Correlation between electronic structure and local ordering in hydrogenated amorphous silicon

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Abstract
Nonparametrized calculations of the electronic structure of hydrogenated amorphous silicon are presented. The method of calculation is based on the cluster Bethe-lattice approximation. The Hamiltonian matrix elements and the equilibrium atomic distances are obtained via ab initio cluster calculations. The calculations are performed for isolated and clustered Si-H units in the silicon Bethe lattice. A two-peak structure in the hydrogen-related electronic density of states is found for dihydride and trihydride bonding. A three-peak structure in the density of states is found for different clustered arrangements of monohydride units. The position and assignment of the structure in the density of states is in qualitative agreement with ultraviolet-photoemission-spectroscopy data. It is shown that finite-cluster calculations produce spurious structure in the density of states, and therefore they are not suited to obtain reliable densities of states. The charge transfer to hydrogen is found to be 0.16, 0.15, and 0.11 electron for the monohydride, dihydride, and trihydride configurations, respectively. The need for a nonparametrized calculation in infinite systems is discussed.