A Critical Test of Defect Creation Models in Hydrogenated Amorphous Silicon Alloys
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A CRITICAL TEST OF DEFECT CREATION MODELS IN HYDROGENATED AMORPHOUS SILICON ALLOYS KIMON C. PALINGINIS*, JEFFREY C. YANG**, S. GUHA** and J. DAVID COHEN* * Department of Physics, University of Oregon, Eugene, OR 97403 U.S.A. ** United Solar Systems Corporation, 1100 W. Maple Road, Troy, MI 48084 U.S.A. ABSTRACT Using the modulated photocurrent method we studied the deep defect creation and annealing kinetics of amorphous silicon-germanium alloys with Ge fractions below 10at.%. The modulated photocurrent spectroscopy clearly discloses the existence of two distinct bands of majority carrier traps in these alloys. The bands were identified as neutral Si dangling bonds and neutral Ge dangling bonds. Our studies show clearly that the Si and Ge defects directly compete with each other during annealing, implying a global reconfiguration mechanism. The creation kinetics reveal the usual t1/3 illumination time dependence for the total deep defect density. However, the individual densities of Si and Ge defects have different time dependencies. The details of the creation and annealing kinetics of Ge and Si defects are used to test predictions of certain defect creation models. INTRODUCTION
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The creation of metastable deep defects upon illumination in hydrogenated amorphous silicon (aSi:H) is a fundamentally very interesting phenomenon. At the same time, it is a major obstacle for the improvement of solar cell technology because it causes the electronic properties of a-Si:H to deteriorate under light-exposure. Since the discovery of the so-called Staebler-Wronski (SW) effect [1], many microscopic models have been proposed to explain the underlying mechanisms [2]. The majority of these theories can be divided into two main categories. One group of models tries to describe the SW effect by local bond breaking or rearrangement. The other group involves more global properties of the amorphous network, such as long-range hydrogen diffusion [3,4] as in the recently proposed “hydrogen collision” model [5]. The early work of Stutzmann et al. was first to show that the degradation process in a-Si:H is roughly proportional to one-third power of the illumination time (t1/3) [6]. This was explained in terms of a bimolecular carrier recombination process leading to the actual defect creation in the material. In this paper we report a detailed study of deep defect creation and annealing kinetics in low Ge content a-Si,Ge:H alloys. We will present experimental results obtained from modulated photocurrent (MPC) measurements which reveal two distinct defect bands in these a-Si,Ge:H films. The defect bands have been identified as neutral Si and neutral Ge dangling bonds [7]. Our light-induced creation studies clearly indicate that the total defect density increases with the usual t1/3 time dependence, while the individual Si and Ge dangling bond densities increase with two distinct time dependencies independent of temperature during degradation. Then we show that the Si and Ge defects compete with each other d
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