Structural stability and electronic structure of bulk semiconductors and superlattices

Abstract

In this thesis, we study the structural property and the electronic structure of ultra-thin semiconductor superlattices and various polytypes of SiC through self-consistent ab initio pseudopotential calculations. The bulk and epitaxial stabilities of ultrathin ternary superlattices for both the lattice-mismatched InP/GaP and the lattice-matched GaP/AlP systems are examined. In both bulk and epitaxial forms of the InP/GaP superlattices, the chalcopyrite-like ordered structure is found to be more stable than the CuAu-I-like and CuPt-like ordered structures, while the CuAu-I-like phase is lower in energy than the CuPt-like ordered structure. Our calculations indicate that the ordered structures in bulk form are unstable against phase segregation into binary constituents at T = O. This instability mainly results from the incomplete release of elastic energy, which is a general feature found in superlattices of lattice-mismatched constituents. For epitaxial growth, the formation enthaply is significantly reduced. We find that the epitaxial formation enthalpy decreases as the substrated lattice constant increases. The chalcopyrite phase is found to be stabilized while the CuPt-like ordered structure is most unstable when grown epitaxially. Thus, the observed [111] or [111]-ordered $In_{0.5}Ga_{0.5}P$ should be related to chemical bonding effect at the growing surface. For the lattice-matched AlP/GaP superlattices, the three ordered structures considered in both bulk and epitaxial forms are unstable against phase segregation into binary constituents at T = O. We find that the instability results from the charge-transfer between the Ga-P and Al-P bonds, contrary to the lattice-mismatched system. The calculated epitaxial formation enthalpies are almost idential to within 0.1 meV/atom to those for the bulk superlattices. For the (GaP)m(AlP)m superlattices with m ranging from 1 to 6, composed of lattice-matched indirect gap semiconductors, the band gap of superlattices te...