Direct-bandgap light-emitting germanium in tensilely strained nanomembranes.

Silicon, germanium, and related alloys, which provide the leading materials platform of electronics, are extremely inefficient light emitters because of the indirect nature of their fundamental energy bandgap. This basic materials property has so far hindered the development of group-IV photonic active devices, including diode lasers, thereby significantly limiting our ability to integrate electronic and photonic functionalities at the chip level. Here we show that Ge nanomembranes (i.e., single-crystal sheets no more than a few tens of nanometers thick) can be used to overcome this materials limitation. Theoretical studies have predicted that tensile strain in Ge lowers the direct energy bandgap relative to the indirect one. We demonstrate that mechanically stressed nanomembranes allow for the introduction of sufficient biaxial tensile strain to transform Ge into a direct-bandgap material with strongly enhanced light-emission efficiency, capable of supporting population inversion as required for providing optical gain.

Strain Engineering of Germanium Nanobeams by Electrostatic Actuation.

Germanium (Ge) is a promising material for the development of a light source compatible with the silicon microfabrication technology, even though it is an indirect-bandgap material in its bulk form. Among various techniques suggested to boost the light emission efficiency of Ge, the strain induction is capable of providing the wavelength tunability if the strain is applied via an external force. Here, we introduce a method to control the amount of the axial strain, and therefore the emission wavelength, on a suspended Ge nanobeam by an applied voltage. We demonstrate, based on mechanical and electrical simulations, that axial strains over 4% can be achieved without experiencing any mechanical and/or electrical failure. We also show that the non-uniform strain distribution on the Ge nanobeam as a result of the applied voltage enhances light emission over 6 folds as compared to a Ge nanobeam with a uniform strain distribution. We anticipate that electrostatic actuation of Ge nanobeams provides a suitable platform for the realization of the on-chip tunable-wavelength infrared light sources that can be monolithically integrated on Si chips.

Strained-germanium nanostructures for infrared photonics.

The controlled application of strain in crystalline semiconductors can be used to modify their basic physical properties to enhance performance in electronic and photonic device applications. In germanium, tensile strain can even be used to change the nature of the fundamental energy band gap from indirect to direct, thereby dramatically increasing the interband radiative efficiency and allowing population inversion and optical gain. For biaxial tension, the required strain levels (around 2%) are physically accessible but necessitate the use of very thin crystals. A particularly promising materials platform in this respect is provided by Ge nanomembranes, that is, single-crystal sheets with nanoscale thicknesses that are either completely released from or partially suspended over their native substrates. Using this approach, Ge tensilely strained beyond the expected threshold for direct-band gap behavior has recently been demonstrated, together with strong strain-enhanced photoluminescence and evidence of population inversion. We review the basic properties, state of the art, and prospects of tensilely strained Ge for infrared photonic applications.