富樫 理恵

The activation energies for Ga and N desorption from a GaN surface were calculated using the density functional theory to understand the detailed decomposition process of the hydrogen terminated GaN(0 0 0 1) Ga and N surfaces under a hydrogen atmosphere. It was found that the Ga atoms on the hydrogen terminated GaN(0 0 0 1) Ga surface desorbed as GaH molecules from the surface while the N atoms on the hydrogen terminated GaN(0 0 0 1) N surface desorbed as NH3 molecules from the surface. The desorption energies of GaH and NH3 on the hydrogen terminated surface were more consistent with the previous experimental values than those on the ideal surface. These results suggest that the initial surface structure of the GaN(0 0 0 1) surface is terminated with hydrogen.

A thin protective AlN layer was grown on a (0001) sapphire substrate at 1065° C for main AlN layer growth at high temperature (> 1300° C) by hydride vapor phase epitaxy (HVPE). The formation of surface pits on the surface of the epitaxial AlN layer, grown directly on the sapphire substrate at 1320° C, could be prevented by growing the protective layer. The full‐width at half‐maximum (FWHM) of X‐ray diffraction (XRD) rocking curves of asymmetric (10 ̄1 0) and symmetric (0002) AlN decreased to 19.8 and 9.6 min, respectively. The concentration of oxygen impurities in the layer grown at 1320° C was also reduced from 3× 1019 to 4× 1018 cm− 3 by protecting sapphire substrate at 1065° C.

The activation energies for Ga and N desorption from a GaN surface were calculated using the density functional theory to understand the decomposition process of the GaN(0 0 0 1) and (0 0 0 -1) Ga- and N-terminated surfaces under a hydrogen atmosphere. It was found that the Ga atoms on the GaN(0 0 0 1) Ga and (0 0 0 -1) Ga surfaces desorbed as GaH molecules from the surface and the Ga desorption energy for a GaN(0 0 0 1) surface was smaller than that for a GaN(0 0 0 -1) surface. In the case of N-terminated surfaces, the N atoms on the GaN(0 0 0 -1) N and (0 0 0 1) N surfaces desorbed as NH3 molecules from the surface and the N desorption energy for a GaN(0 0 0 1) surface was smaller than that for a GaN(0 0 0 -1) surface.

The decomposition processes of AlN from its surface in a hydrogen atmosphere were studied using the ab initio calculation method based on the density functional theory. The activation energies of Al-and N-atoms desorption from (0001) AlN surface were calculated. It was found that Al-atoms desorb from (0001) AlN surface as AlH molecules, and N-atoms as NH 3 molecules. Desorption of AlH molecules is a rate limiting reaction which conforms well with experimental data.

We have investigated the surface structures of AlN and InN using ab initio calculations based on the density functional theory within generalized gradient approximation. We studied the surface energies obtained from total energy calculations for various (2× 2) geometries of cation-terminated (0001) surfaces and anion-terminated (0001) surfaces of AlN and InN, with hydrogen in a carrier gas. It was found that the structures with N–H bonds were favorable under hydrogen ambient, while in the absence of hydrogen, the structures with metal adatoms and N adatoms tended to be stable under metal-rich conditions and N-rich conditions, respectively.

The thermal stabilities of Al- and N-polarity AlN layers grown on (0 0 0 1) sapphire substrates were investigated at temperatures ranging from 1100 to 1400 °C in various gas flows (He, H2 and H2+NH3). Decomposition of AlN occurred in flowing H2, while it did not occur in He or H2+NH3 flow. The decomposition rate in H2 increased with an increase in the temperature over 1200 °C, and the decomposition rate of the Al-polarity AlN layer was found to be lower than that of the N-polarity AlN layer at each temperature. The decomposition reaction of AlN and the relationship between the polarities of the AlN layers and their different decomposition rates are discussed.

In order to fabricate an Fe‐doped semi‐insulating (SI) GaN substrate by hydride vapor phase epitaxy (HVPE) using (111)A GaAs as a starting substrate, the Fe doping mechanism was investigated by two approaches: first‐principles calculation and X‐ray absorption fine structure (XAFS) measurements. First‐principles study clarified that Fe and As atoms are substituting for the Ga and N atoms in the wurtzite GaN lattice, respectively. In addition, incorporation of Fe in GaN was found to be hindered by adjacent As in the N site. XAFS analysis performed for an Fe‐doped SI GaN substrate, obtained by protecting backside of the GaAs substrate, showed that the Fe atoms are substituting for the Ga site in GaN.