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In this study, a numerical simulation of edge-defined film-fed growth (EFG) was performed to determine the appropriate capillary conditions for Ga2O3 melt. Meniscus and capillary rise were significantly influenced by the design of the die in the EFG system. The ratio of the seed crystal and die width was >0.73 for a die width of 4.4 mm. Narrower slit width resulted in higher capillary rise with longer process time compared with wider slit width. Under conditions consistent with the simulation results, highly crystalline (100) β-Ga2O3 single crystals were successfully achieved.

Two-step growth of κ-Ga2O3 thin films on 4H-SiC substrates was attempted with temperature-varied buffer layers via mist chemical vapor deposition. The first-step Ga2O3 buffer layers affect the phase formation and grain size variation depending on growth temperatures. In the second-step thin-film growth, the κ-Ga2O3 thin film was grown at a fixed temperature of 500 °C regardless of various buffer layers. Three zones, namely, amorphous, κ phase, and mixed phase, were categorized in the κ-Ga2O3 thin films according to the buffer growth temperature. High-quality and smooth κ-Ga2O3 thin films could be achieved through the grain growth competition and slight buffer temperature variation in the two-step growth.

We report on the material and optical properties of core–shell InGaN layers grown on GaN nanorod arrays. The core–shell InGaN layers were well grown on polarization-reduced surfaces such as semipolar pyramids and nonpolar sidewalls. In addition, to compensate the biaxial strain between GaN and InGaN layers, we grew interlayers underneath a thick InGaN layer. Here, the interlayers were composed of multiple superlattice structures. We could observe that the indium composition of core–shell InGaN structures increased with the number of interlayers. This indicates that the absorption energy band of InGaN alloys can be better matched to the spectral irradiance of the solar spectrum in nature. We also implemented a simulation of Ga-polar and nonpolar InGaN-based solar cells based on the indium composition obtained from the experiments. The result showed that nonpolar InGaN solar cells had a much higher efficiency than Ga-polar InGaN solar cells with the same thickness of the absorption layer.

Microscale platelet-shaped GaN grains were grown on amorphous substrates by a combined epitaxial growth method of molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). First, MBE GaN was grown on an amorphous substrate as a pre-orienting layer and its structural properties were investigated. Second, MOCVD grown GaN samples using the different growth techniques of planar and selective area growth (SAG) were comparatively investigated by transmission electron microscopy (TEM), cathodoluminescence (CL), and photoluminescence (PL). In MOCVD planar GaN, strong bound exciton peaks dominated despite the high density of the threading dislocations (TDs). In MOCVD SAG GaN, on the other hand, TDs were clearly reduced with bending, but basal stacking fault (BSF) PL peaks were observed at 3.42 eV. The combined epitaxial method not only provides a deep understanding of the growth behavior but also suggests an alternative approach for the growth of GaN on amorphous substances.

In this paper, we discuss the influence of parameters such as type of carrier gas and NH3/HCl flow ratio on the growth of vertical GaN microstructures by selective-area growth (SAG) hydride vapor phase epitaxy (HVPE). On various strain-induced templates such as GaN/sapphire, GaN/Si, and AlN/Si, regular arrays of Ga-polar GaN microrods were properly achieved by adjusting the growth parameters. The photoluminescence and micro-Raman measurements reveal not only the crystal quality of the GaN microrods but also strain distribution. These results will give insight into the control of the morphology of GaN microrods in terms of the strain induced from templates in SAG-HVPE. The precisely controlled arrays of GaN microrods can be used for next-generation light-emitting diodes (LEDs) by realizing InGaN/GaN multi–quantum wells (MQWs) with a radial structure.

We fabricated selectively grown Ga-polar GaN nanorods by optimizing metal organic vapor phase epitaxy (MOVPE) growth parameters using continuous- and pulsed-mode approaches. Nucleation layers were grown using continuous mode with H2 or N2 as carrier gas, which resulted in pyramidal or hexagonal shapes, respectively. The growth mechanism of nanorods was further studied for the nucleation layer grown with the H2 case, in which the pyramidal shape of the nucleation layer was observed to be flattened during the initial step of pulsed-mode growth and then evolved into nanorods by Ga clustering on the top c-plane.

{101̄1} semipolar GaN-based light-emitting diodes (LEDs) grown on 1° miscut m-plane sapphires substrates via metal organic chemical vapor deposition showed undulated surface morphology with ridges and valleys. On the ridge regions, two dominant emission peaks, one at a shorter wavelength (∼438 nm) and one at a longer wavelength (∼490 nm), were observed using electroluminescence and micro-photoluminescence. In the valley regions, the longer peak was observed to be significantly quenched due to the grain boundary. The origin of the longer peak is believed to be not only inhomogeneous distribution of In composition in multiple quantum wells (MQWs) but also strong localization around the ridge region. Moreover, thickness variation of faceted MQWs could be associated with the peak broadening in 101̄1 semipolar LEDs. The results were also confirmed by cathodoluminescence and cross-sectional transmission electron microscopy.

We investigated the photovoltaic performance of InGaN/GaN multiple quantum well (MQW) solar cells by comparing vertical-type and conventional lateral-type solar cells. We found that both bottom reflector and front surface texturing of vertical-type InGaN/GaN MQW solar cells enhanced light absorption by 45%, leading to an enhancement of the short circuit current density (J SC) by 1.6 times, compared to that of a lateral-type structure. For the vertical-type InGaN/GaN solar cell, Ag was used for bottom reflectors and pyramid textured surfaces were formed by KOH etching after a lift-off process, whereas lateral-type structures were fabricated on sapphire substrates having smooth surfaces. As a result, the vertical InGaN/GaN MQW solar cells showed a high fill factor of 80.0% and conversion efficiency of 2.3%; in contrast, the conventional lateral structure produced a fill factor of 77.6% and a conversion efficiency of 1.4%.