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Magnetoresistance of a two-dimensional electron gas in Si- and non-doped InAs/AlGaSb quantum wells was measured for a wide range of temperature. At low temperatures, the Shubnikov-de Haas (SdH) oscillation was observed for both samples, and the Fourier analysis of the SdH oscillation reveals that electrons are populated in two subbands for the Si-doped sample, while only the lowest subband is occupied in the non-doped sample. At high temperatures, a weak oscillation with a fundamental field of T was observed in the non-doped sample, which is attributed to the magnetophonon resonances (MPR). For the Si-doped sample, on the other hand, combined oscillations of magnetoresistance due to the magnetophonon and the magneto-intersubband-scattering (MIS) effects were observed.

Electroreflectance (ER) and photoluminescence (PL) experiments have been performed at 10 K on an asymmetric double-quantum-well (ADQW) structure with coupled narrow and wide quantum wells to investigate optical-phonon assisted tunnelling. At a certain external applied voltage, a distinct quenching of the PL intensity for the narrow-quantum-well luminescence is observed. By calibrating the internal electric field through the ER measurements and calculating subband levels in the ADQW structure, the quenching is attributed to resonant optical-phonon assisted tunnelling of electrons.

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.

Electron tunnelling through donor-related states is discussed. This tunnelling process, which occurs well below the threshold voltage for conventional resonant tunnelling into the two-dimensional continuum states of the quantum well, reveals a new type of Fermi edge singularity effect which arises from the Coulomb interaction between the tunnelling electron on the localized site and the Fermi sea of electrons in the emitter layer. A new means of forming laterally confined resonant tunnelling devices is also described. By studying the effect of an applied magnetic field, the additional structure that appears in the current-voltage characteristics of these devices can be unambiguously associated with a lateral quantum mechanical confinement effect.

N-electron eigenstates in circular, elliptical and triangular quantum dots are calculated by numerically diagonalizing the N-electron Hamiltonian. The addition energy spectrum for a circular quantum dot shows good agreement with the experimental results reported by Tarucha et al. In elliptical quantum dots, the degeneracy of the single-particle states is removed, resulting in a transition of the ground state for a quantum dot with four electrons from the spin-polarized half-filled configuration to the spin-singlet state. The states with 3, 6 and 9 electrons in a triangular quantum dot are found to be slightly more stable compared with a circular quantum dot, which is interpreted in terms of a geometrical effect.

A new model for Monte Carlo simulation is proposed for one-dimensional electron gases. In the model, electron energy and momentum are treated as independent variables, and the Wigner distribution function f(k, omega ,t) is simulated instead of the semiclassical distribution function f(k,t). Using this method, the transport responses of one-dimensional electron gases in a quantum wire at room temperature are evaluated in which electrons are interacting with optical phonons only, and the results reveal that the electron temperature increases monotonically with applied electric field without showing anomalous carrier cooling.

A new mechanism is proposed for the additional structure and the change of sign in magnetophonon resonance extrema at high electric field observed by Eaves and co-workers (1984). The mechanism is based on electric-field-induced inelastic inter-Landau-level scattering, LO phonon scattering, which explains the main features of the experimental results quantitatively. The general expression is derived from the Kubo formula and by using the treatment of Barker.