Metastable Defects in a-SiO x :H and a-SiC x :H
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METASTABLE DEFECTS IN a-SiOx:H AND a-SiCx:H X.-M. DENG, A. HAMED, H. FRITZSCHE AND M. Q. TRAN James Franck Institute, The University of Chicago, 5640 Ellis Ave., Chicago, IL 60637, U.S.A.
ABSTRACT Metastable defects are created in hydrogenated amorphous silicon alloys presumably by the same mechanism as in a-Si:H. We find metastable defects created in a-SiCx:H and a-SiOx:H by hopping injection of photocarriers from adjacent a-Si:H layers. In a-SiCx:H the defects can be created only below T=150K, they anneal at TE=400K. In a-SiOx:H they are created at or below T=300K, they anneal at TE=480K. The anneal temperature is nearly independent of the creation temperature. The defects are detected by their charge exchange with adjacent a-Si:H layers whose conductance is thereby changed. INTRODUCTION The conductance of a layer of hydrogenated amorphous silicon (a-Si:H) sandwiched between insulating layers depends on the charge exchange between these layers which establishes electronic equilibrium, and hence a common Fermi level EF. Any change in the insulating layers such as a change in the number of defects near EF affects its position and thereby the conductance of the a-Si:H layer. The planar conductance of the sandwich can therefore be used to monitor the creation and annealing of metastable defects in the insulators. Such defects can be produced by a variety of means one of which is hopping injection of photocarriers from the semiconducting a-Si:H layer. Depending on the direction of the Fermi level shift one might observe either a metastable increase or decrease in the planar conductance which persists until equilibrium is reestablished. Very large metastable conductance increases have been observed in a-Si:H/a-SiNx:H multilayers following an exposure to light which creates electron-hole pairs in the a-Si:H layers [1, 2]. For historical reasons this phenomenon is called persistent photoconductance or PPC [3]. Although a sandwich suffices for observing the PPC effect, it is convenient to study multilayers because the effect is largest when the a-Si:H is thin (20-30 A). In silicon/silicon nitride multilayers the PPC effect can be excited with equal efficiency at 4.2K and 300K [2]. Equilibrium is restored by annealing at TE=460K. This work explores whether metastable defects can be produced in the same way in other insulating materials such as a-SiOx:H and a-SiCx:H. We also study the temperature dependence of the defect creation process by exposing the multilayers at different temperatures and determine the annealing temperature. EXPERIMENTAL DETAILS The multilayers were prepared in a 13.6 MHz glow discharge chamber. The composition of the reactant gas was changed periodically between Sill4 containing 5 ppm B2 H6 and a mixture containing 3 parts N 20 and 1 part Sill4 (for the silicon oxide multilayers) or between Sill4 and a mixture of 6 parts CH4 and 1 part SiFl 4 (for the silicon carbide multilayers). Corning 7059 sample substrates were held at 510K and covered by a shutter during the change of gases and for one minute
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