F.A.P. Blom

Spatial potential distribution in GaAs/AlxGa1−xAs heterostructures under quantum Hall conditions studied with the linear electro-optic effect

Abstract We apply the linear electro-optic effect (Pockels effect) to investigate the spatial potential distribution in GaAs/AlxGa1−xAs heterostructures under quantum Hall conditions. With this method, which avoids electrical contacts and thus does not disturb the potential distribution, we probe the electrostatic potential of the two-dimensional electron gas locally. Scanning across the width of the sample inside a quantized Hall plateau we observe a steep change of the Hall potential at the edges of the two-dimensional electron gas. This steep change occurs over a distance of about 70 microm, which is the lateral resolution of the experimental set-up. More than 80% of the total Hall voltage is concentrated near the edges. The remainder of the Hall potential is distributed in the interior of the sample and varies linearly with the position. The results are interpreted in terms of unscreened charge at the edges. If the plateau region is left or if the quantized Hall conditions are violated by increasing the temperature or current level the Hall potential becomes a linear function of position.

Subband population and electron subband mobility for two interacting Si-δ-doping layers in GaAs

In this paper we present measurements of the subband population and quantum mobility in the various subbands of GaAs samples that contain two coupled Si-δ-layers and of GaAs samples that contain a single δ-doping layer which was increased in thickness by thermal annealing. The measured subband population will be compared with the subband population obtained from self-consistent solutions of the coupled Poisson and Schrodinger equation. The experimental results on both types of structures are compared and show that the population of the higher subbands is not sensitive to the charge distribution of the ionized donors in the center of the confining potential. The quantum mobility in the highest subbands on the contrary is sensitive to the distribution of the ionized donors.

Effective electron mass in GaAs/AlxGa1-xAs heterostructures under hydrostatic pressure

The authors have determined the effective electron mass in a GaAs/Al0.33Ga0.67As heterostructure from the temperature dependence of the amplitude of the Shubnikov-de Haas oscillations. The effective mass is expected to increase with increasing electron concentration due to the non-parabolicity of the conduction band. The effective mass will also increase when the authors apply a hydrostatic pressure. They evaluated the effective mass in their sample at different electron concentrations and different hydrostatic pressures and have shown that the experimental values fit very well with the theoretically predicted values.

Characterization of silicon δ-doped GaAs grown by MBE at various temperatures

Abstract Si δ-doped GaAs samples have been grown by MBE at 480° C, 530° C and 620° C. The sample grown at 480° C with a doping concentration of 2 × 10 12 cm −2 has a mobility of 6760 cm 2 /V · s at 4.2 K. Shubnikov-de Haas measurements showed that the spreading of the donors in the samples grown at 480° C is near 20 A. On these samples we performed the first reported cyclotron resonance measurements.

Intersubband scattering in GaAs/AlxGa1-xAs heterostructures

We present measurements of both transport and quantum mobility in a GaAs/AlGaAs heterostructure as a function of the electron concentration, in a range where the second subband becomes occupied. From these measurements we observe a drop in the mobilities at the onset of the second subband. The second subband influences the electrical transport properties by intersubband scattering and screening effects. The individual contribution of these effects determines the height and sign of the step in the mobility and depends on the spatial overlap between the wavefunctions of both subbands, as we show with theoretical model calculations.

Imaging of edge channels in the integer quantum Hall regime by the lateral photoelectric effect

We have studied experimentally the widths and positions of edge channels in an inhomogeneous two-dimensional electron gas (2DEG) under quantum Hall conditions. The locations and widths of the edge channels are controlled by a low mobility two-dimensional hole gas (2DHG) in between the GaAs substrate and the 2DEG, which acts as an interior gate. This 2DHG-2DEG system works as a p-i-n photodiode. A lateral photovoltage develops across two contacts attached to the 2DEG, when the structure is illuminated locally. By modulating the illuminated area with respect to position, the differential lateral photovoltage is obtained. We present such measurements as a function of the spot position in a sample under quantum Hall conditions. Two-dimensional images made across large areas of the Hall bar clearly show the edge channels. The measurements are interpreted in terms of macroscopically wide compressible strips alternated with incompressible strips.

The Influence of Impurities on the Shubnikov-De Haas and Hall Resistance of Two-Dimensional Electron Gases in GaAs/AlxGa1-xas Heterostructures Investigated by Back-Gating and Persistent Photoconductivity

It is well-known that in the Quantum-Hall (QH) regime the Shubnikov-de Haas (SdH) oscillations and the Hall resistance show a non-ohmic behaviour, which apparently is spin-dependent. We report on new measurements on this effect, which have been carried out with the help of a back-gate and by the use of the effect of persistent photoconductivity induced by light of out-gap energy.