Carrier Injection in a-Si:H P-I-N Devices: Hydrogen Redistribution and Defect Creation
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CARRIER INJECTION IN a-Si:H P-I-N DEVICES: HYDROGEN REDISTRIBUTION AND DEFECT CREATION J.M. ASENSI, J. ANDREU, J. PUIGDOLLERS, J. BERTOMEU AND J.C. DELGADO. Laboratori de Ffsica de Capes Fines (LFCF). Departament de Ffsica Aplicada i Electrbnica. Universitat de Barcelona. Avda. Diagonal 647, E-08028 Barcelona (Spain) ABSTRACT A straightforward analytical expression of the density-of-states (DOS) of a-Si:H valid in non-equilibrium steady state situation has been obtained. The model is based on a statistical-mechanical treatment of the hydrogen occupation for different defect sites. The broadening of available defect energy levels (defect pool) and the possibility of charged defects are taken into account. This leads to a new explanation of the Staebler-Wronski effect, based on the "conversion" of shallow charge centers to neutrals near the middle of the gap as a consequence of hydrogen redistribution induced by electron-hole recombination. We present results of computer simulations of the behavior of amorphous silicon p-i-n structures including our statistical model of the DOS of a-Si:H. The effect of forward-bias (carrier injection) degradation is studied. INTRODUCTION Defect formation based on solid-state equilibrium has become successful in interpreting a-Si:H behavior [1,2]. There is considerable evidence that hydrogen diffusion is the mechanism of thermal equilibration. In this framework, it is assumed that defect creation can be mediated by hydrogen diffusion which allows the defects to occupy SiH sites from which the hydrogen is removed. Several approaches have been considered, although experimental results and theoretical calculations of hydrogen bonding configurations appear to be most easily reconciled with a model based on the following reaction [3]: 2SiH +SiSi -: 2D + SiHSiH
(1)
where two spatially isolated SiH bonds interact with a weak SiSi bond producing two isolated dangling bonds (D) and a doubly hydrogenated state (SiHSiH). In other words, the weak SiSi bonds are described in terms of a negative-correlation energy for hydrogen. Zafar and Schiff [3] have proposed a microstructural description of reaction (1) based on two known phases (dilute and clustered) of bonded hydrogen in a-Si:H. The analysis of their model in terms of partition functions and hydrogen chemical potential facilitates a general and straightforward viewpoint of chemical equilibrium based on hydrogen-mediated reactions. In a previous paper [4] we presented a model of the gap-state distribution of a-Si:H based on the hydrogen-statistical analysis of reaction (1), which incorporates the defect pool concept and the dependence of the defect formation energies on the Fermi level. The model allows a unified and quantitative description of different defect phenomena in a-Si:H, such as the thermally activated spin density, the gap-state dependence on the Fermi level, and the intensity and temperature dependence of light-induced spin density. This new treatment could have important implications for the understanding of the behaviour of p-
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