228 research outputs found
Single-Electron Transistor in Strained Si/SiGe Heterostructures
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.Comment: 3 page
Positioning of self-assembled Ge islands on stripe-patterned Si (001) substrates
Self-assembled Ge islands were grown on stripe-patterned Si (001) substrates
by solid source molecular beam epitaxy. The surface morphology obtained by
atomic force microscopy (AFM) and cross-sectional transmission electron
microscopy images (TEM) shows that the Ge islands are preferentially grown at
the sidewalls of pure Si stripes along [-110] direction at 650o C or along the
trenches, whereas most of the Ge islands are formed on the top terrace when the
patterned stripes are covered by a strained GeSi buffer layer. Reducing the
growth temperature to 600oC results in a nucleation of Ge islands both on the
top terrace and at the sidewall of pure Si stripes. A qualitative analysis,
based on the growth kinetics, demonstrates that the step structure of the
stripes, the external strain field and the local critical wetting layer
thickness for the islands formation contribute to the preferential positioning
of Ge islands on the stripes.Comment: 10 pages, 7 figures, 1 table, the original paper is in print in J.
Appl. Phy
Initial stage of the 2D-3D transition of a strained SiGe layer on a pit-patterned Si(001) template
We investigate the initial stage of the 2D-3D transition of strained Ge
layers deposited on pit-patterned Si(001) templates. Within the pits, which
assume the shape of inverted, truncated pyramids after optimized growth of a Si
buffer layer, the Ge wetting layer develops a complex morphology consisting
exclusively of {105} and (001) facets. These results are attributed to a
strain-driven step-meandering instability on the facetted side-walls of the
pits, and a step-bunching instability at the sharp concave intersections of
these facets. Although both instabilities are strain-driven, their coexistence
becomes mainly possible by the geometrical restrictions in the pits. It is
shown that the morphological transformation of the pit surface into low-energy
facets has strong influence on the preferential nucleation of Ge islands at the
flat bottom of the pits.Comment: 19 pages, 7 figure
Arrays of Individual DNA Molecules on Nanopatterned Substrates
Arrays of individual molecules can combine the advantages of microarrays and single-molecule studies. They miniaturize assays to reduce sample and reagent consumption and increase throughput, and additionally uncover static and dynamic heterogeneity usually masked in molecular ensembles. However, realizing single-DNA arrays must tackle the challenge of capturing structurally highly dynamic strands onto defined substrate positions. Here, we create single-molecule arrays by electrostatically adhering single-stranded DNA of gene-like length onto positively charged carbon nanoislands. The nanosites are so small that only one molecule can bind per island. Undesired adsorption of DNA to the surrounding non-target areas is prevented via a surface-passivating film. Of further relevance, the DNA arrays are of tunable dimensions, and fabricated on optically transparent substrates that enable singe-molecule detection with fluorescence microscopy. The arrays are hence compatible with a wide range of bioanalytical, biophysical, and cell biological studies where individual DNA strands are either examined in isolation, or interact with other molecules or cells
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
Valley splitting of Si/SiGe heterostructures in tilted magnetic fields
We have investigated the valley splitting of two-dimensional electrons in
high quality Si/SiGe heterostructures under tilted magnetic fields.
For all the samples in our study, the valley splitting at filling factor
() is significantly different before and after the
coincidence angle, at which energy levels cross at the Fermi level. On both
sides of the coincidence, a linear density dependence of on the
electron density was observed, while the slope of these two configurations
differs by more than a factor of two. We argue that screening of the Coulomb
interaction from the low-lying filled levels, which also explains the observed
spin-dependent resistivity, is responsible for the large difference of
before and after the coincidence.Comment: REVTEX 4 pages, 4 figure
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
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
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
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