Kazuhiko Hirakawa

We describe our systematic study of the two-dimensional electron mobilities \ensuremath{\mu} of n-type ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As/GaAs heterojunctions, in particular their dependence on the electron concentration ${N}_{s}$ and the temperatures T, in a variety of field-effect transistors in which the impurity locations are precisely controlled to vary \ensuremath{\mu} over the wide range of 5\ifmmode\times\else\texttimes\fi{}${10}^{3}$ to 1.

We investigate the electron heating process in selectively doped AlGaAs/GaAs heterojunctions from magnetotransport measurements at low temperatures. It is shown that the dominant energy-relaxation mechanism of the degenerate two-dimensional (2D) electrons for the electron temperature below 40 K is the emission of acoustic phonons via deformation potential coupling, and that the energy-loss rate is almost independent of 2D electron densities. From a detailed analysis it is derived that the deform

It is shown for the first time that the controlled deformation of the electron wave function leads to modulation of the two-dimensional electron mobility by as much as 56% even when the electron concentration is kept constant. The deformation is controlled by use of two different gating modes (front gating and back gating) in a novel $n$-AlGaAs-GaAs heterojunction field-effect-transistor configuration. The observed modulation of electron mobility is in accordance with the theoretical prediction

We have investigated the radiative decay of hot two-dimensional (2D) plasmons in Al0.3Ga0.7As/ GaAs heterostructures by far-infrared emission spectroscopy and determined the spectral line shape of the radiation. Narrowband plasmon emission lines are obtained in the terahertz regime. The experimentally observed energies of plasmon emission are in good agreement with the results of a recently developed full grating theory. The plasmon emission intensity is found to increase with increasing input e

The concentration Ns of two-dimensional electrons in N-GaAlAs/GaAs systems is studied; Ns shows a systematic decrease when the thickness Wsp of an undoped GaAlAs ‘‘spacer layer’’ is increased. Such a dependence is shown to be well explained by the theory in which the size quantization is taken into account. Furthermore, Ns is studied as a function of gate voltage in field-effect transistor (FET) structures and found to saturate once Ns reaches its limiting value. This phenomenon is strongly depe

We have investigated the far-infrared (FIR) photoinduced resistance change of the two-dimensional electron systems in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}/\mathrm{GaAs}$ heterojunctions in the integer quantized Hall regime. Sensitive photoinduced resistance change $\ensuremath{\Delta}{R}_{\mathrm{xx}}$ is observed only in the vicinity of the quantum Hall states. It is found that the magnitude and polarity of $\ensuremath{\Delta}{R}_{\mathrm{xx}}$ strongly depend on the L

We investigate the high-field transport of two-dimensional (2D) electrons in selectively doped AlGaAs/GaAs heterojunctions. The dependencies of mobilities on external applied electric fields are systematically studied by pulsed Hall measurements up to 1.5 kV/cm on various samples with mobilities ranging from 4×104 to 1.3×106 cm2/V s. Measured dependencies of mobilities on electric fields are compared with theories. It was found that the simple theoretical approach based on the electron temperatu