R.T. Grimes

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.

Hydrogenic impurities in GaAs/GaAlAs multi-quantum wells in tilted magnetic fields

Far-infrared photoconductivity measurements on a GaAs/GaAlAs multi-quantum well sample in which the normal to the plane of the sample is tipped away from the magnetic field direction are reported. In order to explain these results, the model developed previously by the authors involving a matrix diagonalization procedure is expanded and then applied to the experimental results. Good agreement is obtained for angles of tilt up to about 30 degrees . It is also shown that the model satisfactorily explains results previously obtained by Mueller et al (1991).

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 .

Nanosecond far infra red magnetospectroscopy of GaAs/AlGaAs heterostructures

Short pulses (10–100 ns) of FIR radiation with very sharp cut-off are used to measure the photoconductivity (PC) and transmission of high mobility GaAs/AlGaAs heterostructures. The photoconductive response at cyclotron resonance (CR) does not saturate at intensities up to 100 W cm−2, and corresponds to an increase in αxx. An oscillatory PC (essentially periodic in 1/B) is also observed which is non-resonant in laser wavelength and which can be accounted for by a simple heating mechanism. The decay times for CR and non-resonant PC are the same (∼20 ns) and are slightly sample- dependent. The decay time for transmission CR (< 3 ns) is detector-limited.

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.

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.

Effects of interface donors on far infrared photoresponse at cyclotron resonance in a (AlGa)As/GaAs heterojunction

The low temperature far infrared photoconductive response of several n‐(AlGa)As/GaAs heterojunctions (μ≊2×106 cm2 V−1  s−1 , n≊3×1011 cm−2 ) have been measured at wavelengths between 96 and 255 μm, and at magnetic fields up to 10 T. Under certain bias conditions, splittings of the cyclotron resonance peak are noted, which are attributed to transitions involving an impurity band of energy approximately (1/3)ℏωc below the Landau level. Small anomalies in the de Haas–Shubnikov data are related to these states. Silicon ions, diffusing towards the interface, are thought to be the origin of these states.