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The temperature-dependent polarized photoluminescence spectra of nonpolar ZnO samples were investigated by 263 nm laser. The degree of polarization (DOP) of m-plane quantum wells changes from 76% at 10 K to 40% at 300 K, which is much higher than that of epilayer. The strong anisotropy was presumably attributed to the enhanced confinement effect of a one-dimension confinement structure formed by the intersection of quantum well and basal stacking fault. The polarization of laser beam also has an influence on the DOP. It is assumed that the luminescence polarization should be affected not only by the in-plane strains but also the microstructural defects, which do modify the electronic band structure.

The influence of quantum confinement on the spin polarization of holes in ferromagnetic multiple quantum wells based on (Ga,Mn)As diluted magnetic semiconductor has been investigated. It is shown that the spin polarization of holes in the impurity band is more likely determined by the magnetic properties of GaMnAs rather than the quantum-confinement effect. The model of Mn acceptor in a QW, describing the polarization characteristics of photoluminescence in GaAs: Mn/AlAs QWs, has been developed. Experimental data and theoretical analysis show that the spin polarization of holes in (Ga, Mn)As/AlAs QWs can be explained within a model, which suggests that holes are localized in the impurity band.

The phase transition near 40~$^{\circ }$C of both as-grown thin epitaxial MnAs films prepared by molecular beam epitaxy on GaAs(001) and nanometer-scale disks fabricated from the same films is studied. The disks are found to exhibit a pronounced hysteresis in the temperature curve of the phase composition. In contrast, supercooling and overheating take place far less in the samples of continuous layers. These phenomena are explained in terms of the necessary formation of nuclei of the other phase in each of the disks independent from each other. The influence of the elastic strains in the disks is reduced considerably.

We have studied the effect of stress on the magnetic properties of ferromagnetic (Ga,Mn)As diluted magnetic semiconductor. We show that the magnetization of a thin ferromagnetic (Ga,Mn)As layer can be manipulated by uniaxial stress applied in its plane. The effect of stress manifests itself in spin depolarization of holes and stabilizing the easy axis in the direction of the applied stress. The developed theoretical model, assuming stress induced mixing of Mn acceptor-bound hole (or impurity-band bound hole) wave functions, well describes the observed photoluminescence polarization properties in a magnetic field.

Intra Landau level thermal activation, from localized states in the tail, to delocalized states above the mobility edge in the same Landau level, explains the $B_c(T)$ (half width of the dissipationless state) phase diagram for a number of different quantum Hall samples with widely ranging carrier density, mobility and disorder. Good agreement is achieved over $2-3$ orders of magnitude in temperature and magnetic field for a wide range of filling factors. The Landau level width is found to be independent of magnetic field. The mobility edge moves, in the case of changing Landau level overlap to maintain a sample dependent critical density of states at that energy. An analysis of filling factor $\nu=2/3$ shows that the composite Fermion Landau levels have exactly the same width as their electron counterparts. An important ingredient of the model is the Lorentzian broadening with long tails which provide localized states deep in the gap which are essential in order to reproduce the robust high temperature $B_c(T)$ phase observed in experiment., Comment: 15 pages, 12 figures

The micromagnetic properties of MnAs thin films grown on the (111)B-oriented GaAs surface have been investigated. Compared to films grown on (001) surfaces, these films exhibit completely different domain patterns, as the c axis of the hexagonal unit cell is oriented normal to the surface. In the course of the first order phase transition, ferromagnetic α -MnAs forms a network of quasihexagonal areas separated by Β -MnAs. We present an analysis of the micromagnetic properties based on imaging and simulations. We observe closure domains that either appear as a vortex-like state or a stripe structure. © 2006 American Institute of Physics.

We report on apparently charged domain walls in MnAs-on-GaAs(0 0 1) layers. For head-on domains, described as two domains facing each other with opposite magnetization, the domain walls of greater than or similar to 200 nm thick films exhibit a zig-zag pattern. Depending on the width of the ferromagnetic stripes, which is a function of temperature and thus the strain in the easy axis direction, the zig-zag angle 20 increases from 90 degrees in the case of wide stripes to 180 degrees (i.e., to a straight wall) for narrow stripes. The underlying domain structure was calculated using a three-dimensional micromagnetic simulator. The calculations reveal a number of distinct domain patterns as a result of the system's attempt to reduce its energy through the formation of closure domain-like patterns in the easy plane. A diamond-like state consisting of two intersecting sub-surface domain walls is the underlying magnetic structure resulting in the observed, apparently charged domain walls. The zig-zag pattern of the domain boundary is explained by stray field minimization of the diamond state along the stripe. (C) 2006 Elsevier B.V. All rights reserved.

We report the high-aspect ratio patterning of epitaxial MnAs-on-GaAs(0 0 1) films. The control of strain is key since MnAs-on-GaAs(0 0 1) exhibits a strain-stabilized coexistence of two chemically, elastically and magnetically distinct phases forming a self-organized stripe structure over a temperature range of 10-40 °C. Anisotropic plasma etching allows for high-aspect ratios and good reproducibility. Using Ti films as an etch mask, arbitrarily oriented structures can be transferred into films of up to 300 nm thickness. The removal of the masking material is challenging as MnAs reacts with all common acids, alkalis and even water. Optimum results are obtained by etching the Ti mask in hydrofluoric acid at elevated temperatures (>50 °C), where MnAs is entirely in its β-phase. © 2006 IOP Publishing Ltd.

We have studied the domain behavior of submicrometer wide ferromagnetic stripes by magnetic force microscopy (MFM) in the presence of an in situ magnetic field. MFM images in the demagnetized state show alternatingly magnetized domains fully extended across the stripe. Moreover, domain structures are found to exhibit a substructure across the stripe. Increasing fields drive out the domain walls of the complex domains first, leaving the alternating domains behind. The remaining magnetization process aligns increasing parts of the domains along the field direction by gradually shrinking the width of oppositely magnetized domains rather than by flipping larger areas at once. Micromagnetic simulations confirm the observed behavior. The simulations reveal that flipping of the domains occurs only when a magnetic pinning center is involved. © 2005 American Institute of Physics.

We present variable-temperature magnetic force microscopy (VT-MFM) studies of epitaxially grown MnAs layers on GaAs(001). In MnAs, a structural and magnetic phase transition occurs at Tc ≈ 40 °C from the hexagonal, ferromagnetic α-phase below Tc to the orthorhombic, paramagnetic β-phase above Tc. In the investigated MnAs-GaAs system, both phases coexist over a temperature range of ≈ 30 °C below Tc due to the involved strain. Using MFM we are able to distinguish between the ferromagnetic and the paramagnetic phases by measuring topographic and magnetic contrast of the same sample area. For VT-MFM studies, we have employed a temperature stage that allows heating and cooling in a controlled atmosphere with small thermal drift (for this system, the temperature was varied from below 0 °C to above 45 °C). The ratio of the ferromagnetic to the paramagnetic phases shows a temperature hysteresis, i.e. the ratio is dependent on whether the sample was heated or cooled to reach the measurement temperature. Detailed studies of the domains and their arrangements over the hysteretic temperature cycle are shown and compared. Mainly three different domain types are found that are dominant in their respective temperature ranges.

Zn1-xMgxO films with x = 0.04-0.50 grown on MgO (100) substrates by molecular beam epitaxy retain the rocksalt (rs) crystal structure and grow epitaxially for x ≥ 0.17. In addition, the rs-ZnO epilayer is observed to be stable up to a thickness of 5 nm and also in a ZnO/MgO superlattice sample. However, a portion of the superlattice has transformed to wurtzite (wz)-structure islands in a self-accommodated manner during growth. The transformation is a combination of a Bain distortion, an in-plane rotation of 14.5°, and a Peierls distortion, resulting in an orientation relationship of (100)rs//(101̄0)wz and 〈011〉rs ∼//〈1̄21̄3〉wz. In such a manner, the volume expansion is only necessary along the growth direction and the in-plane strains can be minimized. A negative pressure generated during the transformation of ZnO stabilizes the MgO into a wurtzite structure.

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A strong dependence of the magnetic anisotropy on the growth temperature of Fe 3 + x Si 1 - x Heusler alloy films on GaAs(1 1 3)A substrates is reported for a composition of 17.5 at . % Si ( x = 0.34 ). This composition of Fe–Si alloy lies within the stable phase of the technologically promising Heusler alloy Fe 3 Si . The layers grown at the optimized growth temperature of 250 °C exhibit the expected four-fold magnetic anisotropy, which arises from the magnetocrystalline anisotropy and the large demagnetization energy, similar to Fe films. However, an unexpected strong uniaxial magnetic anisotropy is found for samples grown at 200 and 400 °C with the easy axes along 〈 3 3 2 ¯ 〉 and 〈 1 ¯ 1 0 〉 , respectively. The uniaxial magnetic anisotropy of these samples are shown to be related to the inferior interface quality.

We study the nucleation and growth kinetics on the Ge(0 0 1) surface at elevated temperatures using in situ surface X-ray diffraction. The time evolution of characteristic length scales on the surface is analyzed through the widths of the different components of the integer-order (morphology sensitive) and fractional-order (reconstruction sensitive) diffraction peaks. We find an activation energy of 0.58 eV for Ge island nucleation during homoepitaxy, which implies a diffusion activation energy higher than that obtained for both adatom and dimer diffusion on Ge(0 0 1) in previous studies. Sub-monolayer homoepitaxial Ge islands coarsen according to a power law, with a relatively low time exponent of n =0.2. The coarsening of small 2×1 reconstruction domains on a flat surface prepared by deposition of an integer number of layers shows a strong temperature dependence, whereby the coarsening exponent decreases from 0.41 to 0.2 as the temperature is increased.

We report on a study of the electrical and optical properties of strained M-plane GaN films grown on LiAlO2 substrates. We have electrically characterized Schottky barrier devices fabricated on strained M-plane GaN films. The current leakage found in these devices is probably related to a surface effect. The influence of the current direction for in-plane transport in M-plane GaN was also studied. Polarized photoluminescence spectroscopy shows the effect of residual biaxial strain on the optical selection rules, allowing to identify intrinsic transitions involving holes from the two uppermost valence bands. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular beam epitaxy of Fe 3 Si films on GaAs (001) is studied in situ by grazing incidence X-ray diffraction. Fe 3 Si grows layer-by-layer. During deposition the growth front roughens as indicated by the damping of the X-ray oscillations and corresponding atomic force micrographs. The X-ray oscillations are modified during growth at substrate temperatures of 180 °C and below.

We have investigated the growth, interface structure, and magnetic properties of full-Heusler alloy Co 2 FeSi/GaAs(001) hybrid structures. We found that a L2 1 -ordered Co 2 FeSi layer with an atomically abrupt interface can be achieved up to the growth temperature T G of 200°C. The simulations of the high-resolution transmission electron microscopy interference pattern for Co 2 FeSi/GaAs(001) interface suggests a 1 layer of intermixing at the interface. Both uniaxial and cubic in-plane magnetic anisotropy constants decrease above 200 °C in correspondence with the proceeding interfacial reaction, indicating degradations of the interface perfection as well as the ordering of the layer. The formation of interfacial compounds reduces the saturation magnetization of the Co 2 FeSi layer in higher T G , which can deteriorate its expected half-metallicity.

We have studied the growth temperature dependence of the interfacial reaction of Heusler-alloy Co2FeSi/GaAs(0 0 1) hybrid structures. The reaction proceeds dominantly by Co in-diffusion, resulting in the formation of isolated grains in the GaAs substrate starting at the growth temperature TG of 200–250 °C. The interfacial reaction is classified into two stages: (i) intermediate TG regime (250–300 °C), where a highly oriented single-crystalline phase, most likely ternary Co2GaAs, is formed in the topotaxial relationship of ; and (ii) high TG regime (≥350 °C) where binary CoAs is formed in the asymmetric topotaxial relationship of CoAs[0 0 1](2 1 0) || GaAs[1 1 0](0 0 1).

The geometry of the sample is shown to dictate the importance of strain on the spin-orbit splitting of the conduction band for electrons transported via dynamic quantum dots (DQDs) generated by surface acoustic waves (SAWs). Photoluminescence measurements monitoring the Larmor precession of electron spins in the absence of an applied magnetic field are used to characterize the spin-orbit splitting of the conduction band. We show that, in thick quantum wells (QWs), the Dresselhaus spin-orbit contribution to the precession frequency reduces and may become comparable to the one associated with the strain field induced by the SAW. We attribute deviations in the measured precession frequency to strain effects, which become more pronounced for QWs located closer to the surface because of the larger SAW strain field. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The structure of epitaxially grown Pr2O3 on Si(0?0?1) has been investigated by grazing incidence x-ray diffraction and x-ray reflectivity. We report experimental results from measurements made in situ, after growth, and during annealing to determine the stability of Pr2O3 at temperatures relevant for complementary metal oxide semiconductor (CMOS) processing. Pr6O11 is evaporated and converted to Pr2O3 using an effusion source operating at 1970 ?C. Two different phases of Pr2O3 have been identified in the as-grown films in addition to a silicate layer that forms upon nucleation at the Si interface. The cubic Pr2O3 nucleates as a single crystal and grows in greater proportion to the polycrystalline hexagonal Pr2O3. The crystal structure of the as-grown film is stable during annealing up to about 800 ?C at which point the Pr2O3 is consumed at the expense of the silicate phase.

We explore wet and dry etching processes of thin MnAs layers grown on GaAs(0 0 1) substrates for microstructuring. Most of the common wet chemical etch solutions for GaAs react with MnAs strongly and in a peculiar manner. Unidirectional cracks are generated when the MnAs layers are thicker than 100 nm. We demonstrate that the crack generation can be avoided by choosing a suitable etch solution or etch temperature. We fabricate submicrometer-wide MnAs wires using Ar ion milling. The resistivity of the narrow channels is measured over a temperature range covering the phase transitions in MnAs between the α , β , and γ phases. The resistivity along the MnAs[0 0 0 1] direction is found to be smaller than that along the MnAs [ 1 1 2 ¯ 0 ] direction regardless of the phase. A nearly linear temperature variation of the phase fraction is deduced in the α – β phase coexistence regime. The temperature coefficients of the resistivities are negative for the nonmagnetic phases.

We demonstrate a technique based on wet chemical etching that enables quick and accurate evaluation of edge- and screw/mixed-type threading dislocations (TDs) in GaN. Large and small etch pits are formed by phosphoric acid on the etched surfaces. The large etch pits are attributed to screw/mixed TDs and the small ones to edge TDs, according to their locations on the surface and Burgers vectors of TDs. Additionally, the origin of small etch pits is confirmed by a transmission electron microscopy. The difference in the size of etch pits is discussed in view of their origin and merging. Overetching at elevated temperatures or for a long time may result in merging of individual etch pits and underestimating of the density of TDs. Wet chemical etching has also been proved efficient in revealing the distribution of TDs in epitaxial lateral overgrowth GaN.

InxGa1-xAs1-yNy/Al0.33Ga0.67As multiquantum wells grown by plasma-assisted molecular beam epitaxy are studied by resonant inelastic light scattering. Sharp vibration modes have been observed at 323, 402, 454 and 501 cm-1. Their intensities resonate at the barrier bandgap reduced by the presence of N. Their resonance energies reveal the influence of the N concentration on the barrier gap at the multiquantum well interfaces. These peaks are interpreted in terms of local vibrations involving the pairing of N atoms, which seems to occur mostly at the quantum well interfaces due to preferential bonding of N to Al.

The magnetic properties of full Heusler alloy (Co(2/3)Fe(1/3))(3+x)Si(1-x)/GaAs(001) hybrid structures grown by molecular beam epitaxy have been investigated. The magnetic moment, the coercive field and the in-plane magnetic anisotropy of (Co(2/3)Fe(1/3))(3+x)Si(1-x) films with various Si compositions (-0.46≤x≤1) are discussed. The increase in amount of Si results in a significant reduction in the cubic magnetocrystalline anisotropy constant |K(1)(eff)|. K(1)(eff) changes sign and saturates near the stoichiometric composition of Co(2)FeSi and the easy axis of the cubic component changes from the [Formula: see text] direction to the [Formula: see text] direction accordingly. However, due to the presence of a dominating uniaxial magnetic anisotropy component, the easy axis of magnetization in total is shifted to the [110] direction. The saturation magnetization of stoichiometric Co(2)FeSi films turned out to be 1250 ± 120 emu cm(-3), being equivalent to 6.1 ± 0.57 (μ(B)/formula unit (fu)). The relatively close value of magnetic moment to the theoretically expected integer value (6 μ(B)) suggests that Co(2)FeSi films could be half-metallic ferromagnets.

We report resonant Raman scattering measurements on InGaAsN/AlGaAs multiquantum wells grown by plasma-assisted molecular beam epitaxy. Sharp vibration modes have been observed at 323, 402, 454 and 501 cm -1 . They are resonant in the energy range 1.81-1.87 eV at decreasing energies for increasing N concentration. The resonance energies reveal the influence of the N concentration on the barrier gap at the multiquantum well interfaces. The peaks are interpreted in terms of local vibrations involving GaN and AlN units and pairs. They seem to form mostly at the quantum well interfaces due to preferential N bonding to Al.

We have studied by X-ray absorption spectroscopy the local environment of Mn in highly homogeneous Ga 1-x Mn x N (0.06

We present results on fabrication, and structural and electrical properties of single-crystal Fe 3 Si / GaAs ( 0 0 1 ) heterostructures grown by molecular beam epitaxy. The exact stoichiometry of the Heusler alloy films can be achieved for almost lattice matched films. As evidenced by high-resolution X-ray diffraction, transmission electron microscopy, and resistivity measurements, we find an optimum growth temperature of T G = 250 ∘ C , to obtain ferromagnetic layers with high crystal and interface perfection as well as high degree of atomic ordering. .

We report resonant Raman scattering and secondary ion mass spectrometry measurements on InGaAsN/AlGaAs multiquantum wells grown by plasma-assisted molecular beam epitaxy. The appearance of a strong TO band at resonance with nitrogen (N)-related electronic levels has been observed. The N-induced vibration mode at 470cm -1 changes in intensity and shape with increasing N and In content. A new vibration mode has been observed at 320 cm -1 , whose intensity scales with the N concentration. This mode is not present in InGaAsN films, so it is linked to the presence of Al. Its frequency is close to the B 1 silent mode of wurtzite GaN. It is attributed to the formation of GaN pairs, near the MQW interfaces as a consequence of the preferential Al-N bonding.

Mobile quantum dots (denoted as dynamic quantum dots) formed by the interference of orthogonal beams of surface acoustic waves (SAWs) in an undoped GaAs quantum well coherently transport electron spins over distances exceeding 70 μm. We determine the spin-splitting parameter for a GaAs quantum well to be 7 = 17 ± 2 eV A 3 as well as investigate the effects of the SAW strain field on the spin dynamics.

We have observed an abrupt change at low temperatures in the intrinsic anomalous and planar Hall conductivities of nearly stoichiometric Fe3Si epitaxial thin films grown on GaAs(001). To describe this magnetotransport regime, we phenomenologically take into account the reduction in symmetry resulting from the onset of coherent spin fluctuations. Our theoretical approach can thus account for these different magnetotransport regimes in both the anomalous Hall effect and planar Hall effect as well as the impact on intrinsic magnetotransport of atomic disorder away from stoichiometry.

The electron transport in Ga(As,N) layers is investigated focusing on the influence of potential fluctuations. A metal-insulator transition is observed with increasing electron concentration, in the temperature dependence of the resistivity for a series of samples with 0.8% and 1.7% of N. The observed behavior is discussed in the frame of Anderson transition. A quantitative value for the percolation threshold Vp, at which the transport characteristics will change, is obtained by considering a chemical potential and an activation energy. The large Vp can explain the thermally activated conductivity which can be observed even up to 300 K. We observe an increase of Vp by increasing the N concentration up to 2.2%. Vp can be efficiently reduced by rapid thermal annealing for samples with higher N concentration. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The magnetic anisotropy energy and the resonance linewidth of epitaxial Fe3Si grown on GaAs(001) is studied by ferromagnetic resonance (FMR) technique. For a nearly stoichiometric sample (25.5% Si-content) a clear fourfold anisotropy is observed which is modulated by a small contribution of an uniaxial in-plane anisotropy. This uniaxial component is found to increase with the Fe concentration in the intermetallic compound. The samples show an exceptional narrow resonance linewidth of only 17 Oe at 9 GHz excitation frequency confirming the very high structural quality. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We numerically calculate the conductance of a one-dimensional Aharonov-Bohm loop containing a weakly coupled dot. The Coulomb repulsion is taken into account in the dot within the slave-boson mean-field approximation. We avoid the so-called phase locking by attaching an additional lead to the loop. The phase of the electron wavefunction deduced from the Aharonov-Bohm oscillation changes gradually by π when the Fermi energy sweeps across a transmission resonance in the dot. The asymmetric lineshape of Fano-type resonances is also examined.

We report epitaxial growth of the Heusler alloy Fe3Si on high-index GaAs(1 1 3)A substrates by molecular-beam epitaxy. The growth temperature and growth rate are optimized to 250 ∘ C and 0.13 nm/min, respectively, for producing Fe3Si films with structural properties comparable to that of Fe3Si films on GaAs(0 0 1). The layers grown under these conditions exhibit high crystal quality with smoother interface/surface and maintain the [1 1 3] orientation of the GaAs substrate. The Fe–Si alloy composition is varied around the Fe3Si stoichiometry using these optimized growth conditions. The magnetic properties of a typical Fe3Si layer with the best structural properties exhibit a four-fold magnetic anisotropy, as expected from the magnetocrystalline anisotropy.

We employ superconducting quantum interference device magnetometry to study the thickness dependence of in-plane and out-of-plane magnetic anisotropic properties of Fe films grown on high-index GaAs(113)A substrates by molecular beam epitaxy. The evolution of the in-plane magnetic anisotropy with film thickness is distinguished into two regions. First, for Fe film thicknesses ≤50 MLs, we observe an in-plane uniaxial magnetic anisotropy with the easy axis along the in-plane 〈332〉 axes. Second, for Fe film thicknesses ≥70 MLs, we observe a four-fold magnetic anisotropy with the easy axis along the in-plane 〈031〉 axes. The existence of an out-of-plane perpendicular magnetic anisotropy is also detected in ultrathin Fe films. Similar to Fe on GaAs(001), our results provide evidence for the interfacial origin of the in-plane uniaxial and out-of-plane perpendicular magnetic anisotropy. Both the uniaxial and the perpendicular interface anisotropy are found to be independent of the epitaxial orientation and are hence an intrinsic property of the Fe/GaAs interface.

Spin transport and manipulation in semiconductors have been studied intensively with the ultimate goal of realizing spintronic devices. Previous work in GaAs has focused on controlling the carrier density, crystallographic orientation and dimensionality to limit the electron spin decoherence and allow transport over long distances. Here, we introduce a new method for the coherent transport of spin-polarized electronic wave packets using dynamic quantum dots (DQDs) created by the piezoelectric field of coherent acoustic phonons. Photogenerated spin carriers transported by the DQDs in undoped GaAs (001) quantum wells exhibit a spin coherence length exceeding 100 microm, which is attributed to the simultaneous control of the carrier density and the dimensionality by the DQDs during transport. In the absence of an applied magnetic field, we observe the precession of the electron spin induced by the internal magnetic field associated with the spin splitting of the conduction band (Dresselhaus term). The coherent manipulation of the precession frequency is also achieved by applying an external magnetic field.

A numerical investigation on the transition of surface acoustic waves (SAWs) in AlxGa1−xN/GaN/SiC heterostructures to Lamb-like modes localized in the GaN layers is presented. The significantly large sound velocity in AlN in comparison to that in GaN leads to an expulsion of the SAWs from the surface AlxGa1−xN layer into the buried GaN layer when the SAW wavelength is roughly less than the thicknesses of the AlxGa1−xN and/or GaN layers. The confinement effect is examined as a function of Al composition in the top layer, and is found to be present even when the composition is as low as x ~ 0.3.

The electrical properties in highly doped, weakly strained (In,Ga)(As,N)/(Al,Ga)As MQWs are studied by Hall measurements at various temperatures. With increasing N content in the well and Al content in the barrier, the electron mobility is strongly reduced. Magnetoresistance measurements are performed at low temperatures revealing carrier localization induced by the incorporation of N. From Raman spectroscopy, a preferential incorporation of N at the interface with the (Al,Ga)As barrier is found. Intersubband absorption experiments show only a small shift of the signal with increasing N content, but a strongly reduced and broadened absorption peak.

The growth of digital Mn/GaAs heterostructures with high Mn sheet densities has been studied at widely varied As 4 pressures by in situ RHEED monitoring and microstructure characterization using TEM. In particular for heterostructures with a large number of periods, the evolution of the specular RHEED beam intensity can be advantageously used for growth control. Analysis of the influence of the Mn density and As 4 pressure on the ferromagnetic properties of the heterostructures suggests a correlation with intrinsic defects in the Mn sheets as well as in the GaAs spacers. Stacking faults are found to be the dominant growth defect.

Fe 3 Si/GaAs(001) hybrid structures of high crystal and interfacial perfection are fabricated by molecular beam epitaxy at 200 °C. The composition of the films can be tuned over a wide range of Si content. The Fe 3 Si/GaAs(001) films are robust against thermal annealing up to 425 °C. Room-temperature spin injection is demonstrated from the ferromagnetic metal Fe 3 Si into the semiconductor GaAs by analyzing the circular polarization of the electroluminescence intensity emitted by an n-i-p light-emitting diode.

Manganese arsenide is a promising candidate for new spintronics applications since it is ferromagnetic at room temperature and can be grown with high epitaxial quality on semiconductors. However, the transition temperature of ∼40 °C is a limiting factor for device applications. Since the coupled magnetic and structural transition is of first order, it is in principle possible to shift the transition temperature by changing external parameters. Here we show that by either applying an external magnetic field or by growing the MnAs films on GaAs(1 1 1), i.e., by changing the epitaxial constraints which are equivalent to external pressure, ferromagnetic order can be stabilized well above the bulk-phase transition temperature. © 2004 Elsevier B.V. All rights reserved.

The logic gates of present computers are based on transistors and increasing the computational power means increasing their integration density. Among others, the utilization of magnetism is a promising alternative. A new concept for a magnetic logic gate is discussed, which is based on a single magnetoresistive element addressable by three independent input lines. Each of these gates can represent one out of at least ten logic functions, among them the four elementary functions AND, OR, NAND and NOR, the two more complex funtions XOR and XNOR as well as two reversible ones, CNOT and CCNOT. The functionality of the proposed gate can be pre-programmed with processor speed, its output is non-volatile. As a first application, which takes full advantage of the reprogrammability and non-volatility of the logic elements, a compact design of a magnetic full adder is discussed.

We present an analysis of thin MnAs films on GaAs(001) by the x-ray grazing incidence diffraction during molecular-beam epitaxy and immediately after deposition near the growth temperature. Separate MnAs peaks are detected for average thicknesses starting from ≈1 monolayer, indicating the formation of a relaxed MnAs lattice. The variation of the position and shape of the MnAs peaks during growth yields the time dependence of relaxation and island sizes. The MnAs domains of different orientations are detected and their amount is analyzed quantitatively. A line broadening due to the size and strain effects is observed. Both the effects are separated for each of the main directions along the interface. The lateral domain sizes of 10–40nm and strain values of 0.2%–0.6% are found in the MnAs films. We find that the positions of the misfit dislocations at the interface are correlated.

Systematic studies of as-grown MnAs films deposited by molecular-beam epitaxy on GaAs(001) and GaAs(113)A reveal that their magnetic properties and, in particular, their saturation magnetization are determined by the phase separation into stripes of ferromagnetic α-MnAs and paramagnetic β-MnAs. Using a specific saturation magnetization MS*, which refers to the actual volume of α-MnAs, the thickness dependence of MS* can be described in a universal way. It is due to the variation of the stripe structure and the changing of the intra- and interstripe magnetic interaction. Values well above ∼1100emu∕cm3, obtained for the optimum film thickness at room temperature, are considered as an intrinsic property of a nearly defect-free MnAs in the fully magnetized state.

We investigate the effect of hole localization on the magneto-luminescence emission of a two-dimensional electron system formed in a modulation-doped semiconductor heterojunction. The emission of heterojunctions containing a dilute delta-layer of Be acceptors at a large distance from the interface is compared with that of Be-free samples. A narrow Fermi-edge singularity emission line is observed at low temperature in Be-doped samples, involving electrons in the second confined state. The evolution of this line in a perpendicular magnetic field shows common characteristics with previously reported results in high mobility samples not containing Be. A new emission line appears abruptly at filling factor 2 originating by the recombination of electrons in the lowest Landau level with free valence holes, independent of the presence of Be. The abruptness of this transition, which is also observed in some samples at filling factor 1, reveals a simultaneous change in the electron system over a macroscopic sample area. The new optical emission shows marked deviations with respect to the single-particle behavior, which are tentatively interpreted as the formation of a complex state involving a free photocreated hole and the electron system. This complex unbinds when the Fermi level crosses the mobility edge.

Low-temperature scanning tunneling microscopy was used to assemble monatomic Cu chains on Cu(1 1 1) by atomic manipulation at 7 K and to explore their electronic structure. Tunneling spectroscopy reveals the formation of unoccupied chain-localized states in the pseudogap of the projected Cu bulk bandstructure. The dispersion of these states is fully described by tight binding behavior. Density functional theory calculations confirm the formation of unoccupied states in the relevant energy regime and in addition predict the existence of occupied states trapped in the pseudogap.