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The growth of high-mobility modulation-doped Si/SiGe heterostructures on silicon-on-insulator (SOI) substrates is demonstrated. The structural and electrical properties of the samples are comparable with those of similar samples grown on standard Si substrates. Electron mobilities as high as 2800 cm 2 V −1 s −1 at room temperature and 8.2×10 4 cm 2 V −1 s −1 at 4 K are obtained.

Novel silicon photonics applications requiring heavy n-type doping have recently driven a great deal of interest towards the phosphorous doping of germanium. In this work we report on infrared reflectance spectroscopy measurements of the electron density in heavily n-type doped germanium layers obtained by stacking multiple phosphorous δ-layers. Here, we demonstrate that the conventional Drude model of the electrodynamic response of free carriers in metals can be adapted to describe heavily doped semiconductor thin films. Consequently, the effect of the electron density on the plasma frequency, scattering rate and complex permittivity can be investigated.

Transport characterizations of wires obtained by electron beam lithography and etching of (100) Si/SiGe heterostructures with a high-mobility two-dimensional electron gas are reported. Depending on the wire width, two different regimes for the electrical transport are found. Wires with a width larger than ~200 nm exhibit metallic behaviour in the quasi-ballistic regime. The conductance dependence on the wire width reveals the presence of a depletion layer, ~100 nm thick, on each etched side of the wire. The wires of width smaller than 200 nm have very large resistance and two different behaviours. The first kind of wires exhibit a zero-current region, compatible with a Coulomb blockade effect involving multiple tunnel junctions or with a space-charge limited current. Other wires are insulating up to applied voltages larger than 5-6 V and their I-V characteristics can be fitted by the functional dependence of voltage-induced tunnelling of Fowler-Nordheim type.

We have provided direct evidence for the presence of considerable Si-Ge intermixing in strained and unstrained Ge quantum dots deposited on Si(001) and Si(111). The local structure around Ce was probed by using Ge K-edge X-ray absorption spectroscopy; complementary evidence for intermixing was provided by AFM and STM studies. These results implied that the strain energy in the dots was reduced by Si atoms diffusing into the dots, resulting in a modified form of Stranski-Krastanov growth. (C) 2000 Elsevier Science B.V. All rights reserved.

The presence of misfit and threading dislocations formed by strain relaxation in multilayers of Ge islands grown on Si(100) is investigated by transmission electron microscopy and atomic force microscopy. We find that the dislocations are generated inside the islands and propagate into the silicon spacers remaining confined within the columnar regions above the islands themselves. Thus the dislocations drive the vertical ordering of the stacked relaxed islands in a way similar to the strain induced self-ordering mechanism in the strained islands.

Due to the evergrowing use of optical fibers in communications, for both long distances and local area networks, there is an increasing effort toward the realization of high speed and high efficiency optical detectors, operating in the low-absorption regions ~1.3‐1.55 mm! of silica fibers. Although III‐V semiconductors provide high detection efficiency in the range of interest, incorporating them in the well-established Si technology is difficult and expensive. 1 SiGe structures on silicon represent an alternative solution for near-infrared photodetection, owing to their narrow band gap and full compatibility with silicon technologies. SiGe alloy heterostructures, multiquantum wells, and superlattices have been used as active layers in p-i-n photodetectors and their operation at 1.3 mm has been successfully demonstrated for both normal incidence 2 and waveguide configurations. 3 The fundamental limitation of SiGe-based heterostructures is the vanishing response at 1.55 mm. Pure germanium, indeed, represents the best candidate for 1.3‐1.55 mm photodetectors, due to its direct band gap of 0.8 eV. However, owing to the high lattice mismatch with Si, it is not easy to obtain Ge films with suitable characteristics: thickness appropriate for maximum efficiency, flatness compatible with submicron lithography, minimum defect content for high speed. Thick graded buffers can be used to partially relax the strain due to the lattice mismatch, 4 but the molecular beam epitaxy ~MBE! growth of such a layer is cumbersome and expensive. In the present work we investigate the feasibility of the simplest fabrication approach, i.e., the epitaxial growth of thick layers ~well over the critical thickness! of pure germanium on silicon by chemical vapor deposition ~CVD!. The use of pure Ge on Si for efficient photon detection in the range 1.3‐1.5 mm, first attempted by Luryi et al., 5 requires

We report on Ge Schottky diodes epitaxially grown on Si(001) exhibiting a good responsivity in the near-infrared up to 1.55 mu m. The Ge layers were grown by UHV chemical vapor deposition. It was found that the photocurrent increases upon increasing the reverse bias, reaching a maximum responsivity of 0.12 A/W at 1.3 mu m for 4 V bias. The leakage current density at the saturation voltage is 1 nA/mu m(2).

Heterostructures formed by epitaxial Ge grown in situ on Si(100) substrates were characterized by photoelectric yield spectroscopy, UPS and XPS. It is shown that both substrate and overlayer valence-band tops can be identified in the photoelectric-yield spectrum, thus allowing a direct and precise determination of the band lineup. We find a valence band discontinuity of 0.36 +/- 0.02 eV for heterojunctions whose overlayers were grown according to the Stranski-Krastanov mechanism, A considerably larger offset is obtained from the analysis of the XPS data.

In this letter we present an atomic force microscopy and photoluminescence investigation of two‐ and three‐dimensional Ge structures deposited on Si(100) substrates. The photoluminescence spectra demonstrated the presence of quantum confinement effects in flat and uncapped Ge layers and in Ge islands even for the relatively large size of the structures. Moreover, a correlation between the sample morphology and the features of the photoluminescence is demonstrated.

We use atomic force microscopy, x-ray photoemission spectroscopy, and x-ray absorption spectroscopy to study the effect of the deposition of a Si cap layer by chemical vapor deposition on the morphology and composition of a Ge island layer grown at 600 degrees C. We found that the capping of self-assembled Ge islands under a silicon layer results in high-quality, atomically flat layer only at deposition temperature above 700 degrees C. On the other hand at this temperature Ge-Si intermixing and island coarsening are greatly enhanced, resulting in an increased average island volume. Here we show that the predeposition at low temperature of a thin cap layer preserves island shape, size, and composition when the capped islands undergo a subsequent process at higher temperature up to 750 degrees C. It is shown, therefore, that with a two-step capping process it is possible to combine the benefit of a low temperature capping, which reduces island alloying and coarsening, with that of a high temperature capping which is needed to recover a flat surface. (c) 2006 American Institute of Physics.

We have fabricated and characterized at low temperature, down to 0.32 K, tunnel junctions made by a thin film of heavily doped silicon in contact with superconducting electrodes through Schottky barriers. Doped silicon films were chemical vapor deposited on silicon-on-insulator substrates and laterally dry etched in mesas. Aluminum or, alternatively, niobium contacts were deposited on the mesas. Below the superconducting critical temperature Tc, an energy gap opens in the superconductor and the current–voltage characteristics become nonlinear and strongly sensitive to temperature changes. We have also characterized the heavily doped silicon in terms of the electron–phonon thermal decoupling by cooling the electron gas by means of aluminum–silicon–aluminum structures. With Nb electrodes, we have observed an anomaly of the electrical differential conductance at zero voltage and a larger electron dissipation, as a result of a less opaque barrier.

In this paper we present the observation of ballistic transport in strained silicon cavities defined by etching on a silicon germanium heterostructure, demonstrated by magnetic focusing of conductance of the cavity at T = 50 mK. Numerical simulations, based on a novel approach which allows to include an arbitrary degree of decoherence in mesoscopic transport, show that magnetoconductance features can be related to the semiclassical orbits by means of the local density of states in the cavity. ©2004 IEEE.

In the present work we investigate photo-detectors based on thick relaxed Ge layers, epitaxially grown on silicon after insertion of a low-temperature-grown Ge buffer layer. By using this procedure it was possible to grow films having thicknesses comparable with light penetration depth in the 1.3-1.6 μm spectral range. The films exhibited flatness on the atomic scale. Two kinds of detectors were investigated: vertical heterojunction diodes and a planar Metal-Semiconductor-Metal structure. The detectors show a good responsivity at normal incidence at both 1.3 and 1.55 μm. The photocurrent increases with the voltage applied, reaching a maximum responsivity of 0.24 A/W at 1.3 μm under a bias of 1 V. A complete optoelectronic characterization of the fabricated devices is performed. The results confirm the feasibility of the proposed approach for the fabrication of 1.3-1.55 μm near infrared photodetectors integrated on silicon chips.

Source-drain characteristics of narrow wires, fabricated from high mobility SiGe two-dimensional electron gases, have been investigated at different temperatures and gate biases. The experimental behavior, although reminiscent of Coulomb blockade effects, is instead well accounted for by assuming that the wires are insulating, due to charge depletion, and that the current is due to field-induced tunneling.

In the present work we report on the evolution of the electronic states as a function of Ge overlayer thickness ranging from a fraction of a monolayer to few tens of monolayers. The Ge states above the Si valence band top could be clearly detected at every overlayer thickness d. For d < similar to 6 Angstrom, the Ge state density exhibits a double linear edge. Upon increasing the overlayer thickness the split between the two edges decreases and the double edge structure disappears for d > similar to 6 Angstrom, i.e., at the critical thickness for the transition from strained to unstrained overlayers. For these larger thicknesses, the Ge edge becomes broad and featureless. The energy separation between the Ge onset and the Si valence top increases up to 0.70 eV at larger overlayer thicknesses.

The valence density of states of Ge grown epitaxially on Si(100) is investigated as a function of thickness by yield spectroscopy and photoemission techniques. A double edge is present in the yield data for thicknesses smaller than the lattice relaxation critical thickness. Furthermore, the line-up of the Ge states to the Si valence band varies with overlayer thickness. Photoemission techniques fail to detect this behavior. The causes for the discrepancy are analyzed.

We present a study of the conductance of quantum point contacts fabricated in AlGaN/GaN and Si/SiGe heterostructures. The investigated devices differ for typology (split gates and etched devices, respectively) and for the resulting potential profiles. We observe conductance quantization in multiple of 2e /h units with superimposed anomalous plateaus and/or structures suggesting that correlation effects should be included in the description of our 1D systems. © 2009 IEEE NANO Organizers.

In this article we discuss the use of spectroscopic ellipsometry for an in situ and real time probe of three-dimensional self-organized Ge island growth on Si (100) surfaces. We will show that atomic force microscopy and x-ray photoemission spectroscopy can be combined with spectroscopic ellipsometry to give information on the size and shape evolution of the Ge islands as well as on the amount of Ge deposited on the Si surface.