Investigation of Light-Induced Defect Depth Profile in Hydrogenated Amorphous Silicon Films
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INVESTIGATION OF LIGHT-INDUCED DEFECT DEPTH PROFILE IN HYDROGENATED AMORPHOUS SILICON FILMS S.Shimizu1, P. Stradins, M. Kondo and A. Matsuda National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568 Japan. 1 Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8502 Japan. ABSTRACT We report direct measurement of depth profile of light induced defects (LIDs) in hydrogenated amorphous silicon (a-Si:H) films. These depth profiles were measured by ESR measurements combined with layer-by-layer precise wet etching technique. We discuss those LIDs depth profiles in relation to the spatial depth distribution of photocarriers generation and recombination profiles, as well as the defect creation efficiency. In case of uniformly absorbed light, the light induced defects creation takes place uniformly in depth of the a-Si:H film. In case of nonuniformly absorbed light, the LIDs depth profile is spatially wider than photocarrier generation rate profile G(x), and depends on G(x) sublinearly. Moreover, the LID depth profile agrees well with the photocarrier direct recombination rate profile rather than the profile of total recombination rate or that through defects. The obtained results suggest that the LIDs creation takes place at local site where the photocarriers recombine directly. INTRODUCTION The light induced degradation phenomenon in a-Si:H remains one of the main issues in the a-Si:H field [1]. Although the detailed microscopic mechanism for LIDs creation is not revealed yet, the question has been addressed whether the LIDs creation is a bulk or a surface effect. For example, the earlier study suggested that the degradation took place mostly in the 0.6 µm layer from the surface and the rest of the film was insensitive against LIDs creation [2]. Other work suggested that their observed LID distribution was determined by inhomogeneous photocarrier generation and recombination rate profiles in a-Si:H [3]. Investigating those features might help to understand the mechanism of LID creation. For example, defects might be created by recombination of photocarrier which diffuse deeper into the film after generation. Possible defect precursors [4,5] might diffuse as well. We have developed a novel layer-by-layer wet etching technique which does not introduce extra surface defects as well as surface roughness for investigating the depth profile of LIDs in a-Si:H directly. Our method almost eliminates the contribution of surface dangling bonds. Moreover, compared with studying the films having variety of initial thickness, our method is not affected by the possible variations in photocarrier A14.2.1
generation with thickness as in [3]. Using this technique, we study the LIDs depth profile in a-Si:H under different light absorption conditions. EXPERIMENTAL Intrinsic a-Si:H films were fabricated on quartz substrates using conventional VHF-CVD (100MHz) reactor with SiH4 source gas. The deposition conditions were the following: SiH4 gas flow 20 sccm, total pressure 10
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