Cluster Simulations of Amorfous Silicon, with and without an Impurity Boron Atom
During the past several years there has grown an enormous experimental and theoretical activity on amorphous silicon (a-Si). This is based on the possibility of doping hydrogenated amorphous silicon, which opens the way to many interesting technological applications (solar cells, etc.), despite the limitations due to the difficulty of improving the efficiency of the doping process. On the theoretical side the main effort is aimed at obtaining an understanding of the mechanisms through which the dopant atoms become electrically active. To this purpose a knowledge of the energy levels of the impurity under different configurations is of particular interest. Several theoretical models have been developed to calculate the electronic stucture of amorphous silicon (for a general review see ref. ), and many of them use finite cluster models based on self-consistent quantum chemical methods. The main limitation of these models lies in the small size of the clusters utilized, which cannot be indefinitely increased because of the rapid growth of the computational expense. This can be circumvented by using appropriate boundary conditions, which simulate the surronding infinite medium. In the present paper the electronic structure of a-Si and a-Si doped with an impurity boron atom is studied through ab initio Hartree-Fockplus-correlation calculations on model clusters. The choice of cluster geometries and boundary conditions is discussed in detail.
KeywordsSilicon Atom Amorphous Silicon Boron Atom Angular Distortion Lower Excited State
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