Group IV photonics
A germanium microdisk side coupled to a fiber taper
Germanium pseudo-heterostructure, showing carrier localization in the nanowire
- Develop a CMOS-compatible laser source based on tensile-strained and doped epitaxial germanium on silicon
- Need for CMOS-compatibility for laser sources in optical interconnects
- Germanium can be easily integrated monolithically on silicon
- Can be electrically contacted well
- Understanding optical processes in highly-strained Ge
Germanium is an indirect band gap semiconductor, and as such it is ordinarily a poor optical emitter. However, the indirect band edge is only 0.165 eV below the direct band edge, and this offset can be decreased or even reversed by applying a sufficient tensile strain. Furthermore, even with indirect-gap Ge, n-type doping can be used to fill up states in the indirect valley and make the material quasi-direct-gap. Finding suitable fabrication methods to heavily strain and dope germanium while integrating high-Q optical cavities, minimizing non-radiative recombination, and avoiding free carrier absorption can be difficult, and we are exploring this area in order to address these issues.
Cavity-enhanced emission from highly-strained Ge
- Electroluminescence in Ge-on-Si diodes was demonstrated with heavy n-type doping: Op. Ex. 17, 10019 (2009).
- We also demonstrated cavity coupled photoluminescence from germanium microdisks, collected from a fiber taper: APL
97, 241102 (2010)
- We developed an electrically injected cavity LED that showed direct gap electroluminescence coupled to microdisk whispering gallery modes: App. Phys. Lett. 98, 211101 (2011)
- We achieved >200x increased emission from highly strained Ge nanowires by forming a strain-induced pseudo-heterostructure. We showed both graded and abrupt heterostructure designs on the same chip, achievable with a single lithography step and providing wavelength shifts of up to 500 nm (lithographically controlled): Nano Lett. 13, 7 (2013)
- Recently, we demonstrated high-Q cavity-enhanced direct-band emission from Ge pseudo-heterostructures. We demonstrated quality factors of >2,000 and cavity-enhanced emission over a 500nm wavelength range. To be published.