  The spin-orbit interaction (SOI) of the two-dimensional hole gas in the
inversion symmetric semiconductor Ge is studied in a strained-Ge/SiGe quantum
well structure. We observed weak anti-localization (WAL) in the
magnetoconductivity measurement, revealing that the WAL feature can be fully
described by the k-cubic Rashba SOI theory. Furthermore, we demonstrated
electric field control of the Rashba SOI. Our findings reveal that the heavy
hole (HH) in strained-Ge is a purely cubic-Rashba-system, which is consistent
with the spin angular momentum mj = +-3/2 nature of the HH wave function.
