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 (1.530–1.565μm)+L (1.565–1.625 μm) 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 SiNx external stressor on a narrow Ge strip induces an increased blue shift or an opposite red shift, depending on the stress polarity in SiNx. The results show that it is possible to tune the operating wavelength of modulators and photodetectors of elemental Ge in the C+L band.

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