(Invited) Hexagonal Silicon Germanium As a Direct Bandgap Light Source

Abstract

All SiGe alloys in the cubic crystal phase have an indirect bandgap. Therefore, SiGe alloys are typically considered as unusable as a light source. However, if the Si 1-x Ge x atoms are arranged in a hexagonal crystal structure, the bandgap becomes direct for x>65%. The emission occurs with a nanosecond lifetime, and is tuneable from 1.8 to 3.4um by varying the SiGe composition. The hex-SiGe alloys have a relatively large separation between the direct and indirect conduction band minima, which makes it stand out as a Group IV light source [1]. The hex-SiGe alloys are realized as shells in a core-shell nanowire geometry, using wurtzite GaAs cores as epitaxial templates to copy the hexagonal stacking to the SiGe shells. Recently, photoluminescence studies of these core-shell nanowires resulted in the observation of amplified spontaneous emission (ASE) from hex-Si 0.2 Ge 0.8 , highlighting a positive material gain in this material system. The positive material gain is enabled by the direct bandgap of hex- Si 0.2 Ge 0.8 . Moreover, we have grown nanowires with multiple shells of different Si 1-x Ge x compositions. Thus, these shells contain heterostructures of hex-Si 1-x Ge x , having a type-I band alignment, allowing us to create hex-Ge/Si 0.2 Ge 0.8 quantum wells with light emission up to room temperature. The observation of ASE, in combination with the ability to create type I QWs from hex-Si 1-x Ge x , pave the way towards a laser based on hex-Si 1-x Ge x alloys. [1] Fadaly, E. M. T., Dijkstra, A., Suckert, J. R. et al (2020). Direct-bandgap emission from hexagonal Ge and SiGe alloys. Nature , 580 (7802), 205-209.