Mechanical tensile strain engineering of Ge for gain achievement

We show that the recombination energy of the direct band gap photoluminescence of germanium can be controlled by an external mechanical stress. The deformation is provided by an apparatus used for blister test. This strain leads to a significant change of the room temperature direct band gap recombination of germanium. An energy red-shift up to 60 meV is demonstrated for the room temperature photoluminescence of a thin germanium membrane (125 nm wavelength shift from 1535 to 1660 nm). This photoluminescence shift is correlated to the inplane tensile strain present in the film. A biaxial tensile strain larger than 0.6 % is achieved by this method. The wavelength shift is correlated to the predicted band gap shift as obtained from a 30 band k. p formalism. An excellent agreement is obtained between the experimental band gap shift and the theoretical one. This mechanical deformation allows to approach the direct band gap condition for germanium. We will discuss the possibility to achieve lasing with n-doped layers and the application of an external mechanical stress.