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Inversion-symmetry breaking for second-harmonic generation in centrosymmetric Si and Ge is artificially realized with a pair of asymmetric-coupled SiGe quantum wells. Laser spectroscopy experiments demonstrate a nonlinear susceptibility four orders of magnitude larger than that of natural nonlinear materials in the visible range.

The present work investigates the role of threading dislocation densities (TDD) in the low density regime on the vertical transport in Si0.06Ge0.94 heterostructures integrated on Si(001). The use of unintentionally doped Si0.06Ge0.94 layers enables the study of the impact of grown-in threading dislocations (TD) without interaction with processing-induced defects originating e.g. from dopant implantation. The studied heterolayers, while equal in composition, the degree of strain relaxation, and the thickness, feature three different values for the TDD: 3×106 , 9×106 and 2×107 cm-2 . Current-voltage measurements reveal that leakage currents do not scale linearly with TDD. The temperature dependence of the leakage currents suggests a strong contribution of field-enhanced carrier generation to the current transport, with the trap-assisted tunneling via TD-induced defect states identified as the dominant transport mechanism at room temperature. At lower temperature and at high electric fields, direct band-to-band tunneling without direct interaction with defect levels becomes the dominating type of transport. Leakage currents related to emission from mid-gap traps by the ShockleyRead-Hall (SRH) generation isobserved at higher temperatures (>100 °C). Here, we see a reduced contribution coming from SRH in our material, featuring the minimal TDD (3×106 cm-2 ), which we attribute to a reduction in point defect clusters trapped in the TD strain fields.

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.

The realization of a Ge/SiGe THz emitter is of great interest since it can help to reach room-temperature operation due to the peculiar electron-phonon interaction in nonpolar crystals. Here we present Ge/SiGe quantum-well building blocks epitaxially grown on silicon wafers in order to understand limitations of this material platform in the perspective of realizing a Si-based laser.

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.

In this work we shed light on the early stage of the chemical vapor deposition of graphene on Ge(001) surfaces. By a combined use of m-Raman and x-ray photoelectron spectroscopies, and scanning tunneling microscopy and spectroscopy, we were able to individuate a carbon precursor phase to graphene nucleation which coexists with small graphene domains. This precursor phase is made of C aggregates with different size, shape and local ordering which are not fully sp(2) hybridized. In some atomic size regions these aggregates show a linear arrangement of atoms as well as the first signature of the hexagonal structure of graphene. The carbon precursor phase evolves in graphene domains through an ordering process, associated to a re-arrangement of the Ge surface morphology. This surface structuring represents the embryo stage of the hills-and-valleys faceting featured by the Ge(001) surface for longer deposition times, when the graphene domains coalesce to form a single layer graphene film. (C) 2018 Elsevier Ltd. All rights reserved.

The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1−xGe x THz quantum cascade laser gain media are investigated at 4.79 THz (62.6 μm wavelength). Double metal waveguides demonstrate lower losses than surface plasmonic guiding with minimum losses for a 10 μm thick active gain region with silver metal of 21 cm−1 at 300 K reducing to 14.5 cm−1 at 10 K. Losses for silicon foundry compatible metals including Al and Cu are also provided for comparison and to provide a guide for gain requirements to enable lasers to be fabricated in commercial silicon foundries. To allow these losses to be calculated for a range of designs, the complex refractive index of a range of nominally undoped Si1−xGe x with x = 0.7, 0.8 and 0.9 and doped Ge heterolayers were extracted from Fourier transform infrared spectroscopy measurements between 0.1 and 10 THz and from 300 K down to 10 K. The results demonstrate losses comparable to similar designs of GaAs/AlGaAs quantum cascade laser plasmon waveguides indicating that a gain threshold of 15.1 cm−1 and 23.8 cm−1 are required to produce a 4.79 THz Ge/SiGe THz laser at 10 K and 300 K, respectively, for 2 mm long double metal waveguide quantum cascade lasers with facet coatings.

Strain relaxed Si 1 − x Ge x buffer layers on Si(001) can be used as virtual substrates for the growth of both strained Si and strained SiGe, which are suitable materials for sub-7 nm CMOS devices due to their enhanced carrier mobility. For industrial applications, the threading dislocation density (TDD) has to be as low as possible. However, a reduction of the TDD is limited by the balance between dislocation glide and nucleation as well as dislocation blocking. The relaxation mechanism of low strain Si 0.98 Ge 0.02 layers on commercial substrates is compared to substrates with a predeposited SiGe backside layer, which provides threading dislocations at the edge of the wafer. It is shown that by the exploitation of this reservoir, the critical thickness for plastic relaxation is reduced and the formation of misfit dislocation bundles can be prevented. Instead, upon reaching the critical thickness, these preexisting dislocations simultaneously glide unhindered from the edge of the wafer toward the center. The resulting dislocation network is free of thick dislocation bundles that cause pileups, and the TDD can be reduced by one order of magnitude.

We develop a generalized theory for the scattering process produced by interface roughness on charge carriers and which is suitable for any semiconductor heterostructure. By exploiting our experimental insights into the three-dimensional atomic landscape obtained on Ge/GeSi heterointerfaces obtained by atom probe tomography, we have been able to define the full set of interface parameters relevant to the scattering potential, including both the in-plane and axial correlation inside real diffuse interfaces. Our experimental findings indicate a partial coherence of the interface roughness along the growth direction within the interfaces. We show that it is necessary to include this feature, previously neglected by theoretical models, when heterointerfaces characterized by finite interface widths are taken into consideration.

We explore saturable absorption and terahertz photoluminescence emission in a set of n-doped Ge/SiGe asymmetric coupled quantum wells, designed as three-level systems (i.e., quantum fountain emitter). We generate a non-equilibrium population by optical pumping at the 1→3 transition energy using picosecond pulses from a free-electron laser and characterize this effect by measuring absorption as a function of the pump intensity. In the emission experiment we observe weak emission peaks in the 14–25 meV range (3–6 THz) corresponding to the two intermediate intersubband transition energies. The results represent a step towards silicon-based integrated terahertz emitters.

The exploitation of intersubband transitions in Ge/SiGe quantum cascade devices could pave the way towards the integration of THz light emitters into the silicon-based technology. Aiming at the realization of a Ge/SiGe Quantum Cascade Laser (QCL), we investigate optical and structural properties of n-type Ge/SiGe coupled quantum well systems. The samples have been investigated by means of X-ray diffraction, scanning transmission electron microscopy, atom probe tomography and Fourier Transform Infrared absorption spectroscopy to assess the growth capability with respect to QCL design requirements, carefully identified by means of modelling based on the non-equilibrium Green function formalism.

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.

Theoretical predictions indicate that the n-type Ge / Si − Ge multi-quantum-well system is the most promising material for the realization of a Si -compatible THz quantum cascade laser operating at room temperature. To advance in this direction, we study, both experimentally and theoretically, asymmetric coupled multi-quantum-well samples based on this material system, that can be considered as the basic building block of a cascade architecture. Extensive structural characterization shows the high material quality of strain-symmetrized structures grown by chemical vapor deposition, down to the ultrathin barrier limit. Moreover, THz absorption spectroscopy measurements supported by theoretical modeling unambiguously demonstrate inter-well coupling and wavefunction tunneling. The agreement between experimental data and simulations allows us to characterize the tunneling barrier parameters and, in turn, achieve highly controlled engineering of the electronic structure in forthcoming unipolar cascade systems based on n-type Ge / Si − Ge multi-quantum-wells.

n-type Ge/SiGe terahertz quantum cascade laser are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs QCL design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling operation up to room temperature. The better temperature robustness with respect to III-V is attributed to the much weaker interaction with optical phonons. The effect of lower interface quality is investigated and can be partly overcome by engineering smoother quantum confinement via multiple barrier heights., 5 pages, 5 figures

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.

In this paper we present an atomic force microscopy and X-ray photoemission spectroscopy study of the composition and shape evolution of self-assembled Ge/Si(001) islands upon capping with Si. We found that the islands undergo a reverse Straski-Krastanov shape evolution, with a progressive Si-enrichment of both the wetting layer and the islands. We demonstrate that the island shape evolves at constant volume with silicon atom incorporation occurring in the absence of lateral diffusion of Ge and Si atoms from the wetting layer to the islands themselves. (C) 2008 Elsevier Ltd. All rights reserved.

The effect of the deposition rate on the size, density, and uniformity of Ge islands grown on Si(100) is investigated. Upon changing the growth rate from 4 to 110 ML/min the island density increases by one order of magnitude and the strained dome base decreases from 84 to 55 nm. A narrowing of the island size distribution was also observed. We discuss these experimental findings by taking into account island-island interaction effects. (C) 2003 American Institute of Physics.

Photoemission spectroscopies and atomic force microscopy (AFM) were performed on C-60 evaporated on the monohydride Si (100):H-(2 x 1) surface at different temperatures from room temperature up to 900 degrees C. X-ray photoelectron spectroscopy has revealed that the interaction is with the hydrogenated surface only for temperatures below 250 degrees C. Above this temperature the hydrogen starts to desorb and C-60 interacts directly with the clean silicon surface. The bond between C-60 and the Si(100):H-(2 x 1) surface is van der Waals' in character and is estimated to be of the same order of magnitude or even weaker than the C-60-C-60 interaction. AFM analysis was performed on multilayer samples grown in the range between room temperature and 200 degrees C. Room-temperature samples exhibit small Cs, islands. The order of the islands improves strongly when the temperature of the substrate is increased up to 200 degrees C. At this temperature the islands are all ordered with the hexagonal shape typical of C60 growth along the (111) direction and a lateral size of about 3000 Angstrom. (C) 2000 Elsevier Science B.V. All rights reserved.

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.

Germanium is emerging as the substrate of choice for the growth of graphene in CMOS-compatible processes. For future application in next generation devices the accurate control over the properties of high-quality graphene synthesized on Ge surfaces, such as number of layers and domain size, is of paramount importance. Here we investigate the role of the process gas flows on the CVD growth of graphene on Ge(100). The quality and morphology of the deposited material is assessed by using mu-Raman spectroscopy, X-ray photo emission spectroscopy, scanning electron microscopy, and atomic force microscopy. We find that by simply varying the carbon precursor flow different growth regimes yielding to graphene nanoribbons, graphene monolayer, and graphene multilayer are established. We identify the growth conditions yielding to a layer-by-layer growth regime and report on the achievement of homogeneous monolayer graphene with an average intensity ratio of 2D and G bands in the Raman map larger than 3.

In the context of shallow geothermal applications, geo¬physics can be applied as a design and monitoring tool. One of the most referred applications is monitoring and imaging the time lapse evolution of the Thermal Affected Zone (TAZ). In the last few decades electrical resis¬tivity measurements have been adopted both for the char¬acterization and to image spatial and temporal distribution of temperature within the ground. This paper is therefore devoted to the use of electrical resistivity in this context with a focus on the Italian state of the art. A brief literature review of thermal characteristics influencing the resistivity value and example applications of Electric Resistivity Tomography (ERT) data is reported. Two example case histories, in very different hydrogeological contexts in northern Italy, are also presented.

We have investigated the response of 3D Bi 2 Sr 2 CaCu 2 O 8 vortex structures to a weak perturbation induced by 2D Fe pinning structures acting on one extremity of vortex lines. The pinning patterns were nano-engineered at the sample surface by means of either a Bitter decoration of the vortex lattice or electron-beam lithography. The commensurability conditions between 2D rigid pinning potentials and 3D elastic structures with short-range positional and long-range orientational correlation have been experimentally determined. When the 2D potential is a replica of the nonperiodic vortex structure an amplification of its interaction with the vortex structure takes place. This effect is detected only for the first matching field, becoming negligible for other matching fields. On the other hand, a periodic 2D perturbation is shown to transform the nonperiodic Bragg glass-like structure into an Abrikosov crystal with an effective Debye–Waller factor.

We make a comparative analysis of the response of the three-dimensional vortex lattice in Bi-2 Sr-2 CaCu2O8 and NbSe2 to the presence of square arrays of pinning sites localized at one extremity of the vortex crystals. The absence of the hexagonal to square symmetry transformation and the induction of a distorted hexagonal symmetry in the vortex lattice of NbSe2 contrast to the observed symmetry change in Bi2Sr2CaCu2O8. The dissimilar response in both cases is explained taking into account the vortex structure solidification mechanisms in both materials: The plastic response in the case of the layered material Bi2Sr2CaCu2O8 is suggested to be a result of the simultaneity of solidification and coupling of bidimensional pancake vortices whereas a viscous solidification is the responsible for the three-dimensional elastic response in NbSe2.

We have used Raman spectroscopy, transmission electron microscopy, x-ray diffraction, and x-ray photoemission spectroscopy to investigate strain relaxation mechanism of Si(0.22)Ge(0.78) heteroepitaxial layer deposited on Si substrates in tensile, neutral, and compressive strain conditions. The three regimes have been obtained by interposing between the SiGe layer and the substrate a fully relaxed Ge layer, a partially relaxed Ge layer, or growing directly the alloy on Si. We found that the deposition of a Ge buffer layer prior to the growth of the SiGe is very promising in view of the realization of thin virtual substrates on silicon to be used for the deposition of strain-controlled high Ge content SiGe alloys. We demonstrate that this is mainly due to the strain relaxation mechanism in the Ge layer occurring via insertion of pure edge 90 degrees misfit dislocations (MDs) and to the confinement of threading arms in to the Ge layer due to a second MD network formed at the SiGe/Ge heterointerface.

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.

In this paper, we present a study on the Ge composition and shape evolution of self-assembled Ge/Si(001) islands during the island growth and the subsequent Si capping at 750 degrees C. By combining atomic force microscope images and x-ray photoemission spectroscopy data, we quantitatively determine the Ge distribution in the wetting layer and in the islands, separately. We found that in as-grown sample, the wetting layer is substantially Si-richer than the islands, its average composition being independent of the growth rate. Upon capping, the islands proceed to a reverse Stranski-Krastanov shape evolution, with a progressive Si enrichment of both the wetting layer and the islands. We demonstrate that this evolution occurs at constant island volume. The observed behavior indicates the suppression of the lateral diffusion of both Ge and Si atoms from the wetting layer to the surface of the enlarging islands, and vice versa.

We have investigated the close packed 3√3 x 3√3R30°-C 60 monolayer on the Ge(111) surface using synchrotron radiation photoemission. No filling of the lowest unoccupied molecular orbital (LUMO) derived bands has been observed. The C Is core level and the valence band photoemission spectra point to the presence of a localized covalent bond between the C 60 molecules and the Ge atoms. This bond induces the splitting of the C 60 highest occupied molecular orbital (HOMO) with a new feature appearing at about 2.3 eV. The Ge 3d core level spectra show the quenching of the surface features related to the adatoms/rest atoms complex, suggesting that the observed 3√3×3√3R30° ordering is not due to a Ge(111) reconstruction induced by the C 60 adsorption.

The adsorption of a close-packed ${\mathrm{C}}_{60}$ single layer on the clean $c(2\ifmmode\times\else\texttimes\fi{}8)\ensuremath{-}\mathrm{Ge}(111)$ surface leads to the formation of a $3\sqrt{3}\ifmmode\times\else\texttimes\fi{}3\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{}$ low-energy electron diffraction pattern upon annealing at 450--500 \ifmmode^\circ\else\textdegree\fi{}C. We have investigated this phase using synchrotron-radiation photoemission. The $\mathrm{Ge}3d$ core-level spectra show the quenching of the surface features related to the adatoms/rest-atoms complex of the original $\mathrm{Ge}(111)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}8)$ reconstruction, suggesting that the observed $3\sqrt{3}\ifmmode\times\else\texttimes\fi{}3\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{}$ ordering is not due to a Ge(111) reconstruction induced by the ${\mathrm{C}}_{60}$ adsorption, as instead previously argued on the basis of scanning tunneling microscopy measurements. Moreover, no filling of the lowest unoccupied molecular orbital derived bands has been observed. The C $1s$ core level and the valence-band photoemission spectra point to the presence of a localized covalent bond between the ${\mathrm{C}}_{60}$ molecules and the Ge atoms. This bond induces the splitting of the ${\mathrm{C}}_{60}$ highest occupied molecular orbital with a feature appearing at about 2.3 eV.

We provide a direct experimental proof and the related modeling of the role played by Si overgrowth in promoting the lateral ordering of Ge islands grown by chemical vapor deposition on Si(001). The deposition of silicon induces a shape transformation, from domes to truncated pyramids with a larger base, generating an array of closely spaced interacting islands. By modeling, we show that the resulting gradient in the chemical potential across the island should be the driving force for a selective flow of both Ge and Si atoms at the surface and, in turn, to a real motion of the dots, favoring the lateral order.

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.

In this paper, we show that lateral arrangement of Ge/Si(00 1) self-assembled islands in a square array oriented along the [100]-[0 10] directions can be obtained through the lateral displacement of the islands themselves. We found that when the deposited islands are exposed to an external silicon flux, the impinging silicon atoms induce Ge-Si intermixing resulting in island shape transformation from domes to large pyramids. This transformation leads to an array of closely spaced islands interacting elastically among themselves through the substrate. By means of atomistic simulations we demonstrate that this elastic repulsion drives a net flux of Ge and Si atoms from one side to the other side of the islands, leading to a lateral displacement of the whole island. This displacement ends when the two islands are sufficiently far away, or when another island is approached during the motion. In a dense ensemble of islands, this mechanism drives the tendency to order observed experimentally. (C) 2006 Elsevier Ltd. All rights reserved.

We discuss the effect of the deposition of a Si cap layer on the composition and morphological properties of Ge(Si)/Si(001) self-assembled islands deposited by chemical vapor deposition at 750 degrees C. The morphological evolution of the island shape was investigated by means of atomic force microscopy and the actual island composition has been measured by means of x-ray photoemission spectroscopy and x-ray absorption spectroscopy techniques. At an early stage of Si capping, Si atoms are incorporated in the island layer. As a consequence, we observe a reverse Stranski-Krastanov growth dynamics in agreement with the volume-composition stability diagram proposed for domes, pyramids, and prepyramids in the GexSi1-x/Si(100) system. We find that the island burying begins when the Ge average composition reaches the value x=0.28. Once the islands are buried under a thin silicon layer their composition is unaffected by subsequent silicon deposition. We conclude that strain relief, rather than thermal diffusion, is the main driving force for the observed Ge-Si alloying. (c) 2005 American Institute of Physics. (c) 2005 American Institute of Physics.

The self-patterning of the strain field that arises in the growth of stacked multilayers of heteroepitaxial islands, together with the capability of tuning the island size by acting on the deposition temperature, are here exploited to obtain self-organization, resulting in well-ordered clusters composed of regularly disposed, nanosized islands. Our results show that the island spatial distribution can be tuned from a random one to a well-ordered square lattice of island clusters, and that the number of islands inside each cluster can be selected. Moreover, due to the dipole repulsive interaction between adjacent islands, the islands themselves arrange in an ordered fashion inside a single cluster along the same [010]-[100] crystalline directions of the long-range cluster ordering. (C) 2003 American Institute of Physics.

The LUMO-derived band of the compound was investigated by means of x-ray and UV photoemission and photoelectric yield spectroscopies. A double-peak structure is found, with peak maxima at 1.1 eV and 0.35 eV. The relative magnitude of the two peaks is strongly temperature dependent: a large transfer of spectral weight from the lower- to the higher-binding-energy peak takes place upon cooling to 110 K. The peaks are tentatively attributed to the dimer and the polymeric phase respectively. However the behaviour as a function of temperature does not agree with the metastability of the dimer phase at room temperature. The position of the Fermi level is compatible with a correlated system on the border of the metal - insulating transition.

The shape evolution and the effect of deposition temperature on size and composition of chemical vapor deposition grown Ge/Si(100) islands have been investigated in the deposition temperature range 450-850 degreesC. It is found that the increase of the growth temperature above 600 degreesC entails a strong island enlargement due to an increased Si/Ge intermixing. The crystallographic structure of the islands was investigated by transmission electron microscopy. The analysis of the resulting Moire pattern reveals that the island lattice deformation decreases with increasing island size and that the effective mismatch epsilon between the silicon substrate and the epilayer decreases with increasing deposition temperature. The island nucleation size, the mean size of coherent islands and the critical size for the insertion of misfit dislocations have been found to scale as epsilon(-2), epsilon(-2), and epsilon(-1), respectively. The agreement of our experimental scaling results with the predictions of theoretical calculation performed for homogeneous heterostructures suggests that, although the Si distribution inside the islands is not homogeneous, the island growth is driven by the mean effective strain. (C) 2002 American Institute of Physics.

${\mathrm{Na}}_{x}{\mathrm{C}}_{60}$, ${\mathrm{K}}_{x}{\mathrm{C}}_{60}$, and ${\mathrm{Cs}}_{x}{\mathrm{C}}_{60}$ fullerides in the low-$x$ range were investigated by means of photoemission and yield spectroscopies. The primary aim was the investigation of the lowest unoccupied molecular orbital (LUMO) -derived electronic states as a function of doping. At low doping, double-peak structures in the LUMO-derived spectral weight were consistently found, which are explained in terms of mixed phases present in the specimens. The peak at higher binding energy is attributed to the low-$x$ stable phases, i.e., to ${A}_{1}{\mathrm{C}}_{60}$ ($A$ meaning an alkali atom) for K and Cs doping and to ${A}_{2}{\mathrm{C}}_{60}$ for Na doping. On the assumption that the fulleride electronic states are only weakly affected by the alkali species, the insulating or metallic nature of all stable phases, which can be obtained upon alkali doping, was established. All phases, with the exception of ${A}_{3}{\mathrm{C}}_{60}$, turn out to be insulating in striking contrast with the predictions of band-structure calculations, the separation between the LUMO peak and the Fermi level in the ${A}_{1}{\mathrm{C}}_{60}$ and ${A}_{2}{\mathrm{C}}_{60}$ phases being the largest. The energy position of the Fermi level relative to the highest occupied molecular orbital-derived peak changes little with doping.

In this paper we present a study of the influence of the growth rate and deposition temperature on the growth dynamic of self-assembled Ge/Si(100) islands. By combining AFM and XPS measurements we show that the main effect of rising the deposition temperature is an enhancement of the Ge-Si intermixing. The actual composition of the Ge islands as a function of deposition temperature has been measured in the 450 °C- 850 °C range: the Ge content x in the alloyed epilayer was found to decrease from x=l to x=0.28. By changing the growth rate at fixed deposition temperature we were also able to modify the island density. The influence of the island density on the island size has been investigated. We found that an increase of the island density from 109 to 2×1010 cm-2 at T=600 °C brings about a change of the mean island size from 85 to 55 nm. This reduction is explained in terms of island-island interaction effects on the growth dynamic.

We have applied atomic force microscopy and x-ray photoemission spectroscopy to the study of SiGe intermixing in Ge/Si(100) self-assembled islands. We have quantified the Ge/Si alloying as a function of the deposition temperature in the 500-850 degreesC range. The Si content inside the islands varies from 0% at 550 degreesC up to 72% at 850 degreesC. As a consequence of the reduction of the effective mismatch due to the observed SiGe intermixing, the critical base width for island nucleation increases from 25 nm for T-dep< 600 degreesC up to 270 nm for T-dep=850 degreesC. (C) 2001 American Institute of Physics.

""We present an energy-balance model of the electronic intersubband relaxation in optically excited n-type Ge\\\/SiGe quantum wells with absorption resonance in the THz range. To this aim, the energy relaxation rates of the electron system due to the interactions with both non-polar optical and acoustic phonons are calculated. The time dependence of the relative dierential transmission is also evaluated and compared with experimental data from recent pump-probe measurements. The energy relaxation rates due to acoustic and optical phonon are investigated for different electron temperatures, set by the pump beam intensity. We \\ffind that the relaxation dynamics strongly depends on the intersubband energy spacing when this is close to the optical phonon energy. Finally, our results evidence that in this material system the time dependence of the depolarization shift may have a strong influence on the relative differential transmission signal.""

Ge/Si(001) heterostructures grown by means of ultrahigh vacuum chemical vapor deposition have been investigated by means of variable temperature high resolution x ray diffraction in order to investigate the origin of the residual tensile strain observed in this system. To this purpose, we have simultaneously measured the in- and out-of-plane lattice parameters of the deposited Ge films and of the underlying Si substrate, thus allowing us to directly measure the Ge strain evolution as the epilayer was annealed up to and over the deposition temperature and cooled back to room temperature. We have observed that the tensile strain, resulting from the different Si and Ge thermal expansion coefficient, is partially compensated by the residual compressive heteroepitaxial strain, due to the hardening limit of Ge. This limited the tensile strain observable in these heterostructures to ∼0.002. © 2012 American Institute of Physics.

""We measured the absorption coefficient and the non-radiative lifetime of intersubband transitions in Germanium quantum wells by steady-state and pump-probe terahertz. spectroscopy.""