Magnetotransport of individual rolled-up Fe3Si nanomembranes is investigated in a broad temperature range from 4.2 K up to 300 K in pulsed magnetic fields up to 55 T. The observed magnetoresistance (MR) has the following pronounced features: (i) MR is negative in the investigated intervals of temperature and magnetic field; (ii) its magnitude increases linearly with the magnetic field in a low-field region and reveals a gradual trend to saturation when the magnetic field increases; (iii) the MR effect becomes more pronounced with increasing temperature. These dependences of MR on the magnetic field and temperature are in line with predictions of the spin-disorder model of the spin-flip s-d interaction assisted with creation or annihilation of magnons, which is expected above a certain critical temperature. Comparison of the MR features in rolled-up and planar samples reveals a substantial increase of the critical temperature in the rolled-up tube, which is attributed to a new geometry and internal strain arising in the rolled-up nanomembranes, influencing the electronic and magnetic properties of the material.

This chapter is intended to provide an overview of strain-engineered heterostructures and quantum devices based on SiGe alloys. The growth of SiGe on Si(001) substrates is introduced by focusing on the morphological evolution of SiGe nanostructures and the ways of precisely controlling lateral and vertical ordering. Afterwards, the chapter discusses a revolutionary process technology leading to strain-driven architectures. Finally, a new emerging generation of SiGe-based systems with unique capabilities, ranging from fast field-effect transistors to energy harvesting devices, is reviewed.

We report on a magnetophotoluminescence study of single self-assembled semiconductor nanorings which are fabricated by molecular-beam epitaxy combined with AsBr(3) in situ etching. Oscillations in the neutral exciton radiative recombination energy and in the emission intensity are observed under an applied magnetic field. Further, we control the period of the oscillations with a gate potential that modifies the exciton confinement. We infer from the experimental results, combined with calculations, that the exciton Aharonov-Bohm effect may account for the observed effects.

In this work, we present systematic electron spin resonance (ESR) experiments on SiGe quantum-dot structures. A series of samples with different sizes of quantum dots is prepared by varying growth temperature and spacing between quantum-dot layers. At a frequency of about 9.5 GHz, two ESR signals with g factors around 1.9992 and 1.9994 are observed with magnetic field in growth direction. The signals shift and broaden with magnetic field in the in-plane direction. The estimated dephasing time T(2)(*) amounts up to 500 ns. The saturation behavior yields relaxation time T(1) of about 10 mu s. The relative intensity between the two peaks can be changed with illumination with subband-gap light. The two peaks are interpreted as s- and p-like states of electrons confined in the strained Si around the SiGe nanostructures.

Au/Co/Au trilayers are fabricated by tilted deposition on prestructured polymer sacrificial layers. The metal trilayers are released by selectively dissolving the sacrificial layer and roll-up into microtubes. Magnetization properties are strongly affected by the roll-up process. In addition to a modified shape anisotropy, the magnetostrictive anisotropy due to the anisotropic stress release is reversed. Low temperature measurements support the presence of significant exchange bias in these rolled-up structures.

The production of electron-positron and muon-antimuon pairs in high-frequency laser fields via few-photon absorption is considered. It is assumed that an intense X-ray laser beam collides either with a relativistic ion beam or with a second, equally intense laser beam. We study the generation of free e + e − pairs, free μ+μ− pairs, and bound-free e + e − pairs where in the latter case the electron is born in a low-lying atomic orbit of the projectile nucleus. Effects resulting from the finite nuclear size, the laser’s polarization state, and its magnetic field component are examined, which are testable experimentally by virtue of upcoming X-ray free-electron laser (XFEL) devices.

The process of bound-free pair production of electrons and positrons in combined laser and Coulomb fields is investigated. It is assumed that an ion collides at relativistic speed with an intense x-ray laser beam of linear polarization. The process proceeds nonlinearly due to simultaneous absorption of a few laser photons. The capture of the electron into the ground state and the L shell is considered. The scaling of the total rate, the angular distributions of the emitted positrons, and a comparison to the competing free-free channel are surveyed. Numerical results of pair production rates for parameters for the planned x-ray free-electron lasers at DESY and SLAC are presented. We find that pair production with these laser facilities can become observable in the near future.

24 European Crystallographic Meeting, ECM24, Marrakech, 2007 Page s89 Acta Cryst. (2007). A63, s89 MS37 O1 X-ray Microdiffraction on Individual Self-Assembled Nanostructures C.Mocuta, B.Krause, R.Mundboth, T.H.Metzger, J.Stangl, G.Bauer, I.Vartanyants, C.Deneke, O.G.Schmidt European Synchrotron Radiation Facility (ESRF), Grenoble-France. 2 Johannes Kepler University, Linz-Austria. 3 Hasylab at Desy, Hamburg-Germany.Max-Planck-Instiutut fürFestköperforschung, Stuttgart-Germany.

24 European Crystallographic Meeting, ECM24, Marrakech, 2007 Page s89 Acta Cryst. (2007). A63, s89 MS37 O1 X-ray Microdiffraction on Individual Self-Assembled Nanostructures C.Mocuta, B.Krause, R.Mundboth, T.H.Metzger, J.Stangl, G.Bauer, I.Vartanyants, C.Deneke, O.G.Schmidt European Synchrotron Radiation Facility (ESRF), Grenoble-France. 2 Johannes Kepler University, Linz-Austria. 3 Hasylab at Desy, Hamburg-Germany.Max-Planck-Instiutut fürFestköperforschung, Stuttgart-Germany.

To investigate geometric potentials in low dimensional electron systems, we have conducted first studies on topography dependant electron transport in complete tubes, using built in strain between lattice mismatched semiconductors. Initial studies reveal two regimes of electron transport which are probed by a varying perpendicular magnetic field. At low magnetic field, an increased zero field peak in magneto resistance followed by a negative magneto resistance is observed due to increase in electron scattering along curved regions. At high magnetic field, we find a linear increase in resistance of the curved region as compared to planar regions.

Crystals with cylindrical symmetry, not existing in nature, are mimicked by the roll-up of single-crystalline and highly strained semiconductor bilayers. Exploiting this, the local structure of such individual rolled-up nanotubes is locally probed and quantified nondestructively by x-ray microbeam diffraction. A comparison to simulations, based on the minimization of the elastic energy, allows us to determine layer thicknesses and lattice parameter distributions within the strongly curved bilayers.

All the bilayer films were grown by deposition of Si onto a SiGe strained-layer on insulator (SGOI) using solid-source molecular-beam epitaxy (MBE), except for the one shown in Figure 3 (lower right) which was prepared by ultrahigh-vacuum chemical vapor deposition (UHVCVD) growth of a Si/SiGe bilayer onto SOI. Both MBE and CVD SiGe films had a concentration of ∼ 20 % Ge. The thicknesses of the CVD film were ∼ 30 nm Si/∼ 36 nm SiGe, determined by X-ray diffraction. For the MBE films, we were able to grow five different thickness (20, 40, 50, 70, and 100 nm) of Si layers in situ onto a single piece of SGOI substrate by rotating a shutter in an MBE chamber, allowing the film to fold with different characteristic bending radii. The growth rate was 0.55 Å s at a substrate temperature of 585 °C, measured by an optical pyrometer. The MBE/CVD growth of the Si layer was monitored by reflection highenergy electron diffraction during the entire growth process. The SGOI substrate was a free sample from SOITEC. The thickness and composition of SGOI were about 44 nm Si with 20 % Ge on 190 nm SiO2. The sample was cleaned with 10 % hydrofluoric acid to remove the native oxide grown in air on the original SGOI surface, followed by 10 min of cleaning with piranha (H2SO4/H2O2), and a few seconds 10 % HF etching to remove the oxide layer produced during the piranha treatment. For UHV-CVD growth, we added an additional 15 min SC1 (NH4OH/H2O2/H2O mixture) cleaning step at ∼ 80 °C and deliberately dipped the sample into diluted HF acid to terminate the surface with hydrogen before loading it into the CVD chamber in order to prevent the growth of native oxide in air. We performed photolithography and electron-beam lithography to pattern the thin films into cantilevers. Basically, an array of cantilevers with different dimensions and orientations was created on each side of a 50 lm × 50 lm square, as shown in Figure 3. The common width of the (100) cantilever was 3 lm and the lengths varied from the longest to the shortest as 20, 15, 10, 6, and 3 lm. The spacing in between was 5 lm. The width of the (110) cantilever was 6 lm and the length varied as 36, 26, 16, 6, and 4 lm. The spacing in between was 2 lm. After lithography, the desired patterns were transferred onto the Si/Si0.8Ge0.2 bilayer film by using O2 and SF6 reactive-ion etching. The underlying sacrificial oxide layer was selectively etched off by the vapor of HF acid to release the cantilever, which bended upward and folded into nanorings or nanocoils. The HF vapor releasing process was carried out at a temperature of 40 °C with a time duration of 30–60 min. The most important advantage of this technique is that it is a single process without subsequent rinsing steps, thus preventing the released structure from sticking onto the substrate. Also, for this purpose, it was necessary to create a strain configuration, with a Si film grown on top of SiGe by using the unique SGOI wafer. In the normal configuration, with a SiGe film grown on top of Si by using the conventional SOI wafer, the released bilayer film would bend downward so that it was blocked from folding completely.

We study the formation process of rolled-up InAs/GaAs nanotubes (RUNTs) as a function of etching time and sacrificial layer thickness for tube diameters between 20 and 560nm. Within this diameter range the roll-up velocity strongly depends on the sacrificial layer thickness but is independent of the tube diameter. We also find that the roll-up distance saturates with etching time for distances around 8–16μm. Since we define the starting edge of the roll-up process by optical lithography, we are able to position individual RUNTs on a substrate surface with reasonable accuracy. We also show that the areal density of the tubes on a surface can be doubled if a two-fold stack of strained bilayers is selectively underetched. Finally, we record organic fluid transport within a RUNT in real time and we report intense red light emission from such filled-up nanotubes.

We investigate the wall structure and thermal stability of individual freestanding rolled-up nanotubes (RUNTs) using micro-Raman spectroscopy, transmission electron microscopy, and selected area electron diffraction. Our studies reveal that the walls of the InAs/GaAs RUNTs consist of a radial superlattice comprising alternating crystalline and noncrystalline layers. Furthermore, we locally heated individual RUNTs with a laser beam, and Raman spectroscopy was used in situ to monitor any structural changes. At about 300 °C the heated part of a RUNT starts to oxidize and eventually transforms into crystalline β-Ga2O3. This result shows that RUNTs can serve as nanoreactors that locally synthesize material at intentional places on a substrate surface.

We apply real-time video microscopy to monitor the formation of rolled-up InAs∕GaAs tubes in great detail. The roll-up process is highly nonlinear at the beginning, linear at an intermediate stage, and ceases for long etching times. This self-limitation effect allows precise positioning of the tubes on a substrate surface. We investigate the formation of rolled-up nanotubes (RUNTs) on lithographically patterned substrates as a function of sacrificial layer thickness and tube diameter, and our results are well-described by a simple diffusion model. We fill well-positioned RUNTs of well-defined lengths with dye liquid and we observe strong fluorescence signal of these structures in the red spectral range.

We present a detailed, investigation of novel strain-driven semiconductor nanostructures. Our examinations include self-assembled nanoholes, lateral quantum-dot (QD) molecules, and rolled-up nanotubes. We overgrow InAs QDs with GaAs and apply atomically precise in situ etching to fabricate homogeneous arrays of nanometer-sized holes with diameters of 40 to 60 nm and depths up to 6.2 nm. The structural properties of the nanoholes can be precisely tuned by changing the QD capping thickness and the in situ etching time. We show that strain fields surrounding the buried quantum dots drive the nanohole formation process. We overgrow the mmoholes with 0.2- to 2.5-ML InAs and observe the formation of compact lateral InAs QD molecules. The number of QDs involved in a lateral QD molecule can be tuned from two to six by changing the growth temperature. Our systematic photoluminescence study documents the QD molecule formation process step by step and helps to interpret our structural results. We also present the fabrication of laterally aligned lateral QD bimolecules by growing InGaAs on a GaAs (001) substrate patterned with a square array of nanometer sized holes. Charge carriers in such bimolecules might serve as quantum gates in a future semiconductor based quantum computer. Furthermore, we release strained semiconductor bilayers from their surface to fabricate individual rolled-up semiconductor micro- and nanotubes. We control the diameter of strain-driven In(Ga)As-GaAs tubes from the nanometer to micrometer range by simply,changing the layer thicknesses and built-in strain. We propose to roll in metal strip lines to fabricate nanocoils and nanotransformers. To support our proposition, we fabricate homogeneous single and twin GaInP tubes. We present a straight GaInP microtube of more than 2 mm in length and a length-to-diameter ratio of about 2000, thus, elucidating the great potential of this technology.