Coupled Effects of Light and Temperature on Degradation of a-Si:H
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COUPLED EFFECTS OF LIGHT AND TEMPERATURE ON DEGRADATION OF a-Si:H LISA E. BENATAR, MICHAEL GRIMBERGEN, DAVID REDFIELD, and RICHARD H. BUBE Stanford University, Department of Materials Science and Engineering, Stanford, CA 94305-2205 ABSTRACT The effects of excitation rate and temperature on the kinetics and steady-state behavior of metastable defect formation in hydrogenated amorphous silicon (a-Si:H) have been studied. The dependences on temperature of the lifetime, r, and stretching parameter, P, from a stretched exponential description of the kinetics were measured for one sample. We do not see a linear dependence of 0 on temperature over the entire temperature range studied (270K-370K), and 't increases monotonically with decreasing temperature. Steady-state results show defect density to be dependent on both temperature and excitation rate over the ranges measured (from 395K to 470K and from 6 x 1020 to 2 x 1022 s-1 cm-3 ). The gradual change in temperature dependence is explained by a distribution of barrier heights between the ground and metastable states. INTRODUCTION Both the generation rate and the saturation density of light-induced defects in a-Si:H can vary with light intensity and temperature, as described by recently developed kinetics [1]. In order to test the predictions of this model and in an effort to refine it, we measure the time dependence of the defect density, N(t), and the dependence on light intensity and temperature of the saturation values, Nsat. MODEL We are using a rate equation that includes the effects of both light and temperature on defect formation and anneal[l]. We have assumed that the processes of formation and anneal are dispersive in the same way, which introduces a time dependent term into all rate constants. This type of behavior follows from, for example, a distribution of activation energies for formation and anneal. Such a distribution could be created by small variations in local configuratibn, as would be expected in an amorphous material. The rate equation for defects, N, is the following[l]: dN/dt = (t/P)-a[C1GNL - C 2 GN + VlNL - v 2 N]
(1)
Here G is the carrier excitation rate (assumed to be proportional to the carrier recombination rate which supplies the bond-breaking energy); C's represent the effectiveness of recombination processes; v's are thermally activated frequencies (subscripts 1 and 2 denote generation and anneal, respectively); a (related to the stretching parameter) introduces the dispersive behavior (ax
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