Influence of Annealing Temperature on the Properties of ZnO Thin Films Grown by Sputtering

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1 Available online at Energy Procedia 25 (2012 ) PV Asia Pacific Conference 2011 Influence of Annealing Temperature on the Properties of ZnO Thin Films Grown by Sputtering Jamilah Husna a, M. Mannir Aliyu a, M. Aminul Islam a, P. Chelvanathan a, N. Radhwa Hamzah a, M. Sharafat Hossain a, M.R. Karim c, Nowshad Amin a,b,c* a Electrical, Electronic and Systems Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia Bangi, Selangor, Malaysia. b Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia. c CEREM, College of Engineering, King Saud University, Riyadh, Saudi Arabia Abstract Zinc oxide (ZnO) thin films were deposited by RF magnetron sputtering onto ITO coated soda-lime glass substrates. The effects of annealing in temperature range of 250 to 450 C on the structural and optical properties of the ZnO films have been studied. The crystalline structure, surface topology, morphology, optical properties of the films were determined using X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and UV Visible Spectrometry, respectively. X-ray diffraction measurement showed that the annealed ZnO films were polycrystalline in nature with (002), (101) and (001) oriented crystallites of hexagonal wurtzite structure. Crystalline property and grain size of the films were found to increase after annealing. The optical band gap of ZnO films initially blue shifted ( ev) when annealed at 400 C and further red shifted in the range of 3.23 to 3.1 ev being annealed at 250 to 450 C range. From the UV spectroscopy, the films showed transmittance over 85% in the optical bandgap spectrum. All these results indicate that post deposition annealing improves the film quality with reduced roughness and better crystalline properties that can be utilised as buffer layer in the CIGS or CdTe thin film solar cells Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Solar Energy 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Solar Energy Research Research Institute of Singapore (SERIS) National University of Singapore (NUS). The PV Asia Pacific Institute of Singapore (SERIS) National University of Singapore (NUS). The PV Asia Pacific Conference 2011 Conference 2011 was jointly organised by SERIS and the Asian Photovoltaic Industry Association was jointly organised by SERIS and the Asian Photovoltaic Industry Association (APVIA). (APVIA). Open access under CC BY-NC-ND license. Keywords: Zinc oxide; thin films; annealing; optical properties; RF sputtering * Corresponding author. Tel.: ; fax: address: nowshad@eng.ukm.my Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Solar Energy Research Institute of Singapore (SERIS) National University of Singapore (NUS). The PV Asia Pacific Conference 2011 was jointly organised by SERIS and the Asian Photovoltaic Industry Association (APVIA). Open access under CC BY-NC-ND license. doi: /j.egypro

2 56 Jamilah Husna et al. / Energy Procedia 25 ( 2012 ) Introduction Zinc oxide (ZnO) has been well recognised as one of the most promising oxide semiconductor materials owing to its excellent optical, electrical and acoustic properties. Moreover, it has a direct band gap of 3.36 ev with a high exciton binding energy (60 mev) [1]. Therefore, ZnO has found wider applications in recent years; e.g. as transparent electrodes in optoelectronic devices [2], heat mirrors for energy saving [3], surface acoustic wave devices [4], and in solar cells [5, 6]. Presently, ZnO is one of the prominent materials in solar cell industries being used as window layer, transparent conducting oxide (TCO) and buffer layer. ZnO films are usually prepared by several methods such as molecular beam epitaxy (MBE), chemical vapour deposition (CVD), pulsed laser deposition, metal organic chemical vapour deposition (MOCVD), sol gel processing, spray pyrolysis, spin coating and radio frequency (RF) magnetron sputtering. The sputtering method has many attractive advantages which include better film growth control, repeatability, low temperature deposition, large scale stability and uniform film properties. Thus, RF magnetron sputtering was chosen for the deposition of ZnO thin films. The properties of sputtered ZnO thin films are known to depend not only deposition parameters such as RF power, pressure, substrate temperature and ambient atmosphere [7], but also on the post deposition processes, such as thermal annealing. In this study, we report on the effect of post annealing treatment on the optical properties of the deposited ZnO thin films. The results of crystalline structure, grain size, and the surface morphology of ZnO thin films after post-annealing treatment have been investigated. The main focus of this study is to grow ZnO thin films that can be implemented in thin films solar cells in particular as buffer layer as well as to improve the quality of ZnO films by controlled annealing treatment. 2. Experimental: Deposition of ZnO films and annealing ITO ( cm) coated soda-lime glass substrates were cleaned in acetone, ethanol and DI water using ultrasonic bath for 10 minutes in each solution. Thereafter, these were dried with pure nitrogen gas flow to remove any dust or particle. Usually, after drying the substrate, the samples were placed inside the sputtering chamber to pump down to the base pressure of 10-5 Torr. To avoid any contaminants being deposited on the substrate and to stabilise the plasma, the target was pre sputtered for 10 minutes. Zinc oxide thin films were then deposited by RF magnetron sputtering and substrate temperature was set at room temperature. ZnO target was used at a deposition gas pressure of Torr in argon (Ar) gas, whereas the RF power was kept at 40 W. The thickness of deposited ZnO was found to be 350 nm from cross section SEM as well as referencing to the thickness monitor during deposition. Thickness depends on the total gas pressure during sputtering [8] besides the plasma power. Post deposition annealing was performed at 250, 350, 400 and 450 C in nitrogen-oxygen mixed gas ambient with a vacuum pressure of 25 mtorr. All the films were then subjected to various investigations as mentioned above. 3. Results and discussion X-ray Diffraction (XRD) measurement has been carried out to investigate the crystalline property of the ZnO thin films and the results are shown in Fig. 1. ZnO films were polycrystalline with (101), (102) and (002) oriented crystallites of hexagonal wurtzite structures having the prominent alignment along

3 Jamilah Husna et al. / Energy Procedia 25 ( 2012 ) (002) as shown in Fig. 1 (a-e). The as-deposited ZnO film as shown in Fig. 1(a) has (101), (102) and (002) peaks related to ZnO, with some unknown ones possibly from the ITO beneath. The films treated at annealing temperature of 400 C and above show the dominance of (002) over others. Assuming a homogeneous strain across the films, the crystallite size may be estimated from the full-width at halfmaximum (FWHM) of (002) diffraction peak using Scherer s formula [9]: 0.9 D (1) B cos Here, and B are the X-ray wavelength, Bragg diffraction angle and FWHM of the ZnO (002) diffraction peak, respectively [10]. As can be found in Fig. 2, it is quite obvious that the grain size increases after annealing treatment, which might be due to recrystallisation of the film. ZnO (0 0 2) (e) ZnO (0 0 2) (d) ZnO (0 0 2) ZnO (1 0 1) ZnO (1 0 2) (c) ZnO (0 0 2) (b) ZnO (0 0 2) ZnO (1 0 1) (a) Fig. 1. XRD patterns of ZnO thin films, (a) as-deposited and annealed samples at (b) 250 C, (c) 350 C, (d) 400 C and (e) 450 C in nitrogen-oxygen ambient. Annealed As deposited Fig. 2. Effect of thermal annealing (in nitrogen-oxygen ambient) on the grain size of ZnO thin films.

4 58 Jamilah Husna et al. / Energy Procedia 25 ( 2012 ) Scanning Electron Microscope (SEM) of the films reveal the presence of hexagonal shape of the particles and EDX (Energy Dispersive X-ray spectroscopy) measurements confirm the removal of impurities as a result of the annealing treatment [11]. EDX measurement was performed associated with the SEM inspection, to determine the atomic ratio of Zn to O. As can be observed, when the films are annealed at 250 C and 350 C, atomic percentile value of Zn is getting larger compared to as deposited films, however at higher annealing temperature (e.g. 400 C and 450 C) Zn content reduces making it Zn poor film. Table 1. Percentile ratio of zinc and oxygen in ZnO thin films (as-deposited and annealed) Element (Zn:O) Atomic % of Zn Atomic % of O As deposited C C C C Fig. 3. FESEM images of ZnO thin films. (a) As-deposited and annealed samples at (b) 250 C, (c) 350 C, (d) 400 C and (e) 450 C in nitrogen-oxygen ambient.

5 Jamilah Husna et al. / Energy Procedia 25 ( 2012 ) The surface topography and growth morphology (cross-section images) of the ZnO thin films were investigated by Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FESEM). It is evident that the surface morphology changes upon annealing process. As can be seen from the FESEM images in Fig. 3, annealing treatment changed the surface of films more homogeneously with better coverage and reduction of the porosity of the films (pinhole free films). Figure 4 shows the surface topography and the roughness of the samples that were measured using AFM. It shows that the root mean square (RMS) values of surface roughness of these films were in between 6.83 to nm indicating the better smoothness of the films. The surface roughness reaches the maximum value of nm for the sample annealed at 400 C. However, it was also observed that annealing treatment had little or almost no effect on the surface roughness. RMS = 6.83 nm RMS = 7.26 nm (a) (b) RMS = 7.20 nm RMS = nm (c) (d) RMS = 8.67 nm (e) Fig. 4. AFM images of ZnO thin films. (a) As-deposited and annealed samples at (b) 250 C, (c) 350 C, (d) 400 C and (e) 450 C in nitrogen-oxygen ambient.

6 60 Jamilah Husna et al. / Energy Procedia 25 ( 2012 ) One of the vital parameters for the application of ZnO thin films is to have the higher optical transmittance [12]. The optical transmittance of the film was measured by UV-Visible spectrometer (Perkin Elmer instrument lambda 35), in the range from 350 to 950 nm. Moreover, the optical transmission spectra of the as-deposited and annealed ZnO thin films at different temperatures demonstrate good optical transmittance (over 85%) in the visible and near infrared as shown in Fig. 5. As deposited 250 C 350 C 400 C 450 C Fig. 5. Optical transmittance spectra of ZnO thin films, as-deposited and annealed samples at various temperatures The variation of absorption coefficient, with respect to photon energy (h ) was found to obey the relation: h = A(h -Eg) 1/2 for the allowed direct transition where A is the edge width parameter and Eg is the optical band gap. The optical band gap values are obtained by extrapolating the linear portion of the plots of ( h ) 2 versus h to = 0 [13]. As shown in Fig. 6, the optical band gap of ZnO films initially blue shifted (3.12 to 3.23 ev) as annealed at 400 C and a red shift ( ev) was observed in the annealing temperature range of C. As deposited 250 C 350 C 400 C 450 C Fig. 6. Derivation of ZnO thin film energy bandgap for as-deposited and annealed samples at different temperature

7 Jamilah Husna et al. / Energy Procedia 25 ( 2012 ) Conclusions In this study, intrinsic zinc oxide (ZnO) films have been deposited on top ITO coated soda-lime glass substrates by RF magnetron sputtering technique. After annealing treatment the films surface becomes quite smooth and uniform compared to the samples without annealing. Moreover, the annealing treatment improves the morphology of ZnO film as obvious from AFM and SEM images. Additionally, the films show optical transmission over 85% in the visible wavelength range. XRD results show that the films were polycrystalline with the preferential orientations along (002), (101) and (102) direction, but when the annealing temperature is increased above 400 C, the films show (002) to be more dominant. It is found that the ZnO thin films with post annealing treatment exhibit good structural and optical properties with a smoother surface. Based on the results, it can be concluded that the properties of the ZnO thin films are suitable for solar cell application, especially for the buffer layer in the CIGS and CdTe solar cells, tuning the bandgap to suitable values upon annealing. Acknowledgements This work is supported by NPST Program through King Saud University (KSU) with research grant code 10-ENE Authors would also like to acknowledge the Solar Energy Research Institute (SERI) of The National University of Malaysia (UKM) for other supports. References [1] Laurent K, Yu DP, Tusseau-Nenez S, Leprince-Wang Y. Thermal annealing effect on optical properties of electrodeposited ZnO thin films. J. Phys. D: Appl. Phys. 2008;41: [2] Jin ZC, Hamberg I, Granqvist CG. Optical properties of sputter-deposited ZnO:Al thin films. Thin Solid Films 1988; 164:381. [3] Ma J, Ji F, Zhang DH, Ma HL, Li SY. Optical and electronic properties of transparent conducting ZnO and ZnO:Al films prepared by evaporating method. Thin Solid Films 1999;357:98. [4] Sieber I, Wanderka N, Urban I, Schierhorn IE, Fenske F, Fuhs W. Electron microscopic characterization of reactively sputtered ZnO films with different Al-doping levels. Thin Solid Films 1998;330:108. [5] Kluth O, Rech B, Houben L, Wieder S, Schope G, Beneking C, Wagner H, Loffl A, Schock HW. Texture etched ZnO:Al coated glass substrates for silicon based thin film solar cells. Thin Solid Films 1999;351:247. [6] Weibenrieder KS, Muller J. Thin Solid Films 1997;300:30. [7] Gardeniers JGE, Rittersma ZM, Burger GJ. Preferred orientation and piezoelectricity in sputtered ZnO films. J. Appl. Phys. 1998;83: [8] Sun XW, Wang LD, Kwok HS. Improved ITO thin films with a thin ZnO buffer layer by sputtering. Thin Solid Films 2000;360: [9] Song D, Widenborg P, Chin W, Aberle A. Investigation of lateral parameter variations of Al-doped zinc oxide films prepared on glass substrates by rf magnetron sputtering. Sol. Energy Mater. Sol. Cells 2002;73:269. [10] Fang ZB, Yan ZJ, Tan YS, Liu XQ, Wang YY. Influence of post-annealing treatment on the structure properties of ZnO films. Appl. Surf. Sci. 2005;241: [11] Elilarassi R, Chandrasekaran G. Effect of annealing on structural and optical properties of zinc oxide films. Mater. Chem. and Physics 2010;121: [12] Flickyngerova S, Rehakova A, Tvarozek V, Novotny I. Sputtered of ZnO:Al thin films for application in photovoltaic solar cell. Advances in Electrical and Electronic Engineering 2011; p [13] Chaabouni F, Abaab M, Rezig B. Effect of the substrate temperature on the properties of ZnO films grown by RF magnetron sputtering. Mater. Sci. and Eng. 2004; B109: