Trapping Dynamics and Charged Defects: Light Induced Studies in a-Si:H and a-Si 1-x Ge:H
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TRAPPING DYNAMICS AND CHARGED DEFECTS: LIGHT INDUCED STUDIES IN a-Si:H AND a-Sil.xGe:H J. HAUTALA AND J.D. COHEN University of Oregon, Department of Physics Eugene, OR 97403 ABSTRACT Bandtail defect carrier trapping dynamics in high quality photo-CVD grown a-Sil-xGex:H (x=0.3 and 0.62) and PECVD deposited a-Si:H films were studied using photo-modulated electron spin resonance (PMESR). Comparison of the dark spin densities with junction capacitance (drive-level profiling) methods indicate that a majority of the defects are neutral in the a-Si:H and 30% Ge samples, but the majority of the defects in the 62% Ge sample are charged. Lineshape deconvolution of the PMESR signals in the in-phase and quadrature phase as a function of photo-modulation frequency, excitation energy and excitation intensity for both intrinsic (state A) and light soaked (state B) states revealed three significant insights: in state A all samples behave similarly, state B behavior of the a-Si:H is dramatically different from the state B alloyed samples, and strong evidence for 'safe hole traps' (SHT) is observed in the state B a-Si:H material. INTRODUCTION Light-induced defects in a-Si:H dramatically influence the material's properties and consequently have been extensively researched. When alloyed with Ge, however, the effects of light soaking seem greatly diminished. This may be due to the increased bandtail widths and larger intrinsic defect density or a reduced bandgap; in any case, a better understanding of the defects in these materials is needed. It is particularly useful to directly compare the defect behavior of a-Si:H and a-SiI xGex:H alloys as a function of Ge content. Electron spin resonance (ESR) provides an especially useful probe of the local environment of the defects in a-Si:H and the alloys, where it can determine absolute densities of Si and Ge dangling bonds1 ,2,3 as well as easily distinguish holes from electrons and even (not quite so easily) distinguish electrons trapped in tail states from those residing on a neutral (DO) dangling bonds. When performed in tandem with junction capacitance measurements, such as drive-level profiling, ESR can discern between charged and neutral defects by simply comparing the densities of the two techniques. This has proved useful in studies of deep defect electronic transitions 4 ,5 and influences of impurities 6 on defects in a-Si:H. In this study comparisons are used to determine the relative density of intrinsic and charged defects in Ge alloyed films. Light-induced electron spin resonance (LESR) in a-Si:H has revealed further information about auto-compensation 7 , densities of charged defects associated with interfaces 8 ,9 and light-induced charged dangling bonds 10. For this study photo-modulated ESR (PMESR) has the additional advantage of obtaining dynamic information of the generation and recombination of excited carriers. Specifically, by separating the in-phase and quadrature phase signal components as function of chopping frequency, we have found evidence for "safe hole traps" (S
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