Kazuhiko Horino

BIAXIAL STRAIN DEPENDENCE OF EXCITON RESONANCE ENERGIES IN WURTZITE GAN

We have systematically studied the strain dependence of the free-exciton resonance energies in wurtzite GaN by photoreflectance measurements using well-characterized samples. The experimental data have been analyzed using the appropriate Hamiltonian for the valence bands in wurtzite GaN and determined the values of the crystal field splitting, the spin–orbit splitting, the shear deformation potential constants, and the energy gap in the unstrained crystal. Discussions are given on the strain dependence of the energy gaps, of the effective masses, and of the binding energies for the free-exciton ground states as well as on the valence-band parameters.

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.

Effects of biaxial strain on exciton resonance energies of hexagonal GaN heteroepitaxial layers

Exciton resonance energies of hexagonal (h‐) GaN(0001) epilayers were determined by a combination of high‐resolution modulated photoreflectance methods. The results were analyzed thoretically using the Luttinger‐Kohn type Hamiltonian for the valence bands under the in‐plain biaxial stress, and we obtained the shear deformation potential constants and energy gap in unstrained crystal. Occurrence of the anticrossing of B and C valence bands in tensile biaxially strained h‐GaN was suggested.

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.

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.

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.

Influence of a piezoelectric field on the electron distribution in a double GaN/Al0.14Ga0.86N heterojunction

C–V profiling of Al0.14Ga0.86N/GaN heterojunctions was performed. It was found that a heterojunction with the Al0.14Ga0.86N layer on top increases the electron concentration at the Al0.14Ga0.86N/GaN interface, while the reversed structure with the GaN layer on top decreases it. In accordance with this result, an Al0.14Ga0.86N/GaN double heterojunction was found to experience a strongly asymmetric electron distribution with an enhancement of the electron concentration at the interface closest to the sample surface. This effect is attributed to the presence of a piezoelectric field redistributing the electrons in the heterostructure.

OPTICAL CHARACTERIZATION OF THE E2 DEEP LEVEL IN GAN

The correspondence between the E2 level (∼Ec−0.55 eV) in n-type GaN undergoing thermoionization and photoionization was established. The optical cross section in the vicinity of the threshold for photoionization of this level was measured by means of capacitance transient spectroscopy. Analysis using the formulation of Chantre yielded the optical activation energy, Eo=0.85 eV, and the Franck–Condon parameter, dFC=0.30 eV at 90 K.The correspondence between the E2 level (∼Ec−0.55 eV) in n-type GaN undergoing thermoionization and photoionization was established. The optical cross section in the vicinity of the threshold for photoionization of this level was measured by means of capacitance transient spectroscopy. Analysis using the formulation of Chantre yielded the optical activation energy, Eo=0.85 eV, and the Franck–Condon parameter, dFC=0.30 eV at 90 K.