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Abstract: Lattice-type mismatched heteroepitaxy is demonstrated as a novel concept for the fabrication of almost ideal, highly luminescent nanocrystal quantum dots that are coherently embedded in a single-crystalline matrix. In this approach, the formation of quantum dots is induced by transformation of a metastable epitaxial 2D quantum well into an array of isolated nanocrystals with-highly symmetric shape. This process is driven by the lattice-type mismatch between the constituent materials and the resulting miscibility gap. The investigated PbTe/CdTe heterosystem has a model character because it combines two compounds with different cubic lattice types but almost identical lattice constants. The obtained epitaxial nanocrystals exhibit outstanding properties such as a well-defined symmetric shape, the absence of strain, intermixing and a wetting layer, which is in contrast to the conventional Stranski–Krastanow quantum dots. The small-rhomboedric-cubo-octahedron PbTe/CdTe nanocrystals on GaAs substrates display intense room temperature mid-infrared luminescence as is crucial for device applications. Ab initio density functional theory is used to clarify the interface structure, indicating that the covalent and ionic bonding character of CdTe and PbTe is maintained across the interface. [Copyright &y& Elsevier]

Abstract: We discuss pulsed electron spin resonance measurements of electrons in Si and determine the spin coherence from the decay of the spin echo signals. Tightly bound donor electrons in isotopically enriched 28Si are found to have exceptionally long spin coherence. Placing the donors near a surface or interface is found to decrease the spin coherence time, but it is still in the range of milliseconds. Unbound two-dimensional electrons have shorter coherence times of a few microseconds, though still long compared to the Zeeman frequency or the typical time to manipulate a spin with microwave pulses. Longer spin coherence is expected in two-dimensional systems patterned into quantum dots, but relatively small dots will be required. Data from dots with a lithographic size of 400nm do not yet show longer spin coherence. [Copyright &y& Elsevier]

Abstract: A split gate technique is used to form a lateral quantum dot in a two-dimensional electron gas of a modulation-doped silicon/silicon–germanium heterostructure. e-beam lithography was employed to produce split gates. By applying negative voltages to these gates the underlying electron gas is depleted and a lateral quantum dot is formed, the size of which can be adjusted by the gate voltage. We observe single-electron operation with Coulomb blockade behavior below 1K. Gate leakage currents are well controlled, indicating that the recently encountered problems with Schottky gates for this type of application are not an inherent limitation of modulation-doped Si/SiGe heterostructures, as had been speculated. [Copyright &y& Elsevier]

Abstract: We report on the design and first experiments of Si/SiGe heterostructures that allow gate-operated shifting of a 2D electron gas between two channels with different Landé-factors. This allows gate-operated moving of electrons in and out of resonance in an electron spin resonance (ESR) experiment, which can act as a building block of a proposed solid-state quantum computer. We use MBE-grown modulation-doped quantum-wells (QWs) on SiGe pseudosubstrates with up to 30% Ge and low-temperature electron mobilities up to . A double QW structure with two different Ge contents separated by a thin barrier was optimized for this purpose with self-consistent simulations. The band structure simulations show that by applying gate voltages one can completely shift the wave function from one well to the other. First experiments on pure Si channels show the working of the gate setup. Both carrier density and mobility can be increased by using the back gate which corresponds to shifting the wave function in the channel. [Copyright &y& Elsevier]

The morphology of annealed patterned Si(0 0 1) wire templates was studied by several techniques. We found an enormous Si-mass transport on the Si surface at usual oxide desorption temperatures around 900°C under UHV conditions. Heat treatment of 5 min transforms the initially rectangular wire profiles with a height of 300 nm to flat (<100 nm) and faceted triangular ridges exhibiting thermodynamically preferred {1 1 1}- and {3 1 1}-facets.It was found that the natural SiO2 on the predefined wire pattern must be responsible for the degradation of the wire structure. Removing the SiO2 layer from the Si wires ex situ with an HF dip preserves the rectangular structures during high-temperature annealing. The Si–SiO2 interface was investigated with high-resolution transmission electron microscopy to image the Si wire surface and the natural oxide layer in detail. [Copyright &y& Elsevier]

We report on the growth and properties of Ge islands grown on (0 0 1) Si substrates with lithographically defined two-dimensionally periodic pits. After thermal desorption and a subsequent Si buffer layer growth these pits have an inverted truncated pyramid shape. We observe that on such prepatterned substrates lens-like Ge-rich islands grow at the pit bottoms with less Ge deposition than necessary for island formation on flat substrates. This is attributed to the aggregation of Ge at the bottom of the pits, due to Ge migration from the pit sidewalls. At the later stages of growth, dome-like islands with dominant {1,1,3} or {15,3,23}, or other high-index facets [i.e. {15,3,20} facets] are formed on the patterned substrates as shown by surface orientation maps using atomic force microscopy. Furthermore, larger coherent islands can be grown on patterned substrates as compared to Ge deposition on flat ones. [Copyright &y& Elsevier]

Stacks of Ge islands layers, separated by thin Si spacer layers, have been grown on prepatterned Si (0 0 1) substrates. The sample topography, obtained by atomic force microscopy, exhibits a regular two-dimensional island arrangement. The vertical alignment is confirmed in cross-sectional transmission electron microscopy images. X-ray diffraction reciprocal space maps around the (0 0 4) Bragg point reveal exceptional lateral and vertical ordering of the Ge islands. Photoluminescence spectra taken at 5 K show well-separated peaks of the no-phonon and the transverse-optical phonon replica of these ordered islands, which is achieved too, due to the excellent island size uniformity. [Copyright &y& Elsevier]

We investigate spin relaxation and the g-factor of conduction electrons in modulation doped Si/SiGe quantum wells by means of electron spin resonance. We find that both the transverse- and the longitudinal relaxation times are of the order of microseconds, much longer than in III–V compounds. For high mobility, at carrier densities sufficiently far away from the metal-to-insulator transition, both quantities can be explained consistently in terms of the Bychkov–Rashba field, together with the g-factor anisotropy and its dependence on the carrier density. A single value of the BR coefficient, αBR=0.55×10−12 eV cm, explains all data. This value is by more than three orders of magnitude smaller than for III–V compounds owing to the small spin–orbit coupling of Si. The properties found make Si/SiGe quantum structures interesting candidates for quantum computing. [Copyright &y& Elsevier]

A comparative study is reported of kinetically and thermodynamically driven growth instabilities in the Si/SiGe heterosystem on vicinal Si(0 0 1). Investigating single Si1−xGex-layers and Si1−xGex/Si superlattices, a wide range of the relevant growth parameter space is mapped out. In contrast to earlier reports no evidence for strain-induced step-bunching is found. Single Si1−xGex layers replicate the morphology of the underlying buffer layers and tend to form hut clusters near thermal equilibrium. In Si1−xGex/Si superlattices, no significant influence of the Ge-related strain on the step-bunching morphology can be seen. On the other hand, the effect of growth temperature is very pronounced, again indicating the dominant role of kinetics in the formation of step-bunches. [Copyright &y& Elsevier]

We show that (i) the spin life times of electrons in SiGe quantum wells are much longer than the spin manipulation times required for pulsed ESR under realistic conditions. We demonstrate also (ii) that the g-factor depends on angle, Ge-content and on the kinetic energy of free carriers. The latter two dependencies are sufficiently strong to permit individual manipulation in a controlled NOT gate by clearly separated resonance conditions. [Copyright &y& Elsevier]

Abstract: Tilted field magnetotransport study was performed in a two-valley strained Si quantum well and hysteretic diagonal resistance spikes were observed near the coincidence angles. The spike around filling factor develops into a giant feature when it moves to the high-field edge of the quantum Hall (QH) state and quenches for higher tilt angles. When the spike is most prominent, its peak resistance is temperature independent from T∼20mK up to 0.3K, which is different from the critical behavior previously reported near the Curie temperature of the QH ferromagnet in AlAs quantum wells. Our data suggest a strong interplay between spins and valleys near the coincidence. [Copyright &y& Elsevier]

New X-ray optical methods—two-wave reflecto- and refractometry—have been used for the first time for the determination of GexSi1−x/Si composition and exact measurements of multilayer heterostructure thicknesses. Both techniques are based on simultaneous measurements of two intense characteristic X-ray lines separated from the polychromatic X-ray probe by semitransparent monochromators. X-ray reflectometry of multilayer structures completely eliminates intensity drift influence and geometrical errors at small grazing angles. The refractometry technique provides direct determination of refractive index decrement and the respective value of x in a solid solution with ∼1% accuracy. Refractometry data are not influenced by mechanical strain and by presence of overlaying thin film structure. [Copyright &y& Elsevier]