  Strain engineering has proven to be vital for germanium-based photonics, in
particular light emission. However, applying a large permanent biaxial strain
to germanium has been a challenge. We present a simple, CMOS-compatible
technique to conveniently induce a large, spatially homogenous strain in
microdisks patterned within ultrathin germanium nanomembranes. Our technique
works by concentrating and amplifying a pre-existing small strain into the
microdisk region. Biaxial strains as large as 1.11% are observed by Raman
spectroscopy and are further confirmed by photoluminescence measurements, which
show enhanced and redshifted light emission from the strained microdisks. Our
technique allows the amount of biaxial strain to be customized
lithographically, allowing the bandgaps of different microdisks to be
independently tuned in a single mask process. Our theoretical calculations show
that this platform can deliver substantial performance improvements, including
a >200x reduction in the lasing threshold, to biaxially strained germanium
lasers for silicon-compatible optical interconnects.
