First-principles study of the growth mechanism of cubic GaN film, the ferromagnetism of Co-doped ZnO nanowires, and the defect properties of high-k gate oxides

Abstract

We perform first-principles pseudopotential calculations to study the growth mechanism of cubic GaN film, the ferromagnetism of Co-doped ZnO nanowires, and the defect properties of high-k gate oxides. First, we investigate the effect of Mn delta-doping on the stability of the two polytypes, wurtzite and zinc-blende, of GaN. The total energy difference between the ZB and WZ structures decreases as the concentration of Mn increases and a phase transition from wurtzite to zinc-blende occurs at the Mn concentration of x=0.27 in Mn delta-doped GaN in GGA calculations, while the critical Mn concentration increases to 0.47 in $GGA+\It{U}_d$ calculations. As charge densities around the MnN off-bonds in the WZ structure is higher than for the ZB structure, repulsive interactions between off-bonds increase and weaken the attractive interactions between the GaN bilayers. Using a slab geometry of hexagonal lattices, we find that it costs a very high energy to form inversion domains with Mn exposure, in contrast to Mg doping. As a result of testing the various staking, we find that a stacking fault, which generates cubic bonds, happens easier on the Ga-polar surface than for the N-polar one and the Mn delta-doping helps to make a stacking fault in Ga-polar surface but since the cubic bonds can be formed only in the GaN layers adjacent to the Mn layer, the cubic phase can not occur globally. We note that the Mn layer remaining on the surface is energetically more stable than in the bulk GaN with Ga-polarity, suggesting that Mn may act as a surfactant during the crystal growth of GaN and stabilize the ZB phase globally. We investigate the stability of the zinc-blende (ZB) phase in GaCrN, GaMnN and GaFeN alloys. The GGA calculations show that the ZB phase can be stabilized when the concentrations of the Cr and Mn ions are larger than 21 and 27% in GaCrN and GaMnN alloys, respectively. In GaFeN alloys, the stabilization of the ZB phase occurs at a much higher concentration of t...