A. V. Chernyaev

Low-temperature hopping conduction over the upper Hubbard band in p-GaAs/AlGaAs multilayered structures

The low-temperature 2D variable range hopping conduction over the states of the upper Hubbard band is investigated in detail for the first time in multilayered Be-doped p-type GaAs/AlGaAs structures with quantum wells of 15-nm width. This situation was realized by doping the layer in the well and a barrier layer close to the well for the upper Hubbard band (A+ centers) in the equilibrium state filled with holes. The conduction was of the Mott hopping type in the entire temperature range (4−0.4 K). The positive and negative magnetoresistance branches as well as of non-Ohmic hopping conduction at low temperature are analyzed. The density of states and the localization radius, the scattering amplitude, and the number of scatterers in the upper Hubbard band are estimated. It is found that the interference pattern of phenomena associated with hopping conduction over the A+ band is qualitatively similar to the corresponding pattern for an ordinary impurity band, but the tunnel scattering is relatively weak.

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.

Manifestation of Coulomb gap in two-dimensional p-GaAs–AlGaAs structures with filled lower or upper Hubbard bands

We have studied 2D hopping conductivity within the impurity bands formed by shallow acceptors (Be) in structures of GaAs/AlGaAs. Strong selective doping allowed contributions of both lower (A0) and upper (A+) Hubbard bands to be observed in different samples. The observed conductivity was of the Efros–Shklovskii type, however, the effective temperature (T0) strongly depended on the impurity concentration (while only weak changes of the localisation radii were observed). Since the dielectric constant in 2D does not depend on concentration, the observed variation of T0 can be explained as a result of variation of the coefficient C entering the equation for the above mentioned T0. So the value of C is not a universal one and depends on impurity concentration. In particular the coefficient C differs by an order of magnitude for A0 and A+ bands for samples with the same acceptor concentration. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Manifestation of coulomb gap in two-dimensional p-GaAs-AlGaAs structures with filled upper Hubbard band

The transport properties of multilayer GaAs/AlGaAs structures doped modulationally with Be so as to fill, in equilibrium, the states of upper Hubbard band (A+ centers) with holes were studied. For the concentration of dopants on the order of 5×1011cm−2, the hopping conduction over the states in the Coulomb gap was observed in the temperature range 0.4–4 K. The characteristic temperature (T1) was determined from the temperature dependence of conductance and found to be appreciably lower (by 30 times) than its theoretically predicted value. This discrepancy is assumed to be due to the correlated hopping effect. In the temperature dependence of magnetoresistance, the suppression of negative magnetoresistance was observed with lowering temperature. This is explained by the weakness of underbarrier scattering in the transport via the upper Hubbard band.

Investigation of the Metal–Insulator Transition in n-3C-SiC Epitaxial Films

n-type 3C-SiC films have been grown by sublimation epitaxy on hexagonal silicon carbide substrates. The low-temperature conductivity and magnetoresistance of the films have been studied in relation to their doping level and structural perfection. It was found that a metal--insulator transition occurs in the n-3C-SiC layer at concentrations Nd - Na ≤ 3 1017 cm-3.

Galvanomagnetic Properties of 3C-SiC/6H-SiC Heterostructures

3C-SiC epitaxial layers were grown by method of sublimation epitaxy in vacuum on 6HSiC substrates. It was done investigation of magneto resistance and Hall effect of 3C-SiC/6H-SiC heterostructures in temperature range 1,4 – 300 К. At helium temperatures it was founded low samples resistance and negative magneto-resistance in week magnetic field ( ~ 1 T). Analysis of obtained results shows, that low samples resistance can be connected with metal-isolation junction in 3C-SiC epitaxial films..