Valley spin polarization by using the extraordinary Rashba effect on silicon

The addition of the valley degree of freedom to a two-dimensional spin-polarized electronic system provides the opportunity to multiply the functionality of next-generation devices. So far, however, such devices have not been realized due to the difficulty to polarize the valleys, which is an indispensable step to activate this degree of freedom. Here we show the formation of 100% spin-polarized valleys by a simple and easy way using the Rashba effect on a system with C-3 symmetry. This polarization, which is much higher than those in ordinary Rashba systems, results in the valleys acting as filters that can suppress the backscattering of spin-charge. The present system is formed on a silicon substrate, and therefore opens a new avenue towards the realization of silicon spintronic devices with high efficiency.X114334Nsciescopu

Ge quantum dot arrays grown by ultrahigh vacuum molecular beam epitaxy on the Si(001) surface: nucleation, morphology and CMOS compatibility

Issues of morphology, nucleation and growth of Ge cluster arrays deposited by ultrahigh vacuum molecular beam epitaxy on the Si(001) surface are considered. Difference in nucleation of quantum dots during Ge deposition at low (<600 deg C) and high (>600 deg. C) temperatures is studied by high resolution scanning tunneling microscopy. The atomic models of growth of both species of Ge huts---pyramids and wedges---are proposed. The growth cycle of Ge QD arrays at low temperatures is explored. A problem of lowering of the array formation temperature is discussed with the focus on CMOS compatibility of the entire process; a special attention is paid upon approaches to reduction of treatment temperature during the Si(001) surface pre-growth cleaning, which is at once a key and the highest-temperature phase of the Ge/Si(001) quantum dot dense array formation process. The temperature of the Si clean surface preparation, the final high-temperature step of which is, as a rule, carried out directly in the MBE chamber just before the structure deposition, determines the compatibility of formation process of Ge-QD-array based devices with the CMOS manufacturing cycle. Silicon surface hydrogenation at the final stage of its wet chemical etching during the preliminary cleaning is proposed as a possible way of efficient reduction of the Si wafer pre-growth annealing temperature.Comment: 30 pages, 11 figure

STM and RHEED study of the Si(001)-c(8x8) surface

The Si(001) surface deoxidized by short annealing at T~925C in the ultrahigh vacuum molecular beam epitaxy chamber has been in situ investigated by high resolution scanning tunnelling microscopy (STM) and reflected high energy electron diffraction (RHEED). RHEED patterns corresponding to (2x1) and (4x4) structures were observed during sample treatment. The (4x4) reconstruction arose at T<600C after annealing. The reconstruction was observed to be reversible: the (4x4) structure turned into the (2x1) one at T>600C, the (4x4) structure appeared again at recurring cooling. The c(8x8) reconstruction was revealed by STM at room temperature on the same samples. A fraction of the surface area covered by the c(8x8) structure decreased as the sample cooling rate was reduced. The (2x1) structure was observed on the surface free of the c(8x8) one. The c(8x8) structure has been evidenced to manifest itself as the (4x4) one in the RHEED patterns. A model of the c(8x8) structure formation has been built on the basis of the STM data. Origin of the high-order structure on the Si(001) surface and its connection with the epinucleation phenomenon are discussed.Comment: 26 pages, 12 figure

Initiating and imaging the coherent surface dynamics of charge carriers in real space

The tip of a scanning tunnelling microscope is an atomic-scale source of electrons and holes. As the injected charge spreads out, it can induce adsorbed molecules to react. By comparing large-scale ‘before' and ‘after' images of an adsorbate covered surface, the spatial extent of the nonlocal manipulation is revealed. Here, we measure the nonlocal manipulation of toluene molecules on the Si(111)-7 × 7 surface at room temperature. Both the range and probability of nonlocal manipulation have a voltage dependence. A region within 5–15 nm of the injection site shows a marked reduction in manipulation. We propose that this region marks the extent of the initial coherent (that is, ballistic) time-dependent evolution of the injected charge carrier. Using scanning tunnelling spectroscopy, we develop a model of this time-dependent expansion of the initially localized hole wavepacket within a particular surface state and deduce a quantum coherence (ballistic) lifetime of ∼10 fs

Surface electronic structure study of Au/Si(111) reconstructions: Observation of a crystal-to-glass transition

The alpha-root3xroot3, beta-root3xroot3, and 6x6 surfaces of Au/Si(111) have been studied by angle-resolved photoelectron spectroscopy (ARPES). The alpha-root3xroot3 surface shows four surface state bands, one of which is a dispersing metallic band. Except for one nondispersing band near the Fermi level, the beta-root3xroot3 surface shows surface bands that are similar to those of alpha-root3xroot3. Eight surface state bands have been found on the 6x6 surface instead of two as reported in the literature. Seven of these surface states are broadened into three, when the 6x6 phase is transformed into the quenched beta-root3xroot3 phase just by annealing followed by quick cooling. The transition between the 6x6 and quenched beta-root3xroot3 phases is consistent with a crystal-glass transition

Core-level photoelectron spectroscopy study of the Au/Si(111) 5X2, alpha-root 3X root 3, beta-root 3X root 3, and 6X6 surfaces

Submonolayer coverages of Au on Si(111), known as the 5 X 2, alpha- root3 X root3, beta- root3 X root3, 2root21 X 2root21, and 6 X 6 surfaces, have been investigated by low-energy electron diffraction and photoelectron spectroscopy. Three Si 2p surface components on the 5 X 2 surface, and four surface components on the alpha- root3 X root3, quenched beta- root3 X root3, and 6 X 6 surfaces have been identified by surface sensitive high resolution core-level spectroscopy. The photoemission data of the alpha- root3 X root3, the 6 X 6 and the quenched beta- root3 X root3 phases are discussed in terms of extra Au adatoms on the root3 X root3 surface described by the ideal 1 ML conjugate honeycomb chained trimer model. The similarity between the 6 X 6 and the quenched beta- root3 X root3 surface is obvious from the decomposition of the Si 2p spectra, suggesting an order-disorder relation

Photoelectron spectroscopy study of Ag/Si(111)root 3X root 3 and the effect of additional Ag adatoms

High-resolution core-level spectroscopy has been applied to the Ag/Si(111)root3xroot3 surface. The Si 2p line shape is found to depend critically on the presence of additional Ag adatoms on the surface. A significant broadening caused by the surplus of Ag atoms could be eliminated by careful annealing. The resulting Si 2p spectra are significantly sharper than any published data for this or other Si based surface systems. Two major surface components are identified for the root3xroot3 surface, which find a natural explanation in terms of the honeycomb-chained-trimer model. A small but characteristic contribution to the Si 2p spectrum of the Ag/root3xroot3 surface is tentatively assigned to defects