K. Domen

High‐quality GaN epitaxial layer grown by metalorganic vapor phase epitaxy on (111) MgAl2O4 substrate

We grew GaN crystals by metalorganic vapor phase epitaxy on (111) and (100) MgAl2O4 substrates. We obtained a single‐crystal GaN layer with a specular surface on the (111) substrate. The full width of half‐maximum of the x‐ray rocking curve for a 3.6 μm thick GaN layer was 310 s, comparable to the reported values for GaN on Al2O3 substrates. In the room‐temperature photoluminescence, a band‐edge emission at around 360 nm was dominant. A smooth cleaved (1100) facet of the GaN epitaxial layer was obtained, assisted by the inclined (100) cleavage of the (111) MgAl2O4 substrate. We intend this cleaved facet, which is normal to the surface, to be used as a cavity mirror in a laser diode.

InGaN Laser Diode Grown on 6H–SiC Substrate Using Low-Pressure Metal Organic Vapor Phase Epitaxy

InGaN multiple quantum well (MQW) laser diodes were fabricated on (0001)Si oriented 6H–SiC substrate using low-pressure metal organic vapor phase epitaxy (LP-MOVPE). The laser oscillation was observed above the threshold current of 800 mA at a peak wavelength of 414.3 nm under pulsed current injection at room-temperature. The pulse duration was 300 ns and the repetition frequency was 1 kHz. The threshold current density and differential efficiency were estimated to be 16 kA/cm2 and 0.03 W/A, respectively. The full width at half maximum (FWHM) of the lasing emission lines was between 0.03 nm and 0.21 nm. Streak-shaped far field patterns were clearly observable. The lifetime of the laser diode was more than 5 hours.

Epitaxial Growth of GaN on (1 0 0) β-Ga2O3 Substrates by Metalorganic Vapor Phase Epitaxy

Epitaxial growth of nitride compounds by the metalorganic vapor phase epitaxy (MOVPE) technique is demonstrated for the first time on β-Ga2O3 single crystal substrates, which are near-UV transparent and n-type conductive. High-quality (0 0 0 1) GaN epi-layer with a narrow bandedge luminescence was obtained using a low temperature conductive buffer layer. InGaN multi-quantum well (MQW) structure was also successfully grown. The first blue light-emitting diode (LED) on β-Ga2O3 with vertical current injection is demonstrated.

CRYSTALLOGRAPHIC ORIENTATION DEPENDENCE OF IMPURITY INCORPORATION INTO III-V COMPOUND SEMICONDUCTORS GROWN BY METALORGANIC VAPOR PHASE EPITAXY

This article presents a comprehensive study of the dependence of impurity incorporation on the crystallographic orientation during metalorganic vapor phase epitaxy of III‐V compound semiconductors. We performed doping experiments for group‐II impurities (Zn and Mg), group‐VI impurities (Se and O), and a group‐IV impurity (Si form SiH4 and Si2H6). The host materials were GaAs, Ga0.5In0.5P, and (Al0.7Ga0.3)0.5In0.5P grown on GaAs substrates. We examined the doping efficiency on the surfaces lying between {100} and {111}A/B. Even though we grew epitaxial layers in a mass‐transport‐limited regime, the doping efficiency significantly depended on the orientation, indicating that the surface kinetics plays an important role in impurity incorporation. Comparing our results with other reports, we found that acceptor impurities residing on the group‐III sublattice and donor impurities residing on the group‐V sublattice, respectively, have their own distinctive orientation dependence. Si donors exhibit orientation d...

Analysis of polarization anisotropy along the c axis in the photoluminescence of wurtzite GaN

The polarization of photoluminescence (PL) was investigated on (1100) GaN grown by metalorganic vapor phase epitaxy. We found that the PL intensity and wavelength have polarization dependence parallel and perpendicular to the c axis. We quantitatively analyzed the dependence and found that, since the crystal field of wurtzite GaN along the c axis is strong enough to fix the |z〉 axis of p functions at the c axis, the difference in symmetry between three valence bands appears as the polarization anisotropy in radiative emission, even in bulk GaN.

OPTICAL GAIN FOR WURTZITE GAN WITH ANISOTROPIC STRAIN IN C PLANE

We calculated band structures of (1100)-oriented GaN with various strains. We found that introducing anisotropic strain in the c plane separates heavy- and light-hole bands. We also found that a tensile strain in the (1100) plane makes the light-hole band topmost. These two effects result in a reduction in the density of states at the valence-band edge. In this way we can significantly reduce the transparent carrier density to generate gain.

Origin of nonradiative recombination centers in AlGaInP grown by metalorganic vapor phase epitaxy

We report the first concrete evidence that oxygen causes nonradiative deep levels in (Alx Ga1–x)0.5In0.5P grown by metalorganic vapor phase epitaxy. We doped AlGaInP with O2 and investigated the oxygen and deep level concentrations by secondary ion mass spectroscopy and isothermal capacitance transient spectroscopy. We confirmed that oxygen causes the D3 (thermal activation energy: ET ≅ 1.0 e V for x = 0.7, nonradiative recombination center) and D2 (ET ≅ 0.46 e V) levels, which we previously found in undoped AlGaInP. We demonstrate that the oxygen and nonradiative deep level concentrations are significantly reduced at higher growth temperatures, higher PH3 partial pressures, and substrate offset from (100) toward [011].

Interwell inhomogeneity of carrier injection in InGaN/GaN/AlGaN multiquantum well lasers

A simulation of current flow for an InGaN/GaN/AlGaN multiquantum well (MQW) laser showed that generation of the optical gain is inhomogeneous across the MQW due to inhomogeneous carrier injection. Laser diodes with three and five wells in MQW were compared to experimentally investigate inhomogeneous carrier injection. We found that external quantum efficiency was significantly improved by a reduction in the number of wells from five to three. The result was explained quantitatively by the fact that the five-well laser had a larger internal loss and a poorer internal quantum efficiency due to the inhomogeneous carrier injection.

Room-Temperature Continuous Wave Operation of InGaN Laser Diodes with Vertical Conducting Structure on SiC Substrate

We demonstrated for the first time room-temperature continuous wave operation of InGaN laser diodes with a vertical conducting structure fabricated on a SiC substrate. The threshold current and voltage were 84 mA and 12 V, respectiely, under pulsed operation. The threshold current corresponds to a threshold current density of 5.6 kA/cm2, which is the lowest ever reported with InGaN laser diodes on a SiC substrate. Under continuous wave operation, the threshold current and voltage were 115 mA and 10.5 V, respectiely. The peak lasing wavelength was 408.2 nm. Longitudinal modes of the optical cavity were clearly obsered. The laser oscillation was obsered up to 40°C under CW operation.

Lasing mechanism of InGaN/GaN/AlGaN multiquantum well laser diode

We investigated the lasing mechanism of an InGaN/GaN/AlGaN multiquantum well (MQW) laser diode on SiC grown by low-pressure metalorganic vapor phase epitaxy. Surface emission and edge emission are compared by optical pumping on the laser chip for which electrical pumping was made. Excitation power dependence of photoluminescence (PL) and PL mapping are also taken on the same chip. From the results, we demonstrate that lasing phenomena in our laser are dominated by free carriers and that the observed various stimulated emission lines are caused by the inhomogeneity of the InGaN MQW.