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Valence-band structure and magnetic properties, especially magnetic anisotropies, in the (Ga,Mn)As diluted ferromagnetic semiconductor are shortly discussed. Next, magneto-resistive, cross-like nanostructures fabricated by electron-beam lithography patterning and chemical etching from thin (Ga,Mn)As epitaxial layers are described. The nanostructures, composed of two perpendicular nanostripes crossing in the middle of their length, represent four-terminal devices, in which an electric current can be driven through any of the two nanostripes. In these devices, which make use of the patterning-induced magnetic anisotropy, a novel magneto-resistive memory effect related to a rearrangement of magnetic domain walls in the central part of the device, has been demonstrated. The effect consists in that the zerb-field resistance of a nanostripe depends on the direction of previously applied magnetic field. The nanostructures can thus work as two-state devices providing basic elements of nonvolatile memory cells. (C) 2012 Elsevier B.V. All rights reserved. (Less)

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)

Novel magneto-resistive memory effect is demonstrated, which appears in lithographically patterned Y-shaped, three-terminal nanostructures, consisting of three nanowires joined in one point, formed in the epitaxial layer of a ferromagnetic (Ga,Mn)As semiconductor. The effect, in which the zero-field resistance of a pair of nanowires depends on the direction of the previously applied magnetic field, results from a rearrangement of magnetic domain walls, contributing an extra resistance, between different pairs of nanowires in the structure. Two-state behaviour of the nanostructure resistance provides its usefulness for applications in non-volatile memory devices.

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.

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.

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.

Recently, we have discovered a fine oscillatory structure (FOS) of the current flowing through resonant tunnelling devices consisting basically of two very thin AlAs barriers separated by a GaAs quantum well. In this work we have found two striking properties of FOS in a magnetic field applied normally to the current flow: (i) a phase shift of the FOS on the bias-voltage scale which is proportional to the squared induction of the magnetic field; (ii) rapid oscillatory-type fall-off in FOS amplitude with increasing magnetic field. We demonstrate, using a simple quantitative model, that both properties can be consistently explained in terms of quantum interference between electrons which escaped from the quantum well and those partly reflected at a potential step on the collector side of the device. The physical conditions necessary for the appearance of FOS are discussed.

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.

Interrelations between the EL2 defects and dislocations in GaAs crystals are examined. Firstly, it is discussed, on the basis of recent results of high-pressure experiments, why the dislocation-related EL2 defects can be unquenchable. Next, the experimental data are presented that show a distinct correlation of the EL2 defect concentration with the distribution of indentation-induced dislocations in n-type GaAs crystals. Finally, it is argued that the deformation-induced enhancement of EL2 concentration is caused by dislocation-mediated generation of arsenic antisites, while in as-grown LEC GaAs the correlation between EL2 and dislocations results rather from their common origin. Die Wechselbeziehungen zwischen EL2-Defekten und Versetzungen in GaAs-Kristallen werden untersucht. Auf der Grundlage neuer Ergebnisse aus Hochdruckexperimenten wird diskutiert, warum die versetzungsbezogenen Defekte untilgbar sein konnen. Weiterhin werden experimentelle Fakten angefuhrt, die eine klare Korrelation zwischen der EL2-Defektkonzentration und der Verteilung der Indenter-induzierten Versetzungen in n-GaAs-Kristallen zeigen. Schlieslich wird bewiesen, das die durch Deformation induzierte Erhohung der EL2-Konzentration durch die Erzeugung von As-Antigitterplatzen, vermittelt durch die Versetzungen, verursacht wird, wahrend in gewachsenem LEC-GaAs die Korrelation zwischen EL2 und Versetzungen auf ihren gemeinsamen Ursprung zuruckzufuhren ist.

Some properties of native defects in melt-grown GaAs crystals are discussed. Connection of the dominant native defect, EL2, with the antistructure disorder is analysed. Appearance of the correlation between EL2 and dislocations is discussed in terms of the generation of arsenic antisites through interaction of interstitial arsenic with gliding dislocations. A fundamental role of the precipitation of one component of the crystal in accomodation of its non-stoichiometry and in the generation of excess dislocations is revealed. Vacancies complexed with the arsenic antisites or interrelated with EL2 are considered as possible objects for positron annihilation studies. Einige Eigenschaften der Eigendefekte in aus der Schmelze gezuchteten GaAs Kristallen werden diskutiert. Die Verbindung des Hauptdefekts, EL2, mit antistruktureller Unordnung wird untersucht. Das Modell einer Generation Von AsGa durch eine Wechselwirkung vom Zwischengitterarsen und gleitende Versetzung wird zur Deutung der Korrelation zwischen EL2 und Versetzungen vorgeschlagen. Es wird gezeigt, das in der beobachteten Kristallnichtstochiometrie und bei der Generation Von “grown-in” Versetzungen die Ausscheidung einer Komponente des Kristall eine entscheidende Rolle spielt. Eine Untersuchung der mit AsGa oder mit EL2 verbundenen Leerstellen durch Positronannihilation wird vorgeschlagen.

The effect of a magnetic field on the resistance of a conductor containing a spherical void is calculated for transverse and longitudinal configurations. The results are extrapolated on the case of compensated semiconductors for explaining anomalous magnetoresistance. Es wird der Einflus eines Magnetfeldes auf den Widerstand eines Leiters mit einem kugelformigen Hohlraum in transversaler und longitudinaler Konfiguration berechnet. Die Ergebnisse werden auf den Fall kompensierter Halbleiter extrapoliert zur Erklarung der anomalen Magnetowiderstandsanderung.

The recent experimental results on spin-dependent recombination at dislocations in Si are reviewed. A theoretical model including an exchange interaction between unpaired electrons in the dislocation core is proposed. The numerically calculated exchange integral appears to have a positive value. The model presents a dislocation as consisted of segments of exchange-coupled paramagnetic centres. The magnetization of the segments, calculated in Ising approximation is compared with the experimental results. Kurzliche experimentelle Ergebnisse von spinabhangiger Rekombination an Versetzungen in Si werden erneut diskutiert. Ein theoretisches Modell wird vorgeschlagen, das Austauschwechsel-wirkung zwischen ungepaarten Elektronen im Kern der Versetzung annimmt. Das numerisch berechnete Austauschintegral scheint einen positiven Wert zu haben. Das Modell liefert eine Versetzung, die aus Abschnitten von Austausch-gekoppelten paramagnetischen Zentren besteht. Die Magnetisierung der Abschnitte, die mit der Ising-Naherung berechnet wird, wird mit den experi-mentellen Ergebnissen verglichen.

The influence of the magnetic field on the distortion of current flow around a spherical void in an isotropic conductor is considered. The Hall coefficient and resistance are calculated for such a case up to terms involving the square of the magnetic field. Es wird der Einflus eines Magnetfeldes auf die Verzerrung des Stromflusses um einen spharischen Hohlraum in einem isotropen Leiter untersucht. Hallkoeffizient und Widerstand werden fur einen derartigen Fall bis zu Termen berechnet, die das Quadrat des Magnetfeldes enthalten.

Existing experimental results concerning energetic position of electron states associated with edge dislocations in Ge are critically analysed, and some new data are presented. It is argued that the structure of dislocation states is more complex than assumed previously. Taking the deformation field around a dislocation into account one can expect the effective dislocation levels to be located somewhere in the central part of the forbidden gap. This is really confirmed by photoconductivity studies both in Ge and Si. Bekannte experimentelle Ergebnisse uber die energetische Lage von Elektronenzustanden, die mit Stufenversetzungen in Ge verbunden sind, werden kritisch analysiert und einige neue Werte angegeben. Es wird angenommen, das die Struktur der Versetzungszustande komplizierter ist als fruher vermutet. Durch Berucksichtigung des Deformationsfeldes um eine Versetzung kann erwartet werden, das die effektiven Versetzungsniveaus irgendwo in der Mitte der verbotenen Zone liegen. Dies wird durch Photoleitungsuntersuchungen sowohl an Ge als auch an Si bestatigt.

Formulae for the crystal deformation due to the introduction of charge carriers are obtained using the thermodynamic potential method. This phenomenon is caused by the direct electron-lattice interaction. A change in the energy gap due to this effect is also estimated. for example the band gap in InSb is reduced by about 0.01 eV when 1019 electrons per cm3 are introduced into the crystal. Mit Hilfe der Methode der thermodynamischen Potentiale wird ein Ausdruck fur die durch freie Ladungstrager hervorgerufene Kristalldeformation abgeleitet, die auf der direkten Elektron-Gitter-Wechselwirkung beruht. Es wird die entsprechende Anderung des Bandabstandes abgeschatzt. Es ergibt sich, das sich beispielsweise der Bandabstand in InSb bei der Erzeugung von 1019 Elektronen pro cm3 um etwa 0,01 eV verringert.

The mechanism of creating a stable print at microhardness indentation in diamond-structure crystals is discussed, in which dislocations are quite immobile at room temperature. It is argued that this mechanism consists in the generation of prismatic dislocation loops in the under-indenter region and subsequent removal of the loops from this region, due to a special kind of dislocation motion called conservative climb. A simplified analytical model of such a process is presented and according to that model certain numerical values and dependences associated with microhardness measurement are estimated and compared with those found experimentally for Ge. Der Mechanismus der Erzeugung eines stabilen Abdrucks bei Eindruckmessungen der Mikroharte wird in Kristallen mit Diamantstruktur, in denen Versetzungen bei Raumtempertur vollig unbeweglich sind, diskutiert. Es wird angenommen, das dieser Mechanismus in der Erzeugung von prismatischen Versetzungsschleifen in den Bereichen unter dem Eindruck besteht, der durch eine spezielle Art von Versetzungswanderung (konservatives Klettern) hervorgerufen wird. Ein vereinfachtes analytisches Modell solch eines Prozesses wird vorgeschlagen und in Ubereinstimmung mit diesem Modell werden gewisse numerische Werte und Abhangigkeiten, die mit der Messung der Mikroharte verknupft sind, gemessen und mit denen verglichen, die experimentell fur Ge gefunden wurden.

Steady state and transient photoconductivity are investigated in plastically bent p-type Ge. At low temperatures a trapping effect is observed. All characteristics of photoconductivity are qualitatively the same as for plastically deformed n-type Ge in which recombination occurs through dislocations. The experimental results are explained on the basis of a barrier model of recombination at dislocations assuming that in p-type Ge a dislocation which gives up the unpaired electrons to the valence band becomes positively charged. It is found (from experiment) that the dislocation donor level lies (0.28 ± 0.02) eV above the top of the valence band at 0 °K. The comparison with previous results obtained for n-type Ge shows that the location of the donor as well as of the acceptor dislocation level is approximately the same in the energy gap. This result is consistent with the assumption that the states connected with the broken bonds at dislocation form an one-dimensional energy band. Stationare und nichtstationare Photoleitfahigkeit wird an plastisch gebogenem p-leitendem Germanium untersucht. Bei niedrigen Temperaturen wird ein Haftstelleneffekt beobachtet. Alle Merkmale der Photoleitung sind qualitativ dieselben wie fur plastisch verformtes n-leitendes Ge, wo die Rekombination durch Versetzungen erfolgt. Die experimentellen Ergebnisse werden auf der Grundlage eines Barrierenmodells der Rekombination an Versetzungen erklart, wobei angenommen wird, das in p-leitendem Ge Versetzungen, die die ungepaarten Elektronen an das Valenzband abgeben, positiv aufgeladen werden. Es wird gefunden (aus dem Experiment), das das Donatorniveau der Versetzung bei 0 °K (0,28 ± 0,02) eV oberhalb der Valenzbandkante liegt. Der Vergleich mit fruheren Ergebnissen an n-leitendem Ge zeigt, das die Lage sowohl der Donator- als auch der Akzeptorniveaus in der verbotenen Zone naherungsweise dieselbe ist. Dieses Ergebnis stimmt mit der Annahme uberein, das die Zustande, die mit den aufgebrochenen Bindungen an den Versetzungen verbunden sind, ein eindimensionales Energieband bilden.

A possible mechanism for electron recombination with a hole trapped at an edge dislocation in Ge is considered. It is assumed that the electron transfers a part of its energy to the oscillations of the charge system along the dislocation, being itself captured in one of the vibrational states. The probability of such process is estimated by the harmonic oscillator approximation. The calculated values of the lifetime are in order-of-magnitude agreement with those observed experimentally in plastically deformed germanium. Es wird ein Rekombinationsmechanismus untersucht, bei dem ein Elektron mit einem Loch rekombiniert, das an einer Stufenversetzung im Ge angehaftet ist. Es wird angenommen, das das Elektron einen Teil seiner Energie an die Schwingungen des Ladungssystems entlang der Versetzung abgibt, da es selbst in einen der Schwingungszustande eingefangen ist. Die Wahrscheinlichkeit fur das Auftreten eines derartigen Prozesses wurde in der Naherung harmonischer Oszillatoren berechnet. Die berechneten Werte fur die Lebensdauer stimmen grosenordnungsmasig mit den an plastisch deformierten Ge gefundenen Werten uberein.

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.