Shigehiko Sasa

Breakdown of the quantum Hall effect due to electron heating

Abstract The breakdown of the Quantum Hall effect observed on GaAs-AlGaAs samples is quantitatively explained in terms of peculiar electron-heating characteristic to the Quantum Hall regime. The current instability originates from an S-shaped current-voltage characteristic. Further, exponential dependence of the dissipative conductivity on the electric field, observed in the lower field range, is interpreted as a consequence of the finite size of the wave function of localized states in the presence of potential gradient.

High-reliability InGaP/GaAs HBTs fabricated by self-aligned process

We report the achievement of high-reliability self-aligned InGaP/GaAs heterojunction bipolar transistors (HBTs) with an activation energy of 2.0 eV and a time to failure of 10/sup 6/ hours at a junction temperature of 200/spl deg/C. The reliability level is comparable to other devices in practical use. We also show a possible degradation mechanism of AlGaAs/GaAs HBTs.<<ETX>>

Temperature Dependence of Electron Mobility in InGaAs/InAlAs Heterostructures

Temperature dependence of the electron mobility in modulation-doped InGaAs/InAlAs single heterostructures has been investigated by Hall effect measurements in the temperature range from 15 K to 300 K in order to clarify the scattering mechanisms of the electrons. Two kinds of samples are used with doping densities in the InAlAs barrier layer, ND=3×1017 cm-3 and 1×1018 cm-3. The measured electron mobility is compared with the calculated values by taking into account the scattering by InAs-like and GaAs-like LO phonons in the InGaAs channel layer, in addition to the acoustic deformation potential, piezoelectric, ionized impurity, alloy disorder and interface roughness scatterings. The calculated electron mobility shows a good agreement with the experimental data when the alloy disorder potential is assumed to be about 0.7 eV.

MBE growth of InGaAs-InGaAlAs heterostructures for applications to high-speed devices

Abstract InGaAs-In(Ga 1− x Al x )As (0⩽ x ⩽1) multilayer heterostructures, lattice-matched to InP, were grown with good reproducibility by a new MBE technique using a pulsed molecular beam method. Electronic properties of the InGaAs-In(Ga 1− x Al x )As heterostructures, including selectively doped InGaAs/N-InAlAs heterostructures and InGaAs-InAlAs resonant tunneling barrier structures, are described, and their high potential for applications to high-speed devices is discussed.

Influence of disordered contacts on the four-terminal measurements of integral quantum Hall effects

Abstract Four-terminal resistance measurements are made on GaAs-AlGaAs heterostructure Hall-bar devices with and without a cross gate. When electrons are injected from disordered contacts, anomaly is observed in both longitudinal and Hall resistances in the integer quantum Hall regime. The results are interpreted by an analysis of a contact originally developed by Buttiker.

InGaAs/AWAsSb Heterostructures Lattice-Matched to InP GRown by Molecular Beam Epitaxy

We have successfully grown InGaAs/AIAsSb quantum-well (QW) structures lattice-matched to InP by molecular beam epitaxy for the first time. We studied the band-edge discontinuity and the interface abruptness of these heterostructures. A cross-sectional lattice image of InGaAs/AlAsSb QWs taken along the [100] axis showed atomically smooth heterointerfaces. The photoluminescence (PL) peak energy of the 20-nm-thick InGaAs well (0.758 eV) was lower than that of InGaAs bulk (0.799 eV), indicating that the InGaAs/AlAsSb system has a staggered lineup. The conduction band-edge discontinuity, ΔE c , was evaluated to be 1.74 ± 0.04 eV, which was derived from parameter fitting to the 4.2 K PL peak energy shifts of QWs as a function of InGaAs well width between 2.1 nm and 20 nm. The corresponding valence band-edge discontinuity, ΔEv, was 0.07 ± 0.02 eV. We also fabricated a resonant tunneling barrier structure of InGaAs (4.4 nm)/AlAsSb (2.9 nm), and obtained a very high peak-to-valley current ratio of 15 at 300 K.

Characterization of GaAsSb/InAlAs quantum-well structures lattice-matched to InP grown by molecular beam epitaxy

We have successfully grown GaAsSb/InAlAs quantum-well (QW) structures lattice-matched to InP by molecular beam epitaxy for the first time, to our knowledge, using dual As4 beams. The band-edge discontinuity of the GaAsSb/InAlAs heterointerface was estimated by the dependence of photoluminescence (PL) peak energy on the well width at 4.2 K. We estimated the valence band-edge discontinuity (ΔEv) to be about 0.78 eV. We also studied the interface roughness using PL measurements and transmission electron microscopy (TEM). The PL linewidths from GaAsSb/InAlAs QWs agreed with the linewidth calculated assuming a one-monolayer fluctuation in well width. The interface roughness was expected to be one monolayer in these GaAsSb/InAlAs QWs. We confirmed this by examining the cross section of QW by TEM.

Quantitative analysis of the role of contacts in the measurements of integral quantum Hall effects

Heterostructure (GaAs/AlGaAs) Hall-bar devices with a short cross gate are studied in a quantum Hall regime. Anomalous longitudinal resistances and deviations of the Hall resistance from an expected quantized value are quantitatively analyzed in terms of properties of contacts. Reverse-field reciprocity symmetry is observed to demonstrate symmetric properties of contacts.

Electronic properties of two-dimensional electron gas in pseudomorphic InxGa1−xAs/N-In0.52Al0.48As heterostructures

Abstract The two-dimensional (2D) electron concentration and mobility in pseudomorphic In x Ga 1- x As/N-In 0.52 Al 0.48 As ( x = 0.53, 0.64, and 0.75) heterostructures were studied. The electron concentration increased with the InAs mole fraction, x . This increase agreed with theoretical values when the variation in the conduction band discontinuity including biaxial compressive strain effect and in the 2D density of states was considered. Although the mobility increased with x at 300 K, the low temperature mobility (77 and 4.2 K) had a maximum at x = 0.64. Preliminary 1 μ m gate HEMTs using these heterostructures exhibited transconductances as high as 400 mS/mm for x = 0.53, 460 mS/mm for x = 0.64, and 560 mS/mm for x = 0.75.

Non-equilibrium population of edge states and a role of contacts in the quantum hall regime

Abstract Heterostructure (GaAs/AlGaAs) Hall-bar devices with a short cross gate are studied in a quantum Hall regime. Anomalous longitudinal resistances and deviations of the Hall resistance from quantized values are interpreted as a consequence of non-equilibrium distribution in edge states. The results ar analyzed in terms of properties of contacts.