  We present ab initio calculations of electronic and optical properties of
perturbed hexagonal germanium and demonstrate that it is a superior material
for active optoelectronic devices in the infrared spectral region. It is known
that perfect lonsdaleite Ge is a pseudodirect semiconductor, i.e., with direct
fundamental band gap but almost vanishing oscillator strength for the
lowest-energy optical transitions. Perturbing the system by replacing a Ge atom
in the unit cell with a Si atom boosts of the oscillator strength at the
minimum direct gap by orders of magnitude, with a concurrent blue shift of the
interband distances. This effect is mainly due to the increased s character of
the lowest conduction band because of the perturbation-induced wave function
mixing. A purely structural modification of the lonsdaleite unit cell of
hexagonal Ge yields as well increased optical oscillator strengths, but their
magnitude significantly depends on the actual details of the atomic geometry.
In particular, moderate tensile uniaxial strain can induce an inversion of the
order of the two lowest conduction bands, immediately leading to an extremely
efficient enhancement of optical transitions. In general, chemical and/or
structural perturbations of the lonsdaleite lattice are shown to be the key to
make hexagonal germanium suitable for light emitting devices.
