Spectroscopy of donor-acceptor pairs in nitrogen-doped ZnSe

Selective photoluminescence of donor-acceptor pairs (DAP) is used to study the compensating donors in p-type ZnSe layers grown by molecular-beam epitaxy and doped with nitrogen produced by rf plasma. More than 13 lines are identified in the energy range of optical phonons. We demonstrate that the dominant features are associated with local modes of nitrogen bound phonons. The transitions involving a shallow donor with a binding energy of 29.1+/-0.1 meV are also evidenced. This donor has never been reported before, and, contrary to what it is generally believed, it is not one of the residual impurities usually found in nonintentionally doped layers. Finally, two lines are ascribed to the resonant DAP photoneutralization in the n=2 states of a deep donor. The deep-donor ionization energy is then precisely determined as 45.2+/-0.3 meV.