V. A. Rogozin

Optical and transport properties of short period InAs/GaAs superlattices near quantum dot formation

We have investigated the optical and transport properties of short-period superlattices of InAs/GaAs, grown by molecular beam epitaxy, with different numbers of periods (3 ≤ N ≤ 24) and a total thickness of 14 nm. Band structure calculations show that these superlattices represent a quantum well with average composition In0.16Ga0.84As. The electron wavefunctions are only slightly modulated by the superlattice potential as compared to a single quantum well with the same composition, which was grown as a reference sample. The photoluminescence, the resistance, the Shubnikov–de Haas effect and the Hall effect have been measured as a function of the InAs layer thickness Q in the range of 0.33 ≤ Q ≤ 2.7 monolayers (ML). The electron densities range from 6.8 to 11.5 × 1011 cm−2 for Q ≤ 2.0 ML. The photoluminescence and magneto-transport data show that only one sub-band is occupied. When Q ≥ 2.7 ML, quantum dots are formed and the metallic type of conductivity changes to variable range hopping conductivity.

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.

Persistent IR photoconductivity in InAs/GaAs structures with QD layers

Persistent IR photoconductivity in InAs/GaAs structures with layers of QDs with a p-and n-type conductivity was studied. At the initial stage, after the illumination is switched off, the relaxation of photoconductivity follows a logarithmic law. The relaxation time depends on temperature; it decreases as temperature increases. A simple model of photoconductivity relaxation, based on thermal activation of carriers from the QD layer, is proposed. The model is consistent with the experimental data.

Specific Features of Photoluminescence of InAs/GaAs QD Structures at Different Pumping Levels

Photoluminescence spectra of InAs/GaAs QD structures have been studied at different pumping powers and temperatures. At low pumping levels, one of the spectral lines in an undoped sample is shifted as the power increases. As the temperature increases, the luminescence intensity in the high-energy portion of the spectrum decreases, and the low-energy spectrum is red-shifted. The presence of QDs of two characteristic sizes is demonstrated.

Lateral electronic transport in short period InAs/GaAs superlattices at the threshold of quantum dot formation

Temperature dependences of resistance at 0.7 K<T<300 K, the Hall and Shubnikov-de Haas effects in magnetic fields of up to 40 T, photoluminescence (PL), and morphology of a heterointerface (using an atomicforce microscope) of short-period InAs/GaAs superlattices were investigated. The investigations were carried out for a region of subcritical and critical thickness Q=2.7 monolayers (ML) of InAs. Upon exceeding the critical thickness, the self-organized growth of InAs quantum dots (QDs) set in. The formation of QD layers upon exceeding the critical thickness of InAs Q=2.7 ML is accompanied by a transition of conductivity from metallic to hopping. It is found that at InAs layer thicknesses of Q=0.33 ML and Q=2.0 ML, the PL intensities and electron mobilities in the structures have clearly pronounced maxima. Anisotropy of conductivity, which depends on the thickness of the deposited InAs layers, was observed.

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.

Magnetic-field-induced quantum Hall—insulator transition and persistent photoconductivity in InAs/GaAs quantum dot layers

We have investigated the temperature dependence of resistance in the temperature range T=0.07–300K and in magnetic field up to 35T in InAs/GaAs quantum dot layers. In samples with relatively high carrier concentration quantum Hall effect—insulator transition was observed in high magnetic fields. Two-dimensional Mott variable range hopping conductivity 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. In all samples a positive persistent photoconductivity was observed.

Peculiarities of the electron transport in very short period InAs/GaAs superlattices near quantum dot formation

The photoluminescence, magnetoresistance, Shubnikov-de Haas and Hall effect have been investigated in short period InAs/GaAs superlattices with different numbers of periods (3⩽N⩽24) and a total thickness of 14nm as a function of InAs layer thickness Q in the range 0.33⩽Q⩽2.7 monolayer (ML). These superlattices represent a quantum well with average composition In0.16Ga0.84As. Photoluminescence intensity and electron mobility enhancement occur when the InAs layer thickness Q is equal to 0.33 or 2.0ML. When Q⩾2.7ML, quantum dots are formed. The mobility of electrons and the anisotropy of resistivity do not depend monotonically on the thickness Q of InAs layers.

Peculiarities of conductivity in structures delta-doped by Si on vicinal (111)A GaAs substrate

Novel epitaxial structures delta-doped by Si, grown by a MBE method on a vicinal substrate misoriented by 0.5°,1.5° and 3° from the GaAs plane towards the direction were formed. In this way it is possible to obtain 1D channels, or at least, 1D periodic modulation of the 2D structure. All samples showed p-type conductivity. It was found that the resistivity of structures Rpa along the steps of vicinal surface is lower than that of Rpe across the steps and depends on temperature.

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.