M. Pepper

Comparison of optical and transport measurements of electron densities in quantum wells

We demonstrate that the electron density of a remotely doped quantum well is reduced by continuous illumination, for both photon energies above and below the barrier bandgap. For the latter case we show the depletion is due to the creation of a photovoltage across the GaAs buffer layer deeper in the structure. This results in the electron density determined directly from photoluminescence spectra being significantly lower than that deduced from Hall or Shubnikov - de Haas measurements, either with or without illumination.

Electron conduction characteristics of split-gate structures fabricated on pseudomorphic GaAs-InxGa1-xAs-AlGaAs heterostructures

Sub-micron split-gate structures have been fabricated on three pseudomorphic GaAs-InxGa1-xAs-AlGaAs quantum well structures, with indium fraction, x of 0.04, 0.10, and 0.14. The conductance characteristic of the device on the x=0.04 material showed clear plateaux due to the quantization of ballistic conduction in one dimension. Devices fabricated from the x=0.10 and x=0.14 material did not show plateaux, but exhibited a complex structure that varied quantitatively between nominally identical devices. This structure is thought to be due to both the random nature of the InxGa1-xAs alloy, and the roughness of the InxGa1-xAs interfaces. The spatial variation of the potential between the arms of the split gate due to these effects was probed by applying a differential voltage between the two gates.

The metal-insulator transition in n-type In0.53Ga0.47As

The authors present measurements of the low-temperature conductivity of a series of metallic n-type In0.53Ga0.47As samples, having kFl<3. kF is the Fermi wavenumber and l is the elastic scattering length. The data are extrapolated to zero temperature and the critical behaviour of the zero-temperature conductivity is examined. The temperature dependence of the conductivity appears consistent with terms due to electron-electron interactions and weak localisation. However, a detailed analysis utilising the magnetoconductivity reveals discrepancies with the theory, and it becomes clear that for these samples the corrections to the conductivity due to electron-electron interactions and weak localisation are not independent and additive. The inelastic scattering rate is inferred from the magnetoconductivity and its temperature dependence is found to change from one proportional to T3/2 to one proportional to T as the metal-insulator transition is approach, in agreement with the theories of Isawa and Kaveh and co-workers.

Far-Infrared Transmission of Voltage-Tunable GaAs-(Ga,Al)As Quantum Dots in High Magnetic Fields

We report magneto-capacitance and far-infrared (FIR) magneto-transmission studies of an array of GaAs-(Ga,Al)As quantum dots. With zero gate voltage (V g) applied, the FIR magneto-transmission spectrum shows the cyclotron resonance (CR) of the 2DEG, and a 2D-magnetoplasmon. On applying a negative V g a resonance associated with the dots is observed. The response of the dots can be modelled within either a Maxwell-Garnett or an edge-magnetoplasmon model, to reveal that the effective radius of the dots can be varied by ∼ 50% using reasonable values of V g.

Photoconductive Response of a Quasi-One Dimensional Channel

Much of the physics of quasi-one dimensional (ID) systems has been studied in so-called “ballistic channels” or “quantum point-contacts” defined by a negative voltage applied to two gates a fraction of a micron wide and a similar distance apart on top of a GaAs--(Ga,Al)As heterojunction containing a two dimensional electron gas (2DEG) (van Houten et al, 1990): the channel length is much shorter than the scattering lengths in the 2DEG, and so transport through it is ballistic. At B = 0 the resistance is quantized to a value h/2ie 2, where i is the number of occupied ID subbands (van Wees et al., 1988; Wharam et al., 1988). Much of the interest has focussed on the relationship between the gate voltage (Vg) and the electrostatic potential between the gates, which determines the ID subband spacing in the channel. In transport studies, the potential is determined indirectly by fitting theoretical predictions of the magnetic depopulation of ID subbands to the experimental data (Wharam et al, 1989). An alternative approach is to use a multiple quantum wire system, where an array of ~ 10,000 electron channels ~1 mm long is fabricated (see e.g., Alsmeier et al., 1989; Hansen et al., 1987). Although the large area off such samples means that the transmission of far-infrared (FIR) radiation can be used to detect transitions between ID states, it also leads to poorly defined transport properties, due toj averaging effects (Thornton et al, 1986; Gao et al, 1990). In addition, the transmission spectra of the multiple quantum wire systems are complicated by possible presence of coupling between the quantum wires (plasmons) (Batke et al, 1985; Hansen et al., 1986), and non-local effects such as depolarization shifts (Hansen et al, 1986).

Aspects of One Dimensional Transport Effects in Gallium Arsenide Heterojunction Structures

A brief account is presented on aspects of recent work on one dimensional transport in GaAs-A1GaAs heterojunctions. Application of high resolution electron beam lithography allows the fabrication of a number of structures which illustrate ballistic transport, quantum interference and related effects. The physical origin of the quantisation of the 1D ballistic resistance is discussed as is the current saturation when the applied voltage exceeds the Fermi energy. The “locking” of the plateaux of quantised resistance when resistors are in parallel configuration is described. Results are presented on Aharonov-Bohm oscillations in singly-connected structures when a strong magnetic field forces the current to flow in edge states and a discussion is given on the quenching of the Hall effect and transport in a 1D superlattice.

Ballistic Transport in Quasi-One-Dimensional Structures

The split-gate structure illustrated in Plate 1, and modifications thereof, have been fabricated on a variety of high-mobility heterostructures grown by Molecular Beam Epitaxy at the Cavendish Laboratory. The lithographic length of the split-gate channel was defined to be 0.3 µm whilst the defined channel width was 0.5 µm. The significance of the low temperature high-mobility behaviour is to be seen immediately. The eigenstates of momentum are extremely long-lived and give rise to elastic lengths which can be in excess of several microns. Furthermore the inelastic length, which can be extracted from physical phenomena such as universal conductance fluctuations (Thornton 1987) or Aharanov-Bohm oscillations (Ford 1989), is also of the same order of magnitude. Electrons therefore pass ballistically through the narrow constriction, defined by the application of a negative bias to the gate electrodes, the only scattering being specular scattering from the side walls of the confining potential. Furthermore the Fermi wavelength of the two-dimensional electron gas (2DEG), which is given by \( {{\lambda }_{f}} = \sqrt {{(2\pi /{{n}_{s}})}} \) and is of the order of 50 nm, is comparable to the effective channel width in these devices and hence the quantum nature of the electrons should be significant.

Quantisation of Resistance in One-Dimensional Ballistic Transport

Recent experiments by VAN WEES et al [1] and WHARAM et al [2] on short, narrow constrictions defined in the two-dimensional electron gas (2DEG) of a GaAs-AlGaAs heterostructure have shown that such a constriction possesses a quantised resistance,

Applications of terahertz (THz) technology to medical imaging

R = h/2i{e^2}