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Author: Ortolani, Michele × WGL Institute: IHP ×

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    Intersubband Transition Engineering in the Conduction Band of Asymmetric Coupled Ge/SiGe Quantum Wells
    (Basel : MDPI, 2020) Persichetti, Luca; Montanari, Michele; Ciano, Chiara; Di Gaspare, Luciana; Ortolani, Michele; Baldassarre, Leonetta; Zoellner, Marvin; Mukherjee, Samik; Moutanabbir, Oussama; Capellini, Giovanni; Virgilio, Michele; De Seta, Monica
    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. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
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    Electron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models
    (Basel : MDPI, 2020) Ciano, Chiara; Virgilio, Michele; Bagolini, Luigi; Baldassarre, Leonetta; Rossetti, Andrea; Pashkin, Alexej; Helm, Manfred; Montanari, Michele; Persichetti, Luca; Di Gaspare, Luciana; Capellini, Giovanni; Paul, Douglas J.; Scalari, Giacomo; Faist, Jèrome; De Seta, Monica; Ortolani, Michele
    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-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!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. © 2019 by the authors.
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    Subnanometer Control of the Heteroepitaxial Growth of Multimicrometer-Thick Ge /(Si, Ge) Quantum Cascade Structures
    (College Park, Md. [u.a.] : American Physical Society, 2023) Talamas Simola, Enrico; Montanari, Michele; Corley-Wiciak, Cedric; Di Gaspare, Luciana; Persichetti, Luca; Zöllner, Marvin H.; Schubert, Markus A.; Venanzi, Tommaso; Trouche, Marina Cagnon; Ortolani, Michele; Mattioli, Francesco; Sfuncia, Gianfranco; Nicotra, Giuseppe; Capellini, Giovanni; Virgilio, Michele; De Seta, Monica
    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 Ge content 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.