M B Stanaway

Electrical characterization of molecular beam epitaxial GaAs with peak electron mobilities up to ≊4×105 cm2 V−1 s−1

The effect of varying the temperature (T cr) of an As4→As2 cracker furnace between 600 and 700 °C on the properties of GaAs grown by molecular beam epitaxy has been evaluated using 4–300 K Hall measurements and 4.2 K far‐infrared photoconduction spectroscopy, in an extension of earlier work on high‐mobility material (Ref. 1). The residual donors are silicon and sulphur with mid‐1013 cm−3 concentrations under As2‐growth conditions (T cr=700 °C). By lowering T cr, the silicon concentration is reduced substantially, leaving sulphur as the principal impurity. A 15‐μm‐thick layer grown with T cr=650 °C has measured free‐electron densities of ≊2.8×1013 cm−3 and peak mobilities ≊4×105 cm2 V−1 s−1 at ≊28–42 K, the highest ever recorded in bulk GaAs.

4 × 105 cm2 V−1 s−1 peak electron mobilities in GaAs grown by solid source MBE with As2

Abstract A detailed study into the molecular beam epitaxy of high purity n-GaAs with arsenic dimers has been undertaken, culminating in the growth of a layer with a peak mobility of ≈4.0 × 10 5 cm 2 V −1 s −1 at 28–40 K, the highest ever recorded in bulk GaAs.

Excited-state spectroscopy of confined shallow donor impurities in a multi-quantum well

The far infrared (FIR) photoconductive response of silicon-doped GaAs/AlGaAs multi-quantum wells (MQWS) at 4.2 K reveals evidence of several transitions from the ground state to higher excited states of the confined impurity (e.g. 1s-3d+1, 1s-3p+1, 1s-3d+2, 1s-4d+2 etc.). Assignments are made by comparison with the bulk case and with available theory. Linewidths of the 1s-2p+ transition indicate that in the most favourable case minimum redistribution of the silicon dopant occurs which permits observation of higher state transitions.

Far-infrared photoconductivity spectroscopy of high-mobility n-GaAs grown by MBE

n-GaAs of high peak mobility ( mu =250000 cm2 V-1 s-1) has been grown in a molecular beam epitaxy (MBE) system with modified source-furnace geometry. High-resolution far-infrared magnetospectroscopy has been performed on this material. The photoconductive response at 2-4 K reveals a rich spectrum of excited state transitions. These are identified from theoretical models, and transitions from the ground impurity state up to the N=8 Landau level are noted. For the first time in MBE material, transitions involving D- states are observed.

Subnanosecond far infrared photoconductivity from a GaAs/AlGaAs multiquantum well

The far infrared (FIR) photoconductivity of GaAs/AlGaAs multiquantum wells (MQWs) doped with silicon has been investigated. The spectral response is consistent with extrinsic photoconductivity from shallow donors with an effective Rydberg of approximately 10.5 meV. The time‐resolved photoconductivity due to stimulation with a cavity‐dump FIR laser is measured. Subnanosecond rise and decay times are implied for the MQWs investigated; these times are shorter than for the corresponding bulk cases. Possible effects of geometric confinement on recombination rates are discussed. For a 150‐period MQW, the responsivity at 118 μm is approximately 105 V W−1 .

Resonant subband Landau level coupling in GaAs-(Ga,Al)As quantum wells in tilted magnetic fields

Far-infrared transmission measurements are reported for GaAs-(Ga,Al)As quantum wells of widths 100.0 nm, 30.0 nm and 10.0 nm in magnetic fields of up to 20 T, tilted away from the normal to the quantum well plane by angles of up to theta =50 degrees . Deviations of the cyclotron resonance field from a cos theta law, valid for purely two-dimensional systems, are consistent with a coupling of Landau level and electric subband quantizations by a tiled magnetic field. The varied subband structure of the quantum wells allows access to regimes where the subband separation is less than, comparable to, or much greater than the cyclotron splitting. The shifts in resonant field position are compared with theoretical models, which are found to overestimate the strength of the coupling between the subband and Landau level quantizations. The technique of tilted-field cyclotron resonance is shown to be a method of probing the band bending of a quantum well using asymmetric doping, etc.

The two-dimensional electron gas at the CdTe/InSb interface: Cyclotron resonance and Shubnikov-de Haas investigations

The 2DEG in CdTe/InSb heterojunctions has been investigated using magnetotransport and far-infrared magnetotransmission measurements. Two series of oscillations are seen in the Shubnikov-de Haas effect; however, a comparison of the parallel field magnetoresistance with the universal curves of Reisinger and Koch indicates that six subbands are occupied, and that the total carrier density is around 4.7 × 1012 cm−2. A broad cyclotron resonance is observed in the magnetotransmission, yielding an effective mass of m∗ = 0.045me. This is consistent with theoretical predictions for electrons in InSb inversion layers with a carrier density close to 5 × 1012 cm−2. Published data on other similar systems will be discussed in the light of these measurements.

Magnetic field-induced charge transfer to a GaAs/(Ga,Al)As quantum well interface studied by C(e,A0i) photoluminescence

Abstract The photoluminescence (PL) from a 300 A GaAs((Ga,Al)As quantum well (QW) has been studied for a range of excitation powers, in magnetic fields up to 16 T applied both perpendicular to and in the plane of the QW. Particular attention was paid to the intensity of the (e,A 0 i ) transition due to Carbon acceptors located at one interface of the QW, in the presence of in-plane fields. The low power in-plane field dependence of the PL is a competition between two effects. At fields up to 12 T charge transfer is observed to and from the interface of the QW, resulting in an increase and subsequent decrease of the acceptor PL intensity. For field values exceeding 12 T the acceptor PL intensity is found to increase again. Whereas the first effect is well described by a composite oscillator model, the latter is suggested to be due to the decreased efficiency of electronic traps, located at the QW interface, for in-plane magnetic fields. These suggestions are confirmed by the excitation power dependence of the PL intensity for in-plane fields.

Vertical transport in a GaAs/(Ga,Al)As superlattice containing an enlarged quantum well studied by photoluminescence in high in-plane magnetic fields

Vertical transport in a GaAs/(Ga,Al)As superlattice has been studied using photoluminescence and magnetophotoluminescence measurements with in-plane fields of up to 25 T at temperatures in the range 4-80 K. Vertical transport at 4 K (which is determined by exciton transport) has been found to be less efficient than free-carrier-like transport at high temperatures (47 and 80 K). Application of a high, in-plane magnetic field tends to reduce the hole transport considerably. In addition, a drastic enhancement of the total photoluminescence yield with in-plane field has been observed.

Comments on the origin of low-energy structure observed in the far-infrared cyclotron resonance of ultra-high mobility n-GaAs and n-InP

Recently optically detected cyclotron resonance spectra reported by Ahmed et al. in ultra-pure n-GaAs have revealed low-energy structure associated with the cyclotron resonance. The authors have investigated the far-infrared photoconductive response under conditions of cyclotron resonance in these n-GaAs ( mu approximately=400000 cm2 V-1 s-1) samples and also in n-InP ( mu approximately=170000 cm2 V-1 s-1) samples grown by MBE. The suggestion that the structure arises from transitions involving non-parabolicity and polaron coupling effects is discounted using a five-level P.p calculation as found in the literature. Other possible origins, supported by appropriate theory, are suggested for this structure.