Deposition and Characterization of ZnO:Al Thin Films

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2009 6th International Conference on Electrical Engineering, Computing Science and Automatic Control (CCE 2009) (Formerly known as ICEEE) Toluca, México. November 10-13, 2009

Deposition and Characterization of ZnO:Al Thin Films by Ultrasonic Spray Pyrolysis B. J. Babu, A. Maldonado and S. Velumani Departamento de Ingeniería Eléctrica-SEES, CINVESTAV-IPN, Zacatenco, D.F., C.P.07360, México Phone (00 52 55) 5747 4001 Fax (00 52 55) 5747 4003 Email: [email protected]

Abstract – Al-doped ZnO (AZO) thin films were prepared using simple, flexible and cost-effective ultrasonic spray pyrolysis (USP) technique at different substrate temperatures. Zinc acetate dehydrate (Zn (CH3COO)2.2H2O) and Aluminum acetylacetonate (C15H21AlO6) were used as precursors and the solvent was a mixture of de-ionized water, methanol and acetic acid. Substrate temperatures are varied for 3 at% Al-doped film between 450°C to 500°C. The film’s structural, optical and electrical properties were investigated by X- Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), UV-VIS transmittance spectroscopy, Photoluminescence (PL) and Hall measurements. The obtained films were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (002) crystallographic direction. Grain sizes varied from 21.3 to 25.3 nm based on substrate temperature. FESEM images revealed that the film morphology is strongly affected by the substrate temperature. Transmission measurements showed that for visible wavelength (400-700nm), the AZO films have an average transmission of 75%. Optical band gap of AZO films is varied from 3.26 to 3.29 eV with the increase in substrate temperature. PL spectra showed ZnO:Al films with a low density of native defects. Resistivity of the films varied from 0.7 Ohm-cm to 2×10-2 Ohm-cm. Minimum electrical resistivity was obtained for film deposited at 475°C with film thickness of 602nm. Key words - AZO, EDX, FESEM, PL, USP, UV-VIS, XRD

INTRODUCTION Transparent conductive oxide (TCO) consists of a degenerated wide band-gap semiconductor with low electrical resistivity and high transparency in the visible and near infrared wavelength range [1]. ZnO is an n-type wide band-gap semiconductor with band-gap value of 3.42 eV at 300K and the electrical conductivity of the films is mainly due to intrinsic defects such as interstitial Zinc atoms and Oxygen vacancies [1-3]. The electrical properties of undoped ZnO thin films are unstable, due to the chemisorption of oxygen at the top surface and grain boundaries, which leads to higher resistivity. Two techniques have been used to stabilize these properties, namely, annealing and doping [3]. Extrinsic dopants stabilize the properties of the films. Aluminum has been the most widely dopant in ZnO thin films [2,3]. There are two reasons for this trend: (i) the electronegativity difference between aluminum (1.61) and zinc (1.65) is very small and (ii) the ionic radii of Al (53.0pm and 67.5pm) are slightly smaller than the ionic radii

IEEE Catalog Number:CFP09827 ISBN: 978-1-4244-4689-6 Library of Congress: 2009904789 978-14244-4689-6/09/$25.00 ©2009 IEEE

of zinc (60pm and 71pm) in (IV) and (VI) configurations, respectively [3, 4]. Electrical conductivity of ZnO films can be increased by doping with group III elements such as aluminum, boron, gallium and indium [1, 2, and 4]. Recently AZO thin films have been investigated due to their low resistivity and high transmittance. The AZO films studied so far shows the resistivity ranging from 2 to 5×10-4 Ω cm, depending on the preparation technique, measurement conditions and, quite relevant, the atmosphere of annealing treatment [1, 2]. AZO films have been grown by a variety of techniques. These techniques include sputtering, evaporation, pulsed laser evaporation and chemical vapour deposition. Another type of technique is incorporation of an aluminum-containing compound in a solution (such as in spray pyrolysis or sol-gel) [3]. Among these methods, spray pyrolysis, a nonvacuum technique is useful for large area applications. Also this method is cheaper, simpler, fast, material efficient, easy to use, and permits to obtain films with the required properties for optoelectronic applications [5-9]. Thus, spray pyrolysis of ZnO is not only economical for manufacturing, and also avoiding the need for a two target sputtering systems [6]. In the USP system, an alcoholic solution containing the precursor-salts is nebulised by an ultrasonic actuator and then transported to a heated substrate. Advantage of USP over conventional pneumatic spraying is the droplet uniformity and better control of spray flux with a soft carrier-gas flow, which allows the deposition of very thin layers with homogeneous thickness [9, 10]. This is important for thin buffer layers in CIGS solar cells, where the desired film thickness is in nano-scale [11, 12]. In this study we report the effect of substrate temperature on the electrical resistivity, structural, morphological and optical characteristics of ZnO:Al thin films deposited by ultrasonic spray pyrolysis technique, and the application of the film for solar cell devices as transparent conducting oxide contact was investigated. METHODOLOGY

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Transparent conducting Al-doped ZnO thin films were prepared by ultrasonic spray pyrolysis method. An ultrasonic spray pyrolytic apparatus (vertical configuration type) was used. A schematic experimental set up is given elsewhere [13]. 0.2 M starting solution was prepared from Zinc acetate dehydrate (Zn (CH3COO)2.2H2O) (Merck) dissolved in a mix of methanol (Baker) and acetic acid (Merck). Aluminum acetylacetonate (C15H21AlO6) (Alfa) previously diluted in deionized water and acetic acid (Merck) at 0.2 M was added to the Zn solution in order to dope the initial solution such that [Al/Zn] ratio to be 3 at%. The solution was sprayed onto previously cleaned glass substrates. The droplets of solution produced by the ultrasonic generator were carried to the substrate by nitrogen gas. The films were deposited in air at temperatures ranging from 450 to 500°C for 10 min. Structural development and surface morphology of films were measured by PANalytical X-ray diffractrometer with CuKα (λ=1.5406Å) radiation and JEOL JSM-7401F FESEM respectively. The transmittance spectra were obtained using SHIMADZU UV-VIS double beam spectrophotometer in the wavelength range 300-1100 nm. The photoluminescence properties were studied using Spex double beam spectrophotometer. The film thickness was measured by a KLA Tencor P15 profilometer, etching an edge of the samples to make a reference step. Hall measurements were carried out by the Van der Pauw method using a Walker scientific HV-4H equipment. RESULTS AND DISCUSSION Fig.1. shows the X-ray diffraction patterns of the AZO films. As the substrate temperature increased, the peak intensity and crystal size increased, which may be due to the decrease in stress with increasing temperature. The positions of the peaks fit well with polycrystalline ZnO with hexagonal wurtzite structure. We observed that the (002) peak is very sharp and other reflections (101), (102), (103) appear relatively weak. In all cases a preferential (002) growth appears indicating the crystallite structure of the films is oriented with their c-axis perpendicular to the substrate. This behavior is in good agreement with other reports [2, 8, 14-16]. This particular orientation follows the grain evolutionary selection model proposed by Van der Drift and usually correlates with the energies gained by the incident species on the substrate under appropriate experimental conditions. Regular alternating layers of zinc and oxygen atoms linked along the c-axis as pseudodiatomic molecules thus constitute the ZnO film’s stable hexagonal, closely packed wurtzite crystal structure. However, nonoptimal experimental conditions, defects and other chemical impurities may hinder the (0 0 2)-oriented growth as is the case in low-temperature films, where incomplete growth of Zinc acetate compound into ZnO film is found [16].


Fig.1. XRD patterns for AZO thin films deposited at different temperatures with 3 at% [Al/Zn].

A dominant signal associated with the (101) planes is found for AZO thin films. Using the (002) peak and the Debye-Scherrer formula [8] the crystallites size was estimated. The values obtained for three different substrate temperatures starting from 450°C, 475°C and 500°C is 21.3 nm, 23.8 nm and 25.3 nm respectively. The grain size broadening with substrate temperature correlates with the increase in Eg [16]. Further, the XRD 2θ scattering angle at (002) peak increases slightly from 34.34° to 34.38° as the substrate temperature is varied from 450°C to 500°C. Such shift suggests that some strain and stress are induced in the film due to the film-substrate lattice mismatch [16]. Fig.2 shows FESEM micrographs of AZO thin films. Fig 2(a) is taken for Un-doped ZnO (UZO) film deposited at 475°C.Fig 2(b), (c) and (d) are AZO thin films deposited at 450°C, 475°C and 500°C respectively. The grain size of the AZO thin films increased with increase in temperature. This result is associated not only with the grains growing more easily when the temperature is higher, but also with the Al incorporation. Comparing the crystallization of ZnO with AZO, a large amount of Al dopants resulted in lattice disorder, which is associated with the stress generated. Besides the stress problem, the grains grew more easily when Al dopants were incorporated with ZnO [2, 17]. Optical transmittance of films is known to be dependent upon the surface morphology [15]. The films deposited at 500°C had smooth surface morphology. High resistivity measured in this film indirectly confirms the presence of a highly resistive aluminum oxide phase in the grain boundaries, although at a concentration lower than the limit of resolution of the XRD used, since according to the spectra obtained no aluminum or aluminum oxide phases were detected [14]. Film composition plays a major role in the electrical conductivity of transparent conducting ZnO films.

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wavelengths and as a transparent material at long wavelengths [8].

Fig. 3. EDX of AZO thin films.

Optical transmittance of the films is slightly dependent on the substrate temperature and showed that for visible wavelength (400-700nm), the AZO films have an average transmission of 75%. An increase in substrate temperature leads to a small decrease in optical transmittance. From the transmittance data it is possible to infer the optical band gaps of the films by plotting (αhν)2 vs. hν (where α is the absorption coefficient, and hν the photon energy) and by extrapolating the straight-line portion of this plot to the energy axis. These plots yield optical band gaps of 3.26 3.29 eV. Fig. 2. FESEM images of (a) UZO thin film deposited at 475°C and AZO thin films deposited at (b) 450°C, (c) 475°C and (d) 500°C.

The composition of films was obtained from Energy Dispersive X-ray (EDX) analysis as shown in Fig. 3. The average atomic percentage of zinc and oxygen was 57.49 and 41.89 respectively. This indicates that the films were oxygen deficient in nature. It is known fact that the oxygen deficiency gives rise to electrical conductivity in transparent conducting oxides. In general, the electrical conductivity increases, as the film thickness increases, since the grain size increases with the thickness [7]. The transmission spectrum of AZO films is shown in Fig.4. It was seen that the transmission values of the film are low at short wavelengths and the values are high at longer wavelengths. So, the film behaved as an opaque material because of its high absorbing properties at short Fig. 4. Transmission spectrum of AZO thin films deposited at different temperatures.


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Optical direct band gaps of AZO thin films as a function of a substrate temperature is shown in Fig.5. The quantum confinement of carriers leads to an increase in the optical band gap of the material and the consequent decrease of its optical absorption. Photoluminescence studies may elucidate the quantum confinement effects [16].

is decreased, which is attributed to decrease in intrinsic defects due to aluminum doping.

Fig. 5. Optical Direct band gap of AZO thin films deposited at different temperatures.

Fig. 6. PL spectra of AZO thin film deposited at 475°C with 3 at% [Al/Zn] ratio and comparing UZO thin film deposited at 475°C.

In particular, the increasing trend in optical band gap with substrate temperatures indicates the improvement in the film crystal quality, as also evidenced from the XRD spectra. We see also in Fig. 4 that the UV–VIS transmittance edge of the film shifts toward lower wavelength (higher energy) with the increase in the film’s substrate temperature. This shift is also closely related to the slight increase in the film optical band gap Eg with the substrate temperature [16]. Further energy gap broadening of aluminum doped ZnO films (not the case here) occurs owing to this Burstein–Moss effect [15, 18], where the Fermi level finds itself inside the conduction band.

Fig. 7 shows the electrical properties of the AZO thin films deposited with various substrate temperatures. Resistivity of the films decreased as the substrate temperature increased from 450°C to 475°C. The minimum value of resistivity 2×10-2 Ω-cm was obtained at 475°C. A further increase in substrate temperature resulted in increase of resistivity [2, 14, 20]. The carrier concentration increased as the substrate temperature increased from 450°C to 475°C, reaching the maximum value of 3.11×1020 cm-3 at 475°C. As the substrate temperature increased further, the carrier concentration decreased.

As discussed earlier, Photoluminescence studies may elucidate the quantum confinement effects and optical properties of AZO films. Fig 6 shows Photoluminescence spectra of the AZO and UZO thin films using 325 nm visible light from a 10mW He-Cd laser at room temperature. In this spectrum, the UZO and AZO thin films contain a UV emission band at 380 nm and broad green emission band occurs at about 527 nm. The near band emission of 380 nm was caused by the transition from conduction band to valence band. On the other hand, the green emission peak at 527 nm resulted primarily from intrinsic defects. The intrinsic defects are associated with deep level emissions such as oxygen vacancies or zinc interstitials [13, 17 and 19]. Relatively weak deep-level emission confirms that the films obtained by USP are well close to stoichiometric ZnO and of optically high quality [19]. Notably, the oxygen vacancies and interstitials were induced by the thermal process. After Al doping, the green emission peak intensity


Fig. 7. Electrical resistivity (ρ), carrier concentration (n), and Hall mobility (μ) of AZO films deposited with various substrate temperatures

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[3] M F Al-Kuhaili, M A Al-Maghrabi, SM A Durrani, and I A Bakhtiari, “Investigation of ZnO/Al/ZnO multilayers as transparent conducting coatings”, J. Phys. D: Appl. Phys, Vol. 41, pp. 215302 (8pp), 2008. [4] M. de la L. Olvera, A. Maldonado, R. Asomoza, O. solorza,and D. R. Acosta, “Characteristics of ZnO:F thin films obtained by chemical spray. Effect of the molarity and the doping concentration”, Thin solid films, vol. 394, pp. 242-249, 2001. [5] F. Yakuphanoglu et.al., ¨ The effects of fluorine on the structural, surface morphology and optical properties of ZnO thin films¨, Physica B, vol. 394, pp.86-92, 2007. [6] Sophie G, Alexander G, Nicholas A, Tristan K, Christian C, Martha Lux-Steiner, and Christian-Herbert Fischer, “A spray pyrolysis route to the undoped ZnO layer of Cu(In,Ga)(S,Se)2 solar cells”, Thin solid Films, vol. 517, pp. 2309-2311, 2009. [7] T. Prasada Rao, and M.C. Santhoshkumar, “Effect of thickness on structural, optical and electrical properties of nanostructured ZnO thin films by spray pyrolysis”, Applied Surface Science, vol. 255, pp. 4579-4584, 2009. [8] Bengisu Ergin, Elif Ketenci, and Ferhunde Atay, “Characterization of ZnO films obatined by ultrasonic spray pyrolysis technique”, International journal of Hydrogen Energy, Vol. 34, no.12, pp. 5249-5254, 2009. [9] S. Oktik, “Low cost, non vacuum techniques for the preparation of thick/thin films for photovoltaic applications” in “Progress in crystal growth and characterization”. Ed. J. Brain Mullin. pergamon press plc. 1989, vol. 17, pp. 171-240. [10] Y. LEE, H. Kim and Y. Roh, “Deposition of ZnO Thin films by the Ultrasonic spray pyrolyis technique”, Jpn. J. Appl. Phys., Vol.40, pp. 2423-2428, 2001. [11] K. Ernits , D. Brémaud , S. Buecheler , C.J. Hibberd , M. Kaelin ,G. Khrypunov , U. Müller , E.Mellikov, and A.N. Tiwari , “Characterisation of ultrasonically sprayed InxSy buffer layers for Cu(In,Ga)Se2 solar cells”, Thin Solid Films, vol. 515, pp. 60516054, 2007. [12] S. Buecheler, D. Corica , D. Guettler , A. Chirila , R. Verma , U. Müller , T.P. Niesen, J. Palm, and A.N. Tiwari, “Ultrasonically sprayed indium sulfide buffer layers for Cu(In,Ga)(S,Se)2 thinfilm solar cells”, Thin Solid Films, vol. 517, pp. 2312-2315, 2009. [13] X. Zhang, H. Fan, J. sun and Y. Zhao, “structural and electrical properties of p-type ZnO films prepared by ultrasonic spray pyrolysis”, Thin solid films, vol. 515, pp.8789-8792, 2007. [14] H. Gomez-Pozos, A. Maldonado, and M. de la L. Olvera, “Effect of the [Al/Zn] ratio in the starting solution and deposition temperatura on the physical properties of sprayed ZnO:Al thin films”, Materials Letters, vol. 61, pp. 1460-1464, 2007. [15] J. H. Lee and B. O. park, “Characteristics of Al-doped ZnO thin films obtained by ultrasonic spray pyrolysis: effects of Al doping and an annealing treatment”, Materials Science and Engineering B, vol. 106, pp. 242-245, 2004. [16] L. Hadjeris, L. Herissi, M. Assouar, T. Easwarakhanthan, J. Bougdira, N. Attal and M. Aida, “Transparent and conducting ZnO films grown by spray pyrolysis”, Semiconduct. Sci. Technol., vol. 24, pp. 035006(6pp), 2009. [17] K.J. Chen, T. H. Fang, F. Y. Hung, L.W. Ji, S.J. Chang, S. J. Young, and Y. J. Hsiao, “The crystallization and physical properties of Al-doped ZnO nanoparticles”, Applied surface science, vol. 254, pp. 5791-5795, 2008. [18] M. A. Kaid, and A. Ashour, “Preparation of ZnO-doped Al films by Spray Pyrolysis technique”, Applied surface science, vol. 253, pp. 3029-3033, 2007. [19] J. L. Zhao, X. M. Li, S. Zhang, C. Yang, X. D. Gao and W. D. Yu, “Highly (002)-oriented ZnO film grown by ultrasonic spray pyrolysis on ZnO-seeded Si (100) substrate”, J. Mater. Res, Vol. 21, no. 9, pp. 2185-2190, sep. 2006. [20] Y. K. Moon, S. H. Kim and J. W. Park, “The influence of substrate temperature on the properties of aluminum-doped zinc oxide thin films deposited by DC magnetron sputtering”, J. Mater Sci: Mater Electron, vol.17, pp. 973-977, 2006.

The behavior of resistivity as a function of the substrate temperature has been explained on the basis of energy required for the ZnO film growth. In the case of spray pyrolysis technique, this energy comes from the heater. This in turn causes a decrease in the content of oxygen vacancies or zinc interstitials, leading to a more stoichiometric material with a high resistivity. In addition, the effects of out-diffusion of alkaline impurities coming from the substrate are promoted at high substrate temperature regime compensating the donor character of the Al impurities. These two effects increase the resistivity values at high substrate temperature [2]. CONCLUSION The effect of substrate temperature on the characteristics of Al-doped ZnO (AZO) thin films deposited with Ultrasonic spray pyrolysis (USP) was studied. XRD analysis shows that the sprayed AZO thin films are of polycrystalline texture with a hexagonal structure. FESEM images show the grain size of the AZO thin films increased with the increase in temperature. Transmission measurements showed that for visible wavelength (400-700nm), the AZO films have an average transmission of 75%. Slight variations of the band gap were obtained when the substrate temperature was varied. Furthermore, PL spectra show low signal in deep level transition, indicating a low density of native defects. The lowest resistivity was 2×10-2 Ω-cm for the 3 at% [Al/Zn] film deposited at 475°C. In this study, we demonstrated the possibility of producing TCO films based on Al-doped ZnO with good electrical and optical properties by optimizing the substrate temperature. ACKNOWLEDGMENT The authors wish to thank for technical assistance of Dr. Jaime Vega Perez, Miguel A Avendaño, M. A. Luna-Arias. Also authors like to acknowledge Solid State Physics group, Department of physics, CINVESTAV. B. J. Babu is thankful to CONACyT for the scholarship provided to pursue Doctoral program. REFERENCES [1]W.J. Jeong, S.K. Kim, and G.C. Park, “Preparation and characteristic of ZnO thin film with high and low resistivity for an application of solar cell”, Thin Solid films, Vol. 506-507, pp. 180-183, 2006. [2] H. Gómez, A. Maldonado, R. Castanedo-Pérez, G. TorresDelgado, and M. de la L. Olvera, “Properties of Al-doped ZnO thin films deposited by a chemical spray process”, Materials Characterization, Vol. 58, pp. 708-714, 2007.


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