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Cross-like nanostructures composed of two perpendicular ferromagnetic (Ga,Mn)As nanowires were fabricated using electron-beam lithography patterning. Nanostructures of different orientations with respect to the crystallographic axes of the parent (Ga,Mn)As epitaxial layer were studied. Electrical resistance of individual nanowires as a function of applied magnetic field were investigated at low temperatures. Low-field magnetoresistance (MR) of the nanowires exhibits hysteresis-like behaviour and related remnant resistance in zero magnetic field. These effects are explained in terms of magnetic domain walls (DWs) pinned at the wires intersection, which contribute to the wire resistance. The DW resistivity, which depends on the degree of spin misalignment in the wall, has been determined to be of the order of 1 Omega mu m(2). High-field MR has, in turn, allowed determining the lithography-induced anisotropy field for the nanowires of different crystallographic orientations. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (Less)

We fabricated a simple magnetoresistive microdevice formed by a narrow constriction of submicron width in the epitaxial film of a ferromagnetic (Ga,Mn)As semiconductor, and investigated magnetic properties of the film and the low-temperature charge-carrier transport through the constriction. We have revealed sharp jumps of a lowered conductance in a non-constricted sample and jumps of an enhanced conductance in the constricted one, which appeared when the sweeping magnetic field crossed the regions of the coercive field of the film. We argue that the both features result from a contribution of a magnetic domain wall to the conductance. While the spin-orbit interaction can be responsible for the negative contribution of a domain wall to the conductance, presumably the suppression of the weak localization effects by a domain wall located in the constriction results in the positive contribution to the conductance.

Lattice-mismatch induced defects have been studied by means of deep-level transient spectroscopy (DLTS) in two types of GaAs-based heterostructures, GaAs/InGaAs and GaAs/GaAsSb, and in a GaN-based laser-diode heterostructure grown on a bulk GaN substrate. DLTS measurements, carried out with p-n junctions formed near the heterostructure interfaces, revealed three deep-level traps in the investigated structures. which have been related to dislocations on the grounds of their logarithmic kinetics for capture of charge carriers into the trap states. An electron trap at E C - 0.64 eV and a hole trap at E V + 0.67 eV, found in the GaAs-based structures, have been attributed, respectively, to threading and misfit dislocations. An electron trap at E C - 0.23 eV, found in the GaN-based structure, has been tentatively related to dislocations in a p-type layer of the structure. Detailed investigation of the dependence of DLTS-line amplitude and its shape on the filling time of the traps with charge carriers allowed us to specify the type of electronic states associated with the traps.

Quantum effects characteristic of mesoscopic systems have been revealed in a study of vertical electron transport through GaAs-based heterostructures with a small lattice mismatch. At low temperatures and under strong magnetic field applied parallel to the axes of misfit dislocations generated at the interface, they manifest themselves as regular conductance fluctuations, appearing as a function of applied voltage and magnetic field. We interpret these fluctuations as being caused by trapping of charge carriers on quasi-stationary orbits, formed around the charged dislocations.

Semiconductor devices exploit electron charge to carry and process information, whereas magnetic materials make use of electron spin to store it. Hybridization of both the functions in one device is a goal that could be fully reached if we were able to manipulate magnetic moments by electric voltage. Having this in view we consider a rectangular island etched from a thin layer of a ferromagnetic semiconductor, which represents a single ferromagnetic domain. The island has suitably patterned Schottky gates deposited on the top of the layer. By applying a voltage to the gates, one could locally deplete carrier density beneath the gates and thus tailor ferromagnetic sub-domains of different shapes on the island. We show that it is possible in this manner to reverse the direction of magnetization of the island, which could constitute the principle of novel class of spintronic devices.

Two modes of electron gas injection in resonant tunnelling through GaAs/AlGaAs double-barrier heterostructures were revealed while studying their current—voltage characteristics. Examining peculiarities of the characteristics within the temperature range 4-350 K and under a high magnetic field, we were able to distinguish the contribution to resonant tunnelling of ballistic electrons mjected from a three-dimensional electron gas in the emitter contact and that of electrons injected from a two-dimensional electron gas in the accumulation layer formed near the emitter barrier.

The nature of a small “precursor” of resonance which appears in front of the main resonance peak in the current–voltage characteristic of resonant-tunnelling devices based on GaAs/AlGaAs double-barrier heterostructures is studied. The competition between the precursor and main-peak current is examined within the temperature range 2–400 K and under a magnetic field up to 13 T. The precursor is interpreted as the contribution to resonant tunnelling of ballistic electrons injected from a three-dimensional electron gas in the emitter region. Instead, the main resonance peak is due to the electron injection from a two-dimensional electron gas in a triangular well formed under bias in the emitter spacer layer.

Ring-shaped nanostructures have been designed and fabricated by electron-beam lithography patterning and chemical etching from thin epitaxial layers of the ferromagnetic semiconductor (Ga,Mn)As. The nanostructures, in a form of planar rings with a slit, were supplied with four electrical terminals and subjected to magneto-transport studies under planar weak magnetic field. Magnetoresistive effects caused by manipulation of magnetic domain walls and magnetization reversal in the nanostructures have been investigated and possible applications of the nanostructures as four-terminal spintronic devices are discussed.

The oscillations of tunnel current in double-barrier resonant tunneling structures incorporating a variety of types of spacer layers in both the emitter and the collector have been studied. In some structures the period of oscillations was found to be considerably less than LO-phonon energy. A simple model based on the quantum interference effect arising in structures having a wide spacer layer is proposed to explain the experimental results.

We investigated the current-voltage I(V) characteristics of GaAs AlAs double-barrier heterostructures. A fine periodic structure of the resonant tunnel current has been revealed. We attribute it to a sequence of the collective excitations, presumably of the coupled plasmon-phonon type, that are induced in the heavily doped collector region by hot electrons which escape from the quantum well. An oscillatory structure appears also in the valley regions of the I(V) curve under a high magnetic field parallel to the current. It is due to the off-resonance tunnelling between the Landau-quantized states of the emitter and quantum well. Particular phonon-assisted processes in the tunnelling have been identified.

Perpendicular transport through single- and double-barrier heterostructures consisting of ferromagnetic EuS layers embedded into PbS matrix was investigated. Manifestations of both resonant tunneling and spin filtering through EuS barrier have been observed.

We investigated thoroughly the current-voltage characteristic and the differential conductance of AlGaAs-based resonant-tunneling diodes. We have revealed a fine oscillatory structure of the broad plateau beyond the main resonance peak, displaying a period of 36 mV. We argue that this plateau represents a convolution of several satellite peaks resulting from the resonant tunneling via coupled electron-phonon states in the quantum well.

Some of our recent results obtained with the DLTS method concerning lattice defects in GaAs depending on mechanical stresses are presented and discussed.

Current passing in the forward direction through a GaAs-based heterojunction at liquid helium temperatures exhibits, under a strong magnetic field applied parallel to the junction plane, regular fluctuations. We argue that these fluctuations are associated with the formation of closed Aharonov-Bohm-type orbits encircling misfit dislocations generated at the interface in which the current carriers are temporarily trapped.

Presents experimental results obtained with resonant-tunneling diodes fabricated from (GaAl)As/GaAs and (CdMg)Te/CdTe semiconductor heterostructures grown - respectively - by low-pressure metal-organic vapor-phase epitaxy and molecular-beam epitaxy methods. These results demonstrate, firstly, the appearance of intriguing oscillatory structure on the rising slope of the resonant peak in small-diameter (GaAl)As/GaAs-based devices and, secondly, the improvement of current-voltage characteristics of (CdMg)Te/CdTe-based resonant-tunneling diodes due to reduction of their cross-sectional area.

We analyse conditions for an apapearance of the electrostatic Aharonov- Bohm interference in two systems: a single-channel quantum-wire loop and an open ballistic quantum dot. We show that in the first system the effect will be destroyed by charge fluctuations, which probably is the reason why it has not been clearly observed, while in the second system the effect is still open for exploration.

The effect of a magnetic field in the singular enhancement of tunneling near threshold has been investigated in specifically designed double-barrier resonant tunneling structures. The experimental results prove that an additional increase of tunnel current is due to impurity-assisted tunneling. It is shown that a donor bound state arises as a result of dopant diffusion into the barrier. These states are identified as strongly localized DX centers.

Low-temperature magneto-conductance oscillations, periodic in magnetic field, are observed in GaAsSb/GaAs heterostructures with a small lattice mismatch. Those oscillations appear only under the conditions of a strong magnetic field applied parallel to the axes of misfit dislocations generated at the interface. They are attributed to a trapping of free charge carriers on localised quasistationary orbits encircling the charged dislocations, whose appearance is controlled by the Aharonov-Bohm interference.

In this paper, we demonstrate that dislocations in a macroscopic semiconductor specimen can give rise to specific mesoscopic effects. In the first place, we compare with each other two kinds of closed electron orbits that can appear in small specimens of metals and semiconductors in a magnetic field: the cyclotron orbits and the Aharonov-Bohm orbits. Next, we consider possible Aharonov-Bohm orbits encircling a dislocation in a semiconductor under a strong magnetic field whose direction is aligned with the dislocation axis. Finally, we demonstrate experiments confirming the formation of such orbits around misfit dislocations in semiconductor heterostructures with a small lattice mismatch. They manifest themselves at low temperatures as regular fluctuations of conductance through a heterostructure, appearing as a function of applied voltage and magnetic field.

An effect of magnetic field normal to the tunnel current on the amplitude and phase of the fine oscillatory structure, discovered in the resonance current-voltage curve in double-barrier AlAs/GaAs heterostructures, has been examined. All the obtained results are consistently explained in terms of the interference of ballistic electrons, escaped from the quantum well, in the colfector part of the structure. PACS numbers: 73.40.Gk, 85.30.Mn