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High-quality n-type continuously graded Ge-rich Si1−xGex parabolic quantum wells with different doping level were grown by using ultrahigh-vacuum chemical vapor deposition on Si(001) substrates. A thorough structural characterization highlights an ideal parabolic compositional profile. THz intersubband absorption has been investigated in modulation-doped samples and samples directly doped in the wells. The comparison of experimental absorption data and theoretical calculations allowed us to quantify the impact of electron correlation effects on the absorption resonances in the different doping conditions and for electron sheet densities in the (1÷6)×1011 cm−2 range. A single optical resonance is present in modulation doped samples. Its peak energy and line-shape is independent of temperature-induced variations of the electron distribution in the subbands up to 300 K, in agreement with the generalized Kohn theorem. This achievement represents a relevant step-forward for the development of CMOS compatible optoelectronic devices in the THz spectral range, where thermal charge fluctuations play a key role.

© 2021 Author(s). We report electroluminescence originating from L-valley transitions in n-type Ge/Si0.15Ge0.85 quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of Δ f / f ≈ 0.2. Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. The design is based on a single quantum well active region employing a vertical optical transition, and the observed spectral features are well described by non-equilibrium Green's function calculations. The presence of two peaks highlights a suboptimal injection in the upper state of the radiative transition. Comparison of the electroluminescence spectra with a similar GaAs/AlGaAs structure yields one order of magnitude lower emission efficiency. ISSN:0003-6951 ISSN:1077-3118

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

By combining scanning probe microscopy with Raman and x-ray photoelectron spectroscopies, we investigate the evolution of CVD-grown graphene/Ge(001) as a function of the deposition temperature in close proximity to the Ge melting point, highlighting an abrupt change of the graphene's quality, morphology, electronic properties and growth mode at 930 degrees. We attribute this discontinuity to the incomplete surface melting of the Ge substrate and show how incomplete melting explains a variety of diverse and long-debated peculiar features of the graphene/Ge(001), including the characteristic nanostructuring of the Ge substrate induced by graphene overgrowth. We find that the quasi-liquid Ge layer formed close to 930 degrees is fundamental to obtain high-quality graphene, while a temperature decrease of 10 degrees already results in a wrinkled and defective graphene film., in press

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.

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.

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.

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.

(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 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 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.

We have investigated the structure and the electronic properties of thin SiC films grown on Si(100) by plasma-assisted chemical vapor deposition from CH4 diluted in H-2 It was found that the growth proceeds through the nucleation of cubic and relaxed crystalline SIC islands preferentially oriented in the (100) direction. The average-island size increases with carbonization time up to a maximum size consisting of similar to 300 nm lateral width and similar to 100 nm height. We were able to determine the valence band discontinuity although the SIC overlayers were clusterlike End did not cover uniformly the Si substrate. The found value is 0.77+/-0.08eV. (C) 1998 American Vacuum Society.

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.