First-principles study of defect properties in semiconductors

Abstract

On the basis of first-principles calculations, we present a structural model for the formation of H-induced (111) platelets in Si, which involves a structural transformation from a double-layer-$H_2^*$ configuration of $H_2^*$ aggregates into an H-saturated internal (111)-surface structure. This reaction process preferably occurs at high H plasma treatment temperatures, and subsequently generates H$_2$ molecules in the platelet voids, consistent with experiments. Our model also reveals the important features observed in (111) platelets, such as HRTEM images, step structures, lattice dilation lengths, and H vibrational frequencies. In GaAs, we find that substitutional nitrogens act as hydrogen traps, forming N-H complexes. For low H concentrations, we propose the formation of N-monohydride complexes, where H is positioned at a bond-center site between the N and one of the neighboring Ga atoms, explaining various experimental features such as the Fermi level dependence of the formation of N-H complexes, H vibrational frequencies, isotope shift, and photoconversion of the complexes. For very high H concentrations, the energetically favorable structure is a N-dihydride complex, which corresponds to a configuration of $H_2^*$ in the vicinity of N. This N-dihydride complex is optically inactive, suppressing N-related photoluminescence lines in N-containing GaAs, and induces a blue shift of the band gap in $GaAs_{1-x}N_x$ alloys. We investigate the stability of wurtzite and rocksalt $Mg_xZn_{1-x}O$ random alloys using a first-principles pseudopotential method within the local-density-functional approximation. From the calculated cohesive energies for Mg$_x$Zn$_{1-x}$O alloys in both the wurtzite and rocksalt structures, we find that the wurtzite phase is only stable for x＜0.375. We propose that the solubility limit of Mg in wurtzite $Mg_xZn_{1-x}O$ alloys is about x = 0.375, close to the experimentally measured value of x = 0.33. The band gap ($E_{gap}$) is found to ...