Interimpurity Recombinations Involving the Isoelectronic Trap Bismuth in Gallium Phosphide

A new type of pair spectrum involving the radiative recombination of holes bound to Bi isoelectronic traps with electrons bound to remote shallow group-VI donors has been observed in GaP. The existence of these spectra proves that an isoelectronic impurity forms a stable trapping state for a single electronic particle. The binding energy of this state can be calculated from the positions of the pair spectra, and is 40±1 meV for the Bi hole trap in GaP. The prominent phonon sidebands in the Bi-donor pair spectra and in the luminescence spectrum due to recombination from the $J=1\mathrm{}$ Bi bound exciton state are similar, although the latter contains more detailed structure. The relatively large increase in the transition energy with decreasing pair separation $r$, characteristic of donor-acceptor pair spectra, is not observed in the Bi-donor pair spectra, where the first-order Coulomb interaction is zero because the Bi trap is neutral before the hole is captured. Instead, the Bi-donor pair transition energy decreases slightly with decreasing $r$ as a result of the electrostatic polarization interaction between the charged Bi trap and the neutral donor. The low-temperature time decay of both the Bi-donor and donor-acceptor pair spectra in GaP are slow and nonexponential, confirming that remote Bi-donor pairs of widely variable separation are involved. The decay time of the total luminescence is dramatically reduced, and the slow-decaying Bi-donor pair luminescence is quenched relative to the fast Bi-exciton luminescence, when the temperature is increased above ∼15°K. These changes are attributed to the phonon-assisted tunneling of electrons from the donors to Bi traps already filled by holes.