A. V. Golikov

Electrical transport and persistent photoconductivity in quantum dot layers in InAs/GaAs structures

The conductivity of quantum dot layers is studied in InAs/GaAs structures in the temperature range from 300 to 0.05 K in the dark and using two types of illumination in magnetic fields up to 6 T. Depending on the initial concentration of current carriers, the conductivity of the structures varied from metallic (the Shubnikov-de Haas effect was observed) to hopping conductivity. At low temperatures, the temperature dependence of the resistance changed from the Mott dependence to the dependence described by the Shklovskii-Efros law for hopping conductivity in the presence of the Coulomb gap in the density of states. The conductivity of samples was studied upon their illumination at λ = 791 nm and λ > 1120 nm. All the samples exhibited a positive persistent photoconductivity at T < 250 K. The structures were also studied using photoluminescence and an atomic force microscope.

Quantum Hall effect-insulator transition in the InAs/GaAs system with quantum dots

The InAs/GaAs structures consisting of quantum-dot layers with electronic properties typical of two-dimensional systems are investigated. It is found that, at a low concentration of charge carriers, the variable-range-hopping conductivity is observed at low temperatures. The localization length corresponds to characteristic quantum-dot cluster sizes determined using atomic-force microscopy (AFM). The quantum Hall effect-insulator transition induced by a magnetic field occurs in InAs/GaAs quantum-dot layers with metallic conductivity. The resistivities at the transition point exceed the resistivities characteristic of electrons in heterostructures and quantum wells. This can be explained by the large-scale fluctuations of the potential and, hence, the electron density.

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.

Sn delta-doping in GaAs

We have prepared a number of GaAs structures -doped by Sn using the well known molecular beam epitaxy growth technique. The samples obtained for a wide range of Sn doping densities were characterized by magnetotransport experiments at low temperatures and in high magnetic fields up to 38 T. Hall-effect and Shubnikov-de Haas measurements show that the electron densities reached are higher than for other -dopants, like Si and Be. The maximum carrier density determined by the Hall effect equals 8.4 × 1013 cm-2. For all samples several Shubnikov-de Haas frequencies were observed, indicating the population of multiple subbands. The depopulation fields of the subbands were determined by measuring the magnetoresistance with the magnetic field in the plane of the -layer. The experimental results are in good agreement with selfconsistent bandstructure calculations. This calculation shows that in the sample with the highest electron density also the conduction band at the L point is populated.

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.

Hopping conductivity and magnetic-field-induced quantum hall-insulator transition in InAs/GaAs quantum dot layers

We have investigated the temperature dependence of resistance in the temperature range T = 0.07 - 300 K, the quantum Hall effect (qHe) and the Shubnikov-de Haas (SdH) effect in InAs/GaAs quantum dot structures in magnetic field up to 35 T. Two-dimensional Mott variable range hopping conductivity (VRHC) has been observed at low temperatures in samples with low carrier concentration. The length of localization correlates very well with the quantum dot cluster size obtained by Atomic Force Microscope (AFM). In samples with relatively high carrier concentration the transition qHe-insulator was observed.