Yasuto Miyake

350.9 nm UV Laser Diode Grown on Low-Dislocation-Density AlGaN

We demonstrated a UV-laser diode grown on low-dislocation-density AlGaN. The combination of a low-temperature-deposited AlN interlayer technology and heteroepitaxial lateral overgrowth yielded crack-free and partially low-dislocation-density AlGaN on a grooved GaN substrate. A ridge waveguide was fabricated in the low-dislocation-density region. The lasing wavelength under pulsed current injection at room temperature was 350.9 nm, which is the shortest wavelength ever reported.

High-Efficiency Nitride-Based Light-Emitting Diodes with Moth-Eye Structure

Nitride-based blue light-emitting diodes (LEDs) with a moth-eye structure on the back of a 6H–SiC substrate have been developed. The moth-eye LED has a roughness less than the optical wavelength at the back surface of the SiC substrate fabricated by reactive ion etching (RIE) with CF4 gas. The light extraction efficiency and corresponding output power have been increased to 3.8 times those of a LED with a conventional structure. The experimental findings agree with the results of a theoretical analysis of the effect of the moth-eye structure.

Control of p-type conduction in a-plane GaN grown on sapphire r-plane substrate

Mg-doped p-type a-plane GaN films were grown on high-quality unintentionally doped GaN on +0.5°-off r-plane sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). A maximum hole concentration of 2.0×1018 cm-3 was reproducibly achieved at room temperature, which was higher than the maximum hole concentration of p-type c-plane GaN. The activation energy of Mg acceptors in p-type a-plane GaN with a hole concentration of 2.0×1018 cm-3 was found to be 118 meV by temperature-dependent Hall-effect measurement.

UV laser diode with 350.9-nm-lasing wavelength grown by hetero-epitaxial-lateral overgrowth technology

We have demonstrated a UV-laser diode with a lasing wavelength of 350.9 nm, which has a GaN-AlGaN multiquantum-well (MQW) active layer and was grown on low-dislocation-density Al/sub 0.18/Ga/sub 0.82/N template. The Al/sub 0.18/Ga/sub 0.82/N template was produced by the hetero-epitaxial lateral overgrowth technology on the low-cost sapphire substrate, and has partially low-dislocation density of approximately 2/spl times/10/sup 7/ cm/sup -2/. The lasing operation under pulsed current injection was achieved with the threshold current density of 7.3 kA/cm/sup 2/ and the operating voltage of 10.4 V.

Fabrication of UV devices on various plane substrates

We have fabricated UV-emitters such as UV-light emitting diode (UV-LED) and UV-laser diode (UV-LD) on sapphire substrates. The combination of low-temperature-deposited AlN interlayer and lateral seeding epitaxy (Hetero-ELO) yielded crack-free and low-dislocation-density AlGaN. The light output power of GaN/AlGaN multi-quantum wells active layer based UV-LED monotonically decreased with the increase of threading dislocations. Moreover, we have demonstrated a UV-LD grown on this low-dislocation-density AlGaN. The lasing wavelength under pulsed current injection at room temperature was 350.9 nm. We also present violet and UV-LEDs grown on ZrB2 substrate. The violet LED exhibits excellent linearity of L-I characteristic and sharp single spectrum, and vertical conduction through nitride and ZrB2 interface has been confirmed in the UV-LED. We also present the growth of AlN single crystals by sublimation method.

Physics and simulation of group-III-nitride-based moth-eye LEDs

Group III nitride-based blue-light-emitting diodes having a moth-eye structure at the bottom of the semi-insulating transport 6H-SiC substrate were fabricated. The light extraction efficiency and the corresponding output power were increased 3.8 times compared with those of the LED having the conventional structure. We also study a theoretical simulation using based on the RCWA method with three-dimensional Maxwells equations. The results of theoretical analysis agree with these findings.

Characterization of a-plane AlGaN/GaN heterostructure grown on r-plane sapphire substrate

The anisotropically biaxial strain in a-plane AlGaN on GaN is investigated by X-ray diffraction analysis of the heterostructure of AlGaN and GaN grown on r-plane sapphire. The AlGaN layer with a low AlN molar fraction or small thickness is coherently grown on the GaN layer both along the m-axis and c-axis. An increase in AlN molar fraction or thickness in AlGaN, results in a slight relaxation of AlGaN only in one direction due to tensile stress along the c-axis, which is caused by the underlying GaN layer during the growth. The cause of the relaxation of AlGaN in one direction is thought to be a large anisotropically biaxial stress.

UV laser diode with 350.9 nm-lasing wavelength grown on AlGaN template

We have demonstrated a UV-laser diode with a lasing wavelength of 350.9 nm, which was grown on low-dislocation-density AlGaN template. The lasing operation under pulsed injection was achieved with the threshold current of about 200 mA.

Group III nitrides grown on 4H-SiC (30 3 8) substrate by metal-organic vapor phase epitaxy

The heteroepitaxial growth of a GaN single crystal by metal-organic vapor phase epitaxy on a 4H-SiC (30 3 8) substrate was demonstrated. The crystallographic orientation of GaN was found to be dependent on growth pressure. When the growth pressure was 1000 hPa, the orientation of the GaN single crystal was consistent with that of the SiC substrate, where the c-plane of the GaN was single crystal tilted 54.7° from the surface plane. Then, we fabricated a violet-light-emitting diode (LED) with a GaInN multiple-quantum-well (QW) active layer grown on the GaN layer, which coherently grew on the 4H-SiC (3038 ) substrate. The blue shift of the peak wavelength with increasing injection current of up to 100 mA was confirmed to be two times smaller than that of a conventional LED on a c-plane sapphire substrate due to a low internal polarization.