A Study of the Time Scales of Processes Responsible for the Light-induced Degradation of a-Si:H by Pulse Illumination
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A Study of the Time Scales of Processes Responsible for the Light-induced Degradation of a-Si:H by Pulse Illumination Paul Stradins, Michio Kondo, and Akihisa Matsuda Thin Film Silicon Solar Cells Super Laboratory, Electrotechnical Laboratory Tsukuba, Ibaraki 305-8568, Japan, email [email protected] ABSTRACT Degradation properties of a-Si:H and a-Si:D by pairs of intense light pulse pairs are examined. By varying the delay time between pulses in a pair while keeping the total illumination time and dose constant, the time scales involved in the light-induced defect creation are investigated. In nanosecond pulse case, we find a sharp drop in degradation efficiency with delay time, followed by further gradual decrease at much longer times. For microsecond pulses, the degradation efficiency varies with the delay time in microseconds. The recombination proceeds bimolecularly and is largely completed during the pulse. It is suggested that the first stage of degradation is related to bimolecular recombination that takes place during the pulse. The second stage is possibly related to longer-living metastable species. INTRODUCTION Exposure by intense light pulses greatly accelerates the light-induced degradation in a-Si:H as compared to the same dose of cw light [1]. Since the Si dangling bond defect creation is believed to be triggered by bimolecular recombination events [2], in the case of the intense pulse degradation most of the defects are likely to be created during or shortly after the pulse, due to the very short photocarrier lifetimes. In a detailed study [3], a model for defect creation under pulse degradation was proposed based on the assumption that the concentration of defects controls the fraction of recombination events that proceed bimolecularly. Other authors [4], however, argue that most of the photocarriers recombine bimolecularly already during the laser pulse and that the defects essentially do not control the bimolecular recombination channel. Therefore, defect-controlled recombination would be unlikely to explain the observed sublinear kinetics of defect creation [defect density] ∝ [number of pulses]0.5, which implies a self-limiting process. Branz [5] overcomes this difficulty by considering the diffusion and reactions of mobile H, which is released by recombination during the pulse. In this case, the defect creation may continue long after the light pulse. An experiment [6] is necessary to reveal the time domain at which the defect creation takes place and to relate it with the recombination processes, which is the aim of this work. EXPERIMENTAL a-Si:H films were prepared by PECVD method on fused silica, Corning 7059 glass and sapphire substrates in our standard reactor using SiH4 gas. Growth conditions were optimized for obtaining standard device quality samples (e.g. substrate temperature 250oC, deposition rate ≤ 2Å/s). Film thickness was in the range of 0.8 – 1.2 µm. Dark electrical conductivities at room temperature were below 10-10 Ω-1cm-1. For electrical measurements, coplanar Cr top contacts (
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