Calculations of the Electronic Structure of Vacancies in a-Si:H
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CALCULAWXONS OF THE ELECTROONIC STfRUCTLrDE.: OF VACANCIES IN a-Si:H ARIEL A. VALLADARES and L. ENRIQUE SANSORES Institute de Investigaciones en Materiales, UNAM, Apartado Postal 70-360, Mdxico, D.F., 04510, MEXICO.
ABSIrTACT The elect-onic structure of random clu:;ters has been used in the literature as representative of the electronic structure of random solids. In this work a calculation of the local density of states (LDOS) and charge density contours for clusters of the type XSi 20 H2 8 with X an Si atom, a vacancy or 4 hydrogen atoms, has been carried out. The method used was a pseudopotential SCF Hartree-Fock and the HONDO program. It is found that the generation of a vacancy in the center of the. cluster (removal of the central Si atom), introduces p-like states in the energy gap of the LDOS for the region near the center of the cluster. The saturation of the dangling bonds of the vacancy with 4 hydrogen atoms removes the states within the gap. These results are also borne out by the charge density contours, thereby reinforcing the importance of amorphous cluster calculations in the understanding of the electronic structure of amorphous solids.
UNTIRODUCTION The theory of the electronic properties of amorphous materials has been making steady progress. The versatility of the structures in these materials has impeded the formulation of a formalism that accounts for most of the features revealed by experiment; nevertheless some general conclusions have been reached through the study of random networks, or through the use of model hamiltonians for certain random structures. The use of molecular methods applied to specific samples with definite structures has been an approach frequently used for these materials. At first the main concern was to understand the structure and properties of pure amorphous Si and Ge. but as they became better understood the next step was to attack the hydrogenated amorphous semiconductors both doped and undoped. Recent results 11-31 on the local density of states (LDOS) and the charge density contours of amorphous semiconductor clusters pure and doped with p- and n-type elements of groups ill and V of the Periodic Table have clearly shown that it is possible to obtain theoretical predictions concerning the electronic structunr of amorphous solids by extrapolation of the results found for representative "molecular clusters", within the pseudopotential SCF Hartree-Fock method, utilizing the HONDO program. The fact that systematic studies can be carried out within this approximation for the same geometrical structure of the cluster, allows one to perform calculations that facilitate the comparison of the different results, thereby removing the factors introduced by varying arrangements of the atoms that may mask the essential features of the electronic structure. The electronic structure of these materials has been studied by applying minimal tightbinding methods 141, extended Huckel techniques 151, pseudopotentials 161 and ab initio LCAO 171 to a variety of structurally different sa
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