Gregor Mussler

Band engineering and growth of tensile strained Ge/(Si)GeSn heterostructures for tunnel field effect transistors

In this letter, we propose a heterostructure design for tunnel field effect transistors with two low direct bandgap group IV compounds, GeSn and highly tensely strained Ge in combination with ternary SiGeSn alloy. Electronic band calculations show that strained Ge, used as channel, grown on Ge 1−xSnx (x > 9%) buffer, as source, becomes a direct bandgap which significantly increases the tunneling probability. The SiGeSn ternaries are well suitable as drain since they offer a large indirect bandgap. The growth of such heterostructures with the desired band alignment is presented. The crystalline quality of the (Si)Ge(Sn) layers is similar to state-of-the-art SiGe layers.

Tensely strained GeSn alloys as optical gain media

This letter presents the epitaxial growth and characterization of a heterostructure for an electrically injected laser, based on a strained GeSn active well. The elastic strain within the GeSn well can be tuned from compressive to tensile by high quality large Sn content (Si)GeSn buffers. The optimum combination of tensile strain and Sn alloying softens the requirements upon indirect to direct bandgap transition. We theoretically discuss the strain-doping relation for maximum net gain in the GeSn active layer. Employing tensile strain of 0.5% enables reasonable high optical gain values for Ge0.94Sn0.06 and even without any n-type doping for Ge0.92Sn0.08.

GeSn Heterojunction LEDs on Si Substrates

GeSn on Si light-emitting diodes (LEDs) is investigated for different Sn concentrations up to 4.2% and they are compared with an LED made from pure Ge on Si. The LEDs are realized from in-situ doped pin junctions in GeSn on Ge virtual substrates. The device structures are grown with a special ultra-low temperature molecular beam epitaxy process. All LEDs clearly show direct bandgap electroluminescence emission at room temperature. The light intensity of the compressively strained GeSn LEDs increases with higher Sn concentration. The in-plane strain of the LEDs is determined with reciprocal space mapping. The bandgap energies of the emitting GeSn layer are calculated from the emission spectra.

Photocurrent and transmission spectroscopy of direct-gap interband transitions in Ge∕SiGe quantum wells

The authors studied the direct-gap interband transitions in strain-compensated Ge∕SiGe quantum wells grown by low energy plasma enhanced chemical vapor deposition. A series of excitonic interband absorptions from the quantized hole states to the quantized electronic states at the Γ7c− edge in the Ge wells is observed up to room temperature by photocurrent and transmission spectroscopy. The results are compared with theory. At low temperature, the quantum confined Stark effect is demonstrated in a Schottky diode geometry.

Robust surface electronic properties of topological insulators: Bi2Te3 films grown by molecular beam epitaxy

The surface electronic properties of the important topological insulator Bi2Te3 are shown to be robust under an extended surface preparation procedure, which includes exposure to atmosphere and subsequent cleaning and recrystallization by an optimized in situ sputter-anneal procedure under ultrahigh vacuum conditions. Clear Dirac-cone features are displayed in high-resolution angle-resolved photoemission spectra from the resulting samples, indicating remarkable insensitivity of the topological surface state to cleaning-induced surface roughness.

n-Channel MOSFETs Fabricated on SiGe Dots for Strain-Enhanced Mobility

The silicon germanium dots grown in the Stranski-Krastanow mode are used to induce biaxial tensile strain in a silicon capping layer. A high Ge content and correspondingly high Si strain levels are reached due to the 3-D growth of the dots. The n-channel MOS devices, referred to in this letter as DotFETs, are processed with the main gate segment above the strained Si layer on a single dot. To prevent the intermixing of the Si/SiGe/Si structure, a novel low-temperature FET structure processed below 400°C has been implemented: The ultrashallow source/drain junctions formed by excimer-laser annealing in the full-melt mode of ion-implanted dopants are self-aligned to a metal gate. The crystallinity of the structure is preserved throughout the processing, and compared to reference devices, an average increase in the drain current of up to 22.5% is obtained.

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3.

Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (∼10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi2Se3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se–Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents.

Strain and composition effects on Raman vibrational modes of silicon-germanium-tin ternary alloys

We investigated Raman vibrational modes in silicon-germanium-tin layers grown epitaxially on germanium/silicon virtual substrates using reduced pressure chemical vapor deposition. Several excitation wavelengths were utilized to accurately analyze Raman shifts in ternary layers with uniform silicon and tin content in 4–19 and 2–12 at. % ranges, respectively. The excitation using a 633 nm laser was found to be optimal leading to a clear detection and an unambiguous identification of all first order modes in the alloy. The influence of both strain and composition on these modes is discussed. The strain in the layers is evaluated from Raman shifts and reciprocal space mapping data and the obtained results are discussed in the light of recent theoretical calculations.

High- k gate stacks on low bandgap tensile strained Ge and GeSn alloys for field-effect transistors

We present the epitaxial growth of Ge and Ge0.94Sn0.06 layers with 1.4% and 0.4% tensile strain, respectively, by reduced pressure chemical vapor deposition on relaxed GeSn buffers and the formation of high-k/metal gate stacks thereon. Annealing experiments reveal that process temperatures are limited to 350 °C to avoid Sn diffusion. Particular emphasis is placed on the electrical characterization of various high-k dielectrics, as 5 nm Al2O3, 5 nm HfO2, or 1 nmAl2O3/4 nm HfO2, on strained Ge and strained Ge0.94Sn0.06. Experimental capacitance-voltage characteristics are presented and the effect of the small bandgap, like strong response of minority carriers at applied field, are discussed via simulations.

Electronic structure, surface morphology, and topologically protected surface states of Sb2Te3 thin films grown on Si(111)

We have performed a combined spectroscopy and microscopy study on surfaces of Sb2Te3/Si(111) thin films exposed to air and annealed under ultra-high vacuum conditions. Scanning tunneling microscopy images, with atomic resolution present in most areas of such processed surfaces, show a significant amount of impurities and defects. Scanning tunneling spectroscopy reveals the bulk band gap of ∼170 meV centered ∼65 meV above the Fermi level. This intrinsic p-type doping behavior is confirmed by high-resolution angle-resolved photoemission spectra, which show the dispersions of the lower Dirac cone and the spectral weight of the bulk valence bands crossing the Fermi level. Spin-polarized photoemission revealed up to ∼15% in-plane spin polarization for photoelectrons related to the topologically protected Dirac cone states near the Fermi level, and up to ∼40% for several states at higher binding energies. The results are interpreted using ab initio electronic structure simulations and confirm the robustness of ...