Cluster Computations Related to Silicon Thermal Donors
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CLUSTER COMPUTATIONS RELATED TO SILICON THERMAL DONORS
LAWRENCE C. SNYDER* AND JAMES W. CORBETT** *Chemistry Dept. State University of New York at Albany 1400 Washington Ave. Albany, New York 12222 **Physics Dept. State University of New York at Albany 1400 Washington Ave. Albany, New York 12222
ABSTRACT Ab-initio quantum chemical computations have been applied to a set of molecular clusters derived from Si5 H, 2 to model defects in crystalline silicon involving boron, carbon, nitrogen, oxygen, and hydrogen. In computations of defect structure, hydrogen atoms terminating silicon valencies are fixed at their computed positions in Si6 J-1, to represent forces from the lattice, while the position of other atoms are varied. We have computed the stable bonding structures of boron, carbon, nitrogen and oxygen atoms to a vacancy, as well as interstitial oxygen, the silicon-oxygen ylid and two oxygen atoms bound to a vacancy. The structures of the dipositive ions of the oxygen bearing clusters have been computed as part of a search for candidates for the core of the 450° C' oxygen thermal donor in silicon crystal. The computed cluster energies are employed to give an account of defect thermochem-
istry; the addition of the free atoms to a vacancy, the addition of interstitial oxygen atoms to a vacancy, the reaction of interstitial oxygen atoms to form a vacancy-oxygen complex with the emission of silicon monoxide, and the reaction of interstitial oxygen with the dipositive ion of substitutional oxygen to form the dipositive ion of two oxygen atoms bound to a vacancy. The dipositive ions of the silicon-oxygen ylid and two oxygen atoms bound to a vacancy are found to be possible candidates for the core of the 4500 C oxygen thermal donor. In each case the oxygen atoms draw their their bonded silicon atoms toward themselves making those silicon atoms favorable sites for additional interstitial oxygen atoms. Additional semiempirical calculations with the MNDO method have been made to explore the interaction of additional interstitial oxygen atoms with the central silicon of the ylid dipositive ion. These show that a dative bond is formed by abutting interstitial oxygen atoms with the cationic ylid central silicon. The charge of the dipositive ion is found to be distributed over this chain of three oxygen atoms oriented as in the (ll0) direction of the crystal.
Mat. Res. Soc. Symp. Pro
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