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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. Zöllner, 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.
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