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Electron-optical phonon interaction is the dominant energy-loss mechanism in low-dimensional Ge/SiGe heterostructures and represents a key parameter for the design and realization of electronic and optoelectronic devices based on this material system compatible with the mainstream Si complementary metal-oxide semiconductor technology. Here we investigate the intersubband relaxation dynamics of $n$-type Ge/SiGe multiquantum wells with different symmetry and design by means of single-color pump-probe spectroscopy. By comparing the experimental differential transmittance data as a function of the pump-probe delay with numerical calculations based on an energy-balance rate-equation model, we could quantify an effective value for the optical phonon deformation potential describing the electron-phonon coupling in two-dimensional Ge-based systems. We found nonradiative relaxation times longer than 20 ps even in samples having intersubband energy separations larger than the optical phonon energy, evidencing the presence of a less effective electron-phonon coupling with respect to that estimated in bulk Ge.

We report on the formation of Zn nanoparticles induced by Ga+ focused ion beam on single crystal ZnO. The irradiated materials have been studied as a function of the ion dose by means of atomic force microscopy, scanning electron microscopy, Raman spectroscopy and transmission electron microscopy, evidencing the presence of Zn nanoparticles with size of the order of 5–30 nm. The nanoparticles are found to be embedded in a shallow amorphous ZnO matrix few tens of nanometers thick. Results reveal that ion beam induced Zn clustering occurs producing crystalline particles with the same hexagonal lattice and orientation of the substrate, and could explain the alteration of optical and electrical properties found for FIB fabricated and processed ZnO based devices.

We systematically investigate the chemical vapor deposition growth of graphene on Ge(110) as a function of the deposition temperature close to the Ge melting point. By merging spectroscopic and morphological information, we find that the quality of graphene films depends critically on the growth temperature improving significantly by increasing this temperature in the 910–930 °C range. We correlate the abrupt improvement of the graphene quality to the formation of a quasi-liquid Ge surface occurring in the same temperature range, which determines increased atom diffusivity and sublimation rate. Being observed for diverse Ge orientations, this process is of general relevance for graphene synthesis on Ge.

With the aim of tuning the optical properties of SiC selective solar absorbers, we investigate the surface micro/nano-texturing of porous silicon carbide by using two approaches, namely i) the femtosecond laser treatment and ii) the fluorine based reactive ion etching. It is found that both the techniques are capable of inducing significant surface morphology modification inducing a substantial roughening. The surface features produced are found to be tunable by tailoring the main process parameters, i.e. translation speed and pulse repetition rate for laser treatment, and RF-power, gas pressure and gas mixture for the reactive ion etching process.

Tin doped indium oxide (ITO) coatings have been deposited on epoxy resin and glass substrates by low temperature (70 °C) RF magnetron sputtering. Influence of different process parameters on coating morphological and electrical properties has been studied. Epoxy resin surface roughness influence on coating microstructure was also investigated. We have found that: (i) deposition chamber oxygen content is the main parameter affecting surface electrical resistance, (ii) microstructure and electrical properties are similar in both glass and polymeric smooth substrates; in the latter the substrate stability (type of polymer and degree of cure) is important to prevent microcracks formation as a result of stress release after deposition, and (iii) rough substrates present a non-uniform microstructures with formation of clusters of crystallites with various orientation.

We report on the morphological investigation of nanoscale thick patterns obtained by the scanning probe assisted local oxidation technique on graphene layers grown directly on Ge (100) substrates using CVD technique. Protruding mounds and lines are produced by applying a negative voltage to the atomic force microscope probe while translating the probe tip across the sample surface. The main features of the local oxide produced and the differences with respect to similar experiments conducted on Ge or Si samples are presented.

An extended formation of faceted pit-like defects on Ge(001) and Ge(111) wafers was obtained by thermal cycles to T> 750 {\deg}C. This temperature range is relevant in many surface-preparation recipes of the Ge surface. The density of the defects depends on the temperature reached, the number of annealing cycles performed and correlates to the surface-energy stability of the specific crystal orientation. We propose that the pits were formed by preferential desorption from the strained regions around dislocation pile-ups. Indeed, the morphology of the pits was the same as that observed for preferential chemical etching of dislocations while the spatial distribution of the pits was clearly non-Poissonian in line with mutual interactions between the core dislocations., Comment: European Union (EU), Horizon 2020, Far-Infrared Lasers Assembled using Silicon Heterostructures,FLASH,Grant number: 766719

In this paper we investigate the structural and optical properties of few strain-unbalanced multiple Ge/GeSi quantum wells pseudomorphically grown on GeSi reverse-graded substrates. The obtained high epitaxial quality demonstrates that strain symmetrization is not a mandatory requirement for few quantum-well repetitions. Photoluminescence data, supported by a thorough theoretical modeling, allow us to unambiguously disentangle the spectral features of the quantum wells from those originating in the virtual substrate and to evaluate the impact on the optical properties of key parameters, such as quantum confinement, layer compositions, excess carrier density, and lattice strain. This detailed understanding of the radiative recombination processes is of paramount importance for the development of Ge/GeSi-based optical devices.

Display Omitted Focused ion beam and oxygen plasma etching are used for controlled disruption of ferritin.The detection gain of iron by EDX upon focused ion beam treatment increased by 300%.XPS detection of Fe in ferritin after oxygen plasma etching was enhanced by ~15. Ferritin is the main protein devoted to iron storage in living systems. Several evidences show that ferritin can bind in vivo metals other than iron, such as aluminium. Moreover, recent studies demonstrated that ferritin iron core composition differs between physiological and pathological brain ferritins. The precise evaluation of chemical species present within the ferritin mineral core and their chemical coordination is therefore of paramount importance. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy are powerful tools to perform this chemical analysis: however the protein shell surrounding the mineral core reduces significantly the amount of electrons/photons detected, thus reducing the potentiality of these techniques.In this framework we investigated the use of physical treatments, namely focused ion beam milling and oxygen plasma etching, in order to overcome the limitations imposed by the presence of the protein shell. We demonstrated that the combined use of focused ion beam milling and energy dispersive X-ray spectroscopy provides an enhancement of iron detection by a factor of about two. As for the X-ray photoelectron spectroscopy technique, it has an intrinsic low capability of probing the core metal in ferritin due to the electron escape depth value that is of the order of the protein shell thickness. Using a pre-treatment in oxygen plasma the detection of iron was enhanced by a factor of more than 15. Therefore, the disruption of the protein shell by plasma etching or ion milling emerges as a fundamental prerequisite to perform an accurate chemical characterization of the ferritin core.

We report on the realization of high-mobility two-dimensional electron gas based on modulation-doped Si/SiGe heterostructure grown directly on thin SiGe-On-Insulator (SGOI) substrate. The samples were grown by using low-pressure chemical vapor deposition. A pregrowth procedure for the cleaning of the SGOI surface that preserves the integrity and the composition of the substrate was developed. An electron mobility as high as 10(5)cm(2)V(-1) s(-1) at T = 0.4 K and 2000 cm(2)V(-1) s(-1) at T = 300 K was obtained. (C) 2007 Elsevier B.V. All rights reserved.

Display Omitted Argon and oxygen plasma processes are used for controlled disruption of ferritin.XPS detection of Fe in ferritin film after plasma process was enhanced by ~10.XPS reveals the oxidation state of Fe in the ferritin core after the plasma etching. Ferritin is the main iron storage protein in living systems. Recent studies demonstrated that the chemical species present in the ferritin core and their chemical coordination can be different between physiological and pathological ferritin proteins. Therefore it is of paramount importance revealing the presence of metals other than iron and their oxidation state in the protein core.In this work we investigated and compared the use of argon and oxygen plasma processes for the erosion of the protein shell surrounding the mineral core, which is necessary for the chemical analysis of the ferritin core by surface sensitive technique such as X-ray photoelectron spectroscopy. We find that both the proposed etching processes are effective in the disruption of the protein shell and in the exposure of the Fe inside the core of the molecules. Neither oxygen nor argon plasma induced significant artifacts in the samples nor produce dramatic roughening in the surface allowing for a controlled thinning of the ferritin film. In the plasma treated samples X-ray photoelectron spectroscopy detected the Fe(III) oxidation state of metal present inside the ferritin cavity.

We report on the electrical transport of wire-based devices fabricated on modulation-doped Si/SiGe two-dimensional electron gas. Two different geometries were investigated: straight and bended wires. In both cases, we found typical features of Coulomb blockade regime. Nevertheless, single electron transistor effects were found only on wires with bends. Reported data suggest that the insertion of bends is a suitable tool for obtaining single electron transistor behavior in Si/SiGe wires.

The conductance of strongly confined one-dimensional constrictions fabricated from Si/SiGe two-dimensional electron gases is investigated. Conductance measurements reveal conductance quantization in units of G(0)=2e(2)/h rather than 2G(0)=4e(2)/h, as expected in the presence of valley and spin degeneracy. Furthermore, at temperatures below T=400 mK, small steps and peaklike features, superimposed to the conductance plateaus, become visible. The conductance in the presence of parallel and perpendicular magnetic field shows that significant valley splitting is present even at zero magnetic field. The enhanced valley splitting observed in our etched devices is related to the strong in-plane confinement.

Low field magnetotransport revealing signatures of ballistic transport effects in strained Si/SiGe cavities is investigated. We fabricated strained Si/SiGe cavities by confining a high mobility Si/SiGe 2DEG in a bended nanowire geometry defined by electron-beam lithography and reactive ion etching. The main features observed in the low temperature magnetoresistance curves are the presence of a zero-field magnetoresistance peak and of an oscillatory structure at low fields. By adopting a simple geometrical model we explain the oscillatory structure in terms of electron magnetic focusing. A detailed examination of the zero-field peak lineshape clearly shows deviations from the predictions of ballistic weak localization theory., Comment: Submitted to Physical Review B, 25 pages, 7 figures

We apply a new experimental method for determining band lineups at the Ge Si (100) heterostructure. This method uses a modern version of an old spectroscopy: the photoelectric yield spectroscopy excited with photons in the near UV range. It is shown that both substrate and overlayer valence-band tops can be identified in the yield spectrum, thus allowing a direct and precise determination of the band lineup. We find an offset of 0.36 ± 0.02 eV for heterojunctions whose overlayers were grown according to the Stranski-Krastanov mechanism.

An X-ray scanning microscope study of self-organized morphological defects arising during the growth of thick SiGe virtual substrates grown on Si(001) surface is reported. The 200 nm microscope resolution permitted to probe the local alloy composition. The measured variations of the Ge content in the alloy demonstrate that defect formation and evolution represent a strain relief mechanism. The observed large stress concentration inside the defect and the absence of the cross-hatch pattern on its internal facets are findings in close analogy with the lateral strain relaxation of self-organized islands in heterostructures.

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.

In this letter, we report on the optical properties of multistacked Ge quantum dots grown on Si(001). We demonstrate that the two emission bands observed in the photoluminescence spectra are related to the presence of strained and relaxed islands. Within both types of islands, the experimental findings support the hypothesis of the presence of quantum confining regions whose dimensions are governed by the strain relaxation process.

We have investigated the morphology defects, in particular, pits present in compositionally graded SiGe thick films heteroepitaxially grown on Si(001) surfaces. Besides the pits clearly related to impurities and interface defects, a class of pits intrinsic to the strained-layer growth has been found. Two stable and one metastable pit shapes have been identified that can be related to the alloy film thickness and pit dimension. The influence of the pits on the mobility of the two dimensional electron gas grown on these kinds of substrates has been also investigated.

We report the characterization of a single-electron transistor based on bended wires fabricated on modulation-doped SiGe two-dimensional electron gas. Electrical measurements show a diamond-shaped stability plot and a nonperiodic sequence of conductance peaks. The device behavior suggests the presence of disorder-induced multiple islands along the wire. Conductance oscillations remain well pronounced above liquid helium temperature.

We have investigated the weak antilocalization effect in a high-density two-dimensional electron gas (2DEG) in an AlGaN\\\/GaN heterostructure. The data were analyzed using the most representative theories on the weak antilocalization (WAL) correction to magnetoconductivity. The spin-orbit interaction (SOI) strength and the phase-coherence time were extracted from the fitting to the experimental data. We found that the use of the WAL peak analysis appears to be a reliable way of investigating the SOI strength and that the dependence on charge concentration revealed by the WAL data as reported in the literature can be considered a trustworthy experimental description of the SOI behavior in the AlGaN\\\/GaN 2DEG system.

We report on the use of a carbon-rich Pt-based material, obtained by electron and ion beam assisted deposition from metal-organic precursor, as a mask for pattern transfer processes. Thin and narrow mask patterns subjected to oxygen plasma and reactive ion etching (RIE) of silicon in SF(6) were investigated by atomic force microscopy and energy dispersive X-ray analysis. As for the masks obtained by electron beam assisted deposition, both the pattern and the surrounding halo were found to be etched in oxygen plasma. In contrast, the pattern deposited by assist of ion beam was basically unaffected, likely due to implanted Ga(+) ions during deposition, while the surrounding halo was found to be appreciably thinned. Masks having thickness as low as few nanometers sustained successfully a 200 nm RIE step, producing structures with sub-100 nm size. Mask stripping was achieved in Piranha bath. (C) 2011 Elsevier B.V. All rights reserved.

""We report on the fabrication process of suspended anodes implemented in Schottky-diode devices on epitaxial Ge. An air-bridge technique based on reactive ion etching in SF(6) was developed in order to release the Schottky contact bridge, ensuring higher control respect to wet etching. Metal-semiconductor junctions in the sub-micron range with rectifying behaviour have been obtained. The present fabrication process can be extended as is to the use of silicon on insulator substrates. (C) 2010 Elsevier B.V. All rights reserved.""

Magnetotransport properties of a high-density AlGaN/AlN/GaN two-dimensional electron gas are reported. A quantitative model of amplitude and phase modulation of Shubnikov-de Haas oscillations was developed. This analysis allows the extraction of the spin splitting energy even if nodes and double peaks in the Fourier spectrum are not clearly observed. In this way the presence of two conducting channels was clearly identified. A spin splitting of 1.2 meV was found. © 2011 American Physical Society.

In this work, we investigated the magnetotransport properties of a two dimensional electron gas hosted in an AlGaN/AlN/GaN heterostructure and one-dimensional devices fabricated on it. At cryogenic temperature, high mobility and long mean free path is achieved, allowing ballistic transport experiments. Longitudinal resistivity measured in Hall bar geometry shows well-developed Shubnikov–de Haas oscillations with amplitude modulation. Amongst possible mechanisms, the zero-field spin splitting may be the origin of the observed effects. Split gate quantum point contacts were fabricated by electron beam lithography. Linear conductance measurements at zero magnetic field show clear quantized conductance plateaus at 2e 2/h and 4e 2/h. Non-perfectly quantized conductance values are found for higher plateaus, suggesting the presence of impurity scattering.

We present morphological and electrical characterizations of thin and narrow resistors obtained by focused ion beam assisted deposition of Pt based material. For thin and narrow depositions the measured thickness and width are significantly different from the nominal values. From leakage tests we found that in order to have electrically insulated parallel resistors at room temperature, it is mandatory that the Pt-halo, which results from the deposition procedure, has a thickness well below 6 nm. (C) 2008 Elsevier Ltd. All rights reserved.

We report on the fabrication and electronic transport characterisation of Schottky-gated strongly confined Si/SiGe quantum point contacts (QPC). At zero magnetic field and T=450mK the QPC conductance as a function of gate voltage shows a quantization in units of e^2/h, indicative of transport through 1D modes which appear to lack both spin and valley degeneracy., 2 pages, 2 figure, to be published in Proc. ICPS-28

We fabricated strongly confined Schottky-gated quantum point contacts by etching Si/SiGe heterostructures and observed intriguing conductance quantization in units of approximately 1e2/h. Non-linear conductance measurements were performed depleting the quantum point contacts at fixed mode-energy separation. We report evidences of the formation of a half 1e2/h plateau, supporting the speculation that adiabatic transmission occurs through 1D modes with complete removal of valley and spin degeneracies.

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.

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 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.

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.

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 this letter, we present an atomic-force-microscopy investigation of the Stranski-Krastanov growth of Ge on Si(100). Upon increasing the base width of the islands, two morphology transitions are found. The first transition occurs at a base width of similar to 50-60 nm and marks the: evolution from few-monolayer-thick terraces to square-base pyramidal islands. In the second transition, which takes place when the base width exceeds similar to 300 nm, the island shape changes from square base pyramids to tetragonal truncated pyramids. Both transitions are brought about by the need for the system to minimize the elastic energy.

Exploiting Ge K-edge x-ray absorption spectroscopy we provide direct evidence of Si-Ge intermixing in self-organized strained and unstrained Ge quantum dots on Si, and provide a quantitative measurement of the average composition. For Ge/Si(001) dots with equivalent thickness in the range 5.8-38 nm and morphology ranging from that typical of coherently strained to that associated with relaxed dots we find that the average Si composition is approximately 30%. For Ge/Si(111), we find that the wetting layer has a Si composition near 50%. We discuss these results in terms of the energetics of dot formation and argue that strain-enhanced diffusion of Si into Ge should be considered as an important factor in minimizing the strain energy of the system.