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Theoretical Studies of Structure and Doping of Hydrogenated Amorphous Silicon Bin Cai and D. A. Drabold Department of Physics and Astronomy, Ohio University Athens, OH 45701, U.S.A. ABSTRACT In a-Si:H, large concentrations of B or P (of order 1%) are required to dope the material, suggesting that doping mechanisms are very different than for the crystal for which much smaller concentrations are required. In this paper, we report simulations on B and P introduced into realistic models of a-Si:H and a-Si, with concentrations ranging from 1.6% to 12.5% of B or P in the amorphous host. The results indicate that tetrahedral B and P are effective doping configurations in a-Si, but high impurity concentrations introduce many defect states. For a-Si:H, we report that both B(3,1) and P(3,1) (B or P atom bonded with three Si atoms and one H atom) are effective doping configurations. We investigate H passivation in both cases. For both B and P, there exists a “hydrogen poison range” of order 6 Å for which H in a bond-center site can suppress doping. For B doping, nearby H prefers to stay at the bond-center of Si-Si, leaves B four-fold and neutralizes the doping configuration; for P doping, nearby H spoils the doping by inducing a reconstruction rendering initially tetrahedral P three-fold. INTRODUCTION By introducing B or P, a-Si:H may be doped either n-type or p-type [1], a point of profound technological importance. In c-Si, doping has been extensively studied. Because of translational invariance, impurities are compelled to have the same local tetrahedral environment as Si. According to the 8-N rule, B atoms create a hole when they have Td symmetry, and P similarly donate an electron. The doping efficiency is almost 100%. In c-Si:H, H atoms passivate doping by relaxing strain, and rendering B or P doping-inactive by enabling the impurities to become three-fold [1]. In this short paper, we make no pretense of properly reviewing the substantial literature on the subject and recommend the book of Street[1] as a suitable introduction. In a-Si or a-Si:H, the absence of a unique atomic environment leads to site-dependent doping, as seen in studies with low concentration of Boron [2]. However, theoretical studies on P doping and high concentration of B are still needed. Early experiments show a very low doping efficiency in a-Si:H, with a doping efficiency rollover observed in experiment when impurity density is around 1% [1,3]. NMR [4] shows that, for B doping, 40% of the B has a nearby H at 1.6 Å; for P doping, 50% of P has a H at 2.6 Å. Boyce and Ready have conjectured that the sluggish doping may be due to H passivation [5]. But the atomistic mechanism of H passivation in doped a-Si:H is unclear. In this paper, we report molecular dynamic simulations on B and P doped a-Si and a-Si:H, focusing on the impurity geometry and associated electronic structure. For a-Si, we report the electronic density of states (EDOS) for various impurity concentrations based on substitutional doping. We seek to determine the effective doping and no