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Combining scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we demonstrate how to tune the doping of epitaxial graphene from p to n by exploiting the structural changes that occur spontaneously on the Ge surface upon thermal annealing. Furthermore, using first-principle calculations, we build a model that successfully reproduces the experimental observations. Since the ability to modify graphene electronic properties is of fundamental importance when it comes to applications, our results provide an important contribution toward the integration of graphene with conventional semiconductors.

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.

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

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

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.

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.

""We report on the fabrication process of suspended anodes implemented in Schottky-diode devices on epitaxial Ge. An air-bridge technique based on reactive ion etching in SF(6) was developed in order to release the Schottky contact bridge, ensuring higher control respect to wet etching. Metal-semiconductor junctions in the sub-micron range with rectifying behaviour have been obtained. The present fabrication process can be extended as is to the use of silicon on insulator substrates. (C) 2010 Elsevier B.V. All rights reserved.""

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

We present morphological and electrical characterizations of thin and narrow resistors obtained by focused ion beam assisted deposition of Pt based material. For thin and narrow depositions the measured thickness and width are significantly different from the nominal values. From leakage tests we found that in order to have electrically insulated parallel resistors at room temperature, it is mandatory that the Pt-halo, which results from the deposition procedure, has a thickness well below 6 nm. (C) 2008 Elsevier Ltd. All rights reserved.

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

The growth of high-mobility modulation-doped Si/SiGe heterostructures on silicon-on-insulator (SOI) substrates is demonstrated. The structural and electrical properties of the samples are comparable with those of similar samples grown on standard Si substrates. Electron mobilities as high as 2800 cm 2 V −1 s −1 at room temperature and 8.2×10 4 cm 2 V −1 s −1 at 4 K are obtained.

Novel silicon photonics applications requiring heavy n-type doping have recently driven a great deal of interest towards the phosphorous doping of germanium. In this work we report on infrared reflectance spectroscopy measurements of the electron density in heavily n-type doped germanium layers obtained by stacking multiple phosphorous δ-layers. Here, we demonstrate that the conventional Drude model of the electrodynamic response of free carriers in metals can be adapted to describe heavily doped semiconductor thin films. Consequently, the effect of the electron density on the plasma frequency, scattering rate and complex permittivity can be investigated.

Due to the evergrowing use of optical fibers in communications, for both long distances and local area networks, there is an increasing effort toward the realization of high speed and high efficiency optical detectors, operating in the low-absorption regions ~1.3‐1.55 mm! of silica fibers. Although III‐V semiconductors provide high detection efficiency in the range of interest, incorporating them in the well-established Si technology is difficult and expensive. 1 SiGe structures on silicon represent an alternative solution for near-infrared photodetection, owing to their narrow band gap and full compatibility with silicon technologies. SiGe alloy heterostructures, multiquantum wells, and superlattices have been used as active layers in p-i-n photodetectors and their operation at 1.3 mm has been successfully demonstrated for both normal incidence 2 and waveguide configurations. 3 The fundamental limitation of SiGe-based heterostructures is the vanishing response at 1.55 mm. Pure germanium, indeed, represents the best candidate for 1.3‐1.55 mm photodetectors, due to its direct band gap of 0.8 eV. However, owing to the high lattice mismatch with Si, it is not easy to obtain Ge films with suitable characteristics: thickness appropriate for maximum efficiency, flatness compatible with submicron lithography, minimum defect content for high speed. Thick graded buffers can be used to partially relax the strain due to the lattice mismatch, 4 but the molecular beam epitaxy ~MBE! growth of such a layer is cumbersome and expensive. In the present work we investigate the feasibility of the simplest fabrication approach, i.e., the epitaxial growth of thick layers ~well over the critical thickness! of pure germanium on silicon by chemical vapor deposition ~CVD!. The use of pure Ge on Si for efficient photon detection in the range 1.3‐1.5 mm, first attempted by Luryi et al., 5 requires

We report on Ge Schottky diodes epitaxially grown on Si(001) exhibiting a good responsivity in the near-infrared up to 1.55 mu m. The Ge layers were grown by UHV chemical vapor deposition. It was found that the photocurrent increases upon increasing the reverse bias, reaching a maximum responsivity of 0.12 A/W at 1.3 mu m for 4 V bias. The leakage current density at the saturation voltage is 1 nA/mu m(2).

In this letter we present an atomic force microscopy and photoluminescence investigation of two‐ and three‐dimensional Ge structures deposited on Si(100) substrates. The photoluminescence spectra demonstrated the presence of quantum confinement effects in flat and uncapped Ge layers and in Ge islands even for the relatively large size of the structures. Moreover, a correlation between the sample morphology and the features of the photoluminescence is demonstrated.