Crystal growth of GaN on (0001) face by HVPE: Ab initio simulations

Adsorption of ammonia at GaN(0001) surface in the mixed ammonia/hydrogen ambient - a summary of ab initio data

“…Van de Walle and Neugebauer constructed phase diagram of GaN(0001) surface in ammonia ambient, using coordinates of chemical potential of hydrogen and nitrogen, finding regions of stability of several surface structures. 12,13 These results were confirmed by the DFT determination of the stable structures of polar and nonpolar GaN(0001) surfaces by Ito et al 14 The reaction of ammonia with the bare and H-covered surface was investigated by several groups by ab initio modelling, [15][16][17][18][19][20] Fritsch et al simulated several configurations of GaN(0001) surface showing that ammonia adsorption is dissociative to H and NH 2 radical and transformation of GaN(0001) flat surface into p(2 x 2) vacancy reconstruction. The NH 2 radical was bound to gallium and H atom to gallium and nitrogen broken bonds respectively barrier was found to be of order of 0.5 eV.…”

“…[17][18][19][20] Alternatively, also as reported, ammonia may be adsorbed molecularly in the "'on-top" position. 21 The results concerning the adsorption energy may be understood using the recently formulated theory of the charge transfer at the surface contribution to the adsorption energy. 6,7 According to the theory, the charge transfer may affect the energy even by several electronvolts.…”

“…Similarly, the dissociative adsorption of ammonia at a clean GaN(0001) surface, with the NH 2 radical adsorbed in the asymmetric bridge position, was obtained by Bermudez. 18, 19 Kempisty et al modeled the adsorption of ammonia on a highly H-covered GaN(0001) surface showing that ammonia is adsorbed molecularly in a barrierless process with the adsorption energy equal to 2.5 eV. 21 Recently, intensive investigations of ammonia adsorption were conducted by the same authors. 6 They found that ammonia adsorbed on relatively low H-occupied surface decomposes into a NH 2 radical and a H adatom, which is in accordance with the results of Pignedoli et al and Bermudez. 17-20 The adsorption energy is about 2.8 eV for the hydrogen coverage up to 0.75 ML, but for higher coverage the energy is drastically reduced.…”

Hydride vapor phase epitaxy of GaN boules using high growth rates

“…Different growth rates are reasonable taking into account the different atomistic and energetic structures of these facets [38]. But the adatom kinetics of HVPE is still under controversial discussion even for the (0 0 0 1) plane [39,40,41]. Due to the frequent observation of Ga-droplets forming after cool-down and the low V/III ratio compared to MOVPE [42], we believe that in HVPE under our conditions rather Ga-rich surfaces occur [38].…”

Ab initio calculation for an initial growth process of GaN on (0001) and (000$ \bar 1 $) surfaces by vapor phase epitaxy

“…Nai-Xia et al [3] performed ab initio calculations for NH 3 adsorption and suggested that NH 3 adsorbed to a top site first and dissociated into NH 2 and H on the surface. Kempisty et al [4,5] studied NH 3 adsorption on both bare and H-covered GaN(0001) surfaces and GaCl adsorption on the GaN(0001) surface covered by 1 monolayer nitrogen atoms to understand the epitaxial growth of GaN in HVPE. As a result, it was found that the energy barrier was not present during adsorption of GaCl on an N-covered GaN(0001) surface and adsorption of NH 3 on both bare and H-covered GaN(0001) surface occurs without energy barrier.…”