THEORY OF DOPING OF AMORPHOUS TETRAHEDRALLY BONDED SEMICONDUCTORS

The experimental observation of substitutional doping in amorphous semiconductors has shown that the coordination of atoms in random networks is not solely a function of their number of valence electrons. Using a simple tight-binding Hamiltonian, substitutional sites are found to be stabilized by finite valence and conduction bandwidths. This allows the exertion of non-classical valence without the destabilization usually associated with the incomplete filling of bonding orbitals or the partial occupancy of antibonding states. This in turn allows the dominating covalent interaction to be maximized, which occurs for tetrahedrally hybridized sp3 bonding. Noting the difficulty of quantitative tight-binding calculations, the method is used instead to illustrate the physical processes involved and to find trends in stability with atomic number-in particular it is found that maximizing the atomic number of the impurity and minimizing that of the host will stabilize substitutional sites with respect to the competing saturated bonding or 1 self-compensated ’ sites. A brief discussion of the role and effects of hydrogen is also included. © 1979 Taylor & Francis Ltd.