H. Momose

Magnetophonon resonances in quantum wires.

Magnetophonon resonances are studied in quantum wires with square confinement potential. Magnetoconductivity calculated by using the Kubo formula is found to have two components; one related to the current carried by electron hopping motion between the localized cyclotron orbits through electron-phonon interaction, and the other caused by the current carried by electron motion affected by the confinement potential. The latter, σ po, is found to be two orders of magnitude greater than the former, σ ep, around the fundamental resonance condition in the case of a GaAs quantum wire whose width is 200A and barrier height is 100meV.

Selective Oxidation of AlGaAs for Photonic Crystal Laser

We investigated the feasibility of the selective oxidation of AlGaAs for a novel laser diode (LD) made of two-dimensional photonic crystal (PC). The oxidation was successfully performed in an array of small and deep air holes without any unexpected phenomenon. The oxidation extent was uniform vertically and horizontally around the air holes. It was not affected by crystallographic orientation or steam gas flow direction. Thus, there are no significant difficulties in fabricating the device structures of the proposed PC-LD by selective oxidation.

Reduced Static Dielectric Constant Estimated from Exciton Binding Energy in Dilute Nitride Semiconductors

Dilute nitrides are novel III–V semiconductors that have been developed for photonic and electronic devices. Although they have been investigated, some of their characteristics have not been established yet. In this study, the static dielectric constant of dilute nitrides was experimentally estimated from the exciton binding energy. The results imply that nitrogen incorporation in III–V semiconductors decreases both the static dielectric constant and bandgap. This behavior is opposite to that of conventional semiconductors.

Low-dimensional systems in ultra-high magnetic fields: magnetic-field-induced type I to type II transitions in short-period semiconductor superlattices

We present a review on the recent study of the type I to type II transition in short-period superlattices of GaAs/AlAs by means of cyclotron resonance and interband magneto-optical spectroscopy in pulsed high magnetic fields up to 500 T. In the magneto-photoluminescence spectra of excitons in , the magnetic-field-induced type I to type II transition was observed with and without the simultaneous application of high pressure. The behaviour of the transition varies depending on the thickness of the AlAs layers. In cyclotron resonance of , the resonance peak at the X minima was observed in the type II regime for n smaller than 14, whereas the resonance at the point was observed for n>15. It was found that the angular dependence of the peak position does not obey the simple cosine dependence due to the subband mixing in high magnetic fields. From the angular dependence, the effective masses at the X point were determined. In high-field cyclotron resonance measurements at 129 meV up to 400 T for n = 16 (type I), the resonance of the X minima expected at around 260 T was indiscernible, despite the fact that the transition should have occurred at lower fields.

Angular dependent cyclotron resonance in short period (GaAs)n/(AlAs)n superlattices

Abstract Cyclotron resonance (CR) measurements have been carried out to evaluate the effective masses of electrons at the AlAsX point of the Brillouin zone in short period (GaAs)n/(AlAs)n superlattices (SLs) with n = 8–14. The longitudinal effective mass was deduced to be m1 = 1.04m0 by analyzing a deviation of CR position from the cos φ-dependence for tilted magnetic fields in (GaAs)14/(AlAs)14 SL.

Magnetophonon resonances in quantum wires

The authors have investigated the magnetotransport properties of quantum wires which were fabricated on high-mobility GaAs-GaAlAs modulation-doped heterostructures using laser holography and electron beam lithography. For low temperatures, regular Shubnikov-de Haas oscillations and an additional magnetoresistance peak are observed; the peak occurs each time the cyclotron diameter matches the wire width. For high temperatures (106 K<T<175 K), however, all these peaks are washed out and new magnetoresistance structures are revealed; these are due to the occurrence of magnetophonon scattering in the wire, scattering which is used to determine the 1D quantization energies.

Enhanced electron mobility in novel side-gated quantum wire structures

We have investigated electron transport in side-gated single quantum wires. The wires were produced by deep etching in GaAs/AlGaAs single and quantum well heterostructures. With increasingly negative gate voltage we observed a clear transition from a two-dimensional electron gas (2DEG) to a quasi-one-dimensional electron gas (1DEG). For the first time, enhanced mobility transport is detected in the GaAs/AlGaAs quantum well wire structure whereas a mobility loss occurs in the GaAs/AlGaAs single-heterostructure wire. The controversial behaviour is attributed to electron-density-related screening effects as well as gate-voltage-induced changes of the confinement potential in the growth direction favouring the quantum well wire structure in terms of high-electron-mobility 1DEG transport.

Mobility enhanced 1DEG electron transport in side gated quantum wire structures

Abstract We report on recent investigations of electron transport characteristics in the transition regime from a two-dimensional (2DEG) to a one-dimensional electron gas (1DEG). Electron transport is studied in novel deeply etched side gated quantum wire structures. We observed enhanced electron mobility transport with increasing one dimensionality in a GaAs/AlGaAs quantum well wire structure. However, no mobility enhancement is measured in a GaAs/AlGaAs single heterostructure wire. This controversial behavior is attributed to electron density related screening effects as well as gate voltage induced changes of the confinement potential.

Magnetophonon resonance in quantum wires

The magnetophonon resonance (MPR) effect has been used since the mid-1960s to investigate effective mass, optical-phonon energy and electron-phonon interaction in bulk III–V compounds [1–5]. Several authors have also studied MPR effect in two-dimensional (2D) systems, such as GaAs/AlGaAs, GaInAs/InP, and GaInAs/AlInAs heterostructures [6]. The MPR effect manifests itself as an oscillatory behavior of transverse-conductivity σ xx as a function of applied magnetic field B. At high magnetic fields, electrons move perpendicularly to both electric and magnetic fields, and the transverse-current is carried by electron-hopping motion induced by some scattering mechanisms. The MPR effect arises from the resonant absorption or emission of longitudinal-optical (LO) phonons by electrons when the Landau level is sharp and well defined, and scattering by LO phonons makes a significant contribution to limiting electron mobility. As the magnetic field increases, a small part of σ xx resonantly increases each time the integral multiple of the Landau level spacing becomes equal to the LO phonon energy, because the scattering of the electrons takes place with resonant absorption or emission of LO phonons. In bulk materials and 2D systems, the oscillatory part of σ xx is therefore proportional to the Frohlich coupling constant α, and the resonance condition where σ xx becomes maximal is written as