[en] Coherently strained Ge-clusters on Si(001) were overgrown with Si at temperatures between 300 and 620 deg. C. Using high-resolution transmission electron microscopy the clusters were found to strongly flatten during Si-coverage at temperatures above about 400 deg. C. By contrast, a good preservation of the original morphology of the Ge-clusters was achieved by Si-capping at low temperature (300 deg. C). A flat Si surface finally recovered if the low-temperature overgrowth was followed by Si-deposition ramping the temperature up to growth temperatures typical for the SiGe-system

[en] The combined use of Rutherford backscattering spectrometry and secondary ion mass spectroscopy allowed a complete characterization of a set of SiGe relaxed buffer layers grown by low-energy plasma-enhanced chemical vapor deposition. The Ge contents for the top SiGe constant composition layers have been obtained by RBS. Matrix effects have been studied by using monoatomic and biatomic ions as well as low and high energy O₂⁺ and Cs⁺ primary beam ions. We show that matrix effects are suppressed when an O₂⁺ primary beam ion source is used at 3 keV, and when detecting with ³⁰Si⁺ and ⁷⁰Ge⁺ secondary ions for Ge contents <0.47. For higher Ge contents a better compromise is achieved with Cs⁺ bombardment at 14.5 keV when detecting with ⁷⁴Ge⁷⁶Ge^- secondary ions. The procedure allows to extract the Ge concentration profiles with good accuracy even at very high depths and at very low Ge concentrations

[en] Thick, linearly graded-composition strained Si_1-xGe_x/Si layers were recently developed for proton beam bending and extraction experiments. Such unrelaxed layers which are many microns thick necessitate a low maximum germanium content. Here, graded Si_1-xGe_x epilayers, 5-20 μm thick with maximum Ge compositions of x=0.5-1.7%, grown by low energy plasma enhanced chemical vapour deposition were characterized using a recently developed mode of ion channeling analysis which is capable of quantifying the small lattice rotations along off-normal planar directions. High-quality 10 μm Si_1-xGe_x epilayers with bend angles along off-normal directions which agree very well with those of fully strained layers are successfully grown

[en] Strain-induced roughening and dislocation formation has been studied by high-resolution transmission electron microscopy (HRTEM) in compressively strained Ge quantum wells on linearly graded SiGe buffer layers grown by low-energy plasma-enhanced chemical vapour deposition (LEPECVD). We show that for appropriately chosen plasma densities and substrate temperatures, abrupt interfaces can be achieved on both sides of the Ge channels, when additional hydrogen is supplied to the reactive gases, even for channel widths above the critical thickness for dislocation formation. Optimized modulation doped Ge quantum wells (MODQWs) exhibit the highest hole mobilities observed to date, approaching values of ∼90000 cm² V^-1 s^-1 for a sheet density of ∼6x10¹¹ cm^-2 at liquid He temperatures

[en] Low energy plasma enhanced chemical vapour deposition (LEPECVD) is a relatively new growth method, which has been used to create high quality epitaxial silicon germanium material on conventional Si(001) wafers. This material is eminently suitable for electronic devices. The best performance for n-type and p-type conduction is seen in tensile-strained Si and compressively strained Ge quantum wells, respectively. Since such quantum wells cannot be grown directly on a silicon substrate, a virtual substrate (VS) is first grown. The reactive conditions within the plasma make it possible to grow the VS at rates of up to 10 nms^-1 independent of substrate temperature. The quantum wells were grown using a lower plasma intensity, at growth rates of approximately 0.3 nms^-1. The electrical properties of the material compare very well with molecular beam epitaxy (MBE) references, and hybrid material where the buffer is grown by LEPECVD and the electrically active layers are grown by MBE. In addition, the structural quality of the material is analysed by atomic force microscopy, transmission electron microscopy and defect etching

[en] The current flowing in a homogeneous low-dimensional conductor is shown to be rectified by a gate-controlled asymmetric barrier resembling a Schottky barrier. The barrier shape is set by varying the potential along a nanofabricated nonequipotential gate which allows simple external control over the device function independent of material properties. A forward-to-reverse current ratio of more than 10⁴ is obtained. The merits of diodes fabricated in this way with respect to conventional diodes are discussed

[en] This letter reports on the electrical performance of strained Si-based n-type heterostructure field-effect transistors prepared on 500 nm Si_0.56Ge_0.44 virtual substrates. The method of low-energy plasma-enhanced chemical vapor deposition at low temperature was used for the growth of the relaxed SiGe buffer. The active layers have been deposited by molecular-beam epitaxy. The thin buffer improves the thermal conductivity by a factor of 3 and shows a much lower surface roughness compared to control structures on conventional virtual substrate with a 5-μm-thick graded buffer. Cutoff frequencies of f_T=55 GHz and f_max(U)=138 GHz have been achieved which are very close to the results of the control sample

[en] 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 low-energy plasma-enhanced chemical vapor deposition 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 τ_ε and the deformation potential constant determined.

[en] A detailed spectroscopic and morphological study of GaAs epitaxial layers grown by molecular beam epitaxy on Ge buffer layers deposited by low energy plasma enhanced chemical vapor deposition on Si is presented. The aim is to understand the nature of thermal strain relaxation induced by crack formation in the epilayers. The comparison of the experimental data on the spatial strain relaxation pattern with the theoretical prediction from a purely elastic model indicates that strain relaxation around cracks arises from two contributions. At short distances the main contribution is essentially plastic, due to the presence of extended defects. At large distances, on the contrary, elastic relaxation seems to dominate. It is also shown that GaAs grown on Ge/Si substrates is in a state of metastable strain as a consequence of the fact that cracks relax the thermal tensile strain only locally

[en] A joint theoretical and experimental analysis of the crystalline fraction in nanocrystalline films grown by low-energy plasma enhanced chemical vapor deposition is presented. The effect of key growth parameters such as temperature, silane flux, and hydrogen dilution ratio is analyzed and modeled at the atomic scale, introducing an environment-dependent crystallization probability. A very good agreement between experiments and theory is found, despite the use of a single fitting parameter