New Radiative Recombination Processes Involving Neutral Donors and Acceptors in Silicon and Germanium

Relatively weak extrinsic luminescence bands observed at low temperatures (≲80°K) in silicon doped with several different donors and acceptors are described and identified with the following recombination processes: (1) Phonon-assisted decay of excitons bound to neutral centers involving the momentum-conserving (MC) transverse acoustical phonon; also multiphonon transitions involving the zone-center optical phonon together with the MC transverse optical phonon. The phonon energies and relative intensities of these replicas are shown to be very similar to those observed in the intrinsic luminescence bands in silicon. The relationship between the relative strength of the no-phonon band and the impurity ionization energy is discussed. (2) "Two-electron" transitions, in which an exciton bound to a neutral impurity center decays, leaving the impurity center in a bound excited state. (3) "Two-electron" transitions, observed only for donor centers, in which a free exciton in the vicinity of a neutral donor center decays, leaving the donor in an excited valley-orbit state. (4) Radiative recombination of a free hole (or electron) with the second electron (or hole) in a newly observed ${\mathrm{H}}^{-}$-like complex involving the shallow donor and acceptor centers. (5) Radiative recombination of a free electron with a neutral acceptor center, observed only for the relatively deep acceptors gallium and indium. Processes (1) and (3) are significant only below ∼50°K, while transitions (4) and (5) are observed only above ∼30°K. Process (3) is significant at all temperatures investigated. Unidentified lines reported by Benoit à la Guillaume and Parodi in the low-temperature edge luminescence of doped germanium are shown to be consistent with mechanisms (2) and (3), while other lines previously discussed by them in terms of mechanism (5) are in closer agreement with mechanism (4). These "two-electron" transitions in silicon and germanium all appear to contain donor or acceptor excitations involving no parity change, in contrast to similar transitions in gallium phosphide. The ionization energies of the ${\mathrm{H}}^{-}$-like complexes in silicon and germanium are ∼4 meV and ∼1.5 meV, respectively.