Effective electron mass in high-mobility SiGe/Si/SiGe quantum wells

The effective mass m* of the electrons confined in high-mobility SiGe/Si/SiGe quantum wells has been measured by the analysis of the temperature dependence of the Shubnikov-de Haas oscillations. In the accessible range of electron densities, n _s , the effective mass has been found to grow with decreasing n _s , obeying the relation m*/m _b = n _s /(n _s ? n _c ), where m _b is the electron band mass and n _c ≈ 0.54 × 10¹¹ cm^?2. In samples with maximum mobilities ranging between 90 and 220 m²/(V s), the dependence of the effective mass on the electron density has been found to be identical suggesting that the effective mass is disorder-independent, at least in the most perfect samples.     

Infrared absorption in SiGe/Si multiple quantum wells

Infrared absorption has been used to investigate the subband structures in SiGe/Si quantum wells. The quantum wells are prepared using RRH/VLP-CVD and consist of 20 periods of and 60 periods of . The good periodical and interface sharpness of the SiGe/Si quantum wells have been shown by Auger Electron Spectroscopy (AES). The absorption peaks due to transitions between the hole subbands and the conduction band have been observed in infrared absorption spectra. The transverse photocurrent spectrum parallel to the growth plane have also shown absorption peaks due to transitions between the heavy and light hole band states and the conduction band states in quantum wells.     

Spin manipulation of free two-dimensional electrons in Si/SiGe quantum wells

Understanding the mechanisms controlling the spin coherence of electrons in semiconductors is essential for designing structures for quantum computing applications. Using a pulsed electron paramagnetic resonance spectrometer, we measure spin echoes and deduce a spin coherence time (T2) of up to 3 mus for an ensemble of free two-dimensional electrons confined in a Si/SiGe quantum well. The decoherence can be understood in terms of momentum scattering causing fluctuating effective Rashba fields. Further confining the electrons into a nondegenerate (other than spin) ground state of a quantum dot can be expected to eliminate this decoherence mechanism.     

-Factor tuning of 2D electrons in double-gated Si/SiGe quantum wells

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 g-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.     

Study of valence intersubband absorption in tensile strained Si/SiGe quantum wells

The hole subband structures and effective masses of tensile strained Si/Sil-yGey quantum wells are calculated by using the 6 × 6 k·p method. The results show that when the tensile strain is induced in the quantum well, the light-hole state becomes the ground state, and the light hole effective masses in the growth direction are strongly reduced while the in-plane effective masses are considerable. Quantitative calculation of the valence intersubband transition between two light hole states in a 7nm tensile strained Si/Si0.55Ge0.45 quantum well grown on a relaxed Si0.5Ge0.5 （100） substrates shows a large absorption coefficient of 8400 cm^-1.     

Photoinduced infrared spectroscopy of bound-to-bound and bound-to-continuum transitions in SiGe/Si quantum wells

Infrared spectroscopy of intersubband transitions in the valence band of undoped SiGe/Si quantum wells is presented. Optical pumping of interband transitions is used to generate carriers in the wells. The spectral features of bound-to-bound and bound-to-continuum transitions are analyzed and compared to those of GaAs quantum wells. In samples with only one heavy hole bound level, a ratio of 20:1 is observed between intersubband and free carrier absorption. Room temperature photo-induced absorption is only observed in samples with high germanium content (≈50%). The feasibility of normal-incidence infrared modulators based on s-polarized intersubband absorption is also demonstrated. Resonant dispersion associated with intersubband transitions is evidenced.     

Rashba splitting of kinetically bound states in gated HgCdTe surface quantum wells

The Rashba spin–orbit splitting of 2D electron gas in gated HgCdTe surface quantum wells on n-HgCdTe is studied experimentally (by the magneto-capacitance spectroscopy of Landau level method) and theoretically with emphasis on the peculiarities of spectrum at surface densities N_s corresponding to the onset of 2D subbands occupancy, where the regime of kinetic binding is realized. Although the spin–orbit splitting in kinetic confinement regime is small, the “Rashba polarization” Δn/n can achieve 100% because of strong difference in values of cutoff wave vector k_c for different spin-split sub-subbands.     

Realization of silicon quantum wires based on Si/SiGe/Si heterostructure

We report on the successful fabrication of silicon quantum wires with SiO2 boundaries on SiGe/Si heterostructures by combining Si/SiGe/Si heteroepitaxy, selective chemical etching, and subsequent thermal oxidation. The observational result of scanning electron microscope is demonstrated. The present method provides a well-controllable way to fabricate silicon quantum wires.     

Surface states induced by metal atoms at the Si/electrolyte interface

Metal atoms have been chemically deposited on n-Si and p-Si and the obtained deposits have been characterized with Auger electron spectroscopy. The obtained samples have been used as electrodes in acetonitrile electrolyte. The electrochemical studies heve been performed using classical current-voltage, impedance and Schottky-Mott measurements, and also subgap photocurrent spectroscopy of the surface states. It appears that the deposited metal atoms do induce surface states on the silicon surface. These surface states have a weak effect on the flatband potential (i.e. no strong pinning of the Fermi level is observed even for monolayer coverage) but the subgap photoyield is increased by several orders of magnitude. The shape of the quantum yield versus photon energy curve points to surface states widely distributed through the bandgap. These experiments finally confirm the ability of the subgap photocurrent technique to distinguish between the two kinds of optical processes that may occur between the surface states and the semiconductor bands.     

Effective barriers induced by confined carriers in space charge spectroscopy of Si/Ge quantum wells

Results are presented concerning the characterization of p-Si/Si_1-xGe_x/Si quantum wells (QW) by space charge spectroscopy. Analysis of potential barriers at the QW enables us to determine the valence band offset from such measurements. Admittance spectroscopy data of QWs with 30 nm undoped spacers, acceptor concentration N_A of about 10¹⁷cm^-3 in the cap and buffer layers, x=0.25 and a QW thickness in the range from 1 to 5 nm are in fair agreement with the proposed theoretical model. A decrease of the effective potential barriers due to hole tunneling via shallow acceptor states in the barrier is experimentally confirmed for 5 nm QW structures without spacers.     

Multiparticle states and the factors that complicate an experimental observation of the quantum coherence in the exciton gas of SiGe/Si quantum wells

The measured stationary and time-resolved photoluminescence is used to study the properties of the exciton gas in a second-order 5-nm-thick Si_0.905Ge_0.095/Si quantum well. It is shown that, despite the presence of an electron barrier in the Si_0.905Ge_0.095 layer, a spatially indirect biexciton is the most favorable energy state of the electron–hole system at low temperatures. This biexciton is characterized by a lifetime of 1100 ns and a binding energy of 2.0–2.5 meV and consists of two holes localized in the SiGe layer and two electrons mainly localized in silicon. The formation of biexcitons is shown to cause low-temperature (5 K) luminescence spectra over a wide excitation density range and to suppress the formation of an exciton gas, in which quantum statistics effects are significant. The Bose statistics can only be experimentally observed for a biexciton gas at a temperature of 1 K or below because of the high degree of degeneracy of biexciton states (28) and a comparatively large effective mass (about 1.3m _e ). The heat energy at such temperatures is much lower than the measured energy of localization at potential fluctuations (about 1 meV). This feature leads to biexciton localization and fundamentally limits the possibility of observation of quantum coherence in the biexciton gas.     

Hydrogen-adsorption induced atomic rearrangement of a Pb monolayer on Si(111)

We have observed interesting H-atom adsorption induced atomic rearrangements of a Pb monolayer on the Si(111) with a scanning tunneling microscope. A hexagonal ringlike pattern is formed around the point defect. The interactions among nearby H-adsorbed defects can even produce interferencelike superstructures. Phase boundaries are found to either enhance or suppress the formation of the interference pattern. These phenomena are produced by an intricate interplay between electronic and atomic interactions as perturbed by the adsorbed hydrogen atoms.     

Sharp n-type doping profiles in Si/SiGe heterostructures produced by atomic hydrogen etching

Surface segregation of group V dopant during thin film epitaxy of Si/SiGe heterostructures causes severe limitation on the sharpness of n-type doping profiles in pn junctions. Existing techniques for removal of surface segregated arsenic suffer from either high thermal budget or aggressive (ex situ) wet chemical etching. An in situ low temperature method is clearly desirable, particularly for device structures with high Ge content such as resonant tunnelling diodes, in order to minimize diffusion of the matrix elements as well as maintain structural integrity. In situ etching by atomic hydrogen is shown to be ideal for this purpose. The reaction mechanism ensures that this can only be a low temperature process and the method is shown to be highly effective and selective in the removal of surface segregated As. In comparison with other techniques, atomic hydrogen etching is also shown to be less aggressive and has a smaller impact on the surface/interface quality.     

Confined micro-explosion induced by ultrashort laser pulse at SiO2/Si interface

Ultrashort laser pulses tightly focused inside a transparent material present an example of laser interaction with matter where all the laser-affected material remains inside the bulk, thus the mass is conserved. In this paper, we present the case where the high intensity of a laser pulse is above the threshold for optical breakdown, and the material is ionised in the focal area. We consider in detail a special case where a micro-explosion is formed at the boundary of a silicon surface buried under a 10-micron-thick oxidised layer, providing the opportunity to affect the silicon crystal by a strong shock wave and creating new material phases from the plasma state. We summarise the main conclusions on ultrafast laser-induced material modifications in confined geometry and discuss the prospects of confined micro-explosion for forming new silicon phases.     

Enhancement of valley splitting in (100) Si MOSFETs at high magnetic fields

We report the density and magnetic field dependence of the valley splitting of two-dimensional electrons in (100) Si metal–oxide–semiconductor field-effect transistors, as determined via activation measurements in the quantum Hall regime. We find that the valley activation gap can be greatly enhanced at high magnetic fields as compared to the bare valley splitting. The observation of strong dependence of the valley activation gap on orbital Landau level occupancy and similar behavior of nearby spin gaps suggest that electron–electron interactions play a large role in the observed enhancement.     

Giant Zeeman splitting of light holes in GaAs/AlGaAs quantum wells

We have developed a theory of the longitudinal g-factor of light holes in semiconductor quantum wells. It is shown that the absolute value of the light-hole g-factor can strongly exceed its value in the bulk and, moreover, the dependence of the Zeeman splitting on magnetic field becomes non-linear in relatively low fields. These effects are determined by the proximity of the ground light-hole subband, lh1, to the first excited heavy-hole subband, hh2, in GaAs/AlGaAs-type structures. The particular calculations are performed in the framework of Luttinger Hamiltonian taking into account both the magnetic field-induced mixing of lh1 and hh2 states and the mixing of these states at heterointerfaces, the latter caused by chemical bonds anisotropy. A theory of magneto-induced reflection and transmission of light through the quantum wells for the light-hole-to-electron absorption edge is also presented.     

Local atomic environment of Si suboxides at the SiO2/Si(111) interface determined by angle-scanned photoelectron diffraction

Local environments of Si suboxides at the interface between a thermally grown SiO2 film and Si(111) were studied by angle-scanned photoelectron diffraction. Si 2p core-level spectra containing chemically shifted components were recorded. The components were deconvoluted by least squares fitting and assigned to different Si oxidation states. The obtained diffraction patterns of the various suboxides exhibit different features. Comparison of these patterns with multiple scattering calculations including a multipole R-factor analysis shows that a simple chemical abrupt interface model describes well the environment of the suboxides and indicates ordered SiO2 close to the interface.