Fap Frans Blom

Electric field induced parallel conduction in GaAs/AlGaAs heterostructures

A new mechanism to understand time‐dependent features in the conduction of a two‐dimensional electron gas (2 DEG) in high electric fields is proposed and discussed. The mechanism is based on the idea that not only the properties of the GaAs/AlGaAs heterostructure have to be included, but also the properties of the transition from the ohmic contact to the heterostructure. We show that the ohmic contact to the heterostructure is fundamentally different from the contact to a bulk semiconductor. In low electric fields electrons cannot move from the contact into the AlGaAs since the conduction band normally lies above the Fermi level. However, when high enough electric fields are applied, the barrier between the contact and AlGaAs is pulled down, allowing conduction in both the AlGaAs and the 2 DEG. We propose a model, in which time‐dependent phenomena in the conduction of the 2 DEG can be associated with trapping and detrapping of charge carriers in the AlGaAs. Time‐resolved experiments are shown which confir...

Optical properties of Cd3P2

Absorption measurements at room temperature and 90K on single crystals of Cd3P2 are given. Free carrier absorption can be interpreted in the simple classical model while the interband absorption is interpreted in the Kane band model using exact solutions of the secular equation and without neglecting the free electron term. The best fit of the theory to experimental points is found for Eg(300K)=0.53 eV, Eg(90K)=0.56 eV, P=6.7*10-10 eVm, Delta =0.1 eV and mv1=0.5 m0. Some results of thermomagnetic transport properties are discussed in the same model.

Analysis of the shallow and deep center occupancies in Si‐doped AlxGa1−xAs using a multilevel donor model

The concentration of occupied deep centers in Si‐doped AlxGa1−xAs for x≥0.2 has been calculated from a three‐level donor model, in which the shallow levels are treated as excited states of the deep (DX) ground state. The deep level is assumed to be tied to the L valley, and the shallow levels to the Γ and X valleys. The behavior of the free‐electron density and the thermal activation energy as function of composition is in good agreement with experimental results reported in the literature. In this model of dependent donor levels the deep‐level occupancy can be directly calculated without needing deep‐level transient spectroscopy measurements. A two‐level donor model is used to calculate the pressure dependence of the deep level from a hydrostatic pressure experiment on a GaAs/Al0.3Ga0.7As heterostructure reported in the literature. We assume a shallow level tied to the Γ valley and an arbitrary deep level which is not coupled to any of the conduction bands. The calculation of the position of the deep lev...

Inversion asymmetry spin level splitting in zero-gap semimagnetic semiconductors

We present a new model to interpret recent measurements on Hg1-xMnxSe which show an angular dependence of the nodes in the Shubnikov-de Haas amplitude. The model is based on an infinite Hamiltonian matrix including inversion asymmetry terms. We additionally took into account the exchange interaction, typical for Semimagnetic Semiconductors. By truncating the matrix we can calculate the electron energy levels, allowing us to describe the experimentally observed nodal field positions.

Magnetoplasma reflectivity studies on Cd3As2

Magnetoplasma reflectivity measurements were performed on Cd3As2 in order to obtain more experimental details about the structure of the plasma reflectivity edge. The experimental results can be explained by assuming the existence of a surface layer with a carrier concentration-gradient which is responsible for optical interference effects.

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

Using a low-mobility two-dimensional hole gas in between the GaAs substrate and the two-dimensional electron gas as an interior gate, the edge channels in the quantum Hall regime can be made macroscopically wide. The location and width of the channels are detected by means of the lateral photo electric effect. Two-dimensional images displayed in grey tones made across large areas of a Hall bar clearly show the positions of the channels.

Magneto-oscillations of the gate current in a laterally modulated two-dimensional electron gas

By using a low-field magnetic field modulation technique we are able to detect gate current oscillations in a 1D-laterally modulated two-dimensional electron gas in the magnetic field range where Weiss oscillations occur. The experimental results can be qualitatively well explained by a simple analytical expression for the density-of-states at the Fermi energy.

Dependence of spin-asymmetry in the Shubnikov-de Haas resistance on back-gating, illumination and uniaxial stress

We present measurements of the Shubnikov-de Haas (SdH) resistivity on GaAs/AlGaAs heterojunctions. We compare measurements in which in the same sample the electron density was increased to the same value either by applying a back-gate voltage or illumination. We conclude that both the overlap of the electron wavefunctions in the two-dimensional electron gas with positive and negative scattering centers and the number of positive and negative scatterers determine the asymmetry in the SdH-peaks. The asymmetry in the SdH-peaks depends on the electron density because the scattering on remote and background impurities have a different electron density dependence. After illumination the neutralization rate of the acceptors in the GaAs determines whether the asymmetry increases or decreases. We show that the asymmetry of the SdH-peaks can also be modified by applying uniaxial stress.