Experimental evidence for the formation of stripe phases in Si/SiGe

We observe pronounced transport anisotropies in magneto-transport experiments performed in the two-dimensional electron system of a Si/SiGe heterostructure. They occur when an in-plane field is used to tune two Landau levels with opposite spin to energetic coincidence. The observed anisotropies disappear drastically for temperatures above 1 K. We propose that our experimental findings may be caused by the formation of a unidirectional stripe phase oriented perpendicular to the in-plane field.Comment: 4 pages, 3 figure

Transient-Enhanced Surface Diffusion on Natural-Oxide-Covered Si(001) Templates during Vacuum Annealing

We report on the transient-enhanced shape transformation of nano-structured Si(001) surfaces upon in vacuo annealing at relatively low temperatures of 900 -950 °C for a few minutes. We find dramatic surface mass transport concomitant with the development of low-energy facets on surfaces that are covered by native oxide. The enhanced surface mass transport ceases after the oxide is completely desorbed, and it is not observed on surfaces where the native oxide had been removed by HF before annealing

Efficient room-temperature light-emitters based on partly amorphised Ge quantum dots in crystalline Si

Semiconductor light emitters compatible with standard Si integration technology (SIT) are of particular interest for overcoming limitations in the operating speed of microelectronic devices 1-3. Light sources based on group-IV elements would be SIT compatible but suffer from the poor optoelectronic properties of bulk Si and Ge. Here, we demonstrate that epitaxially grown Ge quantum dots (QDs) in a fully coherent Si matrix show extraordinary optical properties if partially amorphised by Ge-ion bombardment (GIB). The GIB-QDs exhibit a quasi-direct-band gap and show, in contrast to conventional SiGe nanostructures, almost no thermal quenching of the photoluminescence (PL) up to room-temperature (RT). Microdisk resonators with embedded GIB-QDs exhibit threshold-behaviour and super-linear increase of the integrated PL-intensity (IPL) with increasing excitation power Pexc which indicates light amplification by stimulated emission in a fully SIT-compatible group-IV nano-system

Centrosymmetric PbTe/CdTe quantum dots coherently embedded by epitaxial precipitation

A concept for the fabrication of highly symmetric quantum dots that are coherently embedded in a single crystalline matrix is demonstrated. In this approach, the formation of the quantum dots is induced by a transformation of an epitaxial 2D quantum well into an array of isolated precipitates with dimensions of about 25 nm. The formation process is driven by the immiscibility of the constituent materials resulting from their different lattice structures. The investigated PbTe/CdTe heterosystem combines two different cubic lattices with almost identical lattice constants. Therefore, the precipitated quantum dots are almost strain free and near thermodynamic equilibrium they exhibit the shape of small-rhombo-cubo-octahedrons. The PbTe/CdTe quantum dots, grown on GaAs substrates, display intense room temperature luminescence at wavelength around 3.2 micrometer, which makes them auspicious for applications in mid-infrared photonic devices.Comment: 12 pages, 3 figure

The Society for Microelectronics -Annual Report 2003 Spin Relaxation in Si Quantum Wells Suppressed by an Applied Magnetic Field

We investigate spin properties of the two-dimensional electron gas in Si quantum wells defined by SiGe barriers. We find, in contrast to predictions of the classical model of D'yakonov-Perel, a strong anisotropy of spin relaxation and a decrease of the spin relaxation rate with increasing electron mobility. We show that for high electron mobility the cyclotron motion causes an additional modulation of spin-orbit coupling which leads to an effective suppression of spin relaxation rate. In spintronics, the aim is to make use of the spin degrees of freedom in addition to the electronic ones. Therefore, spintronic devices based on spins of carriers in semiconductors appear particularly promising. In such elements carriers can be easily moved by applying external voltages, the well known tool of classical electronics. The utilization of spin properties, however, usually is limited by the fast spin relaxation of conduction electrons. Therefore analysis of the spin relaxation mechanisms and the search for a suitable material and optimum conditions are of primary interest in this field. In III-V compounds the spin relaxation time is below one nanosecond [1]. Silicon based devices, due to much weaker spin-orbit coupling, appear much more promising. 2D Si layers in Si/SiGe structures exhibit a spin relaxation time of the order of a few microseconds by measurements of electron spin resonance (ESR) [2] - The effect of BR coupling on spin, σ, of a conduction electron can be described by an effective magnetic field, B BR . This field is oriented in-plane and perpendicular to electron momentum, ħk. The resulting zero field splitting is given by: The direction of the BR field depends on the direction of electron k-vector, and therefore the spread of k-vectors results in a spread of the BR field. Consequently, the ESR resonance is shifted and broadened. Momentum scattering, described by a rate 1/τ k , causes a modulation of the BR field in time which leads to the so called D'yakonovPerel (DP) spin relaxatio

Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si/SiGe Quantum Wells

Exploiting the spin resonance of two-dimensional (2D) electrons in SiGe/Si quantum wells we determine the carrier-density-dependence of the magnetic susceptibility. Assuming weak interaction we evaluate the density of states at the Fermi level D(E_F), and the screening wave vector, q_TF. Both are constant at higher carrier densities n, as for an ideal 2D carrier gas. For n < 3e11 cm-2, they decrease and extrapolate to zero at n = 7e10 cm-2. Calculating the mobility from q_TF yields good agreement with experimental values justifying the approach. The decrease in D(E_F) is explained by potential fluctuations which lead to tail states that make screening less efficient and - in a positive feedback - cause an increase of the potential fluctuations. Even in our high mobility samples the fluctuations exceed the electron-electron interaction leading to the formation of puddles of mobile carriers with at least 1 micrometer diameter.Comment: 4 pages, 3 figure