[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] We have developed a new instrument which performs simultaneously energy and spin-polarization analysis of an electron beam, to be used in spin resolved electron spectroscopies from magnetic surfaces and thin films. The device consists of a novel polarization detector with high efficiency and analyzing power coupled to a large commercial hemispherical energy analyzer (HEA). The spin polarimeter is based on the spin dependence of the low energy (4-6 eV kinetic energy) reflectivity from a magnetic target, namely a Fe(0 0 1)-p(1x1)O surface. We describe reliable target preparation procedures and the design of the electron-optical system transferring the electrons from the HEA exit slit to the polarimeter target

[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] 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] We apply the principles of Optical Orientation to measure by Mott polarimetry the spin polarization of electrons photoemitted from different group-IV heterostructures. The maximum measured spin polarization, obtained from a Ge/Si_0.31Ge_0.69 strained film, undoubtedly exceeds the maximum value of 50% attainable in bulk structures. The explanation we give for this result lies in the enhanced band orbital mixing between light hole and split-off valence bands as a consequence of the compressive strain experienced by the thin Ge layer

[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] The presence of excited radicals in plasma-assisted chemical vapour deposition silane processes leads to overestimation of radical densities in threshold ionization mass spectrometry analysis. A method to discriminate the signal due to excited radicals is proposed and applied to estimate the relative density trends of the ground-state silane radicals (SiH_x, x < 4) with hydrogen input flow rates (0-50 sccm) in an argon-silane-hydrogen plasma at total pressures of 0.01-0.04 mbar used for the deposition of nano-crystalline silicon (nc-Si) layers for photovoltaic applications. The SiH_x/SiH₄ density trends with hydrogen input show a turning point where SiH becomes dominant, in the process region where nc-Si layers were previously obtained. (fast track communication)

[en] An assessment of main electron-impact and secondary (homogeneous) gas-phase reaction rates of silane in an argon-silane-hydrogen plasma during nano-crystalline silicon deposition is presented. Radially resolved Langmuir probe plasma parameters (electron temperature and density) and electron energy distribution functions (eedfs) have been evaluated for Ar, Ar-H₂ and Ar-SiH₄-H₂ plasma in a low-energy plasma-enhanced chemical vapour deposition reactor. Input flow rates of 50 sccm Ar, 10 sccm SiH₄ and 0-50 sccm H₂ have been used for a reactor pressure range 1-4 Pa. The eedfs are used to evaluate kinetic rate constants for electron-impact dissociative processes of SiH₄ and H₂ and to infer the amount of atomic H available for the silane-hydrogen gas-phase reaction, observing trends with an increase in H₂ input flow. The evolution of silane kinetic rates with an increase in H₂ input indicates that conditions corresponding to nc-Si deposition are characterized by a dominance of silane-hydrogen gas-phase rates over electron-impact dissociation rates up to about two orders of magnitude.

[en] We show the possibility to integrate high quality III-V quantum nanostructures tunable in shape and emission energy on Si-Ge Virtual Substrate. Strong photoemission is observed, also at room temperature, from two different kind of GaAs quantum nanostructures fabricated on Silicon substrate. Due to the low thermal budget of the procedure used for the fabrication of the active layer, Droplet Epitaxy is to be considered an excellent candidate for implementation of optoelectronic devices on CMOS circuits.

[en] We present here the fabrication, via droplet epitaxy, of GaAs/AlGaAs quantum dots with high optical efficiency on Si. The growth substrate lattice parameter was adapted to that of (Al)GaAs via Ge virtual substrates (GeVS). The samples clearly show the presence of quantum dot self-assembly, with the designed shape and density. Photoluminescence measurements, performed at low temperature, show an intense emission band from the quantum dots.