N. V. Agrinskaya

• Senior Researcher at Ioffe Institute

In a previous report, we presented experiments which suggested that ferromagnetic ordering of the spins of localized holes in GaAs/AlGaAs quantum wells could be observed when doped with shallow (Be) acceptors at impurity concentrations near the metal-insulator transition. The compensating impurity (Si) was introduced into a narrow region at the cen...

We suggest a model of synthetic ferrimagnetic semiconductor structure based on GaAs-AlGaAs quantum well doped by two Mn delta-layers. The coupling between the delta-layers is mediated by extra holes, and can be switched between ferro- and antiferromagnetic one by gating the structure. A proper choice of Mn concentrations in the delta-layers and of...

A brief review of theoretical and experimental studies of spin phenomena in various hybrid structures based on magnetically ordered metals and semiconductors is presented. In particular, the following issues are addressed: (i) the effects of electrically driven switching of the magnetization with the minimum energy loss; (ii) ferromagnetic ordering...

We suggest a model of synthetic ferrimagnetic semiconductor structure based on GaAs-AlGaAs quantum well doped by two Mn delta-layers. The coupling between the delta-layers is mediated by extra holes, and can be switched between ferro- and antiferromagnetic one by gating the structure. A proper choice of Mn concentrations in the delta-layers and of...

It is shown that the transport properties of graphitized silicon carbide are controlled by a surface graphene layer heavily doped with electrons. In weak magnetic fields and at low temperatures, a negative magnetoresistance is observed due to weak localization. A crossover in the magnetoresistance from weak localization to weak antilocalization (th...

We have studied experimentally ferromagnetic properties of GaAs-AlGaAs quantum wells where the center of GaAs well was doped by Mn (x<0.1%) while the neighboring barrier was doped by shallow acceptor Be with an aim to occupy (at least, partially) the states of the upper Hubbard band of Mn acceptors in the well (AMn⁺). It was found that while the me...

We observed breakdown behavior for highly doped and low compensated p-type layers located within the central part of GaAs/AlGaAs quantum wells at rather small values of the applied bias. This behavior is attributed to the presence of the band of extended states (BES) within the impurity band. Namely, we believe that the breakdown is due to impact i...

We studied conductivity of AlGaAs-GaAs quantum well structures (where centers of the wells were doped by Be) at temperatures higher than 4K in magnetic fields up 10 T. Throughout all the temperature region considered the conductivity demonstrated activated behavior. At moderate magnetic fields 0.1T < H < 1T we observed negative isotropic magnetores...

We present the results of consistent theoretical analysis of various factors that may lead to influence of temperature and external magnetic field on disorder in semiconductor structures. Main attention is paid to quantum well (QW) structures in which only QWs or both QW and barriers are doped (the doping level is assumed to be close to the value c...

We report experimental studies of conductance and magnetoconductance of GaAs/AlGaAs quantum well structures where both wells and barriers are doped by acceptor impurity Be. Temperature dependence of conductance demonstrate a non-monotonic behavior at temperatures around 100 K. At small temperatures (less than 10 K) where the conductivity has a quas...

We consider in detail the indirect exchange between Mn ions imbedded to GaAs/AlGaAs quantum wells where the barriers are doped with acceptor impurity supported by the carriers of the upper Hubbard band supplied by barriers acceptors. A special attention is paid to an interplay between strong delocalization of the carriers within the upper Hubbard b...

A large positive magnetoresistance peaked at the Curie temperature has been observed in quantum well structures GaAs/AlGaAs doped by Mn. We suggest a new mechanism of magnetoresistance within low T c ferromagnets resulting from a pronounced dependence of spin polarization at the vicinity of T c on the external magnetic field. As a result, any contr...

We present our results obtained for Mn-doped GaAs quantum wells where the evidences of the ferromagnetic transition at relatively high temperatures were found at unusually small Mn concentrations. The observed values of hopping resistance at small temperatures evidenced that the samples are deep in the insulating regime. Thus the corresponding esti...

Theoretical and experimental studies of the conductivity and magnetoresistance of selectively doped structures of GaAs/AlGaAs quantum well structures near a metal-insulator phase transition have been reviewed. Special attention is focused on the role of the structure of impurity bands, which are narrow in the absence of intentional compensation and...

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 ex...

Transport properties of multigraphene layers on 6H-SiC substrates was studied. It was found that the curves of magnetoresistance and Shubnikov- de Haas oscillations shown the features, typical for single-layred graphene. The low temperature resistance demonstrated an increase with temperature increase, which also corresponds to a behavior typical f...

Influence of High vacuum annealing on 6H-SiC(0001) wafers surfaces is studied. Annealing is carried out at the temperature range of 1300-1400°C and at residual pressure of ~10-6 Torr. Auger spectroscopy and RHEED data shows that the used annealing conditions did not lead to any surface reconstruction of the processed wafers. Atomic force microscopy...

Transport properties of multigraphene layers on 6H-SiC substrates fabricated by thermal graphitization of SiC were studied. The principal result is that these structures were shown to contain a nearly perfect graphene layer situated between the SiC substrate and multgraphene layer. It was found that the curves of magnetoresistance and Shubnikov- de...

Earlier we reported an observation at low temperatures of activation conductivity with small activation energies in strongly doped uncompensated layers of p-GaAs/AlGaAs quantum wells. We attributed it to Anderson delocalization of electronic states in the vicinity of the maximum of the narrow impurity band due to the small width of the impurity ban...

Multi-graphene films grown by sublimation on the surface of a semi-insulating 6H_SiC substrate have been studied. It is shown that pre-growth annealing of the substrate in a quasi-closed growth cell improves the structural quality of a multi-graphene film. Ohmic contacts to the film have been fabricated, and the Hall effect has been studied at low...

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 delocalizati...

We demonstrate—both experimentally and theoretically—that the finite size of the current leads does not allow neglecting the role of these leads in the four-probes transport measurements if the properties of these leads are modified in the course of the experiment. We also give a short critical review of other publications, where, to our opinion, a...

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 temp...

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 fe...

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...

We reconsider the theory of magnetoresistance in hopping semiconductors. First, we have shown that the random potential of the background impurities affects significantly preexponential factor of the tunneling amplitude which becomes to be a short-range one in contrast to the long-range one for purely Coulomb hopping centers. This factor to some ex...

We suggest that negative magnetoresistance in small magnetic fields at temperatures lower than 3 K reported in the paper under discussion may be related to superconducting transition in In leads (with Tc = 3.4 K).

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...

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 demons...

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...

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 ( ~...

In this study, we carried out for the first time a galvanomagnetic investigation of 3C–SiC/6H–SiC heterostructures at liquid-helium temperatures and observed in n-3C–SiC low resistance of the samples and the appearance of a negative magnetoresistance in weak fields (~1 T). Analysis of the results we obtained shows that the low resistance is in all...

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, ε4 = (1–3) meV, cannot apparently be ascribed to standard mechanisms. We attribute this behavior to the existence of a narrow band of...

In highly doped uncompensated p-type layers within the central part of GaAs/AlGaAs quantum wells at low temperatures we observed an activated behavior of the conductivity with low activation energies (1-3) meV which cannot be ascribed to standard mechanisms. We attribute this behavior to the delocalization of hole states near the maximum of the nar...

Epitaxial 3C-SiC films have been grown on 6H-SiC substrates by sublimation epitaxy in vacuum. The Hall effect in these heterostructures and their magnetoresistance have been measured in a temperature range from 1.4 to 300 K. At liquid-helium temperatures, the samples are characterized by low resistance and exhibit negative magnetoresistance in weak...

In highly doped uncompensated p-type layers within the central part of GaAs/AlGaAs quantum wells at low temperatures we observed an activated behavior of the conductivity with low activation energies (1-3) meV which can not be ascribed to standard mechanisms. We attribute this behavior to the delocalization of hole states near the maximum of the na...

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...

A crossover from strongly localized behavior to weak localization (SL–WL) was observed in two-dimensional modulation-doped GaAs/Al0.3Ga0.7As structures as the impurity concentration increased. The low-temperature dependence of the conductivity changed its character (from exponential to logarithmic) and the magnetoresistance changed its sign (from l...

Experimental and theoretical studies on transport in semiconductor samples with superconducting electrodes are reported. We focus on the samples close to metal-insulator transition. In metallic samples, a peak of negative magnetoresistance at fields lower than critical magnetic field of the leads was observed. This peak is attributed to restoration...

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 magnet...

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 tempera...

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...

Photoluminescence of GaAs/AlGaAs multiple-quantum-well structures incorporating positively charged beryllium-impurity shallow-level acceptors (the so-called A(+) centers) was investigated. A novel luminescence line, which originated from radiative recombination of free electrons with A(+) centers, was observed. It was shown that its spectral positi...

By selective doping (Be) of the well and barrier regions of p-GaAs/AlGaAs structures we have realized the situation where the upper Hubbard band (A+ centers) has been occupied by holes in the equilibrium. We studied temperature behavior of the Hall effect, variable range hopping conductivity and the photoluminescence spectra of the corresponding st...

By selective doping (Be) of the well and barrier regions of p-GaAs/AlGaAs structures we have realized the situation where the upper Hubbard band (A+ centers) has been occupied by holes in the equilibrium. We studied temperature behavior of the Hall effect, variable range hopping conductivity and the photoluminescence spectra of the corresponding st...

By selective doping of the well and barrier regions of p-GaAs/AlGaAs structures we have realized the situation where the upper Hubbard band (A+ centers) has been occupied by holes in equilibrium. Hall effect and conductivity measurements have shown that the binding energy of A+ centers increases significantly (five times) with respect to the bulk c...

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 Hubb...

The Hall effect and electrical conductivity were studied in the temperature range of 1.7–300 K in Be-doped p-GaAs/AlGaAs multilayer structures with 15-nm-wide quantum wells. Doping of the well itself and the adjacent barrier layer was used to create a situation when the upper Hubbard band (the A + centers) was occupied with holes and electrical con...

To explain the main features of the metal-insulator transition (MIT) in a two-dimensional (2D) electron system, we suggest a simple model, taking into account strongly localized states in the tail of a 2D conductivity band with a specific emphasis on the role of doubly occupied states [the upper Hubbard band (UHB)]. The metallic behavior of the res...

A simple approach describing the crossover from Mott to Coulomb gap regime for hopping conductivity involving percolation arguments and averaging of the DOS over the relevant energy region was suggested. The resulting equation for the hopping exponent (ln R) allows a simple calculation procedure for the universal function describing the crossover....

The role of the upper Hubbard band (D- band) in hopping conductivity is revised. An analysis of magnetoresistance in nearest neighbor hopping related to spin correlations in the D- band is given; the comparison of theoretical predictions both with original experimental data obtained for Ge:Sb samples and with existing experimental data unequivocall...

A simple two-band model is suggested explaining recently reported unusual features for hopping magnetoresistance and the metal-insulator transition in 2D structures. The model implies that the conductivity is dominated by the upper Hubbard band (D^- band). Experimental studies of hopping magnetoresistance for Si delta doped GaAs/AlGaAs heterostruct...

A simple approach to describing crossover between the Mott regime and the Coulomb-gap regime in hopping conductivity with variable-range hopping is described, based on notions of percolation theory and utilizing an interpolation expression for the density of states. The theoretically derived equation for the argument of the exponential of the resis...

A study is reported of the second-order nonlinear optical susceptibility in polycrystalline molecular materials of the chalcone group made by second-harmonic generation at a wavelength of 1.06 µm. Some of the compounds studied exhibit a high nonlinear activity and are transparent in the blue region of the spectrum. The measured macroscopic nonlinea...

Experimental studies of the liquid-helium temperature resistivities of Ge:Sb compounds with degrees of compensation K<0.1, i.e., in the ɛ 2 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...

We consider the magnetoresistance in the nearest neighbor hopping conductivity with an account of both Hubbard and spin correlations. It is shown that no spin magnetoresistance exists for the conductivity within the lower Hubbard band. In contrast, the ε2 conductivity involving the upper Hubbard band states exhibits an increase of activation energy...

The behavior of hopping transport in the region of the crossover from Mott conduction to conduction among Coulomb-gap states is investigated in doped CdTe crystals. Inconsistencies are discovered in individual parameters (the localization radius and the dielectric constant) estimated from the behavior of the conductivity on different sides of the c...

A mechanism to explain the formation of a highly conducting channel in polymer films between metallic electrodes is suggested. We ascribe it to transport of metallic ions leading to a formation of metallic short-circuit; the process is supported by an instability with respect to asperity growth. The predictions of the model are compatible with most...

We discuss the effect of the suppression of negative magnetoresistance in variable range hopping regime with temperature decrease after a crossover to Coulomb gap hopping earlier observed in doped CdTe crystals. Orbital and spin degrees of freedom are taken into account simultaneously. The analytical theory supported by numerical calculations evide...

The method of second-harmonic generation at the wavelength 1.06 µm is used to study the nonlinear second-order optical susceptibility of polycrystalline molecular materials with C 2v molecular symmetry belonging to the group of 2,6-dibenzadiene substituted cyclohexanes. The magnitude of the second harmonic signal is comparable to or greater than it...

An observation of the suppression of negative magnetoresistance in samples of doped CdTe that are far from the metal-insulator transition as the temperature is lowered in the temperature range 3–0.4 K was previously reported [N. V. Agrinskaya, V. I. Kozub, and D. V. Shamshur, JETP 80, 1142 (1995)]. The results of an investigation of samples that ar...

In this work, we report the results of magnetoresistance studies of variable range hopping down to 30 mK in isotopically engineered Ge with low compensation, and in n-CdTe crystals. Experimentally we find a decrease and disappearance of the negative magnetoresistance with decreasing temperature down to 200 mK, in weak magnetic fields. Such behaviou...

An effect of preexponentials on variable range hopping (VRH) conductivity near the crossover from Mott law to Efros-Shklovskii (ES) law is analyzed. These factors ρ0,1(T) are shown to affect significantly the crossover details and may even imitate the crossover. As demonstrated for doped CdTe crystals at 0.4−4K an account for ρ0(T) lifts the crosso...

The temperature behavior of the conductivity and magnetoresistance of CdTe crystals doped by shallow donors (Cl) has been studied at temperatures of 0.4–100 K. The crossover from variable range hopping (VRH) over the states within Coulomb gap to VRH of the Mott type has been observed at some temperatures depending on the samples parameters (donor c...

Four symmetrical derivatives of diacetylenes (1,6-bis-piperidine, 1,6-dimorpholine, 1,6-dimethylphenylamine and 1,6-bis-diphenylamine) of 2,4-hexadiene have been synthesized. The polymerization behavior and third-order non-linear optical susceptibilities (χ(3)) have been investigated as a function of the donor or acceptor properties of the side gro...

We have analysed our experimental data on electronic transport in bulk doped and compensated CdTe crystals. The level of doping has been varied from the weak doping to the high doping limit. It has been shown that the data obtained can be explained by the presence of large-scale potential relief owing to an inhomogeneous impurity distribution. As f...

The self-compensation of wide gap A2B6 semiconductor compounds is a well known phenomenon. The effect is usually due to native defects, generated at high temperatures. Besides, there are some aging processes, pronounced at sufficiently low temperature. After the doped crystals have been kept at room or higer temperature for a long time, the conduct...

The level distribution in semi-insulating CdTe crystals doped with Cl was revealed by two independent methods, i.e., by studying (1) the equilibrium Hall effect and (2) the spectral and intensity dependences of the photo-Hall effect. The pinning of the Fermi level near the gap midpoint is due to a considerable concentration (≈ 1015 cm−3) of the Ev...

In crystals with chlorine concentrations between 1 × 1017 cm -3 and 2 x 1018 cm-3 a high degree of association is found after vacuum annealing at 850 °C and quenching. Electrical measurements and photoluminescence data along with isothermal anneals for varying times at 600 °C are presented which suggest that not only single and double associates [(...

An investigation was made of low-resistivity CdTe:Cl crystals prepared by annealing in cadmium vapor. The room-temperature electron density n was proportional to the chlorine concentration up to n approximately equals 10**1**7 cm** minus **3. The ionization energy of shallow donors (chlorine atoms occupying tellurium sites) was close to that predic...

A study was made of the switching effect in compensated CdTe crystals ; which were either doped with donors or left undoped. The dependences of the ; switching field on the thickness of the sample, delay time, and temperature were ; determined. The switching was nonthermal in thin samples (approximately-less-; than100 $mu$). The current-voltage cha...

Implantation of P** plus and As** plus ions into ″cold″ and ″hot″ n-type CdTe substrates produced inversion of conduction and n-p structures. The acceptor action of the impurities introduced in this way was supported by experiments in which Ar** plus ions were implanted and the photoluminescence spectra were measured. Doping of p-type CdTe by the i...