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 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.

N-polarity InN layers grown on (0001) sapphire substrates by hydride vapor phase epitaxy (HVPE) were annealed in atmospheric N-2, H-2, H-2 + NH3, or N-2 + NH3 ambient at various temperatures. In the N-2 ambient, InN decomposition became significant above 600 degrees C, whereas in the H-2 ambient, InN layers reacted with H-2 and decomposed at very low temperatures above 350 degrees C. The process of InN decomposition was also investigated using a first-principles calculation. In a system free from H-2, the rate-limiting process was found to be desorption of N atoms from the surface as atomic N (N*), whereas in the H-2 ambient, the desorption of In atoms as hi, InH or InH3 is the rate-limiting process. Thus, the decomposition reactions of InN layers in the ambient in the presence and absence of H-2 are different.

The growth of AlN films by the hydrogen vapor phase epitaxy method is generally carried out at a high temperature over a hydrogen atmosphere. The difficulties concerned with the decomposition processes on the surface during the film growth result in necessity of computer modelling of that processes. First principles calculations of the decomposition processes of AlN in a hydrogen atmosphere are reported. The mechanism of desorption of atoms from the surface was deter-mined. Al atoms desorb as AlH from (0001) surface and as Al from (000-1) surface of AlN. And N atoms desorb as NH3 from (0001) surface and as NH from (000-1) surface of AlN. The desorption of Al atoms is a rate limiting reaction. The calculation results correspond well with the experimental date published earlier. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

The influence of hydrogen coverage on a Si(1 1 1) substrate on the initial growth of a GaN buffer layer was investigated using Ga and At adsorption energies obtained by ab initio calculations. It was found that absolute values of the adsorption energies of Ga and At atoms increased as the hydrogen coverage on the substrate surface decreased. Moreover, it was found that the absolute value of Al adsorption energy was larger than that of Ga in any case. These results suggest that it is important to control the substrate surface condition and carrier gas for the growth of a GaN buffer layer on a Si substrate, though an AlN buffer. layer can be grown even under H-2 ambient. (c) 2006 Elsevier B.V. All rights reserved.

An Fe-doped thick GaN layer was grown by hydride vapor-phase epitaxy on a (111)A GaAs starting substrate. By removing the GaAs substrate, a 400-mu m-thick (0001) GaN substrate having a smooth surface and an Fe concentration of 1.5 x 10(19) cm(-3) was obtained. X-ray diffraction rocking curves of the (0002) and (10 (1) over bar0) planes of the GaN substrate had narrow full-widths at half-maximum of 410 and 360 arcsec, respectively. The etch-pit density of the GaN substrate was 8 x 10(6) cm(-2). Extended X-ray absorption fine structure analysis revealed that the Fe atoms are substituting for the Ga in the GaN. The GaN substrate had a high resistivity of 8.8 x 10(12) Omega cm at room temperature. (c) 2006 Elsevier B.V. All rights reserved.