| Inorganic-semiconductor nanomembranes (NMs), i.e., single-crystal sheets with sub-micron thicknesses, have recently emerged as a promising materials platform for a wide range of fundamental studies and device applications. In the context of group-IV semiconductor optoelectronics, a particularly compelling function is the formation of direct-bandgap germanium in mechanically stressed NMs for optical applications. Similar to most other group-IV semiconductors, unstrained Ge has an indirect fundamental energy bandgap, which results in exceedingly low radiative efficiency. Extensive calculations, however, have shown that tensile strain can be used to lower the direct conduction band edge relative to the indirect one, to the point that large optical gain can be established under practical pumping conditions. The fundamental bandgap even becomes direct if the strain exceeds a certain threshold that is generally found to be 1.7%-2% biaxial. In this dissertation, the introduction of 1.8% biaxial strain has been confirmed in sufficiently thin Ge NMs, as well as the emission enhancement and red shift observed in the photolumines-cence measurements. An optical cavity is fabricated and integrated with the Ge NM, in order to enhance the emission intensity in the mid-infrared range. A lateral P-I-N diode is fabricated based on the Ge NMs, to attempt to observe for the first-time electroluminescence as a function of strain in Ge. Finally, I investigate ways to increase the maximum strain that a Ge NM of a given thickness can tolerate. These ways include improving the structural quality of Ge NMs, improving NM releasing methods, and using graphene to protect the Ge NM surfaces from ambient attack. Some improvement in strain tolerance is noted. |
