Photocarrier capture properties of light-induced defects in a-Si:H
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Photocarrier capture properties of light-induced defects in a-Si:H Paul Stradins, Satoshi Shimizu, Michio Kondo, and Akihisa Matsuda Electrotechnical Laboratory, Tsukuba, Ibaraki 305-8568 Japan ABSTRACT Light-induced degradation of photoconductive properties is investigated as a function of lightinduced Si dangling bond defect concentration and subgap absorption. This relation is studied for a variety of sample preparation techniques including a-Si:D, wide range of temperatures during light exposure and annealing, and sample degradation pre-history. Under a given exposure condition, light-induced defects have a fixed effective electron capture coefficient, while its value varies greatly with exposure conditions (temperature during exposure or stepwise anneal) and with sample microstructure. This explains commonly observed non-linear dependences and hysteresis between defect concentration and mobility-lifetime product. Creation efficiency of less-stable defects with large capture coefficients is not influenced by the presence of a large number of stable defects. Structural change accompanying the defect creation, if any, is strongly related to the defect creation and is likely to occur locally around the defects, modifying their capture coefficients. Capture can vary both because of the changes in intrinsic capture probabilities and due to changes in the net defect charge balance. The former may be increased by the presence of H or photocarrier trap (e.g. weak bond tail state) in a close vicinity of Si dangling bond. The latter may be caused by a wide distribution of defect states in the gap or by simultaneous creation of negatively charged floating bond states near VB edge. INTRODUCTION Light induced degradation of photoconductive properties is usually associated with creation of Si dangling bond defects. The extent of degradation of photoconductivity, however, varies greatly for the same amount of defects created under different conditions [1]. Defects created at 4.2 – 80K have particularly damaging effect on the photoconductivity [2]. As a result, the relation between the defect concentration ND and electron mobility-lifetime product µτ appears non-linear and non-unique. Additional larger-scale structural change was proposed to explain these phenomena [3]. Observed changes [4], however, seem too small to strongly affect electrical transport e.g. drift mobility [5]. On the other hand, microstructure around the defects and its thermal relaxation is likely to determine their capture properties [6,7,8]. Finally, other metastable entities such as Si-H complexes [9] or floating bonds [10,11,12] may be created together with defects and indirectly affect the carrier capture. In this work, we investigate the photocarrier capture properties of light-induced defects (LID) for variety of sample preparation conditions and light exposure procedures, trying to distinguish between the above possibilities. EXPERIMENTAL a-Si:H and a-Si:D intrinsic samples were prepared in our RF or very high frequency (VHF, 100MHz) PECVD reactors u
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