Excitonic Molecule Bound to the Isoelectronic Nitrogen Trap in GaP

Two sets of sharp emission lines, associated with the photoluminescence spectrum of the isoelectronic nitrogen trap in GaP, are unambiguously identified as the recombination of a second exciton bound to this center with an energy of 10 meV. This is the first observation of an excitonic molecule bound at a defect. The two electrons and two holes within this complex combine to form two antisymmetric states of angular momentum $J_t=0\mathrm{}$ and $J_t=2\mathrm{}$. The $J_t=2\mathrm{}$ state is split 0.16 meV by the cubic crystal field, and the $J_t=0\mathrm{}$ state lies 0.17 meV above the center of gravity of this doublet. Transitions from these states to the $A$ and $B$ states of a single exciton bound to nitrogen are seen. The complex Zeeman splittings predicted by this model agree in detail with experiment. The excitonic molecule is stable at low temperature; at 1.5°K, the excitonic molecule emission lines increase as the square of the single exciton intensity with increase in pumping power. However, nonradiative Auger recombination reduces the over-all nitrogen emission by a factor of 3 below the intensity at 4.2°K. The binding energy of the second exciton at the nitrogen trap is nearly equal to that of the single exciton. This remarkable fact may be possible only for an isoelectronic trap.