Temperature Dependence of Metastable Defect Creation in a-SiN x :H
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TEMPERATURE DEPENDENCE OF METASTABLE DEFECT CREATION IN a-SiNx:H A. HAMED, H. FRITZSCHE AND S. KOHLER James Franck Institute, The University of Chicago, 5640 Ellis Ave., Chicago, IL 60637, U.S.A.
ABSTRACT Light exposure of multilayers consisting of alternating 2.5 nm thick a-Si:H and about 40 nm thick a-SiNx:H produces a large metastable excess dark conductance. This appears to be due to metastable defects in a-SiNx:H produced by hopping injection of photocarriers from the a-Si:H. They anneal at an equilibration temperature TE-460*K. These metastable defects are created with the same efficiency between 4K and 300K. The efficiency rapidly falls above 350K. The metastable defects have the same anneal temperature regardless of creation temperature. The mechanism of defect creation will be compared with that in a-Si:H. INTRODUCTION Very large light-induced excess conductances have been observed in multilayers consisting of alternating layers of hydrogenated amorphous silicon (a-Si:H) and silicon nitride (a-SiNx:H) [1, 2]. The conductance measured parallel to the layers after illumination can be more than five orders of magnitude larger than that of the annealed state. This phenomenon, also called persistent photoconductance (PPC), has been observed earlier in pnpn doping modulated a-Si:H multilayers [3] and recently in a-Si:H/a-SiCx:H and a-Si:H/a-SiOx:H multilayers [4]. We have shown earlier that the origin of PPC in doping modulated a-Si:H is nothing but the light-induced defect mechanism which gives rise to the Staebler-Wronski effect [5]. While creation of metastable defects moves the Fermi level EF toward the gap center and hence decreases the dark conductance in single a-Si:H layers, the same effect can produce a substantial increase in conductance in multilayers in the following way. Let us assume that the conductance is governed by electrons in one kind of layers. If metastable defects created in the other kind of layers cause EF to move up in energy, then EF moves up also in the conducting layers producing an increase in conductance provided that the whole multilayer comes into electronic equilibrium. Annealing and creation of PPC is thus determined by the annealing and creation of metastable defects in the layers that are adjacent to the conducting layers [5]. How can this explanation be valid for silicon/silicon nitride multilayers in which the silicon nitride layers neither conduct nor absorb any exciting light? We believe that metastable defects are created in the silicon nitride layers by hopping injection of photocarriers from the amorphous silicon layers [2] as illustrated in Fig. 1. The defect creation mechanism is then not dissimilar from that in a-Si:H because the silicon nitride layers contain Si and H except that the H-diffusion coefficient and hence the equilibration temperature TE will depend on the nitrogen concentration and preparation conditions. In this work we investigate how the efficiency of defect creation in a-SiNx:H depends on temperature and whether defects created at different temper
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