Excited states of acceptor-like excitons bound to isolated nitrogen in GaAs

We report on the first observation of the excited states of the hole for acceptor-like excitons bound to isolated nitrogen impurities (N $\cong $ 2 x 10$^{18 }$cm$^{-3})$ in GaAs under pressure. A large absorption-resonance in both transmission and photoluminescence excitation (PLE) spectroscopy leads to identification of the 2S$_{3/2}$ excited-hole-state associated with the ground-state nitrogen isoelectronic bound exciton, known as the N$_{X}$ state from alloy studies. Comparison to the established EM-theory of Baldereschi and Lipari for excited-state acceptors provides good agreement. We thus deduce the hole ground-state ionization energy, finding 19.2 meV -- virtually independent of pressure. These data then also provide direct information about the remaining electron binding energy at isolated N in GaAs, which we find to vary from 0 (level degeneracy with the $\Gamma _{1}$-edge) to some 135 meV, depending upon pressure. These data provide further understanding of the ``deep-level'' isoelectronic N-trap in GaAs and its alloys, including the here-to-fore elusive N$_{\Gamma }$state.