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Studies on the 0.96 and 0.84 eV photoluminescence (PL) emissions at various temperatures in GaAs epilayers grown on Si with [As]/[Ga]=20–50 by metalorganic chemical vapor deposition were made. In terms of an Arrhenius plot and configurational coordinate model, the thermal activation energy and Franck–Condon (FC) shift for the 0.96 eV emission band were obtained by measuring the variations in its PL intensity and full width at half-maximum with temperature, respectively. The dependence on PL intensity versus temperature of the 0.84 eV PL emission could not be fitted with an Arrhenius plot. Instead, it could be fitted with the formula used for amorphous semiconductors or localized states which allowed us to relate this emission with the presence of defects in the heteroepitaxial GaAs layers grown on Si investigated. Taking into account the FC and band-gap shifts, the energy relationships of the transitions from donor to acceptor, from conduction band to acceptor, and from donor to valence band were reformulated. In terms of these transition-energy relationships and experimental data, the 0.96 eV emission was explained as the recombination luminescence of the donor–acceptor pair, composed of an arsenic vacancy and a gallium vacancy, and the 0.84 eV emission as the transition from the localized As interstitial–Ga vacancy complex center to Ga vacancy.

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We have investigated the effects of B impurities on the structure and mechanical properties of NiAl intermetallics by using a first-principles pseudopotential total-energy method, based on the density functional theory with a generalized gradient approximation. We found that the impurity B atoms can either replace Ni atoms or Al atoms or both, depending on the surrounding environment. We demonstrated that the presence of B will cause an increase in brittleness and a decrease in the ductility of NiAl for the Al-substitutional case, while causing an increase in the ductility of NiAl for the Ni-subtitutional case, based on the calculated elastic constants and the empirical criterions. This indicates that the effects of B impurities on the mechanical properties of NiAl intermetallics are quite composition-dependent.