  Relating magnetotransport properties to specific spin textures at surfaces or
interfaces is an intense field of research nowadays. Here, we investigate the
variation of the electrical resistance of Ge(111) grown epitaxially on
semi-insulating Si(111) under the application of an external magnetic field. We
find a magnetoresistance term which is linear in current density j and magnetic
field B, hence odd in j and B, corresponding to a unidirectional
magnetoresistance. At 15 K, for I = 10 $\mu$A (or j = 0.33 A/m) and B = 1 T, it
represents 0.5 % of the zero field resistance, a much higher value compared to
previous reports on unidirectional magnetoresistance. We ascribe the origin of
this magnetoresistance to the interplay between the externally applied magnetic
field and the current-induced pseudo-magnetic field in the spin-splitted
subsurface states of Ge(111). This unidirectional magnetoresistance is
independent of the current direction with respect to the Ge crystal axes. It
progressively vanishes, either using a negative gate voltage due to carrier
activation into the bulk (without spin-splitted bands), or by increasing the
temperature due to the Rashba energy splitting of the subsurface states lower
than $\sim$58 k$_B$. The highly developed technologies on semiconductor
platforms would allow the rapid optimization of devices based on this
phenomenon.
