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IMPURITY-DEFECT COMPLEXES IN HYDROGENATED AMORPHOUS SILICON

LIN H. YANG*, C. Y. FONG**, and CAROL S. NICHOLS*** *Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA. "**Departmentof Physics, University of California, Davis, CA 95616, USA. ***Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA.

ABSTRACT The two most outstanding features observed for dopants in hydrogenated amorphous silicon (a-Si:H) - a shift in the Fermi level accompanied by an increase in the defect density and an absence of degenerate doping - have previously been postulated to stem from the formation of substitutional dopant-dangling bond complexes. Using firstprinciples self-consistent pseudopotential calculations in conjunction with a supercell model for the amorphous network and the ability of network relaxation from the first-principles results, we have studied the electronic and structural properties of substitutional fourfoldcoordinated phosphorus and boron at the second neighbor position to a dangling bond defect. We demonstrate that such impurity-defect complexes can account for the general features observed experimentally in doped a-Si:H.

INTRODUCTION The investigation of whether the shallow donor and acceptor states exit in doped hydrogenated amorphous silicon (a-Si:H) has triggered a number of theoretical and experimental studies of the doping mechanism in a-Si:H.[1-8] Among them, Kocka[7] and Kocka et al.[8] have found from constant photocurrent measurements that the neutral Si dangling bond (D0) in undoped samples has the same position in the energy gap as the D- level in P-doped samples. In B-doped samples, however, they found that the energy of the positively-charged dangling bond (D+) is 0.4 eV closer to the conduction band edge than is the DO level. These authors pointed out that their data were consistent with the pairing model between dangling bonds and dopants, but noted that because of the longrange nature of the coulomb interaction, the defect and dopant need not be nearest neighbors. In this paper, we present theoretical results for the electronic structure of a

Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society

398

fourfold-coordinated () substitutional impurity atom paired at the second-neighbor position to a dangling bond defect in an a-Si:H model as schematically shown in Fig. 1. We choose three different configurations in the supercell model which have varying degrees of local bond-angle and bond-length distortions to examine the effects of local geometries on the electronic structures of the defect states. The results are relevant to the

understanding of the doping mechanism in a-Si:H. dangling bn

S

Fig. 1 The configuration of the impurity-defect complex in a-Si:H model. The dopant can be either P or B. The dashed line is a dangling bond.

METHODS AND MODELS A computer-generated model of a-Si:H with periodic boundary conditions [9] has been used as the basis of our calculations. The model contains 54 Si and 6 H atoms, in w