Fast Deposition of Highly Crystallized Microcrystalline Si Films Utilizing a High-Density Microwave Plasma Source for Si
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0910-A13-03
Fast Deposition of Highly Crystallized Microcrystalline Si Films Utilizing a High-Density Microwave Plasma Source for Si Thin Film Solar Cells Haijun Jia1,2, Hajime Shirai2, and Michio Kondo1 1 Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan 2 Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura, Saitama, 338-8570, Japan
ABSTRACT In this work, we investigate the influences of source gas supply method and dc substrate bias on the deposition rate as well as structural properties of µc-Si films. We demonstrate the high rate synthesis of highly crystallized µc-Si films with improved film density by using a high-density and low-temperature microwave plasma source. A high flux of SiH3 and atomic hydrogen on film growing surface achieved through adjusting the SiH4 gas supply configuration is effective for the fast deposition of highly crystallized µc-Si films and for the improvement of film mass density. Appropriate ion bombardment under moderate negative dc substrate bias along with the suppression of contributions of detrimental species to film growth also account for the improvement in properties of resulting films. As a consequence, a very fast deposition rate of ~65 Å/s was achieved with a high Raman crystallinity Ic/Ia>3 and a low defect density of (1-2)×1016 cm-3 despite a high silane concentration of 67%. INTRODUCTION Microcrystalline silicon (µc-Si) attracts intense attention as a promising material for thin film solar cells with high conversion efficiency and high stability [1]. However, in order to realize sufficient sunlight absorption, a large thickness of over 2 µm of µc-Si film is required due to its indirect band gap nature. In this regard, for mass production of high performance, low-cost thin-film solar cells, uniform and fast deposition of high quality µc-Si is one of the crucial issues in the current research efforts. To date, we have been studying a new high-density and low temperature microwave plasma (2.45 GHz) source utilizing a spoke antenna and its application in µc-Si thin film fabrication. In this paper, the effect of SiH4 gas supply configuration on the film deposition rate, crystallinity and defect density are discussed. Guiding principles for the fast deposition of highly crystallized, high density µc-Si films are proposed. The investigation of influence of dc substrate bias on film growth process is another major objective of this work. EXPERIMENTS Details of the apparatus are described elsewhere [2]. The µc-Si film deposition was performed at a distance of Z=6 cm between the dielectric window and substrate holder (Figure1), wherein
2.45 GHz MW
Spoke antenna
highly crystallized Si films were fabricated as demonstrated in our Al2O3 Window Ring Plasma previous work. The plasma was Z1=6 cm adjusted to favor µc-Si film formation: Showerhead SiH4+H2 (G1) background pressure of lower than Z2=2 cm SiH4 (G2) -6 or H2 (G2) 1×10 To
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