This work has investigated the microstructure characteristics of high-quality alpha-Ga2O3 thin film grown on the Al2O3 single crystal substrate membrane. Hetero-epitaxial alpha Ga2O3 crystals reveal the formation of a three-fold symmetry at the initial stage of the growth by the oxygen template provided by the Al2O3. Inversion domains are found, and they have a 180° inverted configuration from the surroundings. These IDs lead to extra diffraction spots when observed along [110] and [010].

A high-quality alpha-Ga2O3 thin film was grown on an Al2O3 single-crystal thin-wall channel structure. Alpha-Ga2O3 crystals are grown with a 3-fold symmetry template provided by the Al2O3 surface, which forms domain boundaries from the small rotation of the mosaic structure. The anti-phase domain (APD) was confirmed by convergent-beam electron diffraction (CBED), and it is believed to be developed at the stage of nucleation based on the unstable, or uneven Al-O bonding on the Al2O3 surface. The in-phase domain and the APD boundaries exhibit bright contrast in ADF, which originates from a local strain of crystallographic rotation and not from the local elemental fluctuations of gallium. As confirmed by CL measurement in TEM, the bandgap energy of alpha-Ga2O3 is inferred to be 5.56 eV with a near band edge transition of ~ 223 nm. Even though the heights and positions of peaks vary slightly from area to area, 4 distinct luminescence peaks and a long tail of wavelength are commonly observed throughout the crystal. 2-dimensional mapping with the specific wavelength window reveals a clear difference between the inside domain, in-phase domain boundary, and APD boundary. A luminescence peak of distinct 350 nm was observed at the APD boundary. Misorientation between domains was found, which leads to an in-plane mismatch in crystallographic lattices at the boundary when the two misoriented domains grow and the growth front merges. The imperfect lattice structure at the boundary is believed to create strain and intermediate states.

We fabricated van der Waals forces engineered microLEDs and confirmed their transfer method with single-face alignment in a fluidic environment. This transfer and integration results can provide a key solution for the microLED display.

This work investigates the relationship between atomic arrangement and luminescence properties in a high-quality alpha-Ga2O3 thin film grown on an Al2O3 single-crystal membrane. The strain induced by merging domain boundaries shows more significant variability in annular darkfield images even though there is no additional gallium concentration confirmed. The bandgap energy of alpha-Ga2O3 is estimated to be 5.56 eV from the CL measurement in a transmission electron microscope. A peak at 320 nm was observed within the domain, while the domain boundary showed spectrum peaks with 380–480 nm. The anti-phase domain (APD) is formed by the instabilities of Al–O bonding templates provided by the Al2O3 substrate. The APD boundary gives a characteristic wavelength of 350 nm, which is the result of the merging boundary of in-phase and anti-phase domains.

Implementing high-performance ultraviolet C photodetectors (UVC PDs) based on β-Ga2 O3 films is challenging owing to the anisotropic crystal symmetry between the epitaxial films and substrates. In this study, highly enhanced state-of-the-art photoelectrical performance is achieved using single-domain epitaxy of monoclinic β-Ga2 O3 films on a hexagonal sapphire substrate. Unlike 3D β-Ga2 O3 films with twin domains, 2D β-Ga2 O3 films exhibit a single domain with a smooth surface and low concentration of point defects, which enable efficient charge separation by suppressing boundary-induced recombination. Furthermore, a tailored anti-reflection coating (ARC) is adopted as a light-absorbing medium to improve charge generation. The tailored nanostructure, which features a gradient refractive index, not only substantially reduces the reflection, but also suppresses the surface leakage current as a passivation layer. This study provides fundamental insights into the single-domain epitaxy of β-Ga2 O3 films and the application of ARC for the development of high-performance UVC PDs.

In this study, we grew InP heteroepitaxial layers on Si(001) substrates with Ge buffer layers. We found out that surface of InP layer was smoothened by adjusting Ge buffer layer. In addition, the suppression of planar defects in InP leaded to the improved crystallinity and optical properties, in spite of the increased threading dislocations. Our study suggested the possibility of combination of III-V and Ge for the future CMOS technologies.

α-Gallium oxide, with its large band gap energy, is a promising material for utilization in power devices. Sapphire, which has the same crystal structure as α-Ga2O3, has been used as a substrate for α-Ga2O3 epitaxial growth. However, lattice and thermal expansion coefficient mismatches generate a high density of threading dislocations (TDs) and cracks in films. Here, we demonstrated the growth of α-Ga2O3 films with reduced TD density and residual stress on microcavity-embedded sapphire substrates (MESS). We fabricated the two types of substrates with microcavities: diameters of 1.5 and 2.2 μm, respectively. We confirmed that round conical-shaped cavities with smaller diameters are beneficial for the lateral overgrowth of α-Ga2O3 crystals with lower TD densities by mist chemical vapor deposition. We could obtain crack-free high-crystallinity α-Ga2O3 films on MESS, while the direct growth on a bare sapphire substrate resulted in an α-Ga2O3 film with a number of cracks. TD densities of α-Ga2O3 films on MESS with 1.5 and 2.2 μm cavities were measured to be 1.77 and 6.47 × 108 cm-2, respectively. Furthermore, cavities in MESS were certified to mitigate the residual stress via the redshifted Raman peaks of α-Ga2O3 films. Finally, we fabricated Schottky diodes based on α-Ga2O3 films grown on MESS with 1.5 and 2.2 μm cavities, which exhibited high breakdown voltages of 679 and 532 V, respectively. This research paves the way to fabricating Schottky diodes with high breakdown voltages based on high-quality α-Ga2O3 films.

100-nm-thick hexagonal fully and partially crystallized sapphire nanomembrane arrays were demonstrated as growth templates for hexagonal micro-sized GaN arrays for micro-light-emitting diodes (LEDs). By simply controlling the thermal treatment time for crystallization of the amorphous membrane, the partially crystallized nanomembrane which has both single- and polycrystalline alumina on the top surface was fabricated. The unique crystalline structures contributed to unique growth behavior which contains selective and lateral overgrowth of GaN, compared to that of GaN on fully crystallized sapphire nanomembranes, leading to superior structural and optical properties of GaN. Threading dislocation densities were reduced by 75% and 1.29 times higher integrated photoluminescence intensity was achieved compared to GaN on the fully crystallized sapphire nanomembranes. Stress in GaN was also fully relaxed due to the ultrathin membrane structure as a compliant substrate. The superior properties of GaN as well as the truncated inverted pyramid structure are expected to improve the performance of micro-LED devices. Hexagonal partially crystallized sapphire nanomembrane technology also provided advances in the fundamental study of epitaxial growth of GaN-based materials.

For commercialization of micro-light-emitting diodes (micro-LEDs), which have been considered as a next-generation display technology, a novel growth template named sapphire nanomembrane was introduced by our research group previously. However, not only the growth condition but also the transfer of discrete micro-LEDs onto other substrates was limited due to the undesired growth at the bottom substrate region between the membranes. Here, we introduce a new selective area growth technology using amorphous alumina as a growth mask material. Because amorphous alumina consists of the same material with the sapphire substrate, it would not cause contamination or unintentional doping, which is occasionally caused by Si-based mask materials. During the growth of GaN using metal organic chemical vapor deposition, the surface region of an amorphous alumina layer, which was used as a growth mask, was crystallized into polycrystalline gamma-alumina through the "random nucleation and growth" process, while the other region crystallized into single-crystalline alpha-alumina through the "solid phase epitaxy" process. Thereafter, GaN hardly grew on polycrystalline gamma-alumina owing to the large difference in growth rates between GaN islands, which grew on different grains of polycrystalline gamma-alumina. Due to the suppressed growth of GaN on polycrystalline gamma-alumina, highly enhanced selective growth of the micro-GaN array on sapphire nanomembranes could be achieved.

Microlight-emitting diode (Micro-LED) is the only display production technology capable of meeting the high-performance requirements of future screens. However, it has significant obstacles in commercialization due to etching loss and efficiency reduction caused by the singulation process, in addition to expensive costs and a significant amount of time spent on transfer. Herein, multiple-sapphire nanomembrane (MSNM) technology has been developed that enables the rapid transfer of arrays while producing micro-LEDs without the need for any singulation procedure. Individual micro-LEDs of tens of μm size were formed by the pendeo-epitaxy and coalescence of GaN grown on 2 μm width SNMs spaced with regular intervals. We have successfully fabricated micro-LEDs of different sizes including 20 × 20 μm2, 40 × 40 μm2, and 100 × 100 μm2, utilizing the membrane design. It was confirmed that the 100 × 100 μm2 micro-LED manufactured with MSNM technology not only relieved stress by 80.6% but also reduced threading dislocation density by 58.7% compared to the reference sample. It was proven that micro-LED arrays of varied chip sizes using MSNM were all transferred to the backplane. A vertical structure LED device could be fabricated using a 100 × 100 μm2 micro-LED chip, and it was confirmed to have a low operation voltage. Our work suggests that the development of the MSNM technology is promising for the commercialization of micro-LED technology.

α-Ga2O3 of the corundum structure and the large bandgap of 5.3 eV has attracted great interest because it can be grown on a sapphire (α-Al2O3) substrate with the same crystal structure. However, the lattice mismatch (4.3%) between α-Ga2O3 and the sapphire substrate induces a high density of threading dislocations, which act as breakdown leakage paths and lead to deterioration of the crystallinity and electrical properties of the α-Ga2O3 films grown directly on the substrate. Herein, to circumvent this issue compositionally graded α-(AlxGa1-x)2O3 layers are adopted to reduce threading dislocations for a high quality of epitaxial α-Ga2O3 films. The evolution of strain relaxation and the inclination of threading dislocations in graded α-(AlxGa1-x)2O3 layers are confirmed by reciprocal space mapping (RSM) and transmission electron microscopy (TEM). Through RSM and TEM studies, we confirmed that compressive strain enhances the inclination of dislocations and therefore, the dislocations merge and annihilate in the graded α-(AlxGa1-x)2O3 layers. The calculated density of threading dislocations in an α-Ga2O33 films with a graded α-(AlxGa1-x)2O3 buffer layer is reduced by 64.9% compared with that of an α-Ga2O3 films deposited directly grown on a bare sapphire substrate. Furthermore, a fabricated lateral-structure Schottky diode reveals enhanced breakdown voltages and forward current density due to the improved crystalline quality by using the graded α-(AlxGa1-x)2O3 buffer layer. This study provides an attractive approach for obtaining high-quality epitaxial α-Ga2O3 thin films for high voltage power devices.

Alpha-gallium oxide (alpha-Ga2O3) has been considered a promising material for efficient power semiconductors because of its large bandgap. Sapphire can be an appropriate substrate for alpha-Ga2O3 because the two materials have the same crystal structure. However, it is hard to obtain high-quality alpha-Ga2O3 thin films on sapphire due to a large lattice mismatch. Herein, we demonstrate the selective area growth of high-quality and strain-relaxed alpha-Ga2O3 thin films on the top of stripe-patterned sapphire nanomembranes. This process is enabled by the well-understood diffusion of adatoms to the highest surface energy plane of a sapphire nanomembrane at different growth temperatures. High-resolution transmission electron microscopy confirmed that misfit dislocations on the nanomembrane were reduced by 13% compared to those of alpha-Ga2O3 grown directly on a sapphire substrate. Reciprocal space mapping reveals that the sapphire nanomembrane reduces in-plane compressive strain in the film by up to 51.6%. This work paves a way for synthesizing high-quality alpha-Ga2O3 thin films that are promising for optoelectronic applications of high voltage and in deep ultraviolet.

InGaN-based blue light-emitting diodes (LEDs), with their high efficiency and brightness, are entering the display industry. However, a significant gap remains between the expectation of highly efficient light sources and their experimental realization into tiny pixels for ultrahigh-density displays for augmented reality. Herein, we report using tailored ion implantation (TIIP) to fabricate highly efficient, electrically-driven pixelated InGaN micro-LEDs (μLEDs) at the mid-submicrometre scale (line/space of 0.5/0.5 μm), corresponding to 8,500 pixels per inch (ppi) (RGB). Creating a laterally confined non-radiative region around each pixel with a controlled amount of mobile vacancies, TIIP pixelation produces relatively invariant luminance, and high pixel distinctiveness, at submicrometre-sized pixels. Moreover, with the incomparable integration capability of TIIP pixelation due to its planar geometry, we demonstrate 2,000 ppi μLED displays with monolithically integrated thin-film transistor pixel circuits, and 5,000 ppi compatible core technologies. We expect that the demonstrated method will pave the way toward high-performance μLED displays for seamless augmented-reality glasses. Submicrometre-sized InGaN-based light-emitting diodes are fabricated by tailored ion implantation. The devices are free from electrical leakage and show a luminance of 7,440 nit at 4.9 A cm−2 even at the line/space scale of 0.5/0.5 μm (= 8,500 ppi).