Rie Togashi

Abstract The integration of red-green-blue (RGB) light sources is essential for the development of high-resolution micro-light-emitting diode (micro-LED) displays. In this study, we propose a color-tunable device based on self-assembled InGaN-based nanocolumn LEDs without a patterning process. The nanocolumn LEDs exhibited a color shift from red to orange-yellow, pale green, and further to blue as the driving voltage increased. Micro-electroluminescence measurements revealed that a small wavelength shift was observed within individual nanocolumn regions. Instead, emission spots sequentially turned on, transitioning from red to blue (660-435 nm), elucidating the mechanism of color tuning. Combined with pulse-width-modulation driving, these findings open the pathway for the realization of novel monolithic RGB-LED devices.

Herein, triangular‐lattice nanopillar templates are fabricated on sputter‐deposited AlN/Si (111) substrates. Nanotemplate selective‐area growth via radiofrequency‐plasma‐assisted molecular beam epitaxy is employed to grow GaN nanocolumns on the nanopillars. Well‐ordered uniform GaN nanocolumn arrays are obtained by inserting a migration‐enhanced‐epitaxy grown AlN/AlGaN buffer layer, thereby aligning the polarity of GaN to Ga‐polar. Subsequently, bulk InGaN active layers are grown on top of the GaN nanocolumns with increasing growth time (t_g = 10–20 min). In the initial stage of growth (t_g = 10 min), low‐In‐content InGaN grows on the edges of the six‐sided pyramidal top of the GaN nanocolumns. As the growth progresses, low‐In‐composition InGaN fills the sides between InGaN on the edges, while high‐In‐composition InGaN rapidly grows on the top of the c‐plane nanocolumns. High‐angle annular dark‐field scanning transmission electron microscopy reveals the formation of an InGaN core, covered with a low‐In‐composition InGaN shell, on the top of the nanocolumns. At t_g = 20 min, the photoluminescence spectrum exhibits a peak at 669 nm with a full width at half maximum value of 51.7 nm. Thus, the proposed method is suitable for growing red‐light‐emitting well‐ordered InGaN/GaN nanocolumn arrays on Si.

Abstract In this paper, we report achieving extremely high-density packing in high-voltage vertical gallium nitride (GaN) nanocolumn Schottky barrier diodes (NC-SBDs) through the adoption of a bottom-up process. The NC-SBDs were formed via epitaxial growth using Titanium-mask selective area growth (Ti-SAG) by rf-plasma-assisted MBE (rf-MBE), realizing a packing density equivalent to exceeding 10 million columns/mm². Our fabricated NC-SBDs with a period of 300 nm, a diameter of 250 nm, and a drift length of 1.3 μm demonstrated a breakdown voltage (BV) of 260 V with an on-resistance of 2.0 mΩcm², yielding an excellent figure of merit of 33.8 MW/cm² for nanocolumn-based high-voltage devices. We also discuss dielectric reduced surface field effect and impurities within the nanocolumns as potential factors contributing to the achievement of higher BV devices.

Abstract A red InGaN-based nanocolumn micro μLED with an emission diameter of ϕ2.2 μm was demonstrated to achieve an on-wafer external quantum efficiency (EQE) of 2.1% at the peak wavelength of 615 nm. The LED was fabricated by repeating the electrode process on the same nanocolumn pattern area and reducing the emission diameter from ϕ80 to ϕ2.2 μm. The peak EQE, which was maximized at ∼25 A cm⁻², increased by decreasing the emission diameter from 1.2% to 2.1%. This behavior, which differs from that of InGaN-film LEDs, is characterized as a unit of independent nano-LEDs with passivated sidewalls of nanocolumn LEDs.

Abstract In this study, the growth behavior of Indium gallium nitride (InGaN)-based nanocolumn arrays was investigated, and red emission nanocolumn micro-light emitting diodes (μ-LEDs) were fabricated. The internal structure of the InGaN/GaN superlattice (SL) layer under the multiple-quantum-well (MQW) active layers was evaluated using scanning transmission electron microscopy (STEM) analysis. It was revealed that the InGaN crystal plane at the top of the nanocolumn changed from the c-plane, (1-102) plane, to the (10-11) plane as the number of SL pairs increased. A semipolar (10-11) plane was completely formed on top of the nanocolumn by growing InGaN/GaN SLs over 15–20 pairs, where the InGaN/GaN SL layers were uniformly piled up, maintaining the (10-11) plane. Therefore, when InGaN/AlGaN MQWs were grown on the (10-11) plane InGaN/GaN SL layer, the growth of the (10-11) plane semipolar InGaN active layers was observed in the high-angle annular dark field (HAADF)-STEM image. Moreover, the acute nanocolumn top of the (10-11) plane of the InGaN/GaN SL underlayer did not contribute to the formation of the c-plane InGaN core region. Red nanocolumn μ-LEDs with an φ12 μm emission window were fabricated using the (10-11) plane MQWs to obtain the external quantum efficiency of 1.01% at 51 A cm⁻². The process of nanocolumn μ-LEDs suitable for the smaller emission windows was provided, where the flat p-GaN contact layer contributed to forming a fine emission window of φ5 μm.

Abstract Thermodynamic analyses of β-Ga₂O₃ growth by both ozone and plasma-assisted molecular beam epitaxy were performed. In either case, the growth mechanism was found to differ depending on whether the input VI/III ratio was above or below 1.5. Under O-rich conditions (VI/III > 1.5), the driving force for β-Ga₂O₃ growth (ΔP_Ga2O3) was determined to increase linearly with increasing Ga input partial pressure (P°_Ga) because almost all the supplied Ga was used for growth of the β-Ga₂O₃. In contrast, Ga-rich conditions (VI/III < 1.5) caused ΔP_Ga2O3 to decrease. Etching of the β-Ga₂O₃ occurred with increasing P°_Ga due to the formation of volatile Ga₂O. This work also demonstrated that the use of ozone allowed growth at higher temperatures than the use of O radicals. The calculated results were in good agreement with experimental values, indicating that β-Ga₂O₃ growth by molecular beam epitaxy can be explained by thermodynamics.

The thermal and chemical stabilities of group-III sesquioxides (Al2O3, Ga2O3, and In2O3) were comparatively investigated at an atmospheric pressure at heat treatment temperatures ranging from 250 to 1450 °C in a flow of either N2 or H2. In a flow of N2, the thermal decomposition of α-Al2O3 was not observed at the temperatures investigated, while the decompositions of β-Ga2O3 and c-In2O3 occurred above 1150 and 1000 °C, respectively, with no generation of group-III metal droplets on the surfaces. In contrast, the chemical reactions of α-Al2O3, β-Ga2O3, and c-In2O3 began at low temperatures of 1150, 550, and 300 °C in a flow of H2. Thus, the presence of H2 in the gas flow significantly promotes the decomposition of group-III sesquioxides. The order of thermal and chemical stabilities (α-Al2O3 C β-Ga2O3 > c-In2O3) obtained experimentally was verified by thermodynamic analysis, which also clarified dominant decomposition reactions of group-III sesquioxides.

In this work, the first-ever growth of cubic-In2O3 at 1000 °C by halide vapor phase epitaxy (HVPE) was achieved, using gaseous InCl and O2 as precursors in a N2 flow. The growth rates of In2O3 layers on (001) β-Ga2O3 and (0001) sapphire substrates were 4.1 and 5.1μm/h, respectively, even at the high growth temperature applied, which are approximately half the growth rate of β-Ga2O3 homoepitaxially grown by HVPE at 1000 °C. Theoretical thermodynamic analyses were also conducted, and results confirmed the growth of In2O3 at temperatures above 1000 °C by HVPE. The as-grown In2O3 layers were light yellow-green in color. The In2O3 layers grown on the (001) β-Ga2O3 and (0001) sapphire substrates exhibited an optical absorption edge at about 370 nm, in addition to n-type conductivity with electron concentrations of 2.7 ' 1018 and 1.7 ' 1018cm%3, and electron mobilities of 16.2 and 22.7cm2V%1 s%1, respectively.

Tri-halide vapor phase epitaxy (THVPE) of thick GaN using GaCl3 was investigated for fabricating low-cost, high-crystalline-quality GaN substrates instead of the conventional manufacturing method of GaCl-based hydride vapor phase epitaxy (HVPE). The growth rate and upper growth temperature limit of GaN using THVPE were found to be much higher than those obtained using conventional HVPE under the same growth conditions. Drastic reduction in the number of dark spots measured by cathodoluminescence at room temperature was observed for the high-temperature-grown GaN layer on the (000-1) GaN/sapphire template due to the enhancement of precursor migration on the growing surface. It was found that the incorporation of impurities such as O, C, and Cl can be reduced even on the N-polarity GaN by increasing the growth temperature. The possibility of enlargement of the crystal diameter by growing the N-polarity GaN layer using THVPE was also proposed.

The formation mechanism of AlN whiskers on sapphire substrates during heat treatment in a mixed flow of H₂ and N₂ was investigated in the temperature range of 980–1380 °C. AlN whiskers grew above 1030 °C after covering the sapphire surface with a thin AlN layer. The existence of pits on the sapphire surface beneath the thin AlN layer was observed. Both AlN whisker and pit densities of samples were on the same order of 10⁸cm^-2. These results suggested the following mechanism. First, the sapphire surface reacts with H2, and the generated Al gas reacts with N2 to form a thin AlN layer on sapphire. Then, the sapphire surface reacts with H2 diffusing to the AlN/sapphire interface. The Al gas escapes through dislocations in the AlN layer to leave pits on the sapphire surface, and finally reacts with N2 to form AlN whiskers on the top surface.

Thick Si-doped AlN layers were homoepitaxially grown by hydride vapor phase epitaxy on AlN(0001) seed substrates. Following the removal of the seed substrate, an n-type AlN substrate with a carrier concentration of 2.4 ' 1014cm%3 was obtained. Vertical Schottky barrier diodes were fabricated by depositing Ni/Au Schottky contacts on the N-polar surface of the substrate. High rectification with a turn-on voltage of approximately 2.2V was observed. The ideality factor of the diode at room temperature was estimated to be >8. The reverse breakdown voltage, defined as the leakage current level of 10%3A/cm2, ranged from 550 to 770V.

Thick high-purity β-Ga2O3 layers of high crystalline quality were grown homoepitaxially by halide vapor phase epitaxy (HVPE) using gaseous GaCland O2 on (001) β-Ga2O3 substrates prepared by edge-defined film-fed growth. The surface morphology and structural quality of the grown layerimproved with increasing growth temperature. X-ray diffraction ω-rocking curves for the (002) and (400) reflections for the layer grown at 1000 °C had small full widths at half maximum. Secondary ion mass spectrometry and electrical characteristics revealed that the growth of high-purity β-Ga2O3 layers with low effective donor concentration (Nd - Na < 10¹³cm^-3) is possible by HVPE.

The influence of the source gas supply sequence prior to growth and the NH3 input partial pressure (PoNH3) on the nucleation of InN islands during the initial stages of hydride vapor phase epitaxy on a nitrided (0001) sapphire substrate was investigated. The crystalline quality of the InN layer after subsequent lateral growth was also examined. When NH3 was flowed prior to growth, single-crystal hexagonal InN islands formed. When InN was grown with a higher PoNH3, the number of InN islands decreased remarkably while their diameter increased. The crystalline quality of InN grown on the hexagonal islands with a high PoNH3 significantly improved with increasing growth time. A strong PL spectrum was observed only from InN layers grown with a high PoNH3. It was thus revealed that an NH3 preflow and a high PoNH3 are effective for producing InN with high crystalline quality and good optical and electrical properties.

Orientation dependent decomposition of sapphire substrates and resultant AlN formation during heat treatment in an atmospheric-pressure mixedgas flow of H2 and N2 was investigated within the temperature range 980–1480 oC. AlN was formed on sapphire in the temperature range 1030–1430 oC for c-, a-, and m-plane sapphire, and 980–1430 C for the r-plane sapphire. At 1480 C, AlN was not formed, and onlysapphire was decomposed by H2 with the ranking of m- > r- > a- > c-plane. The ranking was contrary to that of the amount of AlN formation at1380 oC, which occurred by the reaction of gaseous Al generated by the sapphire decomposition and N2. This discrepancy was due to the shape of AlN formed on sapphire; whisker-like AlN does not protect c- and a-plane sapphire from decomposing, while layer-like AlN protects r - andm-plane sapphire from decomposing.