R.T.F. van Schaijk

Probing the plateau-insulator quantum phase transition in the quantum hall regime

We report quantum Hall experiments on the plateau-insulator transition in a low mobility In(0.53)Ga(0.47)As/InP heterostructure. The data for the longitudinal resistance rho(xx) follow an exponential law and we extract a critical exponent kappa = 0.55+/-0. 05 which is slightly different from the established value kappa = 0. 42+/-0.04 for the plateau transitions. Upon correction for inhomogeneity effects, which cause the critical conductance sigma(*)(xx) to depend marginally on temperature, our data indicate that the plateau-plateau and plateau-insulator transitions are in the same universality class.

Transport and optical properties of tin-delta-doped GaAs structures

The transport and optical properties of tin δ layers in GaAs are investigated as functions of the Sn concentration. The Shubnikov-de Haas and Hall effects are measured in the temperature range 0.4–12 K in magnetic fields up to 38 T. The band diagrams and quantum mobilities of electrons in the quantum-well subbands are calculated. Features associated with electronic transitions from quantum-well levels are found in the photoluminescence spectra of the structures. Oscillations of the resistance are observed in a magnetic field parallel to the δ layer and are attributed to features in the density of states at the Fermi level.

Wavelength dependent negative and positive persistent photoconductivity in Sn δ-doped GaAs structures

The photoconductivity of GaAs structures δ-doped by Sn has been investigated for wavelengths λ = 650-1200 nm in the temperature interval T = 4.2-300 K. The electron densities and mobilities, before and after illumination, have been determined by magnetoresistance, Shubnikov-de Haas effect and Hall effect measurements, in high magnetic fields. For the heavily doped structures (Hall density nH>2×1013 cm-2) we observe under illumination by light with wavelengths larger than the bandgap wavelength of the host material (λ = 815 nm at T = 4.2 K) first positive (PPPC) and then negative (NPPC) persistent photoconductivity. The NPPC is attributed to the ionization of DX centres and PPPC is explained by the excitation of electrons from Cr impurity states in the substrate. For λ<815 nm, in addition, the excitation of electrons over the bandgap of GaAs contributes to the PPPC. For the lightly doped structures (nH≤2×1013 cm-2) the photoconductivity effect is always positive.

Negative persistent photoconductivity in GaAs (δ-Sn) structures

The effect of illumination with various wavelengths λ (770 nm<λ<1120 nm) on the conductivity of GaAs structures with tin δ-doping of the vicinal faces was investigated in the temperature range 4.2–300 K. Negative persistent photoconductivity was found in strongly doped samples. It was shown on the basis of the results of investigations of the Hall and Shubnikov-de Haas effects that the negative photoconductivity is due to a large decrease in the electron mobility with increasing electron density. The decrease of electron mobility is explained by ionization of DX centers, which destroys the spatial correlation in the distribution of positively charged donors and negatively charged DX centers.