Capacitance Studies of Metastable Defect Creation in Hydrogenated Amorphous Silicon

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CAPACITANCE STUDIES OF METASTABLE DEFECT CREATION IN HYDROGENATED AMORPHOUS SILICON

J.D. COHEN K. MAHAVADI , K. ZELLAMA J.P. HARBISON+, AND A.E. DELAHOY+ *University of Oregon, Department of Physics, Eugene, Oregon 97403 **Present address: Universite Paris VII, 75521 Paris Cedex 05, France +Bell Communications Research, Inc., Murray Hill, New Jersey 07974 ++Chronar Corporation, Princeton, New Jersey 08542

ABSTRACT We have studied the light induced instability problem in hydrogenated These techniques amorphous silicon using junction capacitance techniques. are used to examine specific changes in the density of gap states, and occupation of gap states, for undoped a-Si:H samples after light saturation and for a series of partial anneal "states" which culminate in the original dark annealed state (state A). We find that the observed changes in the metastable occupied and unoccupied defects contradict the Si-Si bond breaking model and indicate at least two defect creation processes. In several samples we also find clear evidence that the metastable defect distribution near midgap has a slightly different energy distribution than the stable deep state (dangling bond) distribution. At the same time, these results seem to be qualitatively consistent with many aspects of recent ESR and optical absorption studies of metastable defect creation. We discuss these findings in terms of alternative possible microscopic models for metastable effects in a-Si:H.

INTRODUCTION The widespread manifestatons of light induced metastable effects in a-Si:H have been widely documented in the literature for over a decade. And, while there is now general agreement that a key element in such phenomena is the increase in the midgap dangling bond (DB) density, there is still considerable disagreement about the fundamental mechanisms of such defect creation. The most popular current model, proposed on the basis of electron spin resonance (ESR) studies [1,2], is that the recombination of photo-induced carriers can break weak Si-Si bonds which then account for the increase in neutral (Si 3) dangling bonds as well as the observed shift toward midgap of the Fermi level in doped samples. Proponents of this model also suggest an important role for a-Si:H film strain [2] in the magnitude of this effect in different samples since a greater film strain is expected to enhance the energetics of the bond breaking process. Not all gap state spectroscopic measurements support this picture, however. Other recent ESR results [3] suggest more the rearrangement of atoms within existing defects rather than Si bond-breaking. Optical absorption studies provide evidence which seems to contradict the proposed strain dependence [4]. That is, whereas one expects film strain to decrease with the number of stable DB defects resulting in a decrease the number of metastable defects, the density of metastable defects is found to be either constant (in intrinsic material) or to increase in proportional to the number of stable defects (for doped films). Additional information

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