The photoluminescence of donor-acceptor pairs, under laser excitation, was recorded at 1.8°K. The spectrum is type II, and is attributed to N-Al pairs. Well-resolved lines were identified up to the 80th shell, and the van der Waals and multiple interactions were evaluated, following procedures used for the very similar GaP pair spectra. The limiting photon energy for distant pairs is $h{\nu }_{\infty }=2.0934\mathrm{}$ eV. As in some GaP spectra, intensity anomalies were observed. With the GaP work serving as a guide, one intensity anomaly was interpreted as a channel cutoff, another as a phonon resonance. This permitted us to evaluate the ionization energies, $E_D=118\mathrm{}$ meV for N, and $E_A=\left(179\mathrm{} \mathrm{meV}\right)+E_x$ for Al, with $E_x$ the still unknown exciton binding energy. With lower resolution the maximum and the half-width of the spectrum envelope were measured as a function of excitation intensity, and phonon replicas were recorded. The band maximum at low excitation yields an impurity level of 1.6×${10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$.

• Received 1 July 1970

DOI:https://doi.org/10.1103/PhysRevB.2.4959