  The controlled manipulation of the charge carrier concentration in nanometer
thin layers is the basis of current semiconductor technology and of fundamental
importance for device applications. Here we show that it is possible to induce
a persistent inversion from n- to p-type in a 200-nm-thick surface layer of a
germanium wafer by illumination with white and blue light. We induce the
inversion with a half-life of ~12 hours at a temperature of 220 K which
disappears above 280 K. The photo-induced inversion is absent for a sample with
a 20-nm-thick gold capping layer providing a Schottky barrier at the interface.
This indicates that charge accumulation at the surface is essential to explain
the observed inversion. The contactless change of carrier concentration is
potentially interesting for device applications in opto-electronics where the
gate electrode and gate oxide could be replaced by the semiconductor surface.
