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.

Tensilely strained germanium nanomembranes as infrared optical gain media.

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.

Distinct Nucleation and Growth Kinetics of Amorphous SrTiO3 on (001) SrTiO3 and SiO2/Si: A Step toward New Architectures.

Integration of emerging complex-oxide compounds into sophisticated nanoscale single-crystal geometries faces significant challenges arising from the kinetics of vapor-phase thin-film epitaxial growth. A comparison of the crystallization of the model perovskite SrTiO3 (STO) on (001) STO and oxidized (001) Si substrates indicates that there is a viable alternative route that can yield three-dimensional epitaxial synthesis, an approach in which STO is crystallized from an amorphous thin film by postdeposition annealing. The crystallization of amorphous STO on single-crystal (001) STO substrates occurs via solid-phase epitaxy (SPE), without nucleation and with a temperature-dependent amorphous/crystalline interface velocity. In comparison, the crystallization of STO on SiO2/(001) Si substrates requires nucleation, resulting in a polycrystalline film with crystal sizes on the order of 10 nm. A comparison of the temperature dependence of the nucleation and growth processes for these two substrates indicates that it will be possible to create crystalline STO materials using low-temperature crystallization from a crystalline seed, even in the presence of interfaces with other materials. These processes provide a potential route for the formation of single crystals with intricate three-dimensional nanoscale geometries.

Electronic Transport Properties of Epitaxial Si/SiGe Heterostructures Grown on Single-Crystal SiGe Nanomembranes.

To assess possible improvements in the electronic performance of two-dimensional electron gases (2DEGs) in silicon, SiGe/Si/SiGe heterostructures are grown on fully elastically relaxed single-crystal SiGe nanomembranes produced through a strain engineering approach. This procedure eliminates the formation of dislocations in the heterostructure. Top-gated Hall bar devices are fabricated to enable magnetoresistivity and Hall effect measurements. Both Shubnikov-de Haas oscillations and the quantum Hall effect are observed at low temperatures, demonstrating the formation of high-quality 2DEGs. Values of charge carrier mobility as a function of carrier density extracted from these measurements are at least as high or higher than those obtained from companion measurements made on heterostructures grown on conventional strain graded substrates. In all samples, impurity scattering appears to limit the mobility.

Quadriceps femoris muscle resistance to fatigue using an electrically elicited fatigue test following intense endurance exercise training.

BACKGROUND AND PURPOSE: Electrical stimulation has been used to assess skeletal muscle resistance to fatigue. The purpose of this study was to test the hypothesis that 12 weeks of intense endurance exercise training on a bicycle ergometer would reduce the percentage of decline in quadriceps femoris muscle torque during an electrically elicited fatigue test. SUBJECTS AND METHODS: Eleven nondisabled subjects performed 12 weeks of high-intensity endurance exercise training, and 6 subjects served as controls and did not exercise. Two electrically elicited fatigue tests, one with and one without prior voluntary fatiguing exercise, were administered to each subject before and after the 12-week training period. RESULTS: The percentage of decline in peak torque of the quadriceps femoris muscle over 50 electrically elicited muscle contractions did not change as a result of endurance exercise training, despite significant improvements in maximal oxygen consumption and quadriceps femoris muscle endurance. The recovery of maximal isometric torque immediately after exhausting voluntary exercise followed by electrical stimulation was significantly greater after 12 weeks of intense exercise training. CONCLUSION AND DISCUSSION: The percentage of decline in peak torque during an electrically elicited fatigue test does not detect improvements in quadriceps femoris muscle endurance induced by endurance exercise training. The percentage of initial torque recovered immediately after fatiguing exercise, however, is improved by endurance training. [Sinacore Dr, Jacobson RB, Delitto A. Quadriceps femoris muscle resistance to fatigue using an electrically elicited fatigue test following intense endurance exercise training.