(Invited) (Si)GeSn Semiconductors for Integrated Optoelectronics, Quantum Electronics, and More
Authors: Assali Simone , Abdi Salim , Atalla Mahmoud , Attiaoui Anis , Bouthillier Étienne , Del Vecchio Patrick , Groell Léonor , Kumar Aashish , Luo Lu , Mukherjee Samik , Nicolas Jérôme , Moutanabbir Oussama 
Published: February 2020

Sn-containing group IV semiconductors (Si)GeSn represent a versatile platform to implement a variety of Si-compatible photonic, optoelectronic, and photovoltaic devices. This class of semiconductors provides two degrees of freedom, strain and composition, to tailor the band structure and lattice parameter thus enabling a variety of heterostructures and low-dimensional systems on a Si substrate. In this presentation, we will discuss the recent progress in the growth of metastable (Si)GeSn semiconductors. We will highlight the current understanding of their fundamental properties and elucidate the effects of the atomistic-level structure on their optoelectronic performance. The relevance of these semiconductors for Si-compatible mid-infrared optoelectronics and quantum information will also be discussed form both materials and device perspectives.
The (Si)GeSn multi-layer heterostructures discussed in this work have a Sn content reaching ~20at.% grown in a chemical vapor deposition (CVD) reactor on 4-inch Silicon wafers, using Monogermane (GeH4) and tin-tetrachloride (SnCl4) precursors, and disilane (Si2H6). By reducing the growth temperature, the Sn content in the alloy is increased, while preserving a high degree of crystal quality for the heterostructure in the topmost Sn-rich layer. No threading dislocations are observed in the upper layers, while misfit and edge dislocations remain mainly confined in the low Sn content layers underneath and in the proximity with the Ge-GeSn interface. By comparing multiple sample series with different strain and composition we demonstrate that the growth rate and the choice of the Ge hydride gas precursor play a limited role in enhancing the incorporation of Sn in the Ge lattice. However, strain minimization and the reduced growth temperature below 350 °C are of paramount importance to achieve Sn-rich GeSn semiconductors.[2,3] Atom probe tomography (APT) measurements will be discussed to address the abruptness of the interfaces in the GeSn multi-layer heterostructure and the composition uniformity.[2] The absence of short-range atomic ordering and Sn precipitates is estimated from the extended statistical APT analyses. Positron annihilation lifetime spectroscopy (PAS) and depth-profiled Doppler broadening measurements will be presented and discussed to elucidate the behavior of point defects in these semiconductors. Based on these analyses, we found that divacancies are the predominant type of point defects in GeSn.[4] Surprisingly, the increase in Sn content in the alloy yields an increase in the concentration of divacancies together with a small reduction in vacancy clusters. The interaction and possible pairing between Sn and vacancies have been proposed to explain the reduced formation of larger vacancy clusters in GeSn with higher Sn content.
Photoluminescence (PL) emission studies will be described. In our GeSn with a Sn content of 18at.% the room temperature PL emission was found to be centered at 0.36 eV (i.e. 3.5 μm wavelength).[2] However, the compressive in-plane strain (-1.3 %) in these GeSn layers reduces the directness of the alloy, leading to a higher energy gap value. By releasing the strain down to -0.2% in the 18 at.% Sn layer using a fully-underetched micro-disk geometry, a 50 meV red-shift of the PL emission energy down to 0.31 eV (i.e. 4.0 μm wavelength) is obtained. Moreover, the strained and relaxed PL emission and reflectance measurements ranging from 300K down to 4K will be shown. These observations will be discussed in the light of photocurrent measurements and photodetector performance.
Finally, we will also discuss the use of GeSn as a platform to develop highly tensile strained Ge quantum wells (QWs. These low-dimensional systems, made possible by the availability of GeSn templates, create a wealth of opportunities to design and fabricate new quantum devices. For instance, unlike the compressively strained QWs, the top of the valence band is of light hole (LH) type under tensile strain, thus corresponding to a much smaller effective mass and ½ spin. These characteristics, combined with high hole mobility and strong spin-orbit coupling in Ge, make tensile strained Ge QWs more attractive for hole spin qubits.

Acknowledgements

The authors thank J. Bouchard for the technical support with the CVD system. O.M. acknowledges support from NSERC Canada (Discovery, SPG, and CRD Grants), Canada Research Chairs, Canada Foundation for Innovation, Mitacs, PRIMA Québec, and Defence Canada (Innovation for Defence Excellence and Security, IDEaS).

References

[1] S. Wirths, D. Buca, and S. Mantl, Prog. Cryst. Growth Charact. Mater. 62, 1 (2016)
[2] S. Assali, J. Nicolas, S. Mukherjee, A. Dijkstra, and O. Moutanabbir, Appl. Phys. Lett. 112, 251903 (2018).
[3] S. Assali, J. Nicolas, and O. Moutanabbir, J. Appl. Phys. 125, 025304 (2019).
[4] S. Assali, M. Elsayed, J. Nicolas, M.O. Liedke, A. Wagner, R. Krause-Rehberg, and O. Moutanabbir, arXiv:1904.09909