Cluster Calculation of B and Al Impurities in Amorphous Silicon
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CLUSTER CALCULATION OF B AND Al IMPURITIES IN AMORPHOUS SILICON J. A. COGORDAN, L. E. SANSORES AND A. A. VALLADARES Instituto de Investigaciones en Materiales Universidad Nacional Aut6noma de M6xico Apdo. Postal 70-360, 04510 M6xico D. F., MEXICO. ABSTRACT We report our results of molecular structure calculations for B and Al impurities. From our unoptimized ab initio calculations, we found the molecular electronic wave function to be unstable for both impurities. This instability was removed through a geometry optimization process. Local densities of states (LDOS) were computed for the optimized geometries. They show a rise of a peak at the tail of the valence LDOS; this feature is due to p orbitals of B and Al. The contribution is slightly higher for B than for A] impurities. Charge contour plots are presented. I. INTRODUCTION Silicon-based semiconductors in amorphous and crystaline form have been used in a wide range of technological devices. Amorphous silicon (a-Si) has proved to be a high quality semiconductor. In addition to this, films of this semiconductor are applicable on a large number of substrates, regardless of their topological form, without any loss in their semiconducting properties. The effect of doping in a-Si has received particular attention, both from the experimental and the theoretical points of view. This is due to the dramatical change in its electronic properties as a result of doping. These studies have been carried out with p- and n-type of dopants. A complete Several models have been proposed to explain different propeties of a-Si; in particular, experimental information has been analyzed and it has been concluded that a reliable model for the microstructure of a-Si is that it should be formed by clusters [2]. This holds independently of the technique used for the formation of the film. The size of these clusters is sensitive to deposition conditions and annealing temperatures. Based on this analysis and on the experience of previous theoretical studies [3,4,5], molecular electronic structure calculations have been undertaken in order to study the effects of B and Al impurities on a-Si. The cluster we consider for this report has the stoichiometry MSi 2 0 H 2 8 , M=B and Al. The article is organized as follows: in section II we give a brief description of the cluster considered; in section III we describe the methods and the strategies used in our calculations; in section IV we describe some of our results, and in section V we discuss these results. II. CLUSTER DESCRIPTION The cluster we have considered has a stoichiometry MSi 2 0 H 2 8 , with M=B and Al. The impurity is positioned at the center of the cluster, removing the corresponding Si atom; in principle, it is coordinated by four Si atoms. The H atoms are used to saturate the Si bondings at the surface of the cluster. This cluster has been used in previous work [3,4,5] and it has proved to give a reasonable representation of the electronic behavior of an impurity in a-Si:H . This cluster posesses tetrahedral symmetry before geome
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