Three Holes Bound to a Double Acceptor:Be+in Germanium

A double acceptor binding three holes has been observed for the first time with photoconductive far-infrared spectroscopy in beryllium-doped germanium single crystals. This new center, ${\mathrm{Be}}^{+}$, has a hole binding energy of ∼5 meV and is only present when free holes are generated by ionization of either neutral shallow acceptors or neutral Be double acceptors. The ${\mathrm{Be}}^{+}$ center thermally ionizes above 4 K. It disappears at a uniaxial stress ≳${10}^9$ dyn ${\mathrm{cm}}^{-2\mathrm{}}$ parallel to [111] as a result of the lifting of the valence-band degeneracy.