Investigation of Isoelectronic Impurity Absorption in n-GaP

The momentum conserving indirect excitonic transitions from the valence band maximum to the conduction band minima close to the X1 points in the Brillouin zone have been investigated for moderately doped n-type GaP using a novel depth- and time-resolved excess free-carrier-absorption (FCA) technique. Different excitonic singularities due to band-to-band and excitonic absorption are observed near the indirect band gap edge at 77 K. Observations clearly yield signatures for sharp no-phonon absorption lines of excitons bound to neutral sulfur (S) and isoelectronic nitrogen (N) impurities, their complexes and phonon replicas. A parallel investigation also reveals emission lines for exciton bound to S and N as well as to their phonon sidebands. The characteristic energies obtained in our experiments are compared with those reported in literature. Optical nonlinearity at the principle no-phonon exciton absorption A line of isoelectronic nitrogen N is observed.