石谷 善博

Local heat transport in two GaxIn1-xN/GaN-heterostructures on sapphire substrates is investigated by microscopic Raman imaging using two lasers of 532 nm (Raman observation) and 325 nm (heat generation and Raman observation), which enables the separation of heat generation and Raman observation positions. It is found that E2(high) and A1(LO) modes of the Ga0.84In0.16N layer exhibit mutually different characteristics, which indicates the analysis of the occupation of the A1(LO) mode is available. E2(high) mode of the GaN layer observed by the 532-nm laser reveals that the transport of the heat energy generated in the Ga0.84In0.16N layer to the GaN under layer is blocked in the high-density area of misfit dislocation in the vicinity of the heterointerface.

© 2019 Author(s). Spatial and temporal diffusive spin dynamics in the transient regime for 10-nm-wide InGaAs/InAlAs quantum wells were investigated using time-resolved optical Kerr rotation (TRKR) microscopy. The transient regime of diffusive spin dynamics is a specific regime which appears after local excitation but before the formation of a spin mode. In this regime, the spin precession frequency induced by the spin-orbit (SO) magnetic field decreases with time since the diffusive velocity decreases as the distribution of spins expands. In this study, by decreasing the spot size of the excitation pulse, we examined the spin dynamics in the transient regime. We carefully analyzed the TRKR signals using time-dependent frequency equations established by Kohda et al. [Appl. Phys. Lett. 107, 172402 (2015)]. All the TRKR signals could be well reproduced by the time-dependent frequency equations. Our analysis reveals that the spin precession frequencies arising from the SO interaction gradually decrease with time, and they allowed the extraction of the Rashba and Dresselhaus SO parameters. We revealed that SO parameters can be extracted accurately via scanning measurements in the diffusive motion of local spin excitation even in the time periods before the appearance of the spin mode.

Longitudinal optical (LO) phonon resonant THz absorption and emission are observed using metal-semiconductor surface stripe structures, where electric dipoles are induced by interface polarization charges. The emission spectrum is affected by the stripe structures, while the peak position is independent. Further, quantum interference of two LO phonon modes and electronic intervalence band transition is found for alloy semiconductors. The present results indicate the feasibility of electro-magnetically induced transparency for optical gain based on LO phonon modes using this composite material.

THz pulse generation for the obliquely crossed optical pump pulses in optical rectification based on the lithium niobate crystal is simulated by finite-difference time-domain method. The beam diameter dependence clearly shows that Cherenkov type radiation with the broad band spectrum is dominant for the beam diameter smaller than 10 mu mathrm{m}, while the non-collinear phase matching with narrow spectrum is dominant for the beam diameter larger than 40 mu mathrm{m}

Population distributions and transition fluxes of the A exciton in bulk GaN are theoretically analyzed using rate equations of states of the principal quantum number n up to 5 and the continuum. These rate equations consist of the terms of radiative, electron-collisional, and phononic processes. The dependence of the rate coefficients on temperature is revealed on the basis of the collisional-radiative model of hydrogen plasma for the electron-collisional processes and theoretical formulation using Fermi's "golden rule" for the phononic processes. The respective effects of the variations in electron, exciton, and lattice temperatures are exhibited. This analysis is a base of the discussion on nonthermal equilibrium states of carrier-exciton-phonon dynamics. It is found that the exciton dissociation is enhanced even below 150 K mainly by the increase in the lattice temperature. When the thermal-equilibrium temperature increases, the population fluxes between the states of n > 1 and the continuum become more dominant. Below 20 K, the severe deviation from the Saha-Boltzmann distribution occurs owing to the interband excitation flux being higher than the excitation flux from the 1S state. The population decay time of the 1S state at 300K is more than ten times longer than the recombination lifetime of excitons with kinetic energy but without the upper levels (n > 1 and the continuum). This phenomenon is caused by a shift of population distribution to the upper levels. This phonon-exciton-radiation model gives insights into the limitations of conventional analyses such as the ABC model, the Arrhenius plot, the two-level model (n = 1 and the continuum), and the neglect of the upper levels.

Although the Fano quantum interference of one longitudinal optical (LO) phonon mode and one electronic continuum of states due to inter-valence band transition has been investigated by the Raman spectra, there is no observation of the interference of multiple phonon modes despite its potential for electromagnetically induced transparency in the far infrared region. In this study, p-type Ga0.5In0.5P films with two main LO modes in the same plane of atomic vibration and one more mode due to CuPt-type ordering are investigated. Asymmetric line profiles, peak energy shifts and peak broadenings in the Raman spectra are analyzed, and the destructive quantum interference of these LO-continuum systems are observed. Discussion with the reference data of p-type GaAs and Si for the one-LO mode system reveals that the asymmetry parameter features the derivatives of the density of the continuum of states in the vicinity of the LO phonon energies and the degree of the electric polarization. It is found experimentally that the interaction between the two main LO modes through the interactions with the continuum states takes place. A(1)(LO) mode due to the ordering affects the spectrum, however the interaction with the main two LO modes through the continuum is small. On the basis of the experimental result, the spectrum features for this alloy with greater hole density are estimated, where the various controls of the absorption spectrum in the phonon energy region are predicted.

The physical mechanism of excitation and deexcitation transitions of nonthermal exciton states in a GaN film is investigated at a measurement temperature of 23 K by time-resolved photoluminescence (PL) analysis involving phonon replica lines of the principal quantum number n = 2 in addition to n = 1 and bound states of the A exciton. A time region of 280 ps after a pulse excitation is mainly analyzed. The emission intensities of the constituent lines are obtained by spectrum fitting. Although the effective exciton temperature of the n = 1 state shows a relaxation time within approximately 150 ps as a previous report, the temperature of the n = 2 state is found to have a longer relaxation time. This is because the n = 2 state strongly couples with the continuum by excitation and deexcitation transfers, while the n = 1 state couples with the donor bound state. These two systems exhibit different dynamic properties. Overall population transfer is the direction of energy relaxation, however, cooling of the upper states is delayed when compared to the lower states by the increase in the excitation transfer rate to the continuum. This dynamics of the exciton has a similarity to that of hydrogen atoms in plasma.

Infrared reflectance spectroscopy is a noncontact measurement method for carrier density and mobility. In this article, the model determination procedure of layer-type nonuniform electron distribution is investigated, since the spectrum fitting hitherto has been conducted on the basis of a multilayer model defined in advance. A simplified case of a high-electron-density GaN layer embedded in a GaN matrix is mainly studied. The following procedure is found to be applicable. The first step is the determination of the high-density layer position in the vicinity of the surface, in the middle region, or in the vicinity of the interface. This is followed by the specification of the sheet electron density and the layer thickness of the high-density region. It is found that this procedure is also applicable to the characterization of two-dimensional electron gases in the vicinity of AlGaN/GaN heterointerfaces. (C) 2016 The Japan Society of Applied Physics

As a basis of the study on exciton stability under a nonthermal equilibrium state, the excitation and deexcitation population fluxes and population densities of several states of the principal quantum number p are calculated using a hydrogen plasma model for various electron excitation densities and temperatures of the lattice, electron, and exciton. It is found that the balance of the excitation and deexcitation population fluxes depends on the p number. At a lower-lattice-temperature region, ladderlike deexcitation flux is dominant for low p states, while the quasi-Saha-Boltzmann relation holds for high p states. At temperatures higher than 150 K, the exciton formation and dissociation fluxes become dominant. Exciton dissociation is enhanced at temperatures higher than approximately 120 K. This process is triggered by the excitation between the states of rho = 1 and 2. High-and low-order states sometimes exhibit different population flow characteristics, which reveal the exciton dissociation dynamics. (C) 2016 The Japan Society of Applied Physics

Interaction of a semiconductor surface or interface and p-polarized infrared light has been investigated from the viewpoint of THz application. In this study the electric dipole absorption at the resonant energy to a longitudinal optical phonon is clearly observed also for s-polarization for lateral Ti/GaN/Ti and Ti/GaAs/Ti stripe structures. Modification of permittivity is found by the analysis of Raman and IR spectra.

Quantum interference between two LO phonon modes and valence band states is investigated by Raman spectra of p-type Ga0.5In0.5P. The spectrum fitting analysis reveals the quantum interference features in asymmetric line shape, broadening, and peak shift. This finding suggests the possibility of the control of the absorption profile in THz frequency region.

We performed a numerical simulation of the spatial behavior of spin precession in a persistent spin helix (PSH) state at high temperatures (>150 K) in a two-dimensional electron gas of GaAs and InGaAs (001)-semiconductor quantum wells (QWs). To describe the spin dynamics of the PSH state at high temperatures, the effect of a cubic Dresselhaus spin-orbit interaction (SOI) that destroys the PSH state was added to the balanced Rashba and linear Dresselhaus SOI. Furthermore, longitudinal optical and acoustic phonon scattering were taken into account in the momentum scattering calculations. The simulation results indicate that the PSH state in the InGaAs QW persists for over 500 ps because of the small effective mass of the electron, even at room temperature. We also reveal that it is closer to the ideal PSH state when the Rashba strength (alpha) is controlled to the renormalized linear Dresselhaus SOI strength (-(beta) over tilde) rather than the linear Dresselhaus SOI strength (-beta). (C) 2015 AIP Publishing LLC.

The increase of InN growth rate by a newly developed two-step precursor generation hydride vapor phase epitaxy (HVPE) was investigated for the preparation of freestanding InN and InGaN substrates. An elevated growth rate was achieved by the complete conversion of InCl generated in the first source zone to InCl3 in the second source zone, by the supply of additional Cl-2. The growth rate reached 12.4 mu m/h at a growth temperature of 600 degrees C, and the rate was observed to decrease above this temperature. Specular lnN layers grown at 650 degrees C exhibited a sharp room temperature photoluminescence peak at 0.73 eV with a bulk electron concentration of 1.2 x 10(18) cm(-3). (C) 2015 Elsevier B.V. All rights reserved.

The interaction of a semiconductor surface or interface and p-polarized infrared (IR) light has been investigated from the viewpoint of terahertz (THz) application. In this study the electric dipole absorption at the resonant energy to a longitudinal optical (LO) phonon of approximately 22 THz achieved using a lateral GaN/Ti stripe structure and s-polarized light, is clearer than the previously reported weak signal obtained using a film structure and p-polarized light. The previously proposed function of permittivity is validated by the analysis of Raman spectra in addition to IR analysis for the present sample structure. In the Raman spectra an additional peak is found at the higher-energy side of the LO phonon peak. The phonon-induced charges at the lateral GaN/Ti interface are greater than those at film interface, causing a greater dipole moment and stronger absorption in the vicinity of the LO phonon energy.

In the present article, we focus our discussion on the carrier dynamics of the scattering and recombination processes of InN films and the related band-edge energy structure. Various reports on this matter are summarized and some issues are reexamined. The analysis result based on the apparent band-band transition matrix element is consistent with a reported effective heavy hole mass of 0.59 (+/- 0.06) m(0). An E-p value related to the transition matrix element of 10-14 eV is thought to be plausible. The ambiguity of the band-edge structure is evaluated by the uncertainty of electron density. The distortion of the conduction band bottom and the ambiguity of the estimation of the many-body effect are discussed. The enhancement of the anisotropic electron-scattering nature with the decrease in residual electron density reveals that the residual electron source has isotropic potentials: point defects or small complexes. Infrared reflectance spectrum analysis reveals the high electron mobility inside grains in spite of the scattering by edge-type dislocations which cause the anisotropic carrier scattering. The recombination processes at low temperatures are dominated by nonradiative processes related to edge-type dislocations, while the thermally activated nonradiative recombination process is independent of the dislocation density. The activation processes and energies of the recombination related to phonon localization are characterized. InN is a peculiar material that has high carrier mobility and a strong electron-phonon interaction, which possibly induces the high nonradiative carrier recombination rate. The control of phonon localization is thus required. (C) 2014 The Japan Society of Applied Physics

We discuss photon-phonon interaction in AlN and GaN thin layer structures by spectroscopic analysis in 20-30 THz region. Optical absorption by the generation of electric dipole moment based on interface polarization charges is found besides interface phonon polariton propagation. The polariton and absorption properties are analyzed.

We investigate the reduction in the efficiency of band-edge radiative recombination in InN by two carrier recombination processes via mid-gap states: radiative recombination via deep states and nonradiative recombination (NR). Because of the small band-gap energy value and the existence of the surface electron accumulation layer, the carrier transition processes via deep states cannot be observed easily. We address this problem by using mid-infrared photoluminescence (PL) measurements, and observe an emission peak around 0.32 eV at room temperature, which we interpret as being caused by transition processes via deep-defect states. Since this emission is weaker than the band-edge emission, the dominant carrier recombination process is concluded to be NR by phonon emission. The NR rate is known to be determined by the NR defect density, carrier transport processes to NR defects, and thermal activation processes of carriers. Carrier transport and capture processes by NR defects are investigated using p-type samples for various carrier mobility values. It is concluded that the NR rate is highly affected by the carrier transport, and that the candidates for the NR defect species are point defects and complexes of acceptor nature. We have also observed the correlation between the thermal conductivity and the band-edge PL intensity. As a result, we have found that the NR rate is highly affected by the carrier transport and thermal activation processes in InN. © 2013 TMS.

Reflectance loss of p-polarized infrared light at longitudinal optical (LO) phonon energies has been reported by Berreman. The origin of this loss has been discussed in view of the absorption by electric dipole moment by the polarization charges at the interfaces and propagation of interface polaritons. However, the existence of the absorption effect is not clear. In this issue, we take an example of GaN films with large polarization charges, and distinguish the two effects on optical spectra by theoretical and experimental analysis. It is found for the first time that the absorption by electric dipoles at the resonant energy with the LO phonon or LO phonon-plasmon coupling (LOPC) mode is identified separately from the interface polariton effect by varying the wavenumber component parallel to the interfaces by attenuated total reflectance analysis. We construct a theoretical expression of the pseudo-dielectric function of thin films around the LO phonon or LOPC mode energies, and show the quantitative proof of the existence of the absorption effect in the experimental spectra. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754555]

Radiative recombination efficiency in InN is lower than the other III nitride binary compounds (GaN, AlN). Particularly, photoluminescence (PL) intensity in p-InN is weaker than that in n-InN by three orders or more. We investigate the nonradiative recombination (NR) processes in InN with respect to carrier density, activation energy of NR processes, and carrier scattering rate of the inside bulk region estimated by infrared (IR) measurements, and show that NR centers of p-InN originate from the same defect species as those of n-type films. We also compare PL properties of N-polar and In-polar samples, and show that NR process is initiated by electron capture by deep levels of accepter nature.