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The hopping ac conductance, which is realized at the transverse conductance minima in the regime of the integer Hall effect, has been measured using a combination of acoustic and microwave methods. Measurements have been made in the p-GeSi/Ge/GeSi structures with quantum wells in a wide frequency range (30-1200 MHz). The experimental frequency dependences of the real part of ac conductance $\sigma_1$ have been interpreted on the basis of the model presuming hops between localized electronic states belonging to isolated clusters. At high frequencies, dominating clusters are pairs of close states; upon a decrease in frequency, large clusters that merge into an infinite percolation cluster as the frequency tends to zero become important. In this case, the frequency dependences of the ac conductance can be represented by a universal curve. The scaling parameters and their magnetic-field dependence have been determined., Comment: 9 pages, 12 figures, 1 table

The effective g-factor of 2D holes in modulation doped \mbox{p-SiGe/Ge/SiGe} structures was studied. The AC conductivity of samples with hole densities from $3.9 \times 10^{11}$~to $6.2 \times 10^{11}~\text{cm}^{-2}$ was measured in perpendicular magnetic fields up to $8~\text{T}$ using a contactless acoustic method. From the analysis of the temperature dependence of conductivity oscillations, the $\mathrm{g}_{\perp}$-factor of each sample was determined. The $\mathrm{g}_{\perp}$-factor was found to be decreasing approximately linearly with hole density. This effect is attributed to non-parabolicity of the valence band., Comment: 4 pages, 5 figures

The high-frequency (ac) conductivity of a high quality modulation doped GeSi/Ge/GeSi single quantum well structure with hole density $p$=6$\times$10$^{11}$cm$^{-2}$ was measured by the surface acoustic wave (SAW) technique at frequencies of 30 and 85~MHz and magnetic fields $B$ of up to 18 T in the temperature range of 0.3 -- 5.8 K. The acoustic effects were also measured as a function of the tilt angle of the magnetic field with respect to the normal of the two-dimensional channel at $T$=0.3 K. It is shown, that at the minima of the conductivity oscillations, holes are localized on the Fermi level, and that there is a temperature domain in which the high-frequency conductivity in the bulk of the quantum well is of the activation nature. The analysis of the temperature dependence of the conductivity at odd filling factors enables us to determine the effective $g_z$ factor. It is shown that the in-plane component of the magnetic field leads to an increase of the cyclotron mass and to a reduction of the $g_z$ factor. We developed a microscopic theory of these effects for the heavy-hole states of the complex valence band in quantum wells which describes well the experimental findings., Comment: 9 pages, 10 figures

We report a procedure to determine the frequency-dependent conductance of quantum Hall structures in a broad frequency domain. The procedure is based on the combination of two known probeless methods -- acoustic spectroscopy and microwave spectroscopy. By using the acoustic spectroscopy, we study the low-frequency attenuation and phase shift of a surface acoustic wave in a piezoelectric crystal in the vicinity of the electron (hole) layer. The electronic contribution is resolved using its dependence on a transverse magnetic field. At high frequencies, we study the attenuation of an electromagnetic wave in a coplanar waveguide. To quantitatively calibrate these data, we use the fact that in the quantum-Hall-effect regime the conductance at the maxima of its magnetic field dependence is determined by extended states. Therefore, it should be frequency independent in a broad frequency domain. The procedure is verified by studies of a well-characterized $p$-SiGe/Ge/SiGe heterostructure., Comment: 7 pages, 8 figures; to be submitted to JAP

The contactless Surface Acoustic Wave (SAW) technique was implemented to probe the high-frequency (ac) conductivity in a high-mobility $p$-SiGe/Ge/SiGe structure in the integer quantum Hall (IQHE) regime. The structure was grown by low-energy plasma-enhanced chemical vapor deposition and comprised a two-dimensional channel formed in a compressively strained Ge layer. It was investigated at temperatures of 0.3 - 5.8 K and magnetic fields up to 18 T at various SAW intensities. In the IQHE regime, in minima of the conductivity oscillations with small filling factors, holes are localized. The ac conductivity is of the hopping nature and can be described within the "two-site" model. Furthermore, the dependence of the ac conductivity on the electric field of the SAW was determined. The manifestation of non-linear effects is interpreted in terms of nonlinear percolation-based conductivity., Comment: 6 pages, 10 figures

Germanium and silicon-germanium alloys have found entry into Si technology thanks to their compatibility with Si processing and their ability to tailor electronic properties by strain and band-gap engineering. Germanium's potential to extend Si functionalities, as exemplified by lasing action of strained-Ge on Si substrates, has brought the material back to attention. Yet despite these advances, non-radiative transitions, induced by crystal defects originating from the Ge/Si interface, continue to be a serious bottleneck. Here we demonstrate the drastic emission enhancement achieved via control and mitigation over the parasitic activity of defects in micronscale Ge/Si crystals. We unravel how defects affect interband luminescence and minimize their influence by controlling carrier diffusion with band-gap-engineered reflectors. We finally extended this approach designing efficient quantum well emitters. Our results pave the way for the large-scale implementation of advanced electronic and photonic structures unaffected by the ubiquitous presence of defects developed at epitaxial interfaces.

Measurement results of the acoustoelectric effects (surface acoustic waves (SAW) attenuation and velocity) in a high-mobility $p$-SiGe/Ge/SiGe structure are presented. The structure was LEPECVD grown with a two dimensional (2D) channel buried in the strained Ge layer. The measurements were performed as a function of temperature (1.5 - 4.2 K) and magnetic field (up to 8.4 T) at different SAW intensities at frequencies 28 and 87 MHz. Shubnikov-de Haas-like oscillations of both SAW attenuation and the velocity change have been observed. Hole density and mobility, effective mass, quantum and transport relaxation times, as well as the Dingle temperature were measured with a method free of electric contacts. The effect of heating of the 2D hole gas by the electric field of the SAW was investigated. Energy relaxation time $\tau_{\varepsilon}$ and the deformation potential constant determined., Comment: 5 pages, 7 figures, 1 table

We investigate the effect of strain on the etching rate of two SiGe wet etchants, namely NH4OH:H2O2 and H2O2. For both etchants, we found that there is no appreciable strain selectivity, 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., Comment: 13 pages, 4 figures, regular paper

The morphology of the first three-dimensional islands appearing during strained growth of SiGe alloys on Si(001) was investigated by scanning tunneling microscopy. High resolution images of individual islands and a statistical analysis of island shapes were used to reconstruct the evolution of the island shape as a function of size. As they grow, islands undergo a transition from completely unfacetted rough mounds (prepyramids) to partially {105} facetted islands and then they gradually evolve to {105} facetted pyramids. The results are in good agreement with the predictions of a recently proposed theoretical model.