Direct Bandgap Control by Narrowing the Germanium Strip Structure on Silicon for C+L Band Photonic Devices

This study reports the bandgap engineering of a Ge epitaxial layer on Si to tune the operating wavelength of optical intensity modulators and photodetectors in the C (1.530–1.565
$\mu \text{m}$
)+L (1.565–1.625
$\mu \text{m}$
) band. A strip structure of elemental Ge is investigated, rather than wider-gap SiGe or narrower-gap GeSn alloy, to achieve the key property of a C band modulation and improved L band detection. By narrowing the strip to the submicron scale, a tensile lattice strain in Ge, induced by a thermal expansion mismatch with Si, is elastically relaxed by an edge-induced relaxation effect. The photoluminescence peak and photodetection spectra show a significant blue shift as the narrowed direct gap of ~0.77 eV is restored to 0.80 eV of unstrained Ge. A standard SiN
_x
external stressor on a narrow Ge strip induces an increased blue shift or an opposite red shift, depending on the stress polarity in SiN
_x
. The results show that it is possible to tune the operating wavelength of modulators and photodetectors of elemental Ge in the
$\text{C}+\text{L}$
band.