Ion Beam Treatment of Functional Layers in Thin-Film Silicon Solar ...

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5 Ion beam treatment of the glass surface (a) no cleaning (b) 15 s dipping in HCl (c) 1 min dipping in HCl (d) with standard-cleaning Figure 5.14: The effect of post HCl etching and re-cleaning of ion beam treated glass substrates on the growth of ZnO:Al films. The AFM images show ZnO:Al films grown on Ar/O 2 beam pretreated glass substrates without post-cleaning (a), with 15 s dipping in HCl solution (b), 1 min dipping in HCl solution (c) and with post-standard cleaning process (d). The AFM measurement scan size for all the films are 4 × 4 µm 2 . The Z range on top of the AFM images means the height of the color bar. treatment. SEM and AFM were taken on the ion beam pretreated glass substrates, and no surface roughness was observed. Therefore, the chemical states of the glass surface must be altered by the ion beam treatments. Post HCl etching of the ion beam treated glass substrate lead the growth of ZnO:Al films back to standard, while the NaCl etching has no effect on the rough growth. This indicates that HCl etching can remove the chemical effects on the glass substrates which are needed for the rough growth of the next ZnO:Al films. Moreover, the etching effect of HCl is time dependent and can be changed gradually. Short time dip in the diluted HCl solution cannot fully remove the chemical effects. As a conclusion for this section, the rough growth of ZnO:Al films was not observed if the ion beam pretreated glass was etched in HCl or re-cleaned by standard cleaning process. These facts indicate the chemical changes on the glass surface caused by the 94
5.2 Reason of rough growth ion beam treatment are not persistent with acidic solution. The observations give a hint to explore the reason of the rough growth. 5.2.3 Sputter ZnO:Al film in different system In the previous sections the glass substrates were changed or modified by post-cleaning process. This section examines the growth of ZnO:Al film in other sputtering systems. In our institute, apart from the large area planar target, small area round target, tube ceramic target, and tube metallic target are available as introduced in Section 3.1.1. As for the large area planar target, a sputtering area of 30 × 30 cm 2 is possible, while sputtering area of 10 × 10 cm 2 is considered in small area sputtering system. The new installed tube target benefits from its high target material utilization and high sputtering rate. Sputtering process has been optimized at high rate sputtering for Si thin-film solar cells. In this experiment, Corning glass substrates were pretreated by mixed Ar/O 2 ion beam under the same condition. Then the glass substrates were transfered to different ZnO:Al sputtering system. ZnO:Al films around 800 nm were sputtered in all systems. In each sputtering system, an untreated glass substrate was added as reference. Finally, the ZnO:Al films were characterized by AFM and 4-point measurement. The characterization of the ZnO:Al films grown in the four sputtering systems are listed in Fig. 5.15. The scan size of AFM measurement for all samples is 4 × 4 µm 2 . The sheet resistance and rms roughness of each sample are marked on the right of the AFM images. For all the samples, with ion beam pretreatment, higher resistance and larger roughness of ZnO:Al films are observed, even though the topographies of ZnO:Al film grown in various system differ from each other. The rough growth of ZnO:Al films on Ar/O 2 treated glass substrates is a universal effect in different sputtering system. Based on this results, the as-grown rough ZnO:Al films might be possible to transfer to other sputtering system, for instance for industrial process. 5.2.4 Sputter ZnO:Al film with different temperature This section examines the rough growth of ZnO:Al films in the standard sputtering system with different sputtering parameters. As introduced by the modified Thorntonmodel in Section 2.2.4.3, the growth of ZnO:Al films depend on the substrate temperature and sputtering pressure. In this section, the deposition temperature was varied to increase the roughness further, while keeping the sheet resistance and total transmittance in a moderate range. Three different heater temperatures were applied during the ZnO:Al deposition. The topography and sheet resistance of the ZnO:Al films change with temperature. On an untreated glass substrate, the ZnO:Al films change from crater-like to pyramid-like feature with decreased compactness as shown in Fig. 2.11 when the temperature decreases. The influence of the rough growth by decreasing the temperature is interested since as-grown rough ZnO:Al films have already pyramide-like features and lower compactness. 95
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Ion Beam Treatment of Functional La
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Forschungszentrum Jülich GmbH Inst
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Contents Abstract 1 Zusammenfassung
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Contents 5.2 Reason of rough growth
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List of Figures 3.4 Schematic diagr
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List of Figures 6.1 2 s HCl etching
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List of Tables 4.1 Energy transfer
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Zusammenfassung In Dünnschichtsola
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1 Introduction Presently, the incre
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study and understanding of as-grown
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2 Fundamentals (a) (b) gas inlet an
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2 Fundamentals When pure Ar is used
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2 Fundamentals in this work was sup
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2 Fundamentals the process is then
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2 Fundamentals where f = 1 2 [(Z 1/
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2 Fundamentals large area. One of t
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2 Fundamentals where m ∗ is the e
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2 Fundamentals where k and T are th
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2 Fundamentals E g0 E g0 + E BM Fig
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2 Fundamentals Vacuum chamber Anode
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2 Fundamentals ation stage [72]. Ho
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2 Fundamentals Feature (Before etch
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2 Fundamentals 2.2.4.5 Surface evol
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2 Fundamentals Figure 2.14: Represe
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2 Fundamentals This paragraph expan
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2 Fundamentals Figure 2.17: Absorpt
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2 Fundamentals Figure 2.18: Schemat
Page 61 and 62: 3 Experimental This chapter describ
Page 63 and 64: 3.2 Ion beam treatment 3.1.1.3 Smal
Page 65 and 66: 3.3 Characterization methods y Carr
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Page 73 and 74: 3.3 Characterization methods the Mi
Page 75 and 76: 3.3 Characterization methods One ch
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Page 79 and 80: 4 Characterization of the ion beam
Page 81 and 82: 4.2 Static etching For Ar ion beams
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Page 85 and 86: 4.3 Dynamic etching 2 E x p e rim e
Page 87 and 88: 4.4 Discussion Table 4.1: Energy tr
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6 Ion beam treatment of the TCO sur
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7 Summary and Outlook Ion beam trea
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Bibliography [1] M. Pelliccione and
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Bibliography [23] A. Shabalin, M. A
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Bibliography [53] D. F. Mitchell, G
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Bibliography [79] J. Venables, G. S
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Bibliography [107] O. Kluth, Textur
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Bibliography [134] H. Sai, H. Fujiw
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Bibliography [158] H. Kato, K. Miya
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Bibliography [181] L. P. Schuler, M
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• Marek Warzecha, Hongbing Zhu, J
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Schriften des Forschungszentrums J
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