Solar Energy Materials & Solar Cells

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1 Solar Energy Materials & Solar Cells 15 (212) Contents lists available at SciVerse ScienceDirect Solar Energy Materials & Solar Cells journal homepage: Letter Highly conductive GaN anti-reflection layer at transparent conducting oxide/si interface for silicon thin film solar cells Dong-Won Kang a, Jang-Yeon Kwon b,n, Jenny Shim c, Heon-Min Lee c, Min-Koo Han a a Department of Electrical Engineering and Computer Science, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul , Republic of Korea b Yonsei University, School of Integrated Technology, 162-1, Songdo-dong, Yeonsu-gu, Incheon 46-84, Republic of Korea c LG Electronics, Seoul , Republic of Korea article info Article history: Received 1 March 212 Received in revised form 22 June 212 Accepted 26 June 212 Available online 18 July 212 Keywords: GaN TiO 2 /ZnO Anti-reflection layer Microcrystalline silicon Thin film solar cells abstract Highly conductive GaN film was prepared by magnetron sputtering and this was applied as an antireflection layer (ARL) between a transparent conducting oxide and microcrystalline silicon (mc-si:h) in order to decrease optical reflection. The efficiency (8.81%) of mc-si:h single junction thin film solar cell with the proposed GaN ARL exceeded that of the cell (8.36%) with the widely used TiO 2 /ZnO bilayer ARL. Moreover, the proposed GaN ARL requires no protection layer against hydrogen plasma such as ZnO overcoating (1 nm) in case of the TiO 2 /ZnO bilayer. GaN ARL can replace the TiO 2 /ZnO bilayer ARL in terms of high performance and simple fabrication process. & 212 Elsevier B.V. All rights reserved. 1. Introduction Recently, photovoltaics are gaining more importance in generating electrical energy under the consideration of environmental sustainability [1]. Silicon (Si) thin film solar cells have gained attention as a promising photovoltaic technology, in terms of low cost and large area fabrication potential [2,3]. In these areas, enhancement of light trapping in photo-active layers is important for increasing the efficiency as light absorption can be more efficient and the Si thickness can also be decreased. Surface texturing of front transparent conductive oxide (TCO) has been widely used for light scattering [4]. However, highly roughened surface texture for enhanced light trapping can cause undesirable local current drain in the cells and degrade the electrical performances such as open circuit voltage (V oc ) and/or fill factor (FF) [5 7]. The roughness of TCO texture is limited to a certain extent for the optimization of the trade-off between the electrical and optical performances [7,8]. Anti-reflection layer (ARL) has been reported in order to reduce the reflection loss and increase the light trapping in Si layer. In Si thin film p i n solar cells, the incident light is reflected at the front interfaces such as air/glass, glass (n¼1.5)/tco (n1.9), and TCO (n1.9)/si (n3.5 5.) before it reaches Si active layer. In the TCO/Si n Corresponding author. Tel.: þ ; fax: þ address: jangyeon@yonsei.ac.kr (J.-Y. Kwon). interface, with significant difference in refractive index, TiO 2 (n2.5) ARL is widely used to decrease the reflection loss [9 11]. However, the ZnO film about1 nm has to be overcoated in order to protect TiO 2 ARL film against hydrogen plasma during the Si deposition process by plasma-enhanced chemical vapor deposition (PECVD). Moreover, the TiO 2 /ZnO bilayer ARL cause also optical reflection loss at TiO 2 /ZnO interface due to the low refractive index of ZnO (n1.9) compared to that of TiO 2 (n2.5). With this perspective, high performance ARL without the protectionlayerisrequiredtosimplify the fabrication process and avoid optical reflection loss at TiO 2 /ZnO interface. The purpose of this work is to report a new MgO/Al-doped ZnO (AZO)/GaN triple-layer antireflective front TCO in order to maximize light trapping and simplify the ARL fabrication process. The GaN single ARL at AZO/Si interface is proposed for replacing state-of-theart TiO 2 /ZnO bilayer ARL. For the replacement of the TiO 2 /ZnO, the refractive index between 1.9 (TCO) and 3.5 (Si) is required such as CuO (n2.58) or GaN (n2.4). High transmittance (high bandgap) and conductivity is also needed for the interface properties. Under these considerations, GaN film was applied for ARL at AZO/Si interface due to its physical properties such as high bandgap (3.4 ev) [12], high refractive index (n2.4) [13], tunable conductivity from 1 11 to 1 3 Scm 1 [14]. GaN thin films with various deposition conditions were prepared and their optoelectronic characteristics were analyzed for the application of ARL. We employed the optimized GaN films that were used as ARL and their performances were evaluated in microcrystalline silicon (mc-si:h) thin film /$ - see front matter & 212 Elsevier B.V. All rights reserved.

2 318 D.-W. Kang et al. / Solar Energy Materials & Solar Cells 15 (212) solar cells in terms of replacing TiO 2 /ZnO bilayer ARL which has been widely used in research and industry. Moreover, MgO ARL was employed at the glass/azo interface in all cells and this was recently developed in our group [15]. Our experimental results suggest that the mc-si:h solar cell with the proposed GaN ARL is more efficient compared to the cell with the TiO 2 /ZnO bilayer and this is due to high electrical performances. 2. Experiment GaN thin films were deposited on glass substrates by RF magnetron sputtering. 4-inch GaN target (4 N) was used to prepare the GaN films and the target was pre-sputtered for 1 min to get rid of possible contaminants on it before main deposition. The GaN films were deposited by sputtering of Ar gas with N 2 -dilution (5%). The substrate temperature was varied from 15 to 35 1C. To compare optoelectronic properties of GaN films with those of typical TiO 2 films, the TiO 2 films were also prepared in a similar manner with TiO 2 ceramic target. After the film deposition, the electrical conductivity (s¼r 1 s t 1 ) was calculated by measuring sheet resistance (R s ) and film thickness (t) of the samples. The R s measurement was carried out by using high resistivity meter (MCP-HT45, MITSUBISHI CHEMICAL) and the t was studied by spectroscopic ellipsometry (M-2 V, J.A.WOONAM). The film thickness obtained by spectroscopic ellipsometry was cross-checked by a-step surface profiler (Nanospec AFT/2, KLA-TENCOR). X-ray diffraction (XRD, D8-Advance, BRUKER) with Cu-K a line was used to characterize the structural property of the GaN films. The electrical property of the GaN films such as Hall mobility and carrier concentration were analyzed by using the Hall measurement with van der Pauw geometry. In addition, optical property of the films was also evaluated as refractive index profile which was also obtained the spectroscopic ellipsometry with Cauchy model [16]. The GaN and TiO 2 films with optimized optoelectronic properties were applied as ARL in mc-si:h solar cells. In terms of the cell fabrication process, MgO (4 nm) was deposited on glass for ARL at the glass/azo interface [15]. AfterAZO (1.1 mm) deposition by DC sputtering, wet chemical etching by HCl (.5%) was performed to characterize light scattering. Then, GaN film (4 nm) without a protection layer against hydrogen plasma was deposited on AZO TCO. The TiO 2 (4 nm)/zno(1 nm) bilayer ARL was also prepared on AZO in order to compare the anti-reflection effect. Then, mc-si:h p i n layers (1.4 mm) and AZO (1 nm) were sequentially deposited by using PECVD and DC magnetron sputtering, respectively. After the deposition, Ag (3 nm)/al (1 nm) were prepared by e-beam evaporation defining the solar cell area of.9 cm 2. In order to investigate the AR effect of the proposed GaN, reflectance of the cells was measured by ultraviolet visible spectrophotometer (UV vis, Lambda 35, PerkinEllmer). Before each series of reflectance measurements, a baseline correction (reference setting) was performed with a white porcelain standard (Certified Reflectance Standard Lab sphere USRS-99-1 AS ). Moreover, quantum efficiency (QE, CEP-25HS, Bunkoh-Keiki) and J V measurement (WXS-2S-2, WACOM) under AM 1.5G spectrum for 1 mw/cm 2 (1 sun) were measured at a constant temperature of 25 1C. In addition, variable intensity measurements (VIM) analysis was carried out on the same set-up by varying light intensity with neutral density filters (CVI Melles Griot) in order to diagnose the performances at the cell level. 3. Simulation procedure In order to determine the optimum thickness of ARL at TCO/Si interface, we have performed an optical simulation using Cell reflectance (%) 8 6 Average reflectance (%) W/O ARL ARL thickness (nm) 1 nm 2 nm 3 nm 4 nm 2 5 nm 6 nm 7 nm ASA (Advanced Semiconductor Analysis, TU Delft) as shown in Fig. 1 below. In the simulation, optical systems with flat interfaces were employed in the ASA reflectance simulations. Previous study reported that the thickness of TiO 2 about 5 nm was optimum thickness because the reflectance at wavelength of 55 nm was minimized by the TiO 2 about 5 nm [9]. However, we have considered the reflectance in wide spectral wavelength regions (4 8 nm) for characterizing the anti-reflection effect. Fig. 1 shows the reflectance of mc-si:h single junction solar cells with TiO 2 ARL at the TCO/Si interface and the inset data exhibits average reflectance in the wavelength regions (4 8 nm) as a function of various ARL thicknesses (1 7 nm). The average reflectance decreased with the increase in the ARL thickness and it was minimized at the thickness of around 4 5 nm. Then, it increased with the thickness increase over 5 nm. In terms of experimental fabrication process, ARL with 4 nm thickness is preferred in order to reduce deposition time and material consumption. Thus, the thickness of ARL was adjusted to 4 nm for demonstrating anti-reflection effect in the following experiments. 4. Results Conductivity of GaN films Fig. 1. Simulated reflectance of mc-si:h thin film solar cells with various TiO 2 ARL thicknesses (1 7 nm). The reflectance decreased with the increase in ARL thickness and it was minimized at 4 5 nm. The conductivity of ARL inserted at the TCO/Si interface is important because the series resistance (R series ) of cells can be increased by the insertion of a highly resistive layer. Thus, the conductivity of GaN and TiO 2 films were investigated with varying deposition temperature. This was a crucial factor that dominantly affected the conductivity of films and it is shown in Fig. 2. The conductivity of GaN films with suitable thickness of about 4 nm applicable for ARL was tunable from 1 7 to 1 2 Scm 1 by controlling the substrate temperature. In order to understand the conductivity variation, XRD and Hall measurement were performed for films with a sufficient thickness of 25 nm. From XRD analysis, the GaN films deposited at various temperatures ( C) were mostly amorphous phase except that the film deposited at 35 1C exhibited very weak diffraction peak located at which corresponds to the (11 ) faces of hexagonal GaN [17]. Hall measurement data indicated that the conductivity of GaN films increased from to Scm 1 with an increase in the substrate temperature from 15 to 35 1C. Hall mobility increased

3 D.-W. Kang et al. / Solar Energy Materials & Solar Cells 15 (212) from.15 to.7 cm 2 /V s along with the temperature. As for carrier concentration, it increased from to cm 3.On the other hand, TiO 2 films showed a conductivity of around 1 7 Scm 1 and they were of a similar level with the results of other group [11]. Conductivity (S cm -1 ) 1-2 GaN TiO Substrate temperature ( C) Fig. 2. Conductivity of the deposited GaN and TiO 2 films with an increase in the substrate temperature. The conductivity of films was increased with the deposition temperature. Refractive index AZO GaN TiO 2 Si Optimum Fig. 3. Refractive indices of the prepared GaN and TiO 2 films as a function of optical wavelength. The theoretical optimum value of the refractive index for minimizing reflectance at AZO/Si interface was estimated, by calculating the geometric mean of the two surrounding indices of AZO and Si Refractive indices of GaN films As optical characteristics for efficient ARL, refractive index matching is a key issue for the decrease in reflectance. Thus, refractive indices of the films deposited at various temperatures were analyzed. It was found out that the refractive index of GaN films with thickness of 4 nm deposited at ranges of C exhibited similar levels from 2.34 (15 1C) to 2.28 (35 1C) at a wavelength of 55 nm. When the conductivity was considered, high temperature deposition (35 1C) was preferred to satisfy the optoelectronic properties. Fig. 3 shows the refractive indices of films deposited at 35 1C as a function of wavelength. The GaN film showed a slightly lower refractive index (n2.28 at 55 nm) compared to that (n2.47 at 55 nm) of TiO 2 film. Theoretical optimum refractive index for minimizing reflectance at the AZO/ Si interface was included by finding out the geometric mean of p the two surrounding indices (i.e. n 1 ¼ ffiffiffiffiffiffiffiffiffiffiffiffi n Un 2 where n ¼ AZO; n 2 ¼ Si). It was found out that the refractive index of TiO 2 film is closer to the optimum profile compared to the GaN film Microcrystalline silicon solar cells with GaN ARL For the evaluation of the AR effect of the proposed GaN ARL, mc-si:h single junction solar cells were fabricated. Basically, the MgO ARLs were applied at the glass/azo interface on all cells for a decreasing reflection loss. Moreover, GaN ARL was applied in mc-si:h cells without a protection layer as the hydrogen plasma test showed no impact on the optoelectronic properties of the GaN films. Fig. 4(a) exhibits the measured reflectance and external quantum efficiency (EQE) of mc-si:h solar cells with and without ARL. In case of mc-si:h cell with the GaN ARL at the AZO/Si interface, it was found out that the reflectance decreased obviously in the whole spectral range compared to mc-si:h cell without ARL. Thus, it should be noted that the proposed GaN film can act as ARL at the TCO/Si interface in silicon p i n type solar cells. On the other hand, the mc-si:h cell with TiO 2 /ZnO bilayer ARL showed a slightly lower reflectance compared to the cell with GaN ARL. As for the EQE curves, it was confirmed that the J sc (23.5 ma/cm 2 ) of a cell employing GaN ARL was higher than that (22.6 ma/cm 2 ) of cell without ARL. However, it was lower than that (24.2 ma/cm 2 ) of cell with TiO 2 /ZnO bilayer ARL. Fig. 4(b) shows internal QE (IQE) as a function of the spectral wavelength and the EQE was also included. The IQE curves are higher than the EQE curves, which are mainly ascribed to the reflectance. In other words, the difference between IQE and EQE (IQE EQE) is principally caused by the reflection loss. In fact, it External quantum efficiency ( a ) MgO/AZO ( b ) MgO/AZO/TiO 2 /ZnO ( c ) MgO/AZO/GaN R Quantum efficiency (%) MgO/AZO (IQE) MgO/AZO (EQE) MgO/AZO/TiO 2 /ZnO (IQE) MgO/AZO/TiO 2 /ZnO (EQE) MgO/AZO/GaN (IQE) MgO/AZO/GaN (EQE) Fig. 4. (a) EQE and 1-R of the fabricated mc-si:h thin film solar cells and (b) IQE combined with EQE of the cells employing various ARLs (TiO 2 /ZnO, GaN) and without ARL. The performance of the solar cells with different ARLs is included.

4 32 D.-W. Kang et al. / Solar Energy Materials & Solar Cells 15 (212) was found out that the (IQE EQE) was highest for the cell without ARL. In the wavelength of 5 nm, the (IQE EQE) of the cell without ARL was 7.%, whereas those of the cells with TiO 2 /ZnO and GaN were 5.2% and 5.9%, respectively. In case of 8 nm, the (IQE EQE) of the cell without ARL was also the highest (8.4%) in all the cells, whereas those of the cells with TiO 2 /ZnO and GaN were 6.2% and 7.1%, respectively. Nevertheless, the cell with GaN ARL showed the highest efficiency (8.81%) in all the cells due to high electrical performance (FF and V oc ). In order to analyze those results, VIM technique was performed by changing the illumination intensity [18,19]. For evaluating the R series of the devices, open circuit differential resistance R oc ¼qV/qJ (at J¼) was plotted as a function of J sc in Fig. 5(a). The R series was obtained from the asymptotic value of R oc for high light intensity region (high J sc ). The R series of the cell without ARL was 2.7 O cm 2, whereas those of the cells with the TiO 2 /ZnO and GaN were 2.61 and 2.11 O cm 2, respectively. It was noted that the V oc and FF of the cells with the ARL was higher than the cell without ARL. In order to investigate the results, i-layer quality of the cells were evaluated by the VIM. From the method, short-circuit differential resistance R sc ¼qV/qJ (at V¼) can be distinguished between the contribution of recombination current (photo shunt) and the contribution of the leakage current (dark shunt or physical shunt). By using the method, R sc was extracted and plotted with the reciprocal of J sc, as shown in Fig. 5(b). From the data acquired, the collection voltage (V coll ) which indicates the quality of an intrinsic mc-si:h layer can be obtained by the relation (R sc ¼J sc 1 V coll ) as included in the table in Fig. 5(b). It was found out that the V coll of a cell with GaN is similar to that of a cell with TiO 2 /ZnO bilayer. Moreover, they are higher than that of a cell without ARL. This can be attributed to the effect of smoothing a highly roughened AZO surface texture by the ARL overcoating. This can be understood from our recent results of additional coating of rough TCO for the improvement of the i-layer quality [19]. 5. Discussion The optoelectronic properties of GaN films have been optimized for ARL application at the TCO/Si interface. We confirmed that the conductivity increased with the deposition temperature. From the XRD and Hall measurement, it is interpreted that the conductivity increase is mainly governed by the increase in the carrier concentration rather than on the improvement of the film structure. The carrier increase of about 1.5 order revealed in the Hall measurement is possibly attributed to nitrogen desorption at high temperature leaving nitrogen vacancies and these act as donors in GaN [2]. Thus, the conductivity gain more than (41 3 Scm 1 ) can be achieved by using the GaN films instead of the TiO 2 films especially in high temperature regions (35 1C). When the simulation and refractive index study are considered, it is expected that the anti-reflection effect of GaN works effectively. On the other hand, it can be more efficient by applying TiO 2 film compared to the GaN film because the refractive index of TiO 2 film that is closer to the optimum profile for minimizing reflectance compared to the GaN film as indicated in Fig. 3. From the deposited solar cells with those ARL, it was confirmed that the reflectance decreased and the EQE increased. It implies that the decrease in reflectance by the effect of the refractive index grading at the TCO/Si interface contributed to the enhancement of light trapping and generation of photoinduced carriers. From IQE combined with EQE data, the antireflection effect of GaN was clearly confirmed. However, it was slightly less effective than the anti-reflection effect of the TiO 2 / ZnO ARL. After the VIM analysis, we found that the introduction of the ARL at the TCO/Si interface increased the R series of the cells. However, the increase in the R series by the GaN was much lower than that by the TiO 2 /ZnO, which can be attributed to the high conductivity of the GaN compared to the TiO 2. The high electrical performance (V oc and FF) of the cells with ARL was related to V coll. The additional coating by using ARL deposition on the textured AZO TCO can give a more smooth morphology in terms of better growth conditions for mc-si:h. The improved mc-si:h quality confirmed by the high V coll can result in an increase of FF and V oc by the ARL (GaN, TiO 2 /ZnO). In the case of a cell with TiO 2 /ZnO bilayer ARL, the electrical performance was lower than the cell with GaN ARL. It can be ascribed to ohmic loss which is related to high R series of the cell employing the TiO 2 film with high resistivity compared to the cell with highly conductive GaN film, as confirmed in the Figs. 2 and 5(a). As a result, the efficiency (8.81%) of a cell with the proposed GaN ARL surpassed that (8.36%) of the cell with the state-of-the-art TiO 2 / ZnO bilayer ARL as high electrical performance exceeds the small increase of J sc. 6. Conclusion We have proposed and successfully fabricated novel MgO/ AZO/GaN triple-layer antireflective front electrode. The overall performance of mc-si:h solar cells with GaN ARL at AZO/Si MgO/AZO MgO/AZO/TiO 2 /ZnO MgO/AZO/GaN 2 15 ( a ) MgO/AZO ( b ) MgO/AZO/TiO 2 /ZnO ( c ) MgO/AZO/GaN V coll (V) R oc (Ω cm 2 ) R sc (Ω cm 2 ) 1 5 (a) 14.1 ( b ) 22.3 (c) J sc (ma/cm 2 ) /J sc (A -1 cm 2 ) Fig. 5. (a) Open circuit differential resistance R oc ¼qV/qJ (at J¼) and (b) short-circuit differential resistance R sc ¼qV/qJ (at V¼) of the fabricated mc-si:h solar cells with various TCO/ARL configurations obtained by the VIM technique. The R series and i-layer quality (V coll ) of the cells with the different TCO/ARL were evaluated.

5 D.-W. Kang et al. / Solar Energy Materials & Solar Cells 15 (212) interface surpassed the cell with the state-of-the-art TiO 2 /ZnO bilayer ARL. Furthermore, the GaN ARL requires no protection layers such as ZnO overcoating (1 nm) as in case of TiO 2 /ZnO. Our results suggest that the proposed GaN films are the promising ARL for replacing the state-of-the-art TiO 2 /ZnO bilayer ARL in terms of high performance and more simple fabrication process. Acknowledgments This work was supported by the Global Leading Technology Program (No. 211T1139) of the Office of Strategic R&D Planning (OSP) funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] V. Fthenakis, Sustainability of photovoltaics: the case for thin-film solar cells, Renewable and Sustainable Energy Reviews 13 (29) [2] K. Yamamoto, A. Nakajima, M. Yoshimi, T. Sawada, S. Fukuda, T. Suezaki, M. Ichikawa, Y. Koi, M. Goto, T. Meguro, T. Matsuda, M. Kondo, T. Sasaki, Y. Tawada, A high efficiency thin film silicon solar cell and module, Solar Energy 77 (24) [3] Armin G. Aberle, Thin-film solar cells, Thin Solid Films 517 (29) [4] R. Dewan, M. Marinkovic, R. Noriega, S. Phadke, A. Salleo, D. Knipp, Light trapping in thin-film silicon solar cells with submicron surface texture, Optics Express 17 (29) [5] M. Python, D. Dominé, T. Söderström, F. Meillaud, C. Ballif, Microcrystalline silicon solar cells: effect of substrate temperature on cracks and their role in post-oxidation, Progress in Photovoltaics: Research and Applications 18 (21) [6] M. Python, E. Vallat-Sauvain, J. Bailat, D. Dominé, L. Fesquet, A. Shah, C. Ballif, Relation between substrate surface morphology and microcrystalline silicon solar cell performance, Journal of Non-Crystalline Solids 354 (28) [7] H.B.T. Li, R.H. Franken, J.K. Rath, R.E.I. Schropp, Structural defects caused by a rough substrate and their influence on the performance of hydrogenated nano-crystalline silicon n i p solar cells, Solar Energy Materials and Solar Cells 93 (29) [8] H. Li, K. van der Werf, J.K. Rath, R.E.I. Schropp, Hot wire CVD deposition of nanocrystalline silicon solar cells on rough substrates, Thin Solid Films 517 (29) [9] T. Fujibayashi, Takuya Matsui, M. Kondo, Improvement in quantum efficiency of thin film Si solar cells due to the suppression of optical reflectance at transparent conducting oxide/si interface by TiO 2 /ZnO antireflection coating, Applied Physics Letters 88 (26) [1] M. Berginski, C. Das, A. Doumit, J. Hüpkes, B. Rech, M. Wuttig, Properties of TiO 2 layers as antireflection coating for amorphous silicon based thin-film solar cells, in: Proceeding of the 22nd European Photovoltaic Solar Energy Conference, 3 7 September 27, Milan, Italy, pp [11] C. Das, A. Lambertz, J. Huepkes, W. Reetz, F. Finger, A constructive combination of antireflection and intermediate-reflector layers for a-si/mc-si thin film solar cells, Applied Physics Letters 92 (28) [12] S. Lawrence Selvaraj, A. Watanabe, T. Egawa, Influence of deep-pits on the device characteristics of metal organic chemical vapor deposition grown AlGaN/GaN high-electron mobility transistors on silicon substrate, Applied Physics Letters 98 (211) [13] T.-Y. Park, Y.-S. Choi., S.-M. Kim, G.-Y. Jung, S.-J. Park, B.-J. Kwon, Y.-H. Cho, Electroluminescence emission from light-emitting diode of p-zno/ (InGaN/GaN) multiquantum well/n-gan, Applied Physics Letters 98 (211) [14] S. Kobayashi, S. Nonomura, T. Ohmori, K. Abe, S. Hirata, T. Uno, T. Gotoh, S. Nitta, S. Kobayashi, Optical and electrical properties of amorphous and microcrystalline GaN films and their application to transparent TFT, Applied Surface Science (1997) [15] D.-W. Kang, J.-S. Woo, S.Y. Lee, H.-M. Lee, M.-K. Han, Effect of MgO antireflection (AR) layer at glass/zno:al interface on characteristics of microcrystalline silicon solar cells, in: Proceeding of the 26th European Photovoltaic Solar Energy Conference, 6 1 September 211, Hamburg, Germany, pp [16] E. Langereis, S.B.S. Heil, H.C.M. Knoops, W. Keuning, M.C.M. van de Sanden, W.M.M. Kessels, In situ spectroscopic ellipsometry as a versatile tool for studying atomic layer deposition, Journal of Physics D: Applied Physics 42 (29) [17] C.G. Zhang, L.F. Bian, W.D. Chen, C.C. Hsu, Effect of growth conditions on the GaN thin film by sputtering deposition, Journal of Crystal Growth 299 (27) [18] A. Shah, Thin-film silicon solar cells, first ed., EPFL Press, Switzerland, 21, pp [19] T. Moon, J.H. Jun, H. Lee, W. Yoon, S. Kim, B.-K. Lee, H.-C. Lee, W. Kim, S.-W. Ahn, S. Lee, H.-M. Lee, Additional coating effects on textured ZnO:Al thin films as transparent conducting oxides for thin film Si solar cells, Progress in Photovoltaics: Research and Applications (211) g/1.12/pip [2] L.F. Lester, J.M. Brown, J.C. Ramer, L. Zhang, S.D. Hersee, Nonalloyed Ti/Al Ohmic contacts to ntype GaN using high temperature premetallization anneal, Applied Physics Letters 69 (1996)