(Invited) Emerging Opto-Electronics and Quantum-Photonics Based on Ultra-Low Temperature Epitaxy of Group-IV Nanolayers
Authors: Brehm Moritz 
Published: August 2024

Germanium has already successfully entered Si technology through electronics and photonics applications [1,2]. However, due to the inherent lattice mismatch between Ge and Si, high-crystalline SiGe epitaxial layers of a sufficient thickness could only be grown for relatively low Ge concentration (<40%). At higher concentrations, the epitaxial SiGe layers relax the strain through either plastic or elastic relaxation. Thus, for about 2% (and 4%) strain between substrate and epilayer, only layer thicknesses of < 2nm (<0.5 nm) can be grown before elastic relaxation leads to quantum dot formation. In principle, such epitaxial Ge-rich QDs on Si(001) substrates can be successfully used to enhance the light emission properties of SiGe material [3-7]. However, planar Ge-rich two-dimensional layers would be preferred for efficient large-scale integration and reliable addressing of the nanostructures.
Here, we demonstrate that ultra-low temperature (ULT) growth, carried out in a temperature window between 100°C and 350°C, is the key to extensive epilayer supersaturation, leading to layer thicknesses that are about one order of magnitude larger than what can be achieved by conventional epitaxy [8]. Thereby, we highlight that molecular beam epitaxy is the suitable choice for ULT growth since, using chemical vapor deposition methods for Si deposition, the ULT range cannot be reached due to the lack of precursor decomposition. However, we also stress that the chamber conditions during the growth need to be excellent in order to limit detrimental point defect formation during epitaxy.
If these preconditions are met, ULT growth can lead to up to now unattainable layer structures. These, in turn, can be used, e.g., for the first available double heterostructure (DHS) light-emitting diodes in the group-IV system. We demonstrate that these LEDs, emitting in the telecom band, work efficiently up to room temperature and above. Light emission above room temperature is eventually limited by minority carrier injection.
Additionally, novel electronic device concepts such as reconfigurable field effect transistors (RFETs) can be realized using several nanometer-thick Ge layers grown directly on silicon on insulator substrates. We argue that these devices have distinct advantages in performance and integration possibilities as compared to conventional nanoelectronics devices based on Ge-rich material, for which typically vertically grown nanowires are employed [9,10].
References
[1] I. A. Fischer, et al., APL Photonics 7, 050901 (2022).
[2] J. Aberl, Opt. Express 27, 32009– 32018 (2019).
[3] M. Grydlik et al., ACS Photonics 3, 298–303 (2016).
[4] M. Grydlik et al., Nano Lett. 16, 6802–6807 (2016).
[5] M. Brehm et al., Nanotechnology 28, 392001 (2017).
[6] P. Rauter et al., ACS Photonics 5, 431-438 (2018).
[7] F. Murphy-Armando et al., Phys. Rev. B 103, 085310 (2021).
[8] A. Salomon et al., Phys. Status Solidi A 219(17), 2200154 (2022).
[9] L. Wind et al., Small 18 (44), 2204178 (2022)
[10] A Fuchsberger et al., Advanced Electronic Materials, 2201259 (2023)