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In this contribution a model for the elastic relaxation of Al0.5Ga0.5As and ZnSe wire structures, respectively, is presented. The wire structures extend along [110] and were characterized by high resolution x-ray diffraction experiments. Based on Monte Carlo simulations, the wire structures, which have been simulated at atomic resolution, were relaxed to reduce the strain caused by the lattice misfit. The x-ray diffraction patterns calculated for the final structures reproduce the observed data. The structures display a two dimensional strain gradient and curved lattice planes. Hence the introduced modeling is a powerful method in particular to resolve the atomic structure of a wet chemically etched or molecular beam epitaxial grown wire by giving a microscopic picture on an atomic scale.

We present a method which increases the versatility of molecular beam epitaxy through multiple application of a stationary shadow mask. The method is based on selected area growth in the overlap of the incidence regions of two molecular beams impinging through different apertures of the mask. The width of the overlap depends on the incidence angles of the beams which can be adjusted in situ. Size-control of CdZnSe quantum-well (QW) islands with a precision of 20 nm by varying the distance between the apertures can be obtained. Without Se beam, a minor quantity of Cd is incorporated in the matrix modulating the band-gap energy by just 5 meV. Cathodoluminescence (CL) of nanoscale QW islands shows dot-like behavior, i.e., sharp emission lines that are attributed to different excited states and a blueshift of the CL on reducing the island dimensions.

Thin layers of MnSi, the first discovered host material of a skyrmion lattice, are epitaxially grown on Si(111) and their crystal properties are investigated by X-ray diffraction and transmission electron microscopy (TEM) measurements. Azimuthal ϕ -scans of asymmetric X-ray reflections reveal the formation of twinned domains with their unit cell rotated ±30° in plane with respect to the Si unit cell. The intensities of corresponding reflections indicate the same volume fractions for both domain types. Cross-sectional TEM confirms the presence of these domains and reveals a typical domain size of about 200 nm.

We investigated random telegraph signals and 1/f noise in a submicron metal–oxide–semiconductor field-effect transistor at low temperatures in the Coulomb-blockade regime. The rich noise characteristics were studied as a function of gate voltage, drain current, and temperature, both in and beyond the Ohmic regime. The results can be understood within a simple model assuming a uniform potential fluctuation of constant magnitude at the location of the dot. Clear signatures of electron heating are found from the noise at higher currents.

We report measurements on submicron metal–oxide–semiconductor field effect transistors equipped with a gate on three sides of the channel. At room temperature, a strong suppression of short-channel effects has been achieved for the narrowest channels. At liquid helium temperatures, the same devices exhibit clear conductance oscillations in the subthreshold regime, indicating that a quantum dot has formed in the disordered channel.

The formation of a Schottky barrier between an eutectic (Ga,In) alloy and a highly doped thiophene oligomer is followed as a function of time using current density–voltage and capacitance–voltage measurements. Within 1 h, the diode characteristics change from almost nonrectifying, leaky behavior into a rectification ratio of 104 with a considerably reduced leakage current. Measurements and energy band diagram calculations show that the depletion width increases with time due to a decrease in the ionizable acceptor density of the semiconductor at the Schottky interface. This is probably caused by a chemical reaction between the in-diffusing metals and the doped oligomer.

Extrinsic disorder strongly affects the performance of graphene-based quantum dots. The standard SiO2 substrate is generally considered to be one major factor besides edge-induced disorder. In this report we present the fabrication of lithographically defined quantum dots on SiO2 with short and narrow constrictions and different central island sizes. Low temperature transport measurements display distinct Coulomb-blockade peaks with amplitudes exceeding what is commonly observed experimentally. The analysis of the normalized Coulomb-blockade peak spacing shows a size dependence, which has not previously been observed for devices on SiO2. Furthermore, a quantitative comparison of the peak spacing distribution to the literature shows that one of the two devices compares favorably to a similar sized dot placed on hexagonal boron nitride, which is known to reduce the substrate disorder. Our findings suggest that the other sources of extrinsic disorder, such as lithography residues, may play an important role ...

We investigate the influence of a spin current generated from a platinum layer on the ferromagnetic resonance (FMR) properties of an adjacent ferromagnetic layer composed of the halfmetallic half-Heusler material NiMnSb. Spin Hall nano-oscillator devices are fabricated, and the technique of spin torque FMR is used to locally study the magnetic properties as in-plane anisotropies and resonance fields. A change in the FMR linewidth, in accordance with the additional spin torque produced by the spin Hall effect, is present for an applied dc current. For sufficiently large currents, this should yield auto-oscillations, which however are not achievable in the present device geometry. QC 20150529

Epitaxial films of the ternary topological insulator Bi2Te2Se were grown on Si(111) substrates and investigated for their surface electronic properties and morphology. We employ a Se-capping procedure allowing for the preparation of clean films in the surface-analysis experimental setups. Using angle-resolved photoelectron spectroscopy, we determine the dispersion of the topological surface state. With time after surface preparation, the spectroscopic features in the surface electronic structure exhibit significant temperature-dependent shifts to higher binding energies. Scanning tunneling microscopy images show terraces with typical step edge separations of 50 nm–150 nm. X-ray photoelectron spectroscopy indicates an increased Se concentration at the surface.

NiMnSb is a ferromagnetic half-metal which, because of its rich anisotropy and very low Gilbert damping, is a promising candidate for applications in information technologies. We have investigated the in-plane anisotropy properties of thin, molecular beam epitaxy-grown NiMnSb films as a function of their Mn concentration. Using ferromagnetic resonance to determine the uniaxial and four-fold anisotropy fields, 2KUMs and 2K1Ms, we find that a variation in composition can change the strength of the four-fold anisotropy by more than an order of magnitude and cause a complete 90° rotation of the uniaxial anisotropy. This provides valuable flexibility in designing new device geometries.

We present a fabrication process for devices on HgTe quantum wells through which the quantum spin Hall regime can be reached without the use of a top-gate electrode. We demonstrate that a nominally undoped HgTe quantum well can be tuned from p-type to n-type, crossing through the quantum spin Hall regime, using only a back-gate hundreds of microns away. Such structures will enable scanning probe investigations of the quantum spin Hall effect that would not be possible in the presence of a gate electrode on the surface of the wafer. All processes are kept below 80 °C to avoid degradation of the heat-sensitive HgTe quantum wells.

We have investigated circularly polarized electroluminescence (EL) from hybrid II-Mn-VI/III–V light emitting diodes (LED’s) at low temperatures in magnetic fields upto 10 T. Both magnetic (the Brillouin paramagnet Cd1−xMnxSe) and nonmagnetic (CdSe) injectors were studied. Electrons, spin unpolarized (n-CdSe) or spin-polarized (n-CdMnSe), were injected into wide InAs quantum wells, where they recombined with unpolarized holes injected from p-type InAs/AlAsSb layers. Detailed measurements and modeling of the circular polarization of the resulting midinfrared EL were carried out to explore and quantify the additional complexities of this materials system compared with the extensively studied GaAs-based spin-LED structures. We show that optical and spin polarization in narrow gap semiconductors such as InAs are not simply related to each other. To analyze the complex relationship, we have developed and used a detailed rate equation model, which incorporates the band-structure of electrons and holes in a magne...

Circularly polarized luminescence of CdMnSe quantum dots in magnetic fields up to 5 T is studied for nominal Mn concentrations of 0%, 1%, and 2% by using a photoelastic modulator technique. The exciton g factors as well as spin relaxation times were determined from the polarized luminescence taking into account the exciton lifetimes, which were also extracted by means of time-resolved photoluminescence spectroscopy. For quantum dots without Mn and with 2% Mn exciton g factors of −1.62 and +1.32, respectively, were found. The quantum dots with 1% Mn show a vanishing small value of g for small excitation powers. For this structure the polarization properties are dominated by the optical orientation. Interestingly, for the 1% Mn quantum dots with increasing excitation power considerable changes of the polarization and the exciton g factor were observed which are interpreted in terms of heating effects. From the power dependence indirect heating via phonons and above a critical value direct heating due to pho...

We present a comprehensive discussion of molecular-beam epitaxy of III-V compound semiconductors through shadow masks. Based on model calculations and growth experiments, we examine how the surface diffusion and the incorporation of group-III adatoms depend on the growth configuration, group-III and group-V fluxes, and the crystal orientation. According to a macroscopic diffusion model, gradients of the group-V flux drive the unidirectional migration of group-III adatoms. Although this effect is generally observed in the experiments, the different growth profiles obtained for [11¯0]- and [110]-oriented samples reflect the different roles of A-type and B-type steps in the incorporation of group-III adatoms. We also demonstrate that during the heteroepitaxial growth of InAs, the dissociation of the GaAs substrate is locally enhanced by the incidence of the In beam. This effect can be exploited for shadow-mask-assisted etching on selected areas. In addition, we show how the positions and sizes of III-V nanos...

This article reports on a spintronics device based on the ferromagnetic semiconductor (Ga,Mn)As. Our transport measurements on a Au∕AlOx∕(Ga,Mn)As tunnel junction yield the surprising result that it is possible to get a spin-valve-like signal using only one magnetic layer. The strong spin-orbit coupling in (Ga,Mn)As creates significant anisotropies in the density of states with respect to the magnetization orientation. This, together with a two-step magnetization reversal creates a bistable magnetoresistive device with properties unattainable in current metal based spin-valves.

The structural and magnetic properties of NiMnSb films, 5-120 nm thick, grown on InGaAs/InP(001) substrates by molecular-beam epitaxy, were studied by x-ray diffraction, transmission electron microscopy (TEM), and ferromagnetic resonance (FMR) techniques. X-ray diffraction and TEM studies show that the NiMnSb films had the expected half-Heusler structure, and films up to 120 nm were pseudomorphically strained at the interface, greater than the critical thickness for this system, about 70 nm (0.6% mismatch to InP). No interfacial misfit dislocations were detected up to 85 nm, however, relaxation in the surface regions of films thicker than 40 nm was evident in x-ray reciprocal space maps. TEM investigations show that bulk, planar defects are present beginning in the thinnest film (10 nm). Their density remains constant but they gradually increase in size with increasing film thickness. By 40 nm these defects have overlapped to form a quasicontinuous network aligned closely with < 100 > in-plane directions. The associated strain fields and or compositional ordering from these defects introduced a reduction in crystal symmetry that influenced the magnetic properties. The in-plane and perpendicular FMR anisotropies are not well described by bulk and interface contributions. In thick films, the in-plane uniaxial and fourfold anisotropies increased with increasing film thickness. The lattice defects resulted in a large extrinsic magnetic damping caused by two-magnon scattering, an increase in the coersive field with increasing film thickness, and a lower magnetic moment (3.6 Bohr magnetons) compared to the expected value for the bulk crystals (4 Bohr magnetons). (C) 2005 American Institute of Physics.

We show that measurements of minority‐carrier lifetimes in thin layer (Al,Ga)As double heterostructures grown by organometallic vapor‐phase epitaxy can be severely influenced by modulation doping of the layer studied. Samples where this effect has been eliminated exhibit a dominantly nonradiative luminescence decay, from which we deduce an interface recombination velocity of only 18 cm s−1, which is among the lowest ever reported for a GaAs/(Al,Ga)As interface.