Guenther Bauer

Initial stage of the two-dimensional to three-dimensional 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.

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.

Coherence and wavefront characterization of Si-111 monochromators using double-grating interferometry

A study of the coherence and wavefront properties of a pseudo-channel-cut monochromator in comparison with a double-crystal monochromator is presented. Using a double-grating interferometer designed for the hard X-ray regime, the complex coherence factor was measured and the wavefront distortions at the sample position were analyzed. A transverse coherence length was found in the vertical direction that was a factor of two larger for the channel-cut monochromator owing to its higher mechanical stability. The wavefront distortions after different optical elements in the beam, such as monochromators and mirrors, were also quantified. This work is particularly relevant for coherent diffraction imaging experiments with synchrotron sources.

Temperature dependence of ordered GeSi island growth on patterned Si (001) substrates

Statistical information on GeSi islands grown on two-dimensionally pit-patterned Si substrates at different temperatures is presented. Three growth regimes on patterned substrates are identified: (i) kinetically limited growth at low growth temperatures, (ii) ordered island growth in an intermediate temperature range, and (iii) stochastic island growth within pits at high temperatures. A qualitative model based on growth kinetics is proposed to explain these phenomena. It can serve as a guidance to realize optimum growth conditions for ordered islands on patterned substrates.

Structure and composition of bismuth telluride topological insulators grown by molecular beam epitaxy

The structure and composition of Bi2Te3-delta topological insulator layers grown by molecular beam epitaxy is studied as a function of beam flux composition. It is demonstrated that, depending on the Te/Bi2Te3 flux ratio, different layer compositions are obtained corresponding to a Te deficit delta varying between 0 and 1. On the basis of X-ray diffraction analysis and a theoretical description using a random stacking model, it is shown that for delta >= 0 the structure of the epilayers is described well by a random stacking of Te-Bi-Te-Bi-Te quintuple layers and Bi-Bi bilayers sharing the same basic hexagonal lattice structure. The random stacking model accounts for the observed surface step structure of the layers and compares very well with the measured X-ray data, from which the lattice parameters a and c as a function of the chemical composition were deduced. In particular, the in-plane lattice parameter a is found to continuously increase and the average distance of the (0001) hexagonal lattice planes is found to decrease from the Bi2Te3 to the BiTe phase. Moreover, the lattice plane distances agree well with the linear interpolation between the Bi2Te3 and BiTe values taking the strain in the epilayers into account. Thus, the chemical composition Bi2Te3-delta can be directly determined by X-ray diffraction. From analysis of the X-ray diffraction data, quantitative information on the randomness of the stacking sequence of the Bi and Te layers is obtained. According to these findings, the layers represent random one-dimensional alloys of Te-Bi-Te-Bi-Te quintuple and Bi-Bi bilayers rather than a homologous series of ordered compounds.

Twin domain imaging in topological insulator Bi2Te3 and Bi2Se3 epitaxial thin films by scanning X-ray nanobeam microscopy and electron backscatter diffraction

Imaging with surface- and bulk-sensitive electron and X-ray diffraction based microscopic techniques enabled identification of the twin domain distribution of Bi2Te3 and Bi2Se3 thin films. Correlations between the surface topography and crystal orientation are established.

Magnetooptical determination of a topological index

When a Dirac fermion system acquires an energy-gap, it is said to have either trivial (positive energy-gap) or non-trivial (negative energy-gap) topology, depending on the parity ordering of its conduction and valence bands. The non-trivial regime is identified by the presence of topological surface or edge-states dispersing in the energy gap of the bulk and is attributed a non-zero topological index. In this work, we show that such topological indices can be determined experimentally via an accurate measurement of the effective velocity of bulk massive Dirac fermions. We demonstrate this approach analytically starting from the Bernevig-Hughes-Zhang Hamiltonian to show how the topological index depends on this velocity. We then experimentally extract the topological index in Pb1-xSnxSe and Pb1-xSnxTe using infrared magnetooptical Landau level spectroscopy. This approach is argued to be universal to all material classes that can be described by a Bernevig-Hughes-Zhang-like model and that host a topological phase transition.Topological matter: magnetooptical characterizationOpening a gap in a Dirac fermion system leads to the formation of a trivial or a non-trivial phase. A non-trivial phase exhibits conductive surface or edge states, and can be attributed to a non-zero topological index related to the parity of the conduction and valence bands. This index is usually inferred from the characterization of the edge or surface states through angle resolved photoemission spectroscopy or quantum spin Hall effect. Now, an international team led by Yves Guldner at Ecole Normale Supérieure reports that the effective velocity of bulk massive Dirac fermions depends on the topological index. Infrared magnetooptical Landau level spectroscopy allows to accurately determining this velocity and therefore to directly extract the topological index, as exemplified with the 3D topological insulators Pb1-xSnxSe and Pb1-xSnxTe.

Strained MOSFETs on ordered SiGe dots

The potential of strained DOTFET technology is demonstrated. This technology uses a SiGe island as a stressor for a Si capping layer, into which the transistor channel is integrated. The structure information is extracted from AFM measurements of fabricated samples. Strain on the upper surface of a 30 nm thick Si layer is in the range of 0.7%, as supported by finite element calculations. The Ge content in the SiGe island is 30% on average, showing an increase towards the top of the island. Based on realistic structure information, three-dimensional strain profiles are calculated and device simulations are performed. Up to 15% enhancement of the NMOS saturation current is predicted.

Processes of Self-Organization during Epitaxial Growth of Semiconductor Superlattices — An X-Ray Scattering Study

Self-organization processes taking place at a surface during epitaxial growth of semiconductor layered systems represent a promising way for the fabrication of low dimensional objects.l Two types of self-organization have been observed during growth. In the so called layer-by layer growth mode, the growth can be described as a lateral movement of monolayer steps at a growing vicinal surface. During the growth, the monolayer steps agglomerate creating a nearly periodic distribution of step bunches divided by nearly atomically flat terraces. In a multilayer, this process creates a template for a nearly periodic sequence of one-dimensional quantum wires. The reason for this bunching tendency is so far not completely clear. In a series of papers2–4 the bunching of monolayer steps has been explained as a consequence of an inhomogeneous strain at the growing surface caused by a rough interface buried underneath and by a lattice mismatch of the growing layer with respect to the substrate. An inhomogeneous surface strain distribution affects the surface diffusion of adatoms and consequently the mobility of migrating monolayer steps. Then, a nucleation of a step bunch is more probable in a region with a large strain gradient. This description of the bunching process was able to explain the major features observed, namely the improving periodicity of the bunches with increasing number of interfaces, and a oblique replication of the position of the bunches at subsequent interfaces.3

Investigation of strain relaxation in short-period SimGen superlattices using reciprocal space mapping

The optoelectronic properties of ultrathin SimGen strained layer superlattices (SLSs) depend strongly on their structural perfection and the strain adjustment of the SLS by a Si1-xGex alloy buffer. We used double crystal and triple axis x-ray diffractometry to characterize the structural properties of short period Si6Ge4 and Si9Ge6 SLSs grown on about 1 micrometers thick step-graded SiGe alloy buffers. As grown SLSs and samples annealed subsequently at 550 degree(s)C, 650 degree(s)C, and 780 degree(s)C for 60 min were investigated, the latter to study effects of post-growth thermal treatments typical for conventional Si device fabrication. Precise strain data were extracted from two-dimensional reciprocal space maps around (004) and (224) reciprocal lattice points. These data were used as refined input parameters for the dynamical simulation of the integrated intensity along the q[004] direction.