Van der Waals epitaxy of GeSn quantum dots on MoS2 enabled by oxygen-plasma pretreatment

GeSn quantum dots (QDs) have garnered much attention as a candidate for monolithically optoelectronic integration due to its direct bandgap and full compatibility with CMOS technology. However, it is still a big challenge to make GeSn QDs because of the low solubility of Sn in Ge and too thick coherently strained GeSn layer grown on Ge or Si substrates at low temperature. In this Letter, van der Waals epitaxy of GeSn QDs with high Sn content of 9.2% on oxygen-plasma modified two-dimensional MoS2 on SiO2/Si substrate is demonstrated using magnetron sputtering. The growth kinetics is proposed, in which oxygen-plasma modification introduces sulfur vacancies on the MoS2 surface, mediating an Sn wetting layer to promote Ge nucleation to form GeSn QDs. The transitions between GeSn QDs and thin films and the precise control over quantum dot density and morphology are performed by simply tuning the sputtering power and durations. As a result, GeSn QDs with high aspect ratio of 1.2 are achieved with excellent crystalline quality. This work provides an intriguing way to construct GeSn QDs on 2D materials for optoelectronic device applications.