Deep hole traps in n -type liquid encapsulated Czochralski GaP

Cathodoluminescence and photoluminescence time-response measurements have been used to study the prominent luminescence bands in n-type GaP grown by the liquid-encapsulated Czochralski (LEC) method. In as-grown crystals, two deep hole traps are identified. An isoelectronic hole trap, ≃0.4 eV above the valence band edge, produces red emission (1.72 eV) due to bound-exciton and pair recombination, while infrared luminescence (1.51 eV) is produced by pair recombination between a deep (≃0.7 eV) acceptorlike center and the donor dopant. In certain heat-treated samples, an additional structured luminescence band is produced at 2.07 eV, which also apparently results from recombination at an isoelectronic center. Resolution of the no-phonon lines of this emission suggests that an exciton is bound by 186 meV to the impurity center created by annealing. Although no positive identification of the chemical nature of these defects has been possible, we find that our results are not inconsistent with previously suggested models in which group-VI donors are complexed with gallium vacancies. Comparisons are also made with similar luminescence bands and defect levels observed in GaP grown by lower-temperature methods. We conclude that the hole traps studied here are not unique to the LEC material, but are apparently greatly enhanced by the LEC growth conditions.