Relaxation of Electron Beam-Induced Metastable Defects in a-Si:II
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RELAXATION OF ELECTRON BEAM-INI)UCEI) METASTABLE DEFECTS in a-Si:II M. GRIMBERGEN, R. McCONVILLE*, D. REDFIELD, and R. H. BUBE Stanford University, Department of Materials Science and Engineering Stanford, CA and *Raychem Corporation, Corporate Technology, Menlo Park, CA
ABSTRACT Relaxation of the metastable defect density in undoped amorphous silicon is observed after keV electron irradiation. The time constant for relaxation has an activation energy close to I eV, similar to that for light-induced defects. Relaxation appears to follow two or more stages. A large initial density relaxes rapidly, followed by slower relaxation more characteristic of light-induced defects. Separation of these components allows for a better comparison of ebeam and light-induced saturation defect density. INTRODUCTION Metastable dangling bond defects limit the performance of many hydrogenated amorphous silicon (a-Si:H) devices. Much emphasis has been placed on light-induced defect creation since it was first observed fifteen years ago [I ]. Since then, additional efforts have focused on dangling bond defects created other ways, such as, by thermal quenching 121 and high energy irradiation 131 to obtain further information. Degradation using keV electron irradiation has the advantage of rapid metastable defect generation without creating irreversible structural changes 141. Saturation defect density has become a primary measure of material stability. Irradiation with keV electrons provides a rapid means to obtain defect density saturation 151, but that saturation level is higher than the defect density normally produced by strong visible light 161. The purpose of this paper is to explain the difference by examining the relaxation properties of electron beam-induced defect density and comparing those with published annealing characteristics of light-induced defect density. We show that electron irradiation creates additional defects beyond those normally formed by light, and that this "excess" density anneals rapidly. Modeling of two sets of defects allows direct comparison between electron beam-induced and light-induced defect density. EXPERIMENT Samples of undoped a-Si:H were I to 2 pm thickness, deposited on quartz substrates using conventional PECVD near 260'C. Coplanar electrodes were evaporated on the substrate prior to film deposition. Defect density was determined using d.c. constant photoconductivity (CPM) 171 with a photocurrent density approximately three to ten times the dark current density. The CPM optical absorption spectra were normalized to match an optical absorption coefficient of 750 cm- 1 at 1.7 eV, which was measured by optical transmission. Defect density was calculated from the CPM-measured subgap optical absorption at 1.2 eV. An absorption coefficient of I cm- 1 was assumed to correspond to a defect density of 1016 cm- 3 . Optical interference effects in the CPM spectra were removed by smoothing. All CPM data on samples from the same substrate were reduced with the same calibration factors, enabling reproducible
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