N. V. Agrinskaya

Low-temperature transport properties of multigraphene films grown on the SiC surface by sublimation

Multigraphene films grown by sublimation on the surface of a semi-insulating 6H-SiC substrate have been studied. It is shown that pregrowth annealing of the substrate in a quasiclosed growth cell improves the structural quality of a multigraphene film. Ohmic contacts to the film have been fabricated, and the Hall effect has been studied at low temperatures. It is found that a 2D electron gas exists in the films. It is concluded that the conductivity of the film is determined by defects existing within the graphene layer or at the interface between the graphene film and a SiC substrate.

Anomalous electron transport in doped uncompensated p-GaAs/AlGaAs quantum wells: evidence of virtual Anderson transition

For highly doped uncompensated p-type layers located within the central part of GaAs/AlGaAs quantum wells, we observed the activated low-temperature behavior of conductivity. The low values of the activation energy, ?4 = (1?3)?meV, cannot apparently be ascribed to standard mechanisms. We attribute this behavior to the existence of a narrow band of extended states near the maximum of the density of states in the impurity band. The Hubbard repulsion prevents metallic transport of holes over these states. However, the minority carriers?electrons?supplied by background defects and situated at low temperatures within the tail of the impurity band can be activated to the above mentioned band of extended states. We refer to this behavior as the virtual Anderson transition since the conductance is maintained by the extended states formed within the impurity band though the conductivity is not metallic. The low-temperature () conductance is strongly non-Ohmic: the I?V curves are S-shaped that leads to a breakdown behavior. We explain the observed low threshold fields () by the fact that we are dealing with the impact ionization of the electrons from the states below the chemical potential to the band of extended impurity states situated close to the chemical potential, the ionization energy being small.

Structure and transport properties of nanocarbon films prepared by sublimation on a 6H-SiC surface

The transport properties of nanocarbon layers on a 6H-SiC substrate, grown by vacuum sublimation, are studied. It is found that these layers consist of a graphene layer adjacent to the substrate and a multigraphene layer coated with a polycrystalline carbine-like phase. In this case, the magnetoresistance and Shubnikov-de-Haas oscillation curves exhibit features inherent to single-layer graphene. The resistance at low temperatures is demonstrated to increase with temperature, which also corresponds to the behavior of single-layer graphene (antilocalization). At the same time, the resistance decreased with temperature at higher temperatures, which corresponds to weak localization. We believe that the observed behavior can be explained by the parallel combination of contributions of single-layer graphene and multigraphene to the conductance.

Transition from strong to weak localization in the split-off impurity band in two-dimensional p-GaAs/AlGaAs structures

A crossover from strongly localized behavior to weak localization (SL-WL) was observed in two-dimensional modulation-doped GaAs/Al0.3Ga0.7 As structures as the impurity concentration increased. In this case, it was observed that the low-temperature dependence of the conductivity changed its character (from exponential to logarithmic) and the magnetoresistance changed its sign (from linear negative to root positive). For 2D structures, it is shown that this transition proceeds in the impurity band separated from the valence band by the mobility gap, whereas the effective mass in the impurity band is larger than in the valence band.

Suppression of the Virtual Anderson Transition in the Impurity Band of Doped Quantum Well Structures

We have previously observed activation-type conductivity with low activation energies of heavily doped p-GaAs/AlGaAs quantum well structures at low temperatures. It has been attributed to the delocalization of the electron states near the maximum of a narrow impurity band in the sense of the Anderson transition. The possibility of this delocalization at a relatively low impurity concentration is associated with the narrowness of the impurity band in the presence of weak disorder. In this case, charge carriers were activated from the tail of the band and their presence was due to the background (weak) compensation. In this work, we study the dependence of the above virtual Anderson transition on the external compensation and impurity concentration. It has been found that an increase in the compensation does not initially affect the Anderson transition; however, at a higher compensation, it leads to the suppression of the transition owing to the growing disorder. An increase in the impurity concentration also initially leads to the suppression of the Anderson transition due to the disorder associated with the partial overlap of the Hubbard bands. However, the conductivity becomes metallic at a fairly high concentration due to the Mott transition.

Slow relaxation of magnetoresistance in doped p-GaAs/AlGaAs layers with partially filled upper Hubbard band

We observed slow relaxation of magnetoresistance in quantum well structures GaAs-AlGaAs with a selective doping of both wells and barrier regions which allowed partial filling of the upper Hubbard band. Such a behavior is explained as related to magnetic-field driven redistribution of the carriers between sites with different occupation numbers due to spin correlation on the doubly occupied centers. This redistribution, in its turn, leads to slow multi-particle relaxations in the Coulomb glass formed by the charged centers.

Metal-insulator transition in n-3C-SiC epitaxial films

The paper reports a study of galvanomagnetic properties of n-3C-SiC/n-6H-SiC heterostructures at liquid-helium temperatures. 3C-SiC epitaxial layers were grown by sublimation epitaxy in a vacuum on the (0001)C face of 6H-SiC substrates produced by the Lely method and 4H-SiC substrates grown by modified Lely method. The x-ray topography demonstrated the high quality structure of the epitaxial layers and the absence of any transition regions between 3C-SiC epitaxial layer and substrate. The low-temperature conductivity and magnetoresistance of the films have been studied as functions of their doping level and structural quality. It was found that the metal-insulator transition occurs in the n-3C-SiC layer at concentrations Nd−Na≤3×1017 cm−3.

Virtual Anderson transition in a narrow impurity band of doped p-GaAs/AlGaAs layers

In highly doped uncompensated layers of p-GaAs/AlGaAs quantum wells, activation conduction with low activation energies is observed at low temperatures and this conduction is not explained by known mechanisms (ε4 conduction). Such behavior is attributed to the delocalization of electron states near the maximum of a narrow impurity band in the sense of the Anderson transition. In this case, conduction is implemented due to the activation of minority carriers from the Fermi level to the indicated delocalized-state band.

Mixed conduction in doped semiconductor structures related to quasi-metallic conduction in the impurity band

Mixed conduction due to simultaneous contributions from allowed states in the valence band and extended impurity (acceptor) states, which occur in the impurity band at high impurity concentrations because of the Anderson transition, is observed in a series of GaAs/AlGaAs structures. Mixed conduction manifests itself in the existence of a minimum in the temperature dependence of the carrier concentration and a noticeable bend in the temperature dependences of the conductivity. Expressions for the mixed conductivity on which the calculations are based were derived taking into account the spectrum of impurity states in the quantum wells (the upper and lower Hubbard impurity bands), their occupancies, and the sign of charge carriers in the valence and the impurity bands; the important assumption was made that the width of impurity bands is much smaller than the spacing from the valence band. The calculation results agree well with the experiment and were used to determine the binding energies for the upper and lower Hubbard bands, the acceptor concentration, and the degree of compensation. It is shown that formulas commonly used for the calculation of mixed conductivity need significant corrections in the case of narrow impurity bands.

Slow relaxation of magnetoresistance in AlGaAs–GaAs quantum well structures quenched in a magnetic field

We observed a slow relaxation of the magnetoresistance in response to an applied magnetic field in selectively doped p-GaAs-AlGaAs structures with a partially filled upper Hubbard band. We have paid special attention to excluding the effects related to temperature fluctuations. Although these effects are important, we have found that the general features of slow relaxation persist. This behavior is interpreted as related to the properties of the Coulomb glass formed by charged centers with account taken of spin correlations, which are sensitive to an external magnetic field. Variation of the magnetic field changes the numbers of the impurity complexes of different types. As a result, it affects the shape and depth of the polaron gap formed at the states belonging to the percolation cluster responsible for the conductance. The suggested model explains both the qualitative behavior and the order of magnitude of the slowly relaxing magnetoresistance.