Studies of field-effect control of the high mobility electrons in MBE-grown selectively doped GaAs/n-Al x Ga1-x As heterojunctions are described. Successful fabrication of a new field-effect transistor, called a high electron mobility transistor (HEMT), with extremely high-speed microwave capabilities is reported.

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Heterostructure (GaAs-Al"Ga~-"As) Hall-bar devices with short cross gates are studied in the quantum Hall regime. A potential barrier introduced by the gate causes back scattering of edge currents, making steplike structures in the curves of the Hall and the diagonal resistances versus the gate bias voltage. The Hall resistance on either side of the gate deviates largely from expected quantized values, indicating a nonequilibrium occupation of edge states and its stability over a macroscopic distance (50 pm). An essential role of imperfect voltage contacts in probing the nonequilibrium edge currents is noted.The most fundamental characteristic of the integral quantum Hall effect (QHE) is that, when current I is transmitted through a channel of two-dimensional electron gas (2D EG) where bulk Landau levels are occupied, a quantized voltage of (It/ve )I appears across the channel, where v is the number of the filled Landau levels, h is the Planck constant, and e the unit charge. ' Significant deviation of the quantized Hall voltage has not been reported even in the studies on small devices of micron and submicron scales or on samples with an electrondensity discontinuity.We report here a significant violation of the QHE, which occurs when a potential barrier is introduced in a 2D EG channel by a short cross gate to cause a back scattering of electrons. According to our interpretation, the deviation of the Hall voltage is a consequence of a nonequilibrium occupation of edge states and imperfect voltage contacts that selectively probe different edge states. The experimental results further indicate that edge currents travel ballistically over a surprisingly long distance (50 pm).

We fabricated non-recessed-gate enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistors (HEMTs) with a gate length L g of 120 nm. As gate metals, Ni/Pt/Au and Mo/Pt/Au were used. The Ni/Pt/Au-gate HEMTs with rapid thermal annealing (RTA) at 500°C were normally-off at a gate-source voltage V gs of 0 V, indicating E-mode operation. Moreover, the Mo/Pt/Au-gate HEMTs also showed E-mode device operation without RTA. The fabricated E-mode HEMTs with both gate metals showed high RF performance. We obtained a cutoff frequency f T of more than 50 GHz and a maximum oscillation frequency f max of approximately 100 GHz.

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GaAs/AlAs quantum wires (QWRs) were found to be naturally formed by a regularly corrugated AlAs/GaAs interface and a flat GaAs/AlAs interface in an AlAs/GaAs/AlAs quantum well with a well width of 3.3 nm grown on a (775)B GaAs substrate by molecular beam epitaxy. The lateral period and vertical amplitude of the AlAs/GaAs interface corrugation were 12 nm and 1.2 nm, respectively. The QWRs were formed side by side with an extremely high density of 8×105 QWRs/cm. A photoluminescence peak at λ=715 nm from the QWRs with a cross section of about 12×3 nm2 showed a polarization degree of (I ∥ - I ⊥) / (I ∥ + I ⊥) = 0.11 and a very small full width at half maximum of 15 meV at 4.2 K.

GaAs/Al0.3Ga0.7As quantum wells (QWs) grown on (411)A-oriented GaAs substrates by molecular beam epitaxy (MBE) showed extremely flat interfaces over a macroscopic area (about 200 µm φ) even for the case of no growth interruption, which is mainly due to the intrinsically large migration of Ga atoms and layer growth in the step-flow mode on the (411)A plane. Photoluminescence linewidths at 4.2K were almost the same as or better than the narrowest linewidths reported for GaAs/AlGaAs and GaAs/AlAs QWs grown on GaAs (100) substrates with growth interruption at each GaAs/AlGaAs(AlAs) interface. Only one sharp luminescence peak was observed for each QW on the (411)A substrates, in contrast with three luminescence peaks for the QWs on the (100) substrates, indicating that extremely flat and uniform interfaces over a macroscopic area of laser excitation (200 µm diameter) are realized in the GaAs/AlGaAs QWs grown on (411)A GaAs substrates.