Radiative recombination of excitons bound to beryllium double acceptors in germanium

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Abstract
Beryllium-doped germanium single crystals exhibit at low temperatures novel sharp-line exciton spectra at photon energies essentially lower than those familiar from studies of excitons localised at shallow donors or acceptors. The spectra consist of two strong principal lines and two weak lines. This structure is observed in no-phonon luminescence as well as when momentum-conserving TA, LA, LO and TO phonons participate. The authors study the lines as a function of excitation power, temperature, doping level and under uniaxial stress along (001). The results strongly suggest that three of the lines are due to multiple bound excitons localised at neutral beryllium acceptors, and one line originates in an excited state of the (single) bound exciton.