Bei Ma

Dielectric absorption of s-polarized infrared light resonant to longitudinal optical phonon energy incident on lateral (0 0 0 1) GaN/Ti stripe structures

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.

Depth profile characterization technique for electron density in GaN films by infrared reflectance spectroscopy

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.

Introducing of biexciton processes into exciton dynamics simulation for GaN based on collisional phononic and radiative model

Excitation and deexcitation dynamics of excitons and biexcitons are calculated on the basis of the collisions of excitons, electrons, and phonons. Fröhlich, deformation, and piezo-electric interactions are taken into account for the phonon collision system. A set of rate equations on populations of exciton states of several principal quantum numbers (p), free electrons, biexciton states of hydrogen like X1Σg+ and several excited states is solved under steady state condition. The temperatures of electron, phonon, and excion are varied independently to represent the non-thermal equilibrium states. As a result, the dissociation of excitons and biexcitons are found to be dominated by phonon temperature rather than other temperatures of electron and exciton. The exciton states p=1 decreases drastically at higher temperatures than approximately 150 K, which is caused by the excitation from p=1 to p=2 and consecutive ladder like excitations to upper states. This dissociation causes the decrease in biexciton population.

Quantum interference of three LO modes in p-type Ga 0.5 In 0.5 P: Contribution of a trigonal phonon mode

Electro-magnetic Induced Transparency (EIT) has been studied as a mechanism of lasers without population inversion. Little attention has been given to EIT on phonon systems, while it has a potential of application to EIT and thus THz laser and modulator. Quantum interference of multiple longitudinal optical (LO) phonon modes and the continuum of inter-valence band transition is studied in this report. p-type Ga0.5In0.5P alloy system is studied by Raman spectroscopy. The detailed analysis by spectrum fitting reveals the superposition of characteristic asymmetric profile and the contribution of a trigonal phonon mode due to ordering effect of Ga0.5In0.5P. These results indicate that Ga0.5In0.5P is a possible material of EIT on phonon systems. It is expected that the change of the degree of ordering leads to the control of quantum interference for application of EIT.

Excitation and deexcitation dynamics of excitons in a GaN film based on the analysis of radiation from high-order states

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.

Raman study of the quantum interference of multiple discrete states and a continuum of states in the phonon energy region of semiconductors: examples of p-type Ga0.5In0.5P films

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.

Theoretical investigation of nonthermal equilibrium exciton dynamics in GaN using hydrogen plasma model

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 p = 1 and 2. High- and low-order states sometimes exhibit different population flow characteristics, which reveal the exciton dissociation dynamics.