Atsushi A. Yamaguchi

Theoretical investigation of optical polarization properties in Al-rich AlGaN quantum wells with various substrate orientations

The optical polarization properties of Al-rich AlGaN thin quantum wells on AlN substrates with various substrate orientations are numerically calculated using a 6×6 k⋅p Hamiltonian. The calculation results show that the predominant polarization direction abruptly switches from the in-plane direction to the c-axis direction at an Al composition of ∼0.76 in quantum wells on c-plane substrates. It is also shown that the polarization characteristics drastically change by small inclination of the substrates due to valence band mixing effects. It is found that the use of the vicinal substrates as well as semipolar and nonpolar substrates could be beneficial in improving optical device performance.

Gain Anisotropy Analysis in Green Semipolar InGaN Quantum Wells with Inhomogeneous Broadening

Polarization switching phenomena in semipolar (1122)-oriented InGaN quantum wells (QWs) were theoretically investigated by a newly formulated model considering the effect of In compositional fluctuation. The theoretical model successfully reproduced the reported polarization switching phenomena for both the emission wavelength and the excitation density in blue-green (1122) QWs, and this showed the importance of inhomogeneous broadening effects to understand polarization properties of semipolar quantum wells. Then, the model was applied for pure-green (1122) QWs, and we predicted that optical polarization was kept in the [1123] direction up to the carrier density high enough to create population inversion in such long-wavelength QWs. These results support the possibility for semipolar-oriented pure green InGaN laser diodes with cleaved facet cavity mirrors.

Theoretical investigation on polarization control of semipolar-oriented InGaN quantum-well emission using (Al)InGaN alloy substrates

The polarization properties of InGaN quantum wells on semipolar AlInGaN alloy substrates are calculated numerically using a 6×6⋅k⋅p Hamiltonian. It is shown that the polarization direction of quantum-well emission changes, depending on the substrate alloy composition and quantum-well width, and that this polarization switching is caused by the strain anisotropy and quantum confinement effects in the quantum wells. The calculation results indicate that InGaN or AlInGaN alloy substrates can be beneficial in obtaining desirable polarization properties for the formation of cleaved-facet cavity mirrors in laser diodes on semipolar substrates.

Comparative study of surface recombination in hexagonal GaN and ZnO surfaces

Surface recombination in GaN and ZnO crystals was comparatively investigated using steady-state and time-resolved photoluminescence (PL) measurements. The measurements were performed for various surface orientations (+c, −c, and m-plane surfaces), and the measured PL intensity and lifetime showed distinct dependence on the surface orientation. The dependence clearly indicates that the surface recombination rate is modified by the effects of surface band bending. The results were also verified by numerical analysis using a rate equation model considering the diffusion of photoexcited carriers and their recombination processes on the surface and inside the crystal.

A simple theoretical approach to analyze polarization properties in semipolar and nonpolar InGaN quantum wells

By using a simple theoretical approach, the previously reported experimental results of the polarization properties in semipolar and nonpolar InGaN quantum wells (QWs) were analyzed. On the basis of the k⋅p-perturbation theory, we derived a useful analytical expression for describing the polarization properties of these QWs, and used this expression to analyze experimental data reported from various research groups. Based on these analyses, we predicted that the negative polarization degree, which is favorable for laser diodes with cleaved-facet cavity mirrors, would appear in the blue- or green-InGaN QWs on the lower-angle semipolar planes (30°–40° inclined from the c-plane).

GaN Lateral Overgrowth by Hydride Vapor Phase Epitaxy through Nanometer-Size Channels Fabricated with Nanoimprint Lithography

Epitaxial lateral overgrowth (ELO) has been used for reducing the dislocation density to grow high-quality GaN crystals. In conventional ELO, micrometer-size channels formed on a mask material such as SiO2, where GaN growth starts, are generally used. In the present study, ELO through nanometer-size (50–80 nm) channels is investigated to improve the dislocation reduction ability. Channels are fabricated using nanoimprint lithography and dry etching. We demonstrate for the first time successful hydride vapor phase epitaxy (HVPE)-based GaN ELO growth through nanochannels. In the growth interface, distinct facet structures appear and coalescence between neighboring facets proceeds. The surface of a 20-µm-thick GaN layer becomes flat by the valleys between facet structures being buried. The dislocation density is shown to decrease to approximately 5×107 cm-2 for a 20-µm-thick GaN layer on sapphire. Photoluminescence measurements show a decay time of over 3 times longer than that of a conventional metalorganic chemical vapor deposition (MOCVD) template.

Surface Morphologies and Optical Properties of Si Doped InGaN Multi-Quantum-Well Grown on Vicinal Bulk GaN(0001) Substrates

Morphological and optical properties of Si doped In0.07Ga0.93N multi-quantum-well (MQW) were studied on a vicinal bulk GaN(0001) substrate with low dislocation density. Surface morphology of InGaN MQW was sensitive to the misorientation direction due to the anisotropic step edge structure peculiar to a hexagonal crystal. Appropriate Si doping was useful to suppress instability of the step front and a well-aligned straight step structure was demonstrated for the misorientation direction of [100] with Si doping of 5×1018 cm-3. Low temperature photoluminescence (PL) indicated that good luminescence properties were maintained under the wide range of doping concentration, while PL degradation was observed for heavily doped MQWs. The luminescence properties were discussed based on a self-consistent calculation of the electronic structure of Si-doped MQWs.

Determination of internal quantum efficiency in GaN by simultaneous measurements of photoluminescence and photo-acoustic signals

Internal quantum efficiency (IQE) is usually estimated from temperature dependence of photoluminescence (PL) intensity by assuming that the IQE at cryogenic temperature is unity. III-nitride samples, however, usually have large defect-density, and the assumption is not necessarily valid. In this study, we developed a new method to estimate accurate IQE values by simultaneous PL and photo-acoustic (PA) measurements, and demonstratively evaluated the IQE values for various GaN samples. The results show that the conventional method cannot give accurate IQE values for low-quality samples although it can be valid for high-quality samples, and that our method can always give accurate values.

Analysis of radiative and non-radiative lifetimes in GaN using accurate internal-quantum-efficiency values estimated by simultaneous photoluminescence and photo-acoustic measurements

Radiative and non-radiative recombination lifetimes in III-nitride semiconductors are usually estimated from time-resolved and temperature-dependent photoluminescence (PL) measurements by assuming that internal quantum efficiency (IQE) at cryogenic temperature is unity. In our recent study, we found that the assumption is not necessarily valid, from simultaneous measurements of PL and photo-acoustic (PA) signals. In this study, we estimate accurate lifetimes from the reliable IQE values estimated by the simultaneous PL/PA measurements, and it is found that radiative lifetime in GaN increases in proportion to the 1.5th power of temperature and that non-radiative lifetime shows little temperature dependence although the non-radiative lifetime itself largely depends on sample quality.

Residual Strain Evaluation by Cross-Sectional Micro-Reflectance Spectroscopy of Freestanding GaN Grown by Hydride Vapor Phase Epitaxy

Distributions of residual stresses in the cross-section of as-grown c-plane freestanding GaN substrates grown by hydride vapor phase epitaxy (HVPE) were precisely measured by low temperature cross-sectional micro-reflectance spectroscopy. Based on the measured residual stresses in the cross-section area, the depth profile of in-plane isotropic biaxial intrinsic strain in the as-grown GaN wafer was also established according to the elastic theory for thin films. It is found that the intrinsic strain increases nearly linearly with the increasing distance to the Ga-face of the freestanding GaN substrate, and is greatly reduced by the introduction of three-dimensional (3D) GaN islands growth in the starting stage of the HVPE growth. According to the relationship between the intrinsic strain and threading dislocation density, the current research suggests that the bending of freestanding GaN is caused by the reduction of threading dislocation density along the film growth direction.