M. Stoffel

Hybrid superconductor-semiconductor devices made from self-assembled SiGe nanocrystals on silicon

The epitaxial growth of germanium on silicon leads to the self-assembly of SiGe nanocrystals by a process that allows the size, composition and position of the nanocrystals to be controlled. This level of control, combined with an inherent compatibility with silicon technology, could prove useful in nanoelectronic applications. Here, we report the confinement of holes in quantum-dot devices made by directly contacting individual SiGe nanocrystals with aluminium electrodes, and the production of hybrid superconductor-semiconductor devices, such as resonant supercurrent transistors, when the quantum dot is strongly coupled to the electrodes. Charge transport measurements on weakly coupled quantum dots reveal discrete energy spectra, with the confined hole states displaying anisotropic gyromagnetic factors and strong spin-orbit coupling with pronounced dependences on gate voltage and magnetic field.

SiGe growth on patterned Si(001) substrates: Surface evolution and evidence of modified island coarsening

The morphological evolution of both pits and SiGe islands on patterned Si(001) substrates is investigated. With increasing Si buffer layer thickness the patterned holes transform into multifaceted pits before evolving into inverted truncated pyramids. SiGe island formation and evolution are studied by systematically varying the Ge coverage and pit spacing and quantitative data on the influence of the pattern periodicity on the SiGe island volume are presented. The presence of pits allows the fabrication of uniform island arrays with any of their equilibrium shapes.

Ge hut cluster luminescence below bulk Ge band gap

We report on the photoluminescence (PL) properties of Ge hut cluster islands on Si(001) that were overgrown at temperatures as low as 250 °C. We find that the island-related photoluminescence systematically redshifts as the overgrowth temperature is reduced from 500 to 360 °C, which is attributed to a reduced Ge segregation. For even lower overgrowth temperatures, the emission energy saturates at 0.63 eV or 1.96 μm, more than 110 meV smaller than the band gap of unstrained bulk Ge. We report a PL peak centered at 2.01 μm at low excitation power, in good agreement with the estimated transition energy for a spatially indirect transition between holes confined in the strained Ge island and electrons confined in the surrounding Si matrix. PL is observed up to a temperature of 185 K and an activation energy of 40 meV is deduced from fitting the temperature-dependent peak intensity. Annealing experiments reveal a systematic blueshift of the hut cluster-related PL, thus verifying unambiguously, that the PL signa...

Electroluminescence of self-assembled Ge hut clusters

We have fabricated Si-based light-emitting diodes operating in the near infrared. The active layers of the devices consist of either one or ten layers of Ge/Si self-assembled hut clusters grown by molecular-beam epitaxy. Luminescence is observed in the spectral range between 1.4 and 1.5 μm. For the ten layer stack of Ge islands, electroluminescence is observed up to room temperature. A direct comparison with a pure Si reference p-i-n diode allows us to attribute the luminescence to radiative recombinations between holes localized in the Ge islands and electrons localized in the strained Si above and below the islands.

Composition and strain in SiGe/Si(001) nanorings revealed by combined x-ray and selective wet chemical etching methods

The authors used x-ray diffraction to investigate strain and composition in SiGe nanorings formed during partial Si capping of self-assembled SiGe/Si(001) islands. The obtained results are corroborated with selective wet chemical etching experiments. Clear evidence is provided that rings are composed of a Ge rich core surrounded by Si richer ridges indicating that a substantial material redistribution occurs during the shape transformation from SiGe islands to rings. The results suggest that SiGe ring formation is driven by strain relief.

SiGe wet chemical etchants with high compositional selectivity and low strain sensitivity

We investigate the effect of strain on the etching rate of two SiGe selective wet etchants, namely NH4OH:H2O2 and H2O2, which are currently used to investigate the composition of strained SiGe layers. We measured the etching rates for relaxed Si1−xGex layers and for layers under a ±0.84% biaxial strain with different nominal Ge fraction x. For both etchants, we found that there is no appreciable strain sensitivity, i.e. the etching rates do not depend on the actual strain state in the SiGe films. Instead, for the NH4OH:H2O2 solution, the rates are primarily determined by the Ge content. Finally, we show that both etchants are isotropic with no preferential etching of particular facets.

Extended wavelength region of self-assembled Ge/Si(001) islands capped with Si at different temperatures

We investigate the emission wavelength region of self-assembled Ge/Si(001) islands. The islands were grown between 360 and 840 °C and subsequently capped with Si at low temperatures (300 °C). Under these conditions, the island morphology is preserved as revealed by atomic force microscopy. By decreasing the capping temperature, photoluminescence measurements evidence a systematic redshift enabling us to discuss the relative contribution of Si intermixing during growth and during capping. We also find that the emission wavelength can be extended up to 2.06 μm for hut clusters grown at 400 °C. By further decreasing the Ge growth temperature to 360 °C, the emission energy evidences a blueshift. This result is explained by enhanced charge carrier confinement in extremely small Ge quantum dots.

Three-dimensional isocompositional profiles of buried SiGe∕Si(001) islands

The authors investigate the composition profile of SiGe islands after capping with Si to form quantum dots, using a two step etching procedure and atomic force microscopy. Initially, the Si capping layers are removed by etching selectively Si over Ge and then the composition of the disclosed islands is addressed by selectively etching Ge over Si. For samples grown at 580°C the authors show that even when overgrowth leads to a flat Si surface and the islands undergo strong morphological changes, a Ge-rich core region is still preserved in the dot. At high growth and overgrowth temperatures (740°C), the experiments show that the newly formed base of the buried islands is more Si rich than their top. Furthermore, the authors find that for the growth conditions used, no lateral motion takes place during capping.

Periodic pillar structures by Si etching of multilayer GeSi∕Si islands

Laterally aligned multilayer GeSi∕Si islands grown on a patterned Si (001) substrate are disclosed by selective etching of Si in a KOH solution. This procedure allows us to visualize the vertical alignment of the islands in a three-dimensional perspective. Our technique reveals that partly coalesced double islands in the initial layer do not merge together, but instead gradually reproduce into well-separated double islands in upper layers. We attribute this effect to very thin spacer layers, which efficiently transfer the strain modulation of each island through the spacer layer to the surface. The etching rate of Si is reduced in tensile strained regions, which helps to preserve sufficient Si between the stacked islands to form a periodic array of freestanding and vertically modulated heterostructure pillars.

Observation of spin-selective tunneling in SiGe nanocrystals

Spin-selective tunneling of holes in SiGe nanocrystals contacted by normal-metal leads is reported. The spin selectivity arises from an interplay of the orbital effect of the magnetic field with the strong spin-orbit interaction present in the valence band of the semiconductor. We demonstrate both experimentally and theoretically that spin-selective tunneling in semiconductor nanostructures can be achieved without the use of ferromagnetic contacts. The reported effect, which relies on mixing the light and heavy holes, should be observable in a broad class of quantum-dot systems formed in semiconductors with a degenerate valence band.