Tensilely Strained Germanium Nanomembranes as Infrared Optical Gain Media

Abstract

The use of tensilely strained Ge nanomembranes as mid-infrared optical gain media is investigated. Biaxial tensile strain in Ge has the effect of lowering the direct energy bandgap relative to the fundamental indirect one, thereby increasing the internal quantum efficiency for light emission and allowing for the formation of population inversion, until at a strain of about 1.9% Ge is even converted into a direct-bandgap material. Gain calculations are presented showing that, already at strain levels of about 1.4% and above, Ge films can provide optical gain in the technologically important 2.1-2.5 μm spectral region, with transparency carrier densities that can be readily achieved under realistic pumping conditions. Mechanically stressed Ge nanomembranes capable of accommodating the required strain levels are developed and used to demonstrate strong strain-enhanced photoluminescence. A detailed analysis of the high-strain emission spectra also demonstrates that the nanomembranes can be pumped above transparency, and confirms the prediction that biaxial-strain levels in excess of only 1.4% are required to obtain significant population inversion.

Spin and energy relaxation in germanium studied by spin-polarized direct-gap photoluminescence

“…Finally, the recently discovered lasing action in Ge on Si heterostructures, 9,67 albeit debated, [68][69][70][71] holds the promise of laser sources monolithically integrated onto the mainstream CMOS platform, 10 thus filling the gap for the development of the active devices needed to ground Si-photonics. At present, however, direct-gap electroluminescence [72][73][74][75] and lasing 67 in Ge-based heterostructures have been achieved only under high current densities and shown to be not efficient yet.…”

Accurate strain measurements in highly strained Ge microbridges

“…Reaching such large strain values while retaining crystal integrity is extremely challenging. Several methods are currently being explored [16][17][18][19] and the highest strains are obtained by strain redistribution 13,20 . The attained strains exceed in a radical way the intrinsic strain limits of conventional epitaxial growth 21,22 .…”

Progress towards Spin-Based Light Emission in Group IV Semiconductors

“…Theory furthermore predicts that the direct gap demonstrates a more pronounced strain-induced redshift than the indirect gap, so that for a tensile strain in the (001) plane of about 2% Ge becomes a direct semiconductor with a bandgap amplitude close to 0.5 eV [20]. The extension of the spectral range of Si photonics into the mid-infrared (MIR) range (e.g., 2-5 μm) is highly desired for many applications, such as chemical and biological sensing, medical diagnostics, environmental monitoring, active imaging and free-space laser communications [42][43][44]. It should be noted that the indirect-to-direct crossover has been predicted to occur also in the case of uniaxial stress, even though this requires strain values as high as ~4% along the (001) and (111) crystallographic directions [45,46].…”