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Proceeding toISTDM/ICSI conference (https://www.istdm-icsi-2023.polimi.it/)

accepted for oral presentation to ECOSS36 (https://www.ecoss36.uni.lodz.pl/) &nbsp

Strain relaxedSi1−𝑥Ge𝑥 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 strainSi0.98Ge0.02 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.

The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1−xGex 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−xGex 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., Optics Express, 28 (4), ISSN:1094-4087

Theoretical predictions indicate that the n-type Ge/SiGe 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 allowed us to characterize the tunneling barrier parameters and, in turn, achieve a highly-controlled engineering of the electronic structure in forthcoming unipolar cascade systems based on n-type Ge/SiGe multi quantum-wells.

We demonstrate a very-low temperature cleaning technique based on atomic hydrogen irradiation for highly (1 %) tensile strained Ge epilayers grown on metastable, partially strain relaxed GeSn buffer layers. Atomic hydrogen is obtained by catalytic cracking of hydrogen gas on a hot tungsten filament in an ultra-high vacuum chamber. X-ray photoemission spectroscopy, reflection high energy electron spectroscopy, atomic force microscopy, secondary ion mass spectroscopy, and micro-Raman showed that an O- and C-free Ge surface was achieved, while maintaining the same roughness and strain condition of the as-deposited sample and without any Sn segregation, at a process temperature in the 100-300 °C range.

Low field magnetotransport revealing signatures of ballistic transport effects in strained Si/SiGe cavities is investigated. We fabricated strained Si/SiGe cavities by confining a high mobility Si/SiGe two-dimensional electron gas in a bended nanowire geometry defined by electron-beam lithography and reactive ion etching. The main features observed in the low temperature magnetoresistance curves are the presence of a zero-field magnetoresistance peak and of an oscillatory structure at low fields. By adopting a simple geometrical model we explain the oscillatory structure in terms of electron magnetic focusing. A detailed examination of the zero-field peak line shape clearly shows deviations from the predictions of ballistic weak localization theory.

We discuss the performance, of a normal metal hot electron bolometer (NHEB) that we have measured at 0.3 K. We found that the noise equivalent power was limited by the amplifier noise. To improve the NHEB power response and to make it more robust and reliable we propose to substitute the normal metal with heavily doped silicon. The heavily doped silicon behaves like a metal with lower carrier concentration and has a smaller electron-phonon thermal coupling. We have fabricated superconductor-doped silicon-superconductor contacts (S-Sm-S) and we have used them as thermometers and coolers. (C) 2003 Elsevier B.V. All rights reserved.

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

The fabrication of complex low-dimensional quantum devices requires the control of the heteroepitaxial growth at the subnanometer scale. This is particularly challenging when the total thickness of stacked layers of device-active material becomes extremely large and exceeds the multi-μm limit, as in the case of quantum cascade structures. Here, we use the ultrahigh-vacuum chemical vapor deposition technique for the growth of multi-μm-thick stacks of high Gecontent strain-balanced Ge/SiGe tunneling heterostructures on Si substrates, designed to serve as the active material in a THz quantum cascade laser. By combining thorough structural investigation with THz spectroscopy absorption experiments and numerical simulations we show that the optimized deposition process can produce state-of-the-art threading dislocation density, ultrasharp interfaces, control of dopant atom position at the nanoscale, and reproducibility within 1% of the layer thickness and composition within the whole multilayer. We show that by using ultrahigh-vacuum chemical vapor deposition one achieves simultaneously a control of the epitaxy down to the sub-nm scale typical of the molecular beam epitaxy, and the high growth rate and technological relevance of chemical vapor deposition. Thus, this technique is a key enabler for the deposition of integrated THz devices and other complex quantum structures based on the Ge/SiGe material system.

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.

n-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron&ndash, phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter- and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1&rarr, 3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3.

We study radiative relaxation at terahertz frequencies in n-type Ge/SiGe quantum wells, optically pumped with a terahertz free electron laser. Two wells coupled through a tunneling barrier are designed to operate as a three-level laser system with non-equilibrium population generated by optical pumping around the 1→3 intersubband transition at 10 THz. The non-equilibrium subband population dynamics are studied by absorption-saturation measurements and compared to a numerical model. In the emission spectroscopy experiment, we observed a photoluminescence peak at 4 THz, which can be attributed to the 3→2 intersubband transition with possible contribution from the 2→1 intersubband transition. These results represent a step towards silicon-based integrated terahertz emitters.

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

The imaging and sensing technology operating in the THz region of the electromagnetic spectrum has a number of applications, with demonstrator products already available on the market for oncology imaging, production monitoring, and non-destructive test [1]. However, the THz sources now at hand are still bulky and too expensive for expanding this technology to other proposed applications, which also include, among other, THz bandwidth photonics and security imaging. A higher level of integration with control electronics, a lower production cost, and a broader wavelength range of emission towards the far-infrared, are all desirable features to expand the fields of application of THz radiation. N-type Ge/SiGe quantum cascade structures grown on top of a Si(001) substrate are particularly promising for realizing a Si based THz source [2]. The low effective mass and long non-radiative relaxation times due to the non-polar nature of the material, are expected to i) provide gain values close to those demonstrated in III-V quantum cascade structures at 4 K, and ii) to potentially enable 300 K operation. In this presentation we will discuss the optical and structural properties of n-type s-Ge/SiGe multi-quantum wells and asymmetric coupled quantum wells grown on Si(001) substrates by means of ultrahigh vacuum chemical vapor deposition [3]. Extensive structural characterization obtained by scanning transmission electron microscopy (STEM), atomic probe tomography (APT) and X-ray diffraction shows the high material quality of strain-symmetrized structures (up to 5 micron active region thickness) and heterointerfaces (featuring interface roughness below 0.2 nm), down to the ultrathin barrier limit (about 1 nm). By performing THz absorption spectroscopy measurements combined to theoretical modeling on different asymmetric coupled quantum well systems (with varying large-well width or barrier thickness), we unambiguously demonstrated inter-well coupling and wavefunction tunneling [3]. The agreement between experimental data and simulations allowed us to characterize the tunneling barrier parameters and, in turn, achieve a highly-controlled engineering of the electronic structure in forthcoming unipolar cascade systems based on n-type Ge/SiGe multi quantum-wells. Furthermore, by pump-and-probe, and time domain spectroscopic data with a thorough theoretical modeling, we will show that this material system is indeed promising as active material in quantum cascade lasers (QCL). We found i) narrow intersubband (ISB) absorption lines; ii) relatively long non-radiative ISB relaxation times at high temperature; iii) relaxation times for different ISB transitions favorable to population inversion. Leveraging on the promising results obtained by spectroscopy experiments, we theoretically investigate an electrically-pumped Ge/SiGe THz QCL through a non-Equilibrium Green Function formalism (nextnano.QCL), using as material parameters to model the scattering processes the values estimated from the analysis of the optical experimental data [4]. As expected, due to the weaker interaction with the phonon field with respect to III-V based devices, we find a lower impact of the temperature on the gain spectrum. In addition, simulations show that the interface roughness values measured on our samples allows to achieve gain overcoming the losses of double-metal waveguides at room temperature. We believe that the present results will motivate new experimental efforts aimed at demonstrating room-temperature operation in group IV QCL THz devices. References [1] D. J. Paul, Laser Photon. Rev. 4, 610 (2010). [2] K. Driscoll, and R. Paiella, J. Appl. Phys. 102, 093103 (2007). [3] C. Ciano, M. Virgilio, M. Montanari, L. Persichetti, L. Di Gaspare, M. Ortolani, L. Baldassarre, M. H. Zollner, O. Skibitzki, G.Scalari, J. Faist, D. J. Paul, M. Scuderi, G. Nicotra, T. Grange, S. Birner, G. Capellini, and M. De Seta, Accepted in Phys. Rev. Appl., 11, 014003 (2019). [4] T. Grange, D. Stark, G. Scalari, J. Faist, L. Persichetti, L. Di Gaspare, M. De Seta, M. Ortolani, D.J. Paul, G. Capellini, S. Birner and M. Virgilio, Submitted to Applied Physics Letters. Preprint available at https://arxiv.org/pdf/1811.12879.pdf.

The efficient integration of electronic and optoelectronic functions on a single chip within a CMOS-compatible material and technology platform is witnessing the next disruptive innovation in the microelectronics industry. Among the proposed approaches to “siliconize” photonics, the most ambitious is to develop a monolithic Si-based light source. The main challenge is the indirect bandgap of Si which hinders efficient light emission. A number of different solutions has been suggested to break down this material limitation, and the optimal strategy depends on the specific spectral range of application. In the THz range, quantum cascade laser (QCL) architectures based on the Group-IV material system have been predicted as viable potential sources up to room temperature1-3, by exploiting intersubband (ISB) transitions in the conduction band of n-type, Ge-rich Ge/SiGe multi-quantum well (MQW) heterostructures3. In this paper, we will show that the interface quality and threading dislocation density4 in this material system have finally reached the level required for possibly enabling, within a wide temperature range, a material gain larger than the cavity losses5. From the structural standpoint, we report on the growth by ultra-high-vacuum chemical vapor deposition (UHV-CVD) of strain-compensated QCL stacks with a thickness up to 10 µm, demonstrating by X-ray diffraction and scanning transmission electron microscopy their high crystalline quality and remarkable growth reproducibility [Fig. 1(a)]. We will also describe optimized designs obtained in waveguide modelling with which waveguide losses comparable to III-V architectures are achieved6. In the model system of n-type Ge-rich asymmetric coupled quantum wells (ACQWs)7,8, being the building block of a QCL structure, we will prove a high degree of control on the engineering at the nanoscale the intersubband electronic spectrum and the wavefunctions relevant for tunneling processes, which allowed us the observation of photoluminescence at 4 THz after optical excitation using a free-electron laser (FEL)9. In addition, we will present a systematic calibration study of the material parameters controlling the interface roughness (IFR) and electron-phonon scatterings in n-type Ge/SiGe MQWs by combining experimental pump-probe data with a numerical model of ISB carrier dynamics including inelastic and elastic scattering channels. The time evolution of subband populations after FEL optical pumping is investigated as a function of the MQW design geometry, comparing symmetric and stepwise configurations, and analyzed by varying the subband energy spacing changing the well width. As a matter of fact, these material parameters critically affect the outcomes of the non-equilibrium Green’s function calculations widely used for predicting quantum transport and, ultimately optical gain of QCL devices10,11 and so to guide the optimization of proposed device designs. Hitherto, however, in most cases, the adopted values for these material parameters were not calibrated against experiments. For a proper calibration of the deformation potentials, we studied the non-radiative lifetimes as a function of the subband spacing on a set of symmetric MQWs of different Ge well widths. The corresponding α2D absorption spectra are reported in the left panels Fig. 1(b-d). Differential transmission spectra from single-color pump-probe experiments and model simulations indicate [Fig. 1(e-g)] that the electron-phonon coupling is much less effective than that expected from the bulk deformation potentials reported by Jacoboni et al.12. In particular, we found that the relaxation mediated by the emission of intervalley OPs is suppressed in low-dimensional multilayer structures while the intravalley OP scattering is described by a reduced effective deformation potential, thus explaining the observation of relaxation times larger than 10 ps above both the OP energy thresholds in single-color pump probe spectroscopy experiments. These results are crucial for developing adequate simulation frameworks and specific design approaches for achieving optical gain in silicon-compatible quantum cascade emitters. [1] K. Driscoll et al., Appl. Phys. Lett. 89, 191110 (2006). [2] K. Driscoll et al., J. Appl. Phys. 102, 093103 (2007). [3] D. J. Paul, Laser & Photonics Reviews 4, 610 (2010). [4] T. Grange et al., Physical Review Applied 13, 044062 (2020). [5] T. Grange et al., Appl. Phys. Lett. 114, 111102 (2019). [6] K. Gallacher et al., Opt. Express 28, 4786 (2020). [7] C. Ciano et al., Physical Review Applied 11, 014003 (2019). [8] L. Persichetti et al., Crystals 10, 179 (2020). [9] C. Ciano et al., Opt. Express 28, 7245 (2020). [10] A. Valavanis et al., Phys. Rev. B 83, 195321 (2011). [11] C. Jirauschek et al., Applied Physics Reviews 1, 011307 (2014). [12] C. Jacoboni et al., Phys. Rev. B 24, 1014 (1981). Figure 1

Regular surface undulations, called cross-hatch patterns, appearing at the free surface of lattice-mismatched heteroepitaxial films are a key signature of plastic relaxation. Here we show that the dynamics of cross-hatch formation is accurately described by a continuum model based on strain-mediated surface diffusion, provided that a realistic distribution of dislocations is considered. We demonstrate quantitative agreement between our time-dependent simulations and dedicated atomic force microscopy experiments on ${\mathrm{Si}}_{0.92}{\mathrm{Ge}}_{0.08}$ films grown on Si(001) at various thicknesses, finally shedding light on the origin and on the dynamical behavior of a widely investigated pattern, first observed more than half a century ago.

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

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

We investigate the distribution of Sn in GeSn nano-heteroepitaxial clusters deposited at temperatures well exceeding the eutectic temperature of the GeSn system. The 600 °C molecular beam epitaxy on Si-patterned substrates results in the selective growth of GeSn nano-clusters having a 1.4 ± 0.5 at% Sn content. These nano-clusters feature Sn droplets on their faceted surfaces. The subsequent deposition of a thin Ge cap layer induced the incorporation of the Sn atoms segregated on the surface in a thin layer wetting the nano-dots surface with 8 ± 0.5 at% Sn. The presence of this wetting layer is associated with a relatively strong photoluminescence emission that we attribute to the direct recombination occurring in the GeSn nano-dots outer region.

We provide an in-depth characterization of the dislocation distribution in partially relaxed Si0.92Ge0.08/Si(001) films. This is achieved by an innovative and general method, combining two state-of-the-art characterization techniques through suitable modeling. After having inferred the dislocation positions from transmission-electron-microscopy images, we theoretically reproduce scanning-x-ray-diffraction-microscopy tilt maps measured on the very same region of the sample. We obtain a nearly perfect match between model predictions and experimental data. As a result, we claim that it is possible to establish a local, direct correlation between the dislocations revealed by the transmission-electron-microscopy analysis and the measured lattice tilt distribution.

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

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.