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n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-rich SiGe tunnel barrier, as a function of the geometry parameters of the design and the P dopant concentration. Through a comparison of THz spectroscopic data with numerical calculations of intersubband optical absorption resonances, we demonstrated that it is possible to tune, by design, the energy and the spatial overlap of quantum confined subbands in the conduction band of the heterostructures. The high structural/interface quality of the samples and the control achieved on subband hybridization are promising starting points towards a working electrically pumped light-emitting device.

Exploiting intersubband transitions in Ge/SiGe quantum cascade devices provides a way to integrate terahertz light emitters into silicon-based technology. With the view to realizing a Ge/SiGe Quantum Cascade Laser, we present the optical and structural properties of n-type strain-symmetrized Ge/SiGe asymmetric coupled quantum wells grown on Si(001) substrates by means of ultrahigh vacuum chemical vapor deposition. We demonstrate high material quality of strain-symmetrized structures and heterointerfaces as well as control over inter-well coupling and electron tunneling. Motivated by the promising results obtained on ACQWs, which are the basic building block of a cascade structure, we investigate, both experimentally and theoretically, a Ge/SiGe THz QCL design, optimized through a non-equilibrium Green’s function formalism

Asymmetric coupled quantum wells in the conduction band of germanium grown on silicon wafers have been investigated by THz pump-THz broadband probe picosecond spectroscopy. The optically-pumped laser gain coefficient is estimated.

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.

Asymmetric quantum well systems are excellent candidates to realize semiconductor light emitters at far-infrared wavelengths not covered by other gain media. Group-IV semiconductor heterostructures can be grown on silicon substrates, and their dipole-active intersubband transitions could be used to generate light from devices integrated with silicon electronic circuits. Here, we have realized an optically pumped emitter structure based on a three-level Ge/Si0.18Ge0.82 asymmetric coupled quantum well design. Optical pumping was performed with a tunable free-electron laser emitting at photon energies of 25 and 41 meV, corresponding to the energies of the first two intersubband transitions 0 → 1 and 0 → 2 as measured by Fourier-transform spectroscopy. We have studied with a synchronized terahertz timedomain spectroscopy probe the relaxation dynamics after pumping, and we have interpreted the resulting relaxation times (in the range 60 to 110 ps) in the framework of an out-of-equilibrium model of the intersubband electron−phonon dynamics. The spectral changes in the probe pulse transmitted at pump−probe coincidence were monitored in the range 0.7−2.9 THz for different samples and pump intensity and showed indication of both free carrier absorption increase and bleaching of the 1 → 2 transition. The quantification from data and models of the free carrier losses and of the bleaching efficiency allowed us to predict the conditions for population inversion and to determine a threshold pump power density for lasing around 500 kW/cm2 in our device. The ensemble of our results shows that optical pumping of germanium quantum wells is a promising route toward siliconintegrated far-infrared emitters.

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.

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.""

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.

A set of recent results concerning lateral and vertical ordering of Ge islands grown on Si(001) is reviewed. Experimental data generated by chemical vapour deposition and analysed by atomic force microscopy and photoelectron spectroscopy are compared with computer simulations and modelling based on atomistic approaches and continuum theory. In particular, we show that it is possible to probe experimentally the detailed strain field generated by buried Ge islands at the surface of the Si capping layer. The observed arrangement of small Ge islands grown over the capping layer is demonstrated to be very close to the one predicted by a simple model where the local chemical potential is inferred from the strain field at the atomic scale, as given by Tersoff- potential molecular dynamics simulations. Moreover, we review recent experimental evidence for lateral ordering, triggered by partial Si capping, in the first layer of Ge islands on Si(001). Theoretical support is given by showing that when two islands lie in close proximity the elastic field is likely to generate a flow of atoms leading to an effective gliding motion along opposite directions of both islands, eventually stopped by the presence of further neighbouring islands.

In this paper we present a study on the influence of the number and the thickness of silicon spacer layer on the optical properties of single- and multi-layers of self assembled Ge/Si (001) islands performed by means of cathodoluminescence spectroscopy, high resolution X-ray diffraction and transmission electron microscopy. In single-layer sample, we do not evidence dependence of the island no-phonon emission peak position on the silicon cap-layer thickness. In multi-layer samples having a thin (33 nm) silicon spacer layer the no-phonon emission energy value progressively blue-shifts for an increasing number of island layers. This is interpreted as an enhanced intermixing driven by the strain interaction existing between island layers. On the contrary, island emission energy position is independent on the number of layers in the sample series having a thicker spacer layer (60 nm). These findings are consistent with the X-ray diffraction observation that islands belonging to different layers have the same composition. As a consequence we can conclude that multilayers with 60-nm spaced islands layer are more homogeneous and ordered.

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.

(Received 4 October 2005; published 16 March 2006)We provide a direct experimental proof and the related modeling of the role played by Si overgrowth inpromoting the lateral ordering of Ge islands grown by chemical vapor deposition on Si(001). Thedeposition 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 gradientin the chemical potential across the island should be the driving force for a selective flow of both Ge and Siatoms at the surface and, in turn, to a real motion of the dots, favoring the lateral order.

We report on the tailoring of the stress field distribution at the surface in a multilayered $\mathrm{Ge}∕\mathrm{Si}$ system in order to have a spontaneous ordering of the self-assembled islands in multi-island clusters. These clusters are ordered along [100]-[010] directions and arranged in a simple planar lattice. We investigate the evolution, from nucleation to ripening, of the Ge islands inside the clusters for two different stress modulations at the surface. The control over the stress at the surface has been obtained by tuning the last Si spacer layer thickness in a stacked-island multilayered structure. An appropriate selection of this spacer layer thickness allowed us to modulate the stress in two different limits of strong and weak island-island vertical interaction. We demonstrate that the stress modulation can be used to influence the island growth dynamics in order to induce an ordered spatial position of islands inside the cluster, to reduce the island size and to narrow the island size distribution.

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 shape evolution and the effect of deposition temperature on the strain status of Ge/Si(100) islands have been investigated in the deposition temperature range 450-750 degreesC. The island crystallographic structure was investigated by transmission electron microscopy. The analysis of the Moire patterns reveals that the island lattice deformation and the elastic energy per unit volume stored in the islands decrease with increasing island size in quantitative agreement with theoretical results. Furthermore, we evidenced that the effective mismatch E between the silicon substrate and the island epilayer decreases upon increasing the deposition temperature. This misfit reduction is fully accounted by the amount of SiGe intermixing in the epilayer. (C) 2003 Elsevier Science B.V. All rights reserved.

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.

The interaction between C/sub 60/ molecules and the Si(100) surface and the preparation of silicon-carbide thin films by thermal reaction of C/sub 60/ molecules with the Si(100) surface has been investigated using X-ray photoelectron spectroscopy, reflection high energy electron diffraction and atomic force microscopy measurements. The effect of annealing temperature on the SiC formation is discussed. It is found that the C/sub 60/ molecules bond covalently with silicon and the number of bonds increase upon increasing the annealing temperature. Annealing at T/spl ges/830/spl deg/C entails the formation of stoichiometric silicon carbide clusters that coalesce to form a continuous SiC. Deep pits acting as silicon diffusion channels are present.

The interaction between C-60 molecules and the Si(100) surface and the preparation of silicon-carbide thin films by thermal reaction of C-60 molecules with the Si(100) surface have been investigated using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, reflection high-energy electron diffraction and atomic force microscopy measurements. The effects of annealing temperature and C-60 coverage on the SiC formation will be discussed. It is found that the C-60 molecules bond covalently with silicon, and the number of bonds increase upon increasing the annealing temperature. Annealing at T greater than or equal to 830 degrees C entails the formation of stoichiometric silicon carbide clusters that coalesce to form a continuous SiC layer when the C-60 coverage is greater than one monolayer. Deep pits acting as silicon diffusion channels are present with dimensions that increase with the amounts of C-60. The interaction of C-60 With the SiC surface was also investigated. It is found that a similar covalent interaction is present in the two systems C-60/Si and C-60/SiC. (C) 2000 Elsevier Science B.V. All rights reserved.

In this paper we present a comparative study of the agglomeration process in silicon-on-insulator (SOI), silicon germanium-om-insulator (SGOI), and strained SOI (SSOI) thin layers under thermal annealing in ultrahigh vacuum. In particular, we provide the first evidence and characterization of agglomeration in SGOI and SSOI substrates. A common agglomeration dynamics is observed in all the substrates investigated, with the semiconductor-on-insulator layer thickness being the main parameter governing it. These findings provide a better understanding of the surface-energy-driven dewetting phenomenon in semiconductor layers and allow us to single out the influence of the surface and stress energies on the void formation and evolution, as well as on the size and density of the agglomerated islands. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3117837]

In this article we present a quantitative study of the influence of the number and the thickness of the silicon spacer layer on the optical and structural properties of single and multilayers of self-assembled Ge/Si(001) islands. By means of cathodoluminescence spectroscopy, high resolution x-ray diffraction, and transmission electron microscopy, we will show that the island composition and strain status of single-layer samples do not depend on the silicon cap-layer thickness. On the contrary, we found that the strain interaction existing between island layers separated by a silicon spacer layer 33 nm thick (i.e., similar to 3 times the mean island height), enhances the SiGe intermixing during the deposition of subsequent layers bringing to a composition inhomogeneity of the intermixed islands of about 18% along the stack. This inhomogeneity is not present in samples having a thicker spacer layer (60 nm) although the strain interaction between different layers is strong enough to drive the island stacking along the vertical direction. We conclude that the latter spacer layer thickness allows us to obtain a highly ordered three-dimensional superlattice of an island having a homogeneous size and composition along the stack itself. (c) 2007 American Institute of Physics.

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.

a-Si:H/a-Si 1− x C x multi-quantum-well structures have been grown and investigated. The density of defects at the interface is measured by photothermal deflection spectroscopy and correlated to the decrease of photoconductivity observed with decreasing silicon layer thickness.

a-Si:H/a-Si1−xCx:H multi-quantum-well structures have been grown and investigated. Optical transmission and electroabsorption measurements show that the optical gap increases with decreasing a-Si:H layer thickness. This behaviour is explained in terms of quantum confinement effects. An increase of the Urbach tail and a decrease of the photoconductivity were also observed with decreasing silicon thickness. Both effects have been attributed to the observed increase of the gap state density.