Two-Step Capacitance Transients From an Oxygen Impurity Defect
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Two-Step Capacitance Transients From an Oxygen Impurity Defect Shouvik Datta1, J. David Cohen1, Yueqin Xu2, and Howard M. Branz2 1 Department of Physics, University of Oregon, 1371 E 13th Avenue, Eugene, OR, 97403 2 Silicon Materials Group, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO, 80401 ABSTRACT This paper describes the study of an electron-trapping defect which underwent significant configurational relaxation in oxygen contaminated hydrogenated amorphous silicon-germanium (a-Si,Ge:H) alloys grown by hot-wire chemical vapor deposition. An unusual two-step electron emission from this relaxed defect is studied using junction-capacitance-based measurements. In this work, we monitor the recovery of the relaxed defect after filling it by photoexcited electrons and also by electrons injected with a voltage filling pulse. The dependence of the transient shape on filling pulse time is described. We have also performed experiments which clearly demonstrate that this is a bulk defect and exclude contributions from any additional blocking junctions. INTRODUCTION We earlier reported [1,2] the discovery of an oxygen related defect with a large configurational relaxation energy [3,4] (roughly 0.8 eV) in hydrogenated amorphous silicongermanium (a-Si,Ge:H) alloys grown by hot-wire chemical vapor deposition (HWCVD) with roughly 30at.% Ge and high levels (~5×1020 cm-3) of intentional oxygen contamination. The existence of this defect was revealed by a negative feature in the transient photocapacitance (TPC) spectra [5-6] near 1.35 eV, which indicated that valence band electrons photoexcited into these oxygen defects remained deeply trapped even when the optical excitation threshold is very close to the conduction band. The detailed behavior of those defects in these a-Si,Ge:H alloys were further characterized [2] using a variety of techniques including transient photocurrent (TPI) spectroscopy, as well as by monitoring the capacitance recovery in the dark following the excitation with 1.2 eV light. These latter studies [2] of the time evolution of the capacitance transients due to release of electrons from these oxygen defects into the conduction band as a function of temperature clearly demonstrated that significant configurational relaxation must have occurred. This may be the best documented & most clear cut observation to date of such a substantial relaxation of defect energy following the change of charged state of a defect level in amorphous silicon or related materials. Recently, we have been studying in greater detail the kinetics of this oxygen-related defect relaxation in samples with Ge fractions of both 30% and 15% with different levels of oxygen. For example, we have examined dark capacitance recovery due to electron emission transients obtained after selective light pulses or voltage filling pulses. Indeed, under certain conditions we can observe a distinct ‘two-step’ capacitance recovery transient in the dark. This suggests an additional configurational rearrangement of the am
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