T. A. Polyanskaya

Energy relaxation of 2D electrons at an AlGaAs/GaAs heterojunction at helium temperatures

Abstract The peculiarities of 2D electron gas heating are derived from the comparison of temperature and field dependences of galvanomagnetic effects for one or two occupied size-quantized subbands. The main energy relaxaton mechanism at helium temperatures is shown to be the scattering by piezoelectric potential of acoustic phonons. Experimental data and theory show that while the carrier density increases, the temperature dependence of energy loss rate changes due to contribution of electrons in second subband to the energy relaxation processes.

Quantum transport effects in a two-dimensional electron gas as a tool for the investigation of heterointerfaces

We have studied the influence of interface defects on low temperature galvanomagnetic effects in a two-dimensional electron gas (2DEG). Experiments have shown a non-ideal character of the InP/In 0.53 Ga 0.47 As interface in modulation-doped structures with 2DEG grown by liquid phase epitaxy. That is why such structures have been chosen for investigation. The following galvanomagnetic effects were investigated at liquid helium temperatures: the Hall mobility; the oscillatory magnetoresistance in a magnetic field perpendicular to the heterointerface; the negative magnetoresistance in a weak magnetic field parallel to 2DEG. The quantitative analysis of these effects allowed one to determine the interface parameters: the amplitude δ=7-10 A and the lateral size Λ=50-70 A of small-scale interface fluctuation, the interface charge density N z =(8±2) ×10 19 cm -2 and the amplitude of large-scale 2DEG density fluctuations δn s /n s 5%

Elemental composition and electrical properties of (a-C:H):Cu films prepared by magnetron sputtering

Amorphous hydrogenated carbon films with varied copper concentration were prepared by dc co-sputtering of graphite and copper targets in an argon-hydrogen atmosphere. The relative atomic concentrations of carbon, copper, and oxygen were determined using proton Rutherford backscattering and the method of nuclear reactions. The dc conductivity of the films was studied in the in-plane and transverse geometries. The conductivity data are discussed in terms of the model of a medium in the form of a dielectric matrix containing two types of conducting inclusions in the form of graphite-like and copper nanoclusters.

Evidence for ɛ2-conductivity in the magnetoresistance of multivalley semiconductors

Experimental studies of the liquid-helium temperature resistivities of Ge:Sb compounds with degrees of compensation K<0.1, i.e., in the ɛ2 conductivity range, reveal that the resistivity is determined by hopping of carriers activated to the upper Hubbard D− band. The experimentally observed positive magnetoresistance, which is exponential in the magnetic field, arises from field-induced changes in the occupancy of the spin subbands by electrons. Evidence for the ɛ2-conductivity mechanism is discussed on the basis of certain features of the magnetoresistance associated with g-factor anisotropy of different valleys, which is therefore specific to multivalley semiconductors.

Weak antilocalization and spin-orbit interaction in a In0.53Ga0.47As/InP quantum well in the persistent photoconductivity state

Low-field quantum magnetoresistance of two-dimensional electron gas at the In0.53Ga0.47As/InP interface was studied in the persistent photoconductivity state. The sign-alternating property of the dependences of the magnetoresistance on the magnetic field indicates that the spin-orbit interaction affects the quantum well conductivity. The mechanism caused by the electric field built in at the interface was shown to contribute dominantly to the spin-orbit scattering frequency 1/τso. This is the Rashba mechanism, which is linear in the electron wave vector. These data allowed us to estimate the parameters of spin-orbit splitting of the energy spectrum as α=(84±10) Å2 (by the Rashba mechanism) and γ=(73±5) eV Å3 (by the Dyakonov-Perel and Dresselhaus mechanisms).

Weak-field magnetoresistance of two-dimensional electrons in In0.53Ga0.47As/InP heterostructures in the persistent photoconductivity regime

The weak-field magnetoresistance of the two-dimensional electron gas (2DEG) in a modulation-doped In0.53Ga0.47As/InP heterostructure is studied as the state of the system is converted to a state with persistent photoconductivity by illuminating the sample with interband light. The concentration dependences of the parameters that characterize the phase (Hϕ) and spin (Hs coherence are investigated, both in the low-concentration regime where only the first quantum-well subband is occupied by carriers, and in the regime where the second subband is occupied. A qualitative description of all the features observed in experiment is obtained by taking into account the redistribution of charge in the persistent photoconductivity state and the importance of processes that take place in the second quantum-well subband even when its occupation is small.

Electron phase and spin decoherence in the vicinity of the second subband edge in an asymmetrical quantum well

Weak antilocalization of a two-dimensional electron gas formed at a In0.53Ga0.47As/InP heterointerface was studied. The Fermi level was varied from below, to above, the energy minimum of the second subband. A model for quantum coherence with two conducting subbands and fast intersubband scattering was used to extract the characteristic phase and spin decoherence rates from experimental magnetoresistance data. Taking into account the spatial inhomogeneity of the energy associated with the subband minimum, the first and second subband decoherence contributions were separated. It was shown that phase decoherence in the second subband is much faster than in the first subband and it decreases with increasing occupation of the second subband. By contrast, spin dephasing due to scattering in the second subband and intersubband scattering does not play a noticeable role.

Spin relaxation and weak localization of two-dimensional electrons in asymmetric quantum Wells

The anomalous alternating-sign magnetoresistance in a two-dimensional electron gas on an In0.53Ga0.47As/InP heterostructure was investigated experimentally at liquid-helium temperatures in a wide range of electron densities, including the case of two filled quantum-well subbands. The data obtained are analyzed in terms of a theory that takes into account terms in the spin splitting of the electron spectrum which are cubic and linear in the wave vector. The linear term is related to the asymmetry of the quantum well, i.e., the presence of an electric field at the heterojunction. It is shown that the new theoretical model describes the experiment better.

Electron localization in sound absorption oscillations in the quantum Hall effect regime

The absorption coefficient for surface acoustic waves in a piezoelectric insulator in contact with a GaAs/Al0.25Ga0.75As heterostructure (with two-dimensional electron mobility μ=1.3×105 cm2/(V·s) at T=4.2 K) via a small gap has been investigated experimentally as a function of the frequency of the wave, the width of the vacuum gap, the magnetic field, and the temperature. The magnetic field and frequency dependences of the high-frequency conductivity (in the region 30–210 MHz) are calculated and analyzed. The experimental results can be explained if it assumed that there exists a fluctuation potential in which current carrier localization occurs. The absorption of the surface acoustic waves in an interaction with two-dimensional electrons localized in the energy “tails” of Landau levels is discussed.

Density-of-states anomaly and tunneling conductance of Au/p-GaAs0.94Sb0.06 contacts near the metal-insulator transition

The tunneling-current anomaly in Au/p-GaAs0.94Sb0.06 contacts at zero bias voltage (V) → 0) is investigated. Epitaxial layers of the solid solution GaAs0.94Sb0.06 are doped with germanium and have a conductivity close to that at the metal-insulator transition. The square-root dependence of the differential conductance G(V)=(dV/dI)−1 at small values of eV>kT TT predicted by the Al’tshuler-Aronov theory of quantum corrections to the density of states at the Fermi level in disordered conductors is observed. Satisfactory agreement between the experimental data and theory is observed at hole densities (p) in the layers greater than the critical density for the metal-insulator transition pc, but the relative magnitude of the anomaly is sharply smaller at p<pc. This confirms the specificity of the condition kFl⩾1 (instead of ) for applicability of the theory for the density-of-states anomaly appearing as a result of electron-electron interactions in a three dimensional electron gas.