8/3)1/2 and u = 3/8 for an ideal crystal [4]. ZnO crystals deviate from this ideal arrangement by changing both of these values. This deviation occurs in such a way that the tetrahedral distances roughly remain constant in the lattice. At room temperature, ZnO is characterized by a direct energy bandgap (≈ 3.37 eV) and large exciton binding energy (≈60 meV) [4, 5, 6]. Therefore, at room temperature, the stable existence of excitons makes it a versatile material with good electrical and optical properties along with thermal and chemical stability. Moreover, low-cost, non-toxicity and abundance of ZnO in nature has drawn considerable attention of researchers to investigate pure and doped ZnO in various forms such as thick and thin films and nanocrystals for newer and newer range of applications. One of the most demanding applications of doped ZnO thin films is in the form of transparent conducting oxide (TCO), considered as a potential candidate material for the next-generation display devices. Aluminum-doped zinc oxide (Al:ZnO) is expected to be useful alternative to expensive tin-doped indium oxide (In2 O3 :Sn) popularly known as ITO. Moreover, Al:ZnO has high transmittance in ultraviolet-visible (UV-VIS) spectral region and good electrical conductivity and it is relatively cost effective in contrast to ITO [7, 8]. A significant improvement of film conductivity can be observed by doping ZnO with group III elements like B, Al, Ga, In [9, 10, 11, 12, 13]. This is due to the difference in ionic radii and charge of these atoms with respect to those of Zn atom. Reported results suggest that among the group III elements, Al is an excellent dopant due to low resistivity of Al:ZnO thin films [14, 15]. Limited incorporation of Al atoms (2.07 at.%) into the ZnO lattice yields an optimum value of carrier concentration n ≈ 2 × 1020 cm−3 and lowest resistivity µ ≈ 6 × 10−3 Ωcm) [15]. There are many reports about the influence of carrier gas, dopant concentration and growth temperature over the properties of Al:ZnO films [16, 17, 18]. It is worth noting here that excessive dopant concentration does not enhance carrier concentration. On the contrary, it reduces the mobility perhaps due to higher probability of impurity scattering and poor crystalline quality. High conductivity, high transparency and low resistivity of doped ZnO leads to its application in various electronic and optoelectronic devices such as Mott-barrier diodes and solar cells [19, 20]. Various methods like spray deposition [21], pulsed laser deposition [22], magnetron sputtering [23], molecular beam epitaxy [24] and chemical vapour deposition (CVD) [25] have been used for the deposition of TCOs. Among these, aerosol assisted CVD (AACVD), a variant of CVD, has proved itself to be a simple and inexpensive method, particularly useful for large area applications [26, 27]. It allows rapid formation of the deposition phase at temperature around 480 o C. In the present work, doping induced optical, electrical, structural and morphological changes in the properties of AACVD grown 2

ACCEPTED MANUSCRIPT Al:ZnO thin films have been studied. Optical transmittance data have been used for the calculation of thickness and bandgap of semiconductor thin films. For the structural and surface morphology of thin films, x-ray diffraction (XRD) and atomic force microscopy (AFM) have been used. Van der Pauw technique is employed to measure resistivity, carrier concentrations and mobility of these films. 2. Experimental Methodology:

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In the present paper, we report the optical, electrical, structural and morphological properties of Al:ZnO polycrystalline thin films developed by using AACVD technique. The experimental setup consists of a horizontal SS-304 cylindrical reactor having a heated reaction zone, where at atmospheric pressure the growth of oxide semiconductor thin films occurred. Solvent and the solutes used as a liquid precursor in the present work are listed in table-1. The liquid precursor was injected into the reactor through a fine nozzle having an orifice ≈ 0.5 mm diameter. Oxygen was used as the carrier gas for the injection of aerosol of liquid precursor inside the reactor. The flow rate of the precursor was controlled by varying the flow of O2 . Inside the reactor the aerosol of precursor flows close to the horizontally placed heated substrate placed ≈ 50 cm away from the nozzle. There it cracks homogeneously by thermo chemical reactions and subsequently form the thin layer of required material. Liquid precursor used for the growth of Al:ZnO thin films have different concentrations of aluminum acetyl acetonate (cAl ) and zinc acetyl acetonate (cZn ) dissolved in isopropyl alcohol (IPA). Al-fraction (Al/Zn ratio) in the precursor solution were calculated in at.% by the expression (cAl × 100)/(cZn × D). Here, D is the ratio of molecular weight of aluminum acetyl acetonate (324.21 g/mol) to that of the zinc acetyl acetonate (263.61 g/mol). In order to have various Al-fraction in the precursor solution, first we increased cAl from 0.0 to 0.75 g/l with constant cZn (10 g/l), then cZn was reduced from 12.5 to 5 g/l with constant cAl (1.25 g/l). Precursor concentration, C is defined as cAl + cZn . For dissolving the solute, IPA was heated to its boiling point and the solution was stirred for about 15 minutes. Before growth, the deposition chamber was purged with carrier gas for 10 minutes at a constant flow rate. Soda lime glass substrates (100 × 100 ) were used for the deposition of Al:ZnO thin films. Prior to deposition, the substrates were cleaned by boiling in trichloroethylene, acetone and methanol for 2 minutes each. Prior knowledge of the growth parameters such as precursor flow-rate, substrate temperature and its orientation to the horizontal plane, concentration of the precursor etc. is necessary while analyzing the film characteristics for different device applications. Table 1: Film deposition conditions of AACVD system for the deposition of Al:ZnO films.

Process Parameters Substrate Substrate Temperature Substrate Tilt Carrier Gas Flow Rate Zn Source Material Al Source Material Solvent

Optimized Values 100 × 100 × 0.0200 soda lime glass slip 480±10 o C (10 ± 1)o w. r. to horizontal plane 4.00 liter std./min. Zn(C5 H7 O2 )2 .H2 O (purity 99%) Al(C5 H7 O2 )3 (purity 99%) 400 ml Isopropyl alcohol (IPA)

The detailed description of present AACVD system as developed by the authors along with its advantages and limitations have been described in ref. 26. Gledhill et al. [27] 3

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have used aluminium acetylacetonate precursor to deposit Al:ZnO films grown by spray pyrolysis technique which is analogous to AACVD and obtained free charge carrier concentrations larger than 1020 cm−3 . To have appreciable optical, electrical and structural properties of the films deposited by the present AACVD system, it is very important to optimize the process parameters of the system. Among the process parameters, carrier gas flow rate and substrate temperature play the most critical role. The selection of optimized flow rate of carrier gas depends on the criteria that the minimum amount of aerosol reaches in the vicinity of heated substrate. Since, at high flow rate cloudy environment of aerosol around the substrate makes the deposition hazy. Further, at low substrate temperature the adhesion of ZnO atoms over the substrate is poor that again gives hazy deposition. On the other hand at high growth temperatures (≥ 550 o C), the film quality deteriorates mostly due to the re-evaporation of ZnO atoms. The optimized film deposition conditions are listed in table-1. Swanpoel envelope method has been adopted for calculation of film thickness and absorption coefficients from optical transmittance [28, 29]. The optical transmission spectra of the films were obtained using a UV-Visible-NIR spectrophotometer (CARY50, Varian) in the range of 250 nm to 1000 nm. For the analysis of elemental composition of Al:ZnO thin films, energy dispersive x-ray (EDX) of five samples were carried out by using JEOL JSM 5600 EDX spectrophotometer. The crystalline structures of the various films were determined from the θ−2θ patterns obtained by using a Brukers D8000 system. The system uses a 2.2 kW x-ray generator and the measurements were carried out with copper Kα source in the Bragg-Brentano geometry. Atomic force microscopy (AFM) measurements were carried out in a SOLVER-PRO setup (NT-MDT) using Si cantilever tips of radius of curvature ≈ 20 nm and resonant frequency ≈ 190 kHz in tapping mode of measurement. Variation of surface morphology and roughness of the films with respect to Al-composition in Al:ZnO thin films were determined. For the measurement of electrical properties of the films, Van der Pauw geometry was used in which we made four In contacts symmetrically placed at the corners of thin films. In this geometry, an AC current ≈ 1 - 10 µA at 1 kHz frequency was passed through the films. For carrier concentration the films were placed in a magnetic field (0.8T), applied normal to the film surface. The calculated value of resistivity and carrier concentrations are subsequently used to obtain the mobility of films. Finally, the modified Burstein-Moss relation [30] was employed to determine the energy bangap of the thin film samples. 3. Results and Discussion: We analyze the effect of Al doping in ZnO thin films grown by AACVD for characterization of optical, electrical, structural and morphological properties. We have calculated the various parameters like deposition rate (DR), precursor concentration (C), Al-fraction, thickness (t), average optical transmittance (Tavg ), bandgap (Eg ), crystallite size (δ), resistivity (ρ), carrier concentration (n), mobility (µ) and figure of merit (FOM) as shown in table-2. The variation of Al-fraction in the film (xAl ) as a function of Al-fraction in precursor solution is shown in fig. 1. In table-2, xAl values of ten samples are shown with five being measured by EDX and remaining five (with asterisk marks *) are observed from fig. 1. For these observation we note the xAl values corresponding to the Al-fraction in precursor solution. It can be observed that for small Al-fraction in precursor, upto 6 at.%, the variation of xAl is much faster in the sample, while beyond this value the incorporation 4

ACCEPTED MANUSCRIPT Table 2: Table for various measured and calculated parameters of Al:ZnO thin films. C (g/l) 10.00 10.03 10.06 10.13 10.25 10.75 13.75 11.25 8.75 6.25

Al-fraction Precursor 0.0 0.3 0.5 1.0 2.0 6.1 8.1 10. 13.6 20.3

(at.%) EDX 0.0 0.8* 1.8* 3.4 4.9* 10.9 11.9* 12.7 13.9* 16.4

DR (nm/s) 0.29 0.17 0.15 0.11 0.10 0.15 0.16 0.27 0.23 0.11

t (nm) 867 647 490 409 371 479 467 887 841 430

Tavg (%) 83 83 86 83 83 84 83 85 80 84

δ (nm) 23.2 22.5 26.0 29.6 31.9 25.2 23.5 22.0 21.4 15.9

ρ (Ω-cm) 8.18E00 1.50E00 8.90E-2 2.92E-2 1.89E-2 2.12E-2 3.60E-2 5.12E-2 9.56E-2 1.43E00

n (cm−3 ) 3.40E16 3.01E17 5.65E18 3.95E19 1.66E20 2.67E20 2.74E20 1.89E20 1.20E20 1.94E18

µ (cm2 /Vs) 22.50 13.90 12.42 5.43 1.99 1.10 0.63 0.65 0.54 2.26

Eg (eV) 3.20 3.22 3.29 3.31 3.34 3.41 3.44 3.50 3.47 3.37

FOM (Ω−1 ) 2.07E-4 8.41E-4 2.73E-2 2.73E-2 3.83E-2 5.53E-2 2.53E-2 5.69E-2 1.06E-2 7.37E-4

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of Al in ZnO lattice becomes slower. Interestingly, the trend of higher Al-fraction in the sample than in the precursor reverses for Al-fraction greater than 14 at. %. We can see from fig. 1 that the Al-fraction in the sample film is 16.4 at. % for around 20.0 at. % of Al-fraction in the precursor. This nonlinear at. % variation can be attributed to the fluctuations in the Zn occupancy in the vicinity of the heated substrate. In the present study, we have restricted ourselves to a maximum xAl of 16.4 at. %. 18

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Figure 1: Variation of Al-fraction in the film (xAl ) as a function of Al-fraction in precursor.

3.1. Transmittance and bandgap of Al:ZnO thin films: The specular transmittance spectra of the glass substrate and the thin films of Al:ZnO are shown in fig. 2. The presence of interference fringes and a sharp absorption edge can be seen in all the curves. Shifting of bandgap with increasing xAl have been highlighted in the inset of fig. 2. Large transmittance in some parts of the spectrum for xAl = 1.8 at.% than that of the bare substrate may be due to refractive index inhomogeneity along the growth direction. The calculated values of average transmittance Tavg of films measured in the wavelength range from 400 to 1000 nm, is tabulated in table-2. Although no regular trend has been found in Tavg but for all the films, it lies in the range between 80 to 86 % which is comparable to the transmittance obtained in films grown by atomic layer deposition and RF magnetron sputtering with much smaller film thickness [10, 11, 31]. In the visible region the films exhibited low reflectance ≈ 12% and low absorbance ≈ 5%. 5

ACCEPTED MANUSCRIPT Thus it may be inferred that the optical quality of the Al:ZnO films grown by AACVD as reported here is in no way inferior to those reported in literatures. 100

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Tauc plot has been used to determine the bandgap Eg of the grown films using the relation αhν = A(hν − Eg )1/2

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ln( 100 ) T α= (2) t with T as the film transmittance; t being the film thickness [31]. For direct bandgap semiconductors, the curves in the inset of fig. 2 tend asymptotically towards a linear section. Consequently, Eg is the intersection with the hν axis of a fit to the linear section of the (αhν)2 vs hν curve. The observed values of bandgap of all the films have been listed in table-II. The bandgap shifts in the present films range from 3.2 to 3.5 eV. In figure 3, we plot the variation of measured bandgap Eg and calculated values of carrier concentration n of Al:ZnO thin films for different doping values of xAl . It may be noted that the curves have three different regions as shown in fig. 3 by I, II and III. In region-I with xAl varying from 0.0 to 4.0 at.% both Eg and n increase sharply. For region-II at xAl between 4.0 to 12.0 at.% the rate of increase of both Eg and n slows down but the same increasing trend continues with respect to xAl . Beyond this, in region-III, xAl increases from 12.0 to 18.0 at.% whereas both Eg and n values decrease. In region-I, xAl is within the solid solubility limit (≤ 3 mol%) [32, 33] of Al in ZnO due to which the abundance of xZn as compared to xAl the rate of incorporation of Al+3 ions in the lattice site of Zn is large which is responsible for the sharp increase of bandgap. In this region improvement of crystalline quality of Al:ZnO films have also been observed. In region-II, due to higher Al concentration, Al+3 occupies interstitial sites of ZnO as well, thereby not manifesting as a dopant and hence may not influence the bandgap as observed in region-I. We find fall in the slope of bandgap that increases with increasing xAl in 6

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this region. However, the finite increase of bandgap with xAl does not rule out incorporation of some Al+3 in the lattice site of ZnO. The Al+3 in the interstitial decrease the overall quality of the films which is manifested in the form of increase in roughness and decrease in grain size. Using the effective mass model, Hamberg et. al. [34] have shown that the optical bandgap widens progressively with doping of oxide semiconductors. Therefore, it is expected that by doping with xAl the aluminum atoms enter substitutionally into the zinc sites in the ZnO lattice so that they act as ionized donors. With increasing dopant concentration, the donor electrons occupy the states at the bottom of the conduction band leading to bandgap widening which is referred to as Burstein-Moss (BM) shift or blue shift. In region-III, the bandgap diminishes with increasing xAl . The reason for such response in semiconductors like Al:ZnO may be that as the doping becomes very high the host conduction band is occupied by additional electrons. This bandgap narrowing is a consequence of many-body effects such as exchange and coulomb interactions in the conduction and valence bands [35]. This bandgap shrinkage as discussed above contradicts the BM effect which predicts a bandgap widening as a result of the blocking of the lowest states in the conduction band. In this case, bandgap narrowing occurs at carrier concentration higher than Mott’s critical value (≈ 1019 cm−3 for Al:ZnO) [36]. As suggested by Hamberg et. al. [34], the shrinking of bandgap may be due to electron-electron and electron-impurity scattering. Lu et. al. [30, 37] have shown that narrowing of bandgap after attaining the maximum value of dopant (Mott’s critical value) may be due to the bandgap renormalization effect. According to Sagar et. al. [14], at high doping concentration the interstitial occupancy of Al+3 in the ZnO lattice may lead to the bandgap narrowing. Tewari et. al. [21], suggested the possibility of a disorderness in ZnO lattice at high doping due the difference in the ionic radii of Zn+2 (0.74 ˚ A) and Al+3 (0.54 ˚ A) which in turn enhances the efficiency of scattering mechanism and causes bandgap shrinking.

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3.2. Doping dependance of resistivity, carrier concentration and mobility of Al:ZnO thin films: Hall measurements were carried out for all the films. Experimentally measured values of ρ, n and µ of Al:ZnO thin films are shown in table-2. The variations of these parameters as functions of xAl are shown in fig. 4. We find that undoped ZnO thin film has n ≈ 3.4 × 1016 cm−3 . This value increased with the addition of Al in precursor and attains the maximum value ≈ 2.74 x 1020 cm−3 for xAl ≈ 12 at.%. In the present paper, measured carrier mobility µ for the undoped ZnO thin film is 22.5 cm2 /Vs. This value compares well with the reported values of electron mobility in polycrystalline ZnO films [14, 38]. As expected, electron mobility drops significantly with the increase in Al doping concentration. This is primarily due to the rise in the ionized impurity scattering with increasing Al doping in the ZnO films. A steady fall in the resistivity of the films with increasing xAl upto 6.0 at.% is observed. The origin of the same being in the increased carrier concentration with increasing xAl as shown in region-I of fig. 4. 3.0

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With further increase in xAl , we find that the carrier concentration shows a decreasing trend for xAl ≥ 12 at.%. This indicates that out-diffusion of the Al atoms from the ZnO lattice (possibly to the grain boundaries) takes place at higher Al incorporation. The presence of Al in the form of AlOx at the grain boundaries create additional scattering centers, thereby further decreasing the carrier mobility below 1 cm2 /Vs, for xAl ≥ 12 at.%. Under the present deposition conditions, Al:ZnO thin film with xAl ≈ 12 at.% yields n ≈ 2.7 × 1020 cm−3 , µ ≈ 0.6 cm2 /Vs and resistivity ρ ≈ 3.60 × 10−2 Ωcm. Ideally, an effective TCO should have high electrical conductivity combined with high transmission and low absorption of visible light. Gordon [39] discusses the criterion for choosing a transparent conductor by the knowledge of its figure of merit (FOM) which is the the ratio of the electrical conductivity to the visible absorption coefficient and given by F OM = − [Rs .ln(T + R)]−1 (3) where, Rs is the sheet resistance, T is the visible transmission and R is the visible reflectance of TCO thin films. A larger value of FOM is the indicator for the better performance of TCO thin films. The calculated values of present Al:ZnO thin films are shown 8

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in table-2. The order of FOM of the present thin films is 10−2 Ω−1 that indicate the applicability of AACVD grown Al:ZnO thin films as a transparent electrodes [40, 41]. It appears worthy to mention that the trend in variation of n is similar to that of the bandgap. According to Burstein [42], the bandgap broadening is proportional to n2/3 and inversely proportional to the effective mass (m∗ ) of charge carrier. For heavily doped semiconductors, Lu et. al. [30] have suggested that the influence of many-body effects on the narrowing of energy gap can be analyzed by perturbation theory and governed by the equation (3~3 π 2 n)2/3 − An1/3 − Bn1/4 − Cn1/2 (4) 2m∗ where A, B, and C are coefficients. The typical values of these coefficients as suggested by Lu et. al. [30] for n-type ZnO are 6.86 x 10−9 eV-cm , 1.60 × 10−7 eV-cm3/4 and 7.76 × 10−12 eV-cm3/2 , respectively. By using equ. (3) we calculate the values of bandgap of Al:ZnO thin films at different m∗ of the charge carrier (e− for Al:ZnO). For these calculations we use experimentally measured values of n. The bandgaps calculated by using Tauc plot (Experimental) are shown in table-2. Figure 5 shows the variation of bandgap as a function of n. Here, we find that up to n ≈ 3 × 1019 cm−3 the values of bandgap calculated at m∗ = 0.20m (m is the electron rest mass) are in close agreement with that of the experimental values. At higher carrier concentration (n ≥ 1020 cm−3 ) the calculated values of bandgap deviates from that of the experimental values. This deviation may be perhaps due to the inhomogeneity in thin films having high doping concentration. This leads to increase in the effective mass of charge carrier in the conduction band of Al:ZnO thin films having high Al doping.

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3.3. XRD spectra, lattice constants and crystallite size of Al:ZnO thin films: Figure 6a shows x-ray diffraction of Al:ZnO thin films having varying xAl . All the strong diffraction peaks of ZnO are clearly seen, indicating that the films are randomly oriented and polycrystalline in nature with wurtzite structure. The most prominent peak 9

ACCEPTED MANUSCRIPT observed in the Al:ZnO film corresponds to the (1011) plane which is in very good agreement with those available in literature. Other planes corresponding to (1010), (0002), (1012), (1120), (1013) and (1122) have relatively low intensities. With the incorporation of Al+3 in the lattice site of ZnO, no significant change in the texture is observed. In figure 6a, absence of (0006) peak at 33o corresponding to the formation of Al clusters [30] and (0002) peak at 42o corresponding to the formation of crystalline Al2 O3 phase [10]. 10.0k

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Figure 6b shows the XRD data of Al:ZnO thin films for 2θ in the range between 31o to 38o with three major reflections (1010), (0002) and (1011). A small systematic shift in the position of these three peaks with xAl is observed as shown by the arrows in fig. 6b. The lattice constants a and c of Al:ZnO films are evaluated from the XRD data by using the expression for the inter planer spacing (d) of hexagonal geometry given by [12]   1 4 h2 + hk + k 2 l2 = . (5) + d2 3 a2 c2 a c

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In region-I, both a and c decrease monotonically with increasing xAl . This can be explained by invoking the fact that the Al+3 ionic radius (0.54 ˚ A) is about 30 % smaller than that of Zn+2 ions (0.74 ˚ A). As Al atoms enter inside the ZnO lattice, they replace Zn atoms and a strain in the lattice occurs that leads to the monotonic decrease in the lattice parameters as observed in the present case. In region-II, the change in a and c with xAl is very slow as compared to region-I. This is primarily due to the fact that the Al atoms not only replace the Zn atoms (Zn sites) but some of them also settle at the interstitial sites. This is because the net change in strain under this regime is small as compared to region-I. As a result, the lattice constants reduce. This observation is also consistent with the trends seen in the variation of bandgap with xAl as shown in fig. 3. In region-I, the incorporation of Al in the lattice of ZnO is high, resulting in large strain, and a rapid decrease in lattice constants with Al incorporation. Correspondingly, there is a large rate of increase in the bandgap with Al incorporation is observed. On the contrary, in region-II the incorporation of Al in the lattice of ZnO is reduced resulting in a slow rate of decrease of lattice constants. Similarly, in region-II the rise in bandgap with xAl is less as compared to region-I. In region-III, on the other hand, reduction of lattice constants cannot be attributed to the above mentioned reason as there is no corresponding increase in the carrier concentration as shown in fig. 3. The rapid decrease in lattice parameters in region III is attributed to the presence of amorphous oxides at the grain boundaries which provide compressive stress resulting in the observed decrease in the lattice parameters. Although in this region xAl values are very high but the presence of Al clusters can be ruled out due to the growth mechanism in the AACVD system with the growth being carried out at atmospheric pressure in O2 rich environment. Moreover, the absence of Al clusters and crystalline Al2 O3 or AlOx have been confirmed by XRD. It is reported in literature that semiconductors like ZnO and InN show increase in bandgap with increase in strain [43, 44]. But in region III of fig. 7, we observe decrease in both bandgap and carrier concentration with decrease in lattice constants and hence with increase in strain. One may infer from this observation that in region-III, Al is not incorporated in the lattice of ZnO while it is forming amorphous Al2 O3 or AlOx clusters at the grain boundaries of ZnO that contribute to a compressive stress in ZnO lattice. Furthermore, in region-III the presence of Al2 O3 or AlOx at grain boundaries form an insulating potential barrier that enhance the resistance of the thin films. Hence reduced carrier concentration increased resistivity and almost constant mobility has been observed in region-III of fig. 4. The most prominent peak of x-ray diffraction, (1011) for Al:ZnO thin films, is used to calculate crystallite size (δ) by using Scherrer’s formula given by [45] δ=

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˚) is the wavelength of the incident x-ray and θ where β is the FWHM, λ (=1.54 A is the angle at which the maximum peak occurs. The calculated values of δ are given in table-2 where we find that the obtained crystallite size is of the order of tens of nm, justifying the polycrystalline nature of thin films. Figure 8 showing the dependance of crystallite size δ on Al-doping concentration xAl can also be separated into three regions similar to figs. 3 and 7. It is observed that the crystallite size in Al:ZnO films sharply increases in region-I and slowly decreases in region-II. In region-I δ increases from about 22 nm (xAl = 0.0 at.%) to about 32 nm (xAl 11

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= 4.0 at.%) i.e. by ≈ 45 % signifying improvement in film crystallinity. On the contrary in region-II, δ decreases by ≈ 45 % for xAl varying from 4.0 to 12 at.%. This is attributed to the replacement of relatively bigger Zn+2 ions by the Al+3 ions during the formation of the Al:ZnO films. The minimum crystalline size of 15.9 nm is found for 16.4 at.% Al-doped film. It is worthy to note that the crystallite size in regions-II and III does not vary in any regular pattern with xAl may be due to the lattice disorderness produced in the films at higher dopant concentration and variation in the ionic radii of Zn+2 and Al+3 .

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3.4. Effect of doping on the surface morphology of Al:ZnO thin films: Figure 9 shows the two-dimensional AFM images (2 × 2 µm) of pure and Al:ZnO thin films. Roughness of undoped ZnO film is high (average ≈ 24 nm and rms ≈ 31 nm) and several grains can be seen. It is apparent from fig. 9 that there is a systematic change in surface morphology with increasing xAl from 0 to 16.4 at.%. Slight incorporation of Al (xAl = 1.8 at.%) in ZnO lattice (substitutional doping) changes the morphology, which can be clearly seen. Surface roughness decreases by a factor of 3. This is associated with decrease in deposition rate. As xAl is increased by an order of magnitude (xAl ≈ 11 at.%), surface morphology remains nearly same and roughness slightly decreases. Beyond this xAl , roughness starts increasing. Closely packed grains can be seen on the surface. Highest Al-fraction in the lattice of ZnO thin film (xAl = 16.4 at.%) results in the formation of granular surface with rectangular shaped grains and drastically increased roughness. In the presence of aluminum acetyle acetonate, surface chemistry changes in such a way that it removes randomness and increases surface diffusion [46, 47]. AFM measurements were carried out at 5 places on each set of sample (10 × 10 mm2 ) and the variation of average (avg) and root mean square (rms) roughness of the reported films as functions of xAl are shown in fig. 10. Standard deviation observed in the measurements lie within ± 1nm of average value. This figure also has three distinct regions as discussed in fig. 3. In region-I (0 at.% ≤ xAl ≤ 4 at.%) sharp decrease in roughness is observed (≈ 4 fold). In region-II (4 at.% ≤ xAl ≤ 12 at.%) roughness remains nearly constant and afterwards, in region-III (12 at.% ≤ xAl ≤ 16.4 at.%) roughness again increases. In this region, rate of increase of roughness (≈ 2 fold) is less than the rate of decrease of roughness in region-I. These experimental observations are in very good 12

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agreements with the results reported by Banerjee at. al.[48]. The observed feature shown in fig. 10 suggests that in the range of 4 to 11 at.% of Al, incorporation of Al+3 ions dominate the growth process. Granular surface morphology of 5 at.% Al doped films [49] grown at 325 o C by ALD is quite similar to the film grown by us using AACVD at 16.4 % of xAl . The only difference one may note is that the ALD grown films have roughness ≈ 7.3 nm while AACVD grown films have about 26 nm of roughness. Interestingly, average transmittance is more than 83 % for the AACVD grown films with rms roughness in the range of 8 to 12 nm which is much higher then that of ALD grown films with roughness in the range of 1.0 to 0.3 nm as well as the RF sputtered films with roughness in the range of 3.4 to 1.6 nm. As a consequence, one may infer that the AACVD grown films have good optical quality. Figure 11 illustrates the variation of deposition rate and roughness of Al:ZnO thin films as functions of xAl (≤ 6 at.%, region-I). From this figure, we come across a very interesting observation that as xAl increases, both deposition rate and roughness decrease sharply. This decrease in the growth rate clearly indicates that at high concentration of Al in the precursor solution, Al plays a major role in the growth mechanism along 13

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with Zn precursors. This is in contrast to the standard observation in MOVPE growth technique where group-III or group-II precursors primarily determine the growth rate and the concentration of dopant precursor plays a trivial role. 4. Conclusions:

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In this work, we have presented the experimental results of a detailed study of the properties of Al:ZnO thin films prepared by AACVD process with a wide range of Aldoping concentration (xAl ). ZnO films with high average transmittance ≥ 80 % have been obtained by this growth technique. The transmission spectra of films show that Al doping is responsible for the blue shift of band edge. Depending upon the xAl range, the actual shift in bandgap is decided by three competing mechanisms viz., (i) bandgap narrowing which is a consequence of many-body effects on both the conduction and valence bands, (ii) the conventional Burstein-Moss effect yielding a bandgap widening as a result of the blocking of the lowest states in the conduction band and (iii) the compressive strain generated in the lattice due to the presence of amorphous Al2 O3 or AlOx . Mechanisms (i) and (ii) are applicable in the complete concentration range studied in this work, and mechanism (iii) has appreciable effect only for xAl ≥ 16 at.%. The experimentally measured values of carrier concentration (≈ 1020 cm−3 ) exceed Mott critical density (≈ 1019 cm−3 ). Increase in the effective mass of charge carrier in the conduction band of Al:ZnO thin films having high xAl has been confirmed by comparing experimentally obtained bandgap to that of the theoretically calculated values. The highest carrier concentration of 2.75 × 1020 cm−3 with resistivity = 3.6 × 10−3 Ωcm and mobility < 1 cm2 /Vs have been found. For all the films, we have obtained the resistivity ≈ 10−2 Ωcm. The minimum resistivity of 1.9 × 10−2 Ωcm, mobility 2 cm2 /Vs and bandgap 3.34 eV are obtained for the film with xAl = 1 at.%. The best figure of merit of AACVD grown Al:ZnO thin films ranges from 1.06 × 10−2 to 5.69 × 10−2 Ω−1 which justify the use of the films as a transparent conductor. In the range of xAl (0 to 16.4 at.%) studied in this work, absence of crystallite Al2 O3 phase and Al cluster has been confirmed by XRD. Morphology of Al:ZnO films is smoother for xAl ≤ 11 %. Beyond this, the roughness of film surface begins to increase. This suggests that during the growth, an optimum value of xAl may enhance the migra14

ACCEPTED MANUSCRIPT tion of atoms on the surface of film. The results suggest the possibility of producing TCO films based on ZnO with good electrical, optical, structural and morphological properties by using low-cost AACVD technique. 5. Acknowledgment:

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The authors acknowledge Mr. Amit Das of RRCAT, Indore for their support during electrical measurements. The authors thank Mr. Rajneesh Dhavan and Dr. S. K. Rai of RRCAT, Indore for XRD measurements and Dr. A. K. Ahire and Dr. D. M. Phase of UGC-DAE-CSR, Indore for EDX measurements. 6. References:

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• Carrier concentration exceed Mott critical density, figure of merit is ≈ 10−2 Ω−1 .

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• Morphology of Al:ZnO films became smoother with increasing Al doping.

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