Excitation Spectra and Photo-Ionization of Neutral Mercury Centers in Germanium

The infrared absorption cross section of mercury-doped germanium was measured in wavelength regions in which occur: (1) bound-bound transitions terminating on loosely bound excited states, and (2) photo-ionization to the valence bands. The binding energies of the excited states are quite similar to those observed for the Group III acceptors in germanium. The ground state is split into two levels separated by 0.7 meV. The absorption resonances broaden rapidly above 12°K; lifetime broadening due to phonon absorption is proposed to explain this phenomenon. The integrated absorption cross sections for the resonances predict electric dipole spontaneous lifetimes in the range ${10}^{-4\mathrm{}}$ to ${10}^{-2\mathrm{}}$ sec for the bound-bound transitions. The spectral dependence of the photo-ionization cross section can be fitted by a zero-range square-well potential. Photo-ionization to the spin-orbit split-off valence band was observed. There was no evidence for optical-phonon-assisted transitions.