  We report a magneto-transport study of a two-dimensional hole gas confined to
a strained Ge quantum well grown on a relaxed Si0.2Ge0.8 virtual substrate. The
conductivity of the hole gas measured as a function of a perpendicular magnetic
field exhibits a zero-field peak resulting from weak anti-localization. The
peak develops and becomes stronger upon increasing the hole density by means of
a top gate electrode. This behavior is consistent with a Rashba-type spin-orbit
coupling whose strength is proportional to the perpendicular electric field,
and hence to the carrier density. By fitting the weak anti-localization peak to
a model including a dominant cubic spin-orbit coupling, we extract the
characteristic transport time scales and a spin splitting energy of ~1 meV.
Finally, we observe a weak anti-localization peak also for magnetic fields
parallel to the quantum well and attribute this finding to a combined effect of
surface roughness, Zeeman splitting, and virtual occupation of higher-energy
hole subbands.
