Tohoru Matsubara

Origin of lattice bowing of freestanding GaN substrates grown by hydride vapor phase epitaxy

This paper describes a mechanism to explain the lattice bowing of freestanding GaN substrates grown by hydride vapor phase epitaxy on sapphire substrates. The freestanding GaN substrates typically exhibit a concave shape. It is revealed that the radius of curvature and lattice constant of the top surface are almost the same as those of the bottom surface. This is indicative of the complete relaxation of the GaN lattice, even though the freestanding GaN substrate exhibited a curvature. It is shown that dislocations are present in a plane normal to the growth direction in addition to conventionally known threading dislocations; these are referred to as in-plane dislocations. Based on these results, it is proven quantitatively that the extra-half planes related to the in-plane dislocations are primarily responsible for the phenomenon of lattice bowing.

Transmission Electron Microscopy Investigation of Local Atomic Environment of Nitrogen inside Voids Formed at GaN/Sapphire Interface

The local atomic environment of nitrogen inside voids was investigated by transmission electron microscopy (TEM) and high-angle annular dark-field (HAADF)-scanning TEM (STEM) imaging, electron energy loss spectroscopy (EELS), and in situ electron beam irradiation experiments. Voids were formed on the sapphire side at the interface between GaN and the sapphire substrate. EELS analyses revealed that nitrogen existed inside the voids and its bonding type differed from GaN. Electron beam irradiation caused nitrogen inside the voids to be released into a vacuum as gaseous nitrogen. By assessing the amount of nitrogen inside the voids, the density of nitrogen was estimated as 1.9±0.4 g/cm3, and the volume as 15.6±3.0 cm3/mol, which means that nitrogen inside the voids may be in the molecular state of solid nitrogen as δ-N2. Based on the results, a mechanism of formation of voids is proposed.

Atomic-scale investigation of structural defects in GaN layer on c-plane sapphire substrate during initial growth stage

Structural defects in the initial growth stages of GaN on sapphire, including stacking faults (SFs), threading dislocations (TDs), and mosaic structure containing grain boundaries, are investigated at the atomic scale. Individual grains in the as-deposited low temperature-GaN buffer layer are found to have twists correlated with those of the adjacent grains. These grains have little similarity on the stacking sequences, and the atomic arrangement on each side of the grain boundaries may be rearranged by annealing to achieve higher similarity in the stacking sequence. The TD identified as a-type at the top of the SFs-rich interfacial region is thought to originate from Frank partial dislocations. The Frank partial dislocation produces a distorted wurtzite-type structure. At the intermediate region of the basal-plane stacking fault between Frank and Shockley partial dislocations, the TD relieves the distortion in the wurtzite-type structure. In the TD, the wurtzite structure slips relative to the surrounding wurtzite.

V-shaped pits in HVPE-grown GaN associated with columnar inversion domains originating from foreign particles of α-Si3N4 and graphitic carbon

The v-shaped pits (so-called V-pits) observed in hydride-vapor-phase-epitaxy-grown GaN and associated with the columnar inversion domains originating from foreign particles were investigated. The inversion domains on the front and back surfaces of the test sample were recognized after chemical mechanical polishing. It was found that the V-pits originate from the columnar inversion domains. The inversion domains, in turn, arise from the particles that exist on a low-temperature GaN buffer layer on sapphire substrate. Using transmission electron microscopy, these particles were found to be of α-Si3N4 and graphitic carbon. Such particles are attributable to the components of the reactor and adhere to the low-temperature GaN buffer layer, which has a surface roughness of the order of several nanometers. Thus, an effective way of obtaining HVPE-grown thick GaN layers without the V-pits associated with columnar IDs is to maintain the parts of the HVPE chamber properly to prevent foreign particles from being generated.

Mechanism for the formation of nitrogen-filled voids after annealing of GaN on a sapphire substrate

This study investigated the formation of nitrogen-filled voids at the interface between a GaN layer grown on a sapphire substrate by metalorganic vapor phase epitaxy. These voids were formed in the sapphire substrate at the interface after annealing and previous research shows that they can induce an inversion domain in AlN, which affects the film quality and the device performance. We investigated the void formation using scanning electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray spectrometry, and electron energy loss spectroscopy. The voids are known to originate from the decomposition of sapphire in the presence of ammonia and hydrogen. Our analysis confirmed that the outgassing reaction between the decomposing sapphire and the low temperature GaN buffer layer resulted in the diffusion of aluminum and oxygen into the GaN buffer layer. During the annealing process, oxygen ions replaced nitrogen ions and created nitrogen-filled voids at the interface between the GaN buffer layer and the sapphire substrate. The presence of molecular nitrogen in the voids was confirmed by scanning transmission electron microscopy and electron energy loss spectroscopy.This study investigated the formation of nitrogen-filled voids at the interface between a GaN layer grown on a sapphire substrate by metalorganic vapor phase epitaxy. These voids were formed in the sapphire substrate at the interface after annealing and previous research shows that they can induce an inversion domain in AlN, which affects the film quality and the device performance. We investigated the void formation using scanning electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray spectrometry, and electron energy loss spectroscopy. The voids are known to originate from the decomposition of sapphire in the presence of ammonia and hydrogen. Our analysis confirmed that the outgassing reaction between the decomposing sapphire and the low temperature GaN buffer layer resulted in the diffusion of aluminum and oxygen into the GaN buffer layer. During the annealing process, oxygen ions replaced nitrogen ions and created nitrogen-filled voids at the interface between the GaN ...

Direct observation of inclined a -type threading dislocation with a -type screw dislocation in GaN

We investigated both the atomic arrangements in the core structure of threading dislocations (TDs) and their behaviors in unintentionally doped c-plane-GaN layers grown by metalorganic vapor phase epitaxy and hydride vapor phase epitaxy using high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The extra image contrast near the core was attributed to an extra displacement in a-type TDs in addition to the core structures revealed in previous reports; we used the notation “with displacement” to describe the new core structure. We found that TDs incline towards both the m- and a-directions from the c-direction. The transition of a-type TDs from the conventional core structure to the structure with displacement was deduced from its relationship to the TD inclination. We also found similarities between a-type screw dislocations and a-type TDs with displacement in the atomic-scale HAADF-STEM images. We concluded that a-type TDs could incline towards the a-direction via a-type sc...