  We demonstrate the crossover from indirect- to direct band gap in
tensile-strained germanium by temperature-dependent photoluminescence. The
samples are strained microbridges that enhance a biaxial strain of 0.16% up to
3.6% uniaxial tensile strain. Cooling the bridges to 20 K increases the
uniaxial strain up to a maximum of 5.4%. Temperature-dependent
photoluminescence reveals the crossover to a fundamental direct band gap to
occur between 4.0% and 4.5%. Our data are in good agreement with new
theoretical computations that predict a strong bowing of the band parameters
with strain.
