Verifying the band gap narrowing in tensile strained Ge nanowires by electrical means.

Group-IV based light sources are one of the missing links towards fully CMOS compatible photonic circuits. Combining both silicon process compatibility and a pseudo-direct band gap, germanium is one of the most viable candidates. To overcome the limitation of the indirect band gap and turning germanium in an efficient light emitting material, the application of strain has been proven as a promising approach. So far, the experimental verification of strain induced bandgap modifications were based on optical measurements and restricted to moderate strain levels. In this work, we demonstrate a methodology enabling to apply tunable tensile strain to intrinsic germanium 〈111〉 nanowires and simultaneously in-situ optical as well as electrical characterization. Combining I/V measurements and µ-Raman spectroscopy at various strain levels, we determined a decrease of the resistivity by almost three orders of magnitude for strain levels of ~ 5%. Thereof, we determined the fundamental band gap narrowing as a result of the strain induced increase of the charge carrier density. We found remarkable accordance to recently published simulation results for moderate strain levels up to 3.6 %. Deviations for ultrahigh strain are discussed with respect to surface reconfiguration and reduced charge carrier lifetime.