Metastability in Doped Hydrogenated Amorphous Silicon: A Bistable Charge-Trapping Defect Model
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METASTABILITY IN DOPED HYDROGENATED AMORPHOUS SILICON: A BISTABLE CHARGE-TRAPPING DEFECT MODEL HOWARD M. BRANZ Solar Energy Research Institute, Golden, CO 80401
ABSTRACT An analysis of the phenomenology of a class of bistable charge-trapping defects is undertaken and it is proposed that such defects explain the dominant metastable effects observed in doped hydrogenated amorphous silicon (a-Si:H). The stable configuration of such a defect changes with its charge state and a substantial barrier retards changes in configuration. Reconfiguration is accompanied by carrier trapping or emission. Published data on metastabilities observed in quenched, depletion-bias annealed and light-soaked films of both P-doped and B-doped a-Si:H are reviewed. The experimental observations are consistent with the proposed model.
INTRODUCTION Metastability in hydrogenated amorphous silicon (a-Si:H) is widely studied [11 for its scientific interest and because metastable states induced by illumination or carrier injection degrade the performance of electronic and photovoltaic devices. Among the observed metastable effects are large increases in the conductivity and/or shallow carrier density of doped a-Si:H films which can be induced by quenching [2,3], annealing under depletion bias [4], or light-soaking [5] the films. These and other experiments support the proposal [6] that structural e~uilibrium of defects in a-Si:H is obtained above a fictive temperature, T , but the structure below T* depends upon the sample's history. Adler [7] and Crandall [8] previously suggested that charge trapping is responsible for In this the light-induced metastability observed in undoped a-Si:H. paper, I propose that bistable charge-trapping defects are responsible for the observed metastabilities in doped a-Si:H. By analyzing the properties of a class of these defects, the metastabilities induced by quenching, depletion-bias annealing and illuminating n-type and p-type a-Si:H are explained and unified. Specific bistable defects that can explain the experimental data and may be present in phosphorous-doped a-Si:H are discussed in detail elsewhere [9]. These reactions involve activation of the dopant atom by simple bond-formation or bond-switching reactions accompanied by charge trapping.
BISTABLE CHARGE-TRAPPING DEFECT MODEL Bistable charge-trapping defects have been observed in a variety of doped crystalline semiconductors. Examples include the iron-acceptor pair in Si:AI [10], the C center (likely the boron-vacancy pair) in Si:B [11], the interstitial-carbon-substitutional-carbon pair in both n-type and ptype Si [12], the DX centers in III-V alloys [13], the M center in n-type InP [14] and the MFe center in n-type InP:Fe [15]. Each of these defects is bistable because its stable configuration changes with the defect's charge state and a substantial barrier retards changes in configuration. Song et al. [12] showed, for example, that the C interstitial near a C substitutional site in n-type Si is found in one of two structural configurations and each can
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