(Invited) Epitaxy of Direct Bandgap Group IV Si-Ge-Sn Alloys towards Heterostructure Light Emitters
Authors: von den Driesch Nils , Stange Daniela , Rainko Denis , Povstugar Ivan , Breuer Uwe , Ikonic Zoran , Hartmann Jean-Michel , Schubert Markus Andreas , Capellini Giovanni , Sigg Hans , Mantl Siegfried , Grützmacher Detlev , Buca Dan 
Published: February 2020

Silicon (Si)-based group IV materials form the backbone of our digital society, being found in every smartphone and tablet. They, however, lack the possibility to efficiently emit and detect light, because of their indirect bandgap. A truly Si-compatible direct bandgap material allows merging of state-of-the-art CMOS technology on-chip with photonic components to tackle bottlenecks of modern electronics by radically reducing the required power consumption or increasing the available data bandwidth [1]. Alloying the group IV elements germanium (Ge) and tin (Sn) has turned out to be a viable solution, as a direct bandgap semiconductor – even suitable for lasing – is formed for relaxed epilayers with Sn incorporation above ∼9 at.% [2]. The value of the bandgap can further be controlled by adding Si into GeSn, which can be exploited for carrier confinement inside heterostructures [3].
In this contribution, we discuss the epitaxy of various Sn-based group IV alloys. Different heterostructure designs combine direct bandgap active layers, ternary barriers and a GeSn buffer technology. In Figure 1a, a TEM micrograph of a SiGeSn/GeSn/SiGeSn double heterostructure (DHS) is presented, overlapped with the elemental distribution taken by energy-dispersive X-Ray spectroscopy (EDX). A Sn incorporation of roughly 14.5 at.% ensures a direct bandgap inside the active region. Another approach facilitates direct bandgap GeSn multi quantum wells (MQW), sandwiched between SiGeSn barriers. Atom Probe Tomography (APT) allows to obtain precise elemental distribution within this complex heterostructure stack, as is demonstrated in Figure 1b. Techniques such as temperature-dependent photoluminescence are used to compare the optical properties of the grown heterostructures. Moreover, the impact of quantum confinement on MQW emission is revealed, backed up by bandstructure calculations.
Optically pumped microdisk resonators are formed to demonstrate lasing from different types of heterostructures, as shown in Figure 1c. As depicted in the inset, the superiority of MQW structures over DHSs (and bulk GeSn layers) is demonstrated, leading to an order of magnitude reduced lasing thresholds at cryogenic temperatures.
In the next step of integrated light emitters, carriers need to be injected electrically, rather than optically. We will show first experiments on epitaxially grown p-i-n heterostructure diodes. In Figure 1d, incorporation of phosphorus and boron for the formation of n- and p-doped carrier injection layers is shown by SIMS. We will further point towards possible improvements that may pave the way towards future energy efficient opto-electronic integrated circuits (OEICs).
[1] L. Vivien, Computer technology: Silicon chips lighten up, Nature. 528 (2015) 483–484.
[2] S. Wirths, R. Geiger et al., Lasing in direct-bandgap GeSn alloy grown on Si, Nat. Photonics. 9 (2015) 88–92.
[3] N. von den Driesch et al., Advanced GeSn/SiGeSn Group IV Heterostructure Lasers, Adv. Sci. (2018). doi:10.1002/advs.201700955.

Figure 1