Search

Ge/SiGe heterostructures are promising in the view of silicon-foundry compatible THz optoelectronic applications. Here we present a structural and optical characterization of Ge/SiGe parabolic quantum wells that represent ideal systems for room-temperature THz emission and strong light-matter coupling investigation.

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.

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 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.

The process of capping Ge islands with Si overlayers is known to have a strong influence on their composition and shape. In this work we have investigated Ge islands on Si produced by chemical vapor deposition covered with Si layers of different thickness. The structural characterization was carried out by X-ray absorption spectroscopy at the Ge-K edge. A noticeable Si uptake by the islands is evident upon capping. Bond length for the first three shells have been analyzed by comparison with models based on the valence force field method. The results evidence that the islands have, on the average, a relaxed state with presumably strained parts in contact with the Si matrix. 2005 Elsevier B.V. All rights reserved.

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., Comment: to appear in Physical Review B

Highly efficient and transparent multicomponent catalysts for Dye Solar Cells (DSCs) were obtained by simultaneous electrodeposition of platinum and cobalt-sulfide (Pt:CoS) on conducting glass substrates from a mixture solution of both precursors. At equal charge transfer resistances (RCT), the superior transparency of electrodeposited (ED) Pt:CoS compared to ED Pt-only and ED CoS-only was attributed to the synergic combination of a very thin CoS layer and a fine dispersion of Pt nanoparticles. The highly catalytic properties of Pt:CoS with less electrodeposited material consumption, carried out at room temperature, can represent a fast and energy saving fabrication process, also compatible with flexible plastic substrates. In fact, DSCs with the ED Pt:CoS counterelectrodes showed the highest power conversion efficiency in back illumination, which is the configuration used for flexible metallic and bifacial DSCs during operation, gaining on average almost 14% and 7% in device efficiency with respect to ED CoS-only and ED Pt-only based cells. These results are important for all applications where transparency represents a key factor (e.g. BIPV, metal based devices) and may also point at blending different precursor solutions as a more general route for electrodeposition of layers with added functionality.

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.

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 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.""

The beam of planar THz Schottky detectors is strongly concentrated in the dielectric half-space. With the goal of illuminating arrays from the free-space side, we started to investigate substrate removal from both GaAs and Ge-on-Si diode wafers. © 2011 IEEE.

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.

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(2)/h units with superimposed anomalous plateaus and/or structures suggesting that correlation effects should be included in the description of our 1D systems

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 report on the growth and characterization of high‐mobility 2DEGs on tensile strained Si grown on cubic SiGe alloy on SOI substrates. The electrical properties were investigated in the 4.2–300 K temperature range as a function of growth conditions and sample structure. Hall mobilities as high as 82000 cm2/V.s at T=4.2 K were obtained in the best 2DEGs on SOI. We found that the main mechanism limiting the mobility at low temperature is the ionized impurity scattering.

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.

We have investigated the pit formation and evolution 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 was found. We demonstrate that a mechanism of self-organization drives the nucleation and the growth evolution of these intrinsic pits. Two stable and one metastable pit shapes have been identified that can be related to the alloy film thickness and pit dimension.

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

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.

Employing electronic transitions in the conduction band of semiconductor heterostructures paves a way to integrate a light source into silicon-based technology. To date all electroluminescence demonstrations of Si-based heterostructures have been p-type using hole-hole transitions. In the pathway of realizing an n-type Ge/SiGe terahertz quantum cascade laser, we present electroluminescence measurements of quantum cascade structures with top diffraction gratings. The devices for surface emission have been fabricated out of a 4-well quantum cascade laser design with 30 periods. An optical signal was observed with a maximum between 8-9 meV and full width at half maximum of roughly 4 meV.

Terahertz (THz) semiconductor based quantum cascade lasers (QCLs) represent powerful and compact integrated light sources. To date they have been realized with various III-V materials where LO phonon emission becomes very effective as the temperature is increased, limiting the maximum operating temperature to 200K. To overcome this limitation, non-polar material systems are attractive because of their weaker e-phonon interaction. Theoretical studies have indicated n-type Ge/SiGe heterostructures where transport is associated to L electrons, as the most promising architecture [1]. Experimentally, sharp intersubband transitions in n-type strain compensated Ge/SiGe QWs have been observed in the 20–50 meV region [2]. Recently, non-equilibrium Green's functions calculations proved to be efficient in optimising high temperature performance of GaAs/AlGaAs THZ QCLs [3]. In the effort of realizing a Ge/SiGe THz QCL, we present here a QCL structure designed by means of non-equilibrium Green's functions [4], whose the bandstructure is reported in Fig.1(a). Epitaxial growth of strain-compensated Ge/SiGe heterostructure (≈1.5 μm thick) has been carried out by means of UHV-CVD yielding very good sample quality as visible from Fig.1(b). Mesa devices for transport experiments have been fabricated by dry etching and metallization. In Fig.1(c) a series of I-V curves as a function of the heatsink temperature display a thermally activated behaviour with E act ≃ 10 meV. From low temperature (4 K) magnetotransport [5] in a magnetic field parallel to the current direction shows characteristics oscillations. we can deduce an energy distance between the lasing states of E 23 =19.5 meV, in good agreement with the theoretical calculation. Optical experiments are ongoing and experimental results will be presented.