Reaction Control in Amorphous Silicon Film Deposition by Hydrogen Chloride
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A15.2.1
Reaction Control in Amorphous Silicon Film Deposition by Hydrogen Chloride Akihiro Takano1, Takehito Wada1, Shinji Fujikake1, Takashi Yoshida1, Tokio Ohto1 and Eray S. Aydil2 1 Fuji Electric Corporate Research and Development, Ltd., 2-2-1 Nagasaka, Yokosuka, Kanagawa 240-0194 Japan 2 Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106, U.S.A. ABSTRACT HCl was added to SiH4 containing plasmas to grow a-Si:H(Cl) films with dangling bonds terminated with Cl instead of H. Bulk and surface infrared spectra, film thickness and optical band gap were examined by in situ multiple total internal reflection Fourier transform infrared spectroscopy and in situ spectroscopic ellipsometry. SiH2Cl2 was also used as a conventional Cl source for reference a-Si:H(Cl) film deposition experiments. The introduction of HCl does not affect the deposition rate significantly, and the deposited a-Si:H(Cl) films contain over 1021cm-3 Cl atoms. HCl addition to the gas phase changes the surface compositions of the growing films drastically from higher silicon hydride to chlorinated lower hydride. The surface reaction control eliminates unfavorable hydride bonding structures such as SiH2 and/or SiH in voids in the deposited films. The a-Si:H(Cl) films deposited from mixtures of SiH4 and HCl do not show significant optical band gap widening in spite of containing over 1021cm-3 Cl atoms, a concentration that is comparable to that of hydrogen. In contrast, a conventional chlorine source of SiH2Cl2 increases the deposition rate significantly compared to HCl. The increase in the deposition rate results in monotonic decrease of the refractive index and the optical band gap widening.
INTRODUCTION Alternative dangling bond terminating atoms, such as D, F and Cl, have been extensively investigated aiming at the suppression of the photoinduced degradation in hydrogenated amorphous silicon (a-Si:H) solar cells [1-9]. These higher mass atoms are expected to have lower diffusion coefficients in the amorphous bulk network resulting in the suppression of the photoinduced degradation. Although SiH2Cl2 and SiCl4 have been used as Cl sources for terminating dangling bonds by Cl, drastic improvements have not been reported. On the contrary, an excess introduction of SiH2Cl2 and SiCl4 deteriorates the film properties and widens the optical band gap [3,10], which may be caused by excess SiCl bond introduction and formation of clustered SiCl and nanovoids. Despite its simplicity and an applicability to conventional solar cell fabrication processes, HCl introduction as a chlorine source during film deposition had not been reported until we reported that chlorine containing hydrogenated amorphous silicon (a-Si:H(Cl)) film deposition from SiH4/HCl mixed gases without band gap widening is possible [11]. In this study, we investigate HCl introduction effects on amorphous silicon film deposition focusing on surface hydride and chloride composition, the resulting bonding features
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