Takaaki Mano

We investigated the effects of insertion layers at the initial stage of growth on the crystalline and electrical properties of lattice-relaxed (metamorphic) InAs films grown on GaAs(111)A substrates. Introducing thin, low-temperature InAs layers at the initial stage, followed by high-temperature growth, led to a noticeable reduction in the peak width of the x-ray rocking curve (XRC) compared with samples grown at fixed (high) temperatures. By introducing hetero-material insertion layers, such as AlSb and InAs/InAlAs superlattice (SL), more pronounced changes were achieved. While the insertion of an AlSb layer realizes a large reduction in the XRC peak width, it also has undesirable effects, namely, an increase in the threading-dislocation density and a decrease in the electron mobility. The electron mobility was improved by the insertion of InAs/InAlAs SL, while the XRC peak width was not reduced.

We grew GaN/InxGa1−xN superlattices (SLs) on GaN/sapphire substrates by atmospheric-pressure metal-organic vapor phase epitaxy (MOVPE) and determined the InxGa1−xN growth rate using high-resolution x-ray diffraction analysis. The use of SL structures substantially reduced the influence of strain relaxation, phase separation, and mixed 2D/3D growth modes, enabling reliable extraction of the effective GaN and InN growth rates within the alloy. By maintaining identical temperature, pressure, and gas-flow balance, we precisely estimated the individual growth rates and proposed a model that explicitly separates incorporation and desorption processes. For low-In composition (xIn < 0.25), the GaN and InN rates exhibit negligible mutual interaction and depend solely on temperature under constant pressure and flow balance. The model successfully predicts InxGa1−xN growth rates using independent incorporation and desorption terms, and its applicability is confirmed for In vapor-phase ratios <0.7 and sufficiently high V/III ratios, while limitations are identified under excess-In or low-temperature conditions. These results provide practical guidelines for optimizing MOVPE growth, contributing to the efficient design of active and SL layers for high-power light-emitting diodes and green laser diodes.

We have studied the oxidation states and photoluminescence (PL) properties of GaAs(100) samples that incorporated buried InGaAs quantum well (QW) structures prepared by molecular-beam epitaxy (MBE). The surfaces of the MBE-grown GaAs(100) samples were controlled so that they were either As-rich c(4 × 4)α, Ga-rich (4 × 6) or Se-terminated (2 × 1) structures prior to oxidation. We found that the As/Ga composition ratio at the initial GaAs surface strongly affects the oxidation processes and the resultant PL properties. An oxidized sample whose initial As-rich surface contains a large amount of As oxide, has a significantly lower PL intensity than As-deficient samples. The reduction of the surface As coverage simply by preparing Ga-rich (4 × 6) and Se-terminated (2 × 1) surfaces has a positive effect on the PL properties, which is maintained even after the oxidation has progressed into deeper layers after longer air exposure, e.g., six months.

This study explores the relationship between spin diffusion, spin lifetime, electron density, and lateral spatial confinement in two-dimensional electron gases hosted in GaAs quantum wells. Using time-resolved magneto-optical Kerr effect microscopy, we analyze how Hall-bar channel width and back-gate voltage modulation influence spin dynamics. The results reveal that the spin diffusion coefficient increases with reduced channel widths, a trend further amplified at lower electron concentrations achieved via back-gate voltages, where it increases up to 150% for the narrowest channels. The theoretical model developed in this work suggests that the spin diffusion coefficient is spatially inhomogeneous across the channel cross section. Near the channel edges, where the electron density is reduced, the spin diffusion coefficient is enhanced due to weaker electron–electron scattering. As a result, narrower channels, which contain a relatively larger proportion of these low-density edge regions, exhibit overall faster spin diffusion. Our results underscore the importance of tuning electron density and spatial geometry to optimize spin transport and coherence, providing valuable design considerations for spintronic devices where efficient spin manipulation is crucial.

Theoretical analysis of the electrostatic force between a metallic tip and semiconductor surface in Kelvin probe force microscopy (KPFM) measurements has been challenging due to the complexity introduced by tip-induced band bending (TIBB). In this study, we present a method for numerically computing the electrostatic forces in a fully three-dimensional (3D) configuration. Our calculations on a system composed of a metallic tip and GaAs(110) surface revealed deviations from parabolic behavior in the bias dependence of the electrostatic force, which is consistent with previously reported experimental results. In addition, we show that the tip radii estimated from curve fitting of the theory to experimental data provide reasonable values, consistent with the shapes of tip apex observed using scanning electron microscopy. The 3D simulation, which accounted for the influence of TIBB, enables a detailed analysis of the physics involved in KPFM measurements of semiconductor samples, thereby contributing to the development of more accurate measurement and analytical methods.

Sulfur (S) termination of III–V semiconductor surfaces is an effective technique for passivating surfaces to prevent oxidation. In this study, we systematically investigated the effects of S termination by (NH₄)₂S _x treatment on enhancement of the photoluminescence (PL) properties of buried InGaAs/GaAs(001) quantum wells (QWs). X-ray photoelectron spectroscopy (XPS) and PL measurements revealed that the (NH₄)₂S _x treatment suppresses the formation of surface oxides, especially arsenic oxides, and enhances the PL intensity of QWs. Clear correlation between the PL and XPS results suggests that greater PL intensity is attributable to a reduction in the number of nonradiative recombination centers at the surface caused by arsenic oxide formation. In addition, from the observed temporal changes in PL intensities, we found that the S-terminated surfaces exhibit long-term and high resistance to surface oxidation by air exposure.

We present our recent development of the surface-emitting quantum cascade laser with a PC (photonic crystal) resonator and a strain-compensated MQW (multiple quantum well) active layer operating at around 4.3 μm. We describe the laser performance mainly from the viewpoint of the design and analysis of the PC resonators, which include both numerical calculations by FEM (finite element method) and analytical calculations using the k·p perturbation theory and group theory. We analyze the resonance quality factor, overlap factor, extraction efficiency, and far-field pattern, and show how the output power and beam quality have been improved by the appropriate design of the PC resonator.

Surface morphology in molecular-beam epitaxy of InAs(001), (111)A, and (111)B has been studied using scanning tunneling microscopy. The surface morphologies of InAs strongly depend on substrate temperature, substrate orientation, and As/In flux ratio. The size and density of two-dimensional InAs islands on the (001) surface decreases and increases, respectively, as the As/In flux ratio is increased. On the other hand, island size (density) is increased (decreased) with the increase in As flux on (111)A and (111)B surfaces. Surface reconstructions on growing surfaces strongly affect the diffusion and incorporation kinetics of In atoms, thereby resulting in the observed surface morphologies.