Bonding and transport properties in amorphous semiconductor alloys with ordered compositions

Abstract

For those amorphous or liquid alloys which are represented by random networks of covalent bonds and in which the coordination number of each atom is constant and equal to its valence, there is a fully occupied valence band and the Fermi level is near the middle of the pseudo-gap. A maximum in resistivity and thermopower may be found at the composition at which chemical ordering takes place. It is shown that the electronic properties change little with increasing bond ionicities until, at a critical ionicity, the local coordination takes on an ionic form. In this new structure, the shear rigidity characteristic of a covalent bond required for the validity of the continuous random network (CRN) is lost, and the coordination numbers are allowed to be composition-dependent. This ionic model applies to a range of binary alloys, such as amorphous MgBi, and composition changes away from stoichiometry (Mg₃Bi₂) can be interpreted in terms of the doping of this ordered composition. The structural and bonding properties of these alloys are discussed and differentiated from both the covalent and the idealized Faber-Ziman binary metallic alloys.