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Here we show by phonon spectroscopy with superconducting tunnelling junctions that one-phonon excitations from the ground state to the conduction band (CB) are measurable as phonon induced conductivity (PIC) changes in the special case of donors in Ge. For the conductivity thresholds we find somewhat smaller binding energies than evaluated from optical measurements whereas for the 1s-singlet to 1s-triplet ground state splitting as seen by elastic phonon scattering we obtain the optical values.

One-phonon ionization of donors in germanium via intervalley scattering by slow TA phonons at the X point of the Brillouin zone is demonstrated by the stress dependence of phonoconductivity. Superconducting aluminum junctions are used as tunable quasimonochromatic phonon sources up to these extreme frequencies. The ionization probability is found to be increased when donor levels associated with stress-shifted higher valleys of the conduction band cross the lowest valley(s).