Electronic Properties and Microhardness of Semiconductor Alloys.

The results of theoretical investigations of the properties of semiconductor alloys are presented. First, the effects of alloy disorder on the electronic properties of four III-V quaternary alloys are investigated using an extension of the bonding-antibonding coherent potential approximation (CPA) formalism. The alloys considered have two disordered sublattices so that their chemical formulae have the general form A_{x}B _{1-x}C_ {y}D_{1-y}. Some alloys of this type are used in optoelectronic devices. Results are presented for the densities of states, the self energies, and the band gap bowing for the alloys Al_ {x}Ga_{1-x}As _{y}P_{1 -y}, Al_{x}Ga _{1-x}As_ {y}Sb_{1-y}, Ga_{x}In_ {1-x}As_{y}P _{1-y}, and Ga _{x}In_{1-x }As_{y}Sb _{1-y}. Comparisons of these CPA results are made with Virtual Crystal Approximation (VCA) results, and with experimental data where available. Second, the electronic properties of the II-VI alloy Hg _{1-x-y}Cd_{x }Zn_{y}Te are calculated. To do this, an extension is made of a previously developed CPA formalism for quaternary alloys of the form A_{1-x-y}B _{x}C_{y }D to include spin-orbit coupling in the input bandstructures. This study is motivated by recent results that indicate improvement in the structural properties of the widely used infrared material Hg_ {1-x}Cd_{x}Te on the addition of few per cent zinc. Results for the effective masses and band gap variations with compositions are presented. Lastly, a formalism for the calculation of the alloy microstructural hardness is developed. This formalism combines a solid-solution hardening model, an analysis of the composition dependence of critical stress, and an empirical relation for the alloy hardness. Results for the hardness variations with composition for the II -VI alloys Cd_{1-x}Zn _{x}Te, Hg_{1 -x}Cd_{x}Te, Hg_{1-x}Zn_ {x}Te, and Hg_{1-x -y}Cd_{x}Zn _{y}Te are presented and compared with available data. The results for Hg _{1-x-y}Cd_{x }Zn_{y}Te verify improved structural properties of Hg_ {1-x}Cd_{x}Te on the addition of zinc. Results for the composition dependence of hardness are also presented for the III-V alloys In _{x}Ga_ {1-x}As, Ga_{x} In_{1-x}Sb, Al _{x}In_ {1-x}Sb, Ga_{x} In_{1-x}P, and Al_{x}In_ {1-x}As and compared with experimental data where available.