Indirect transitions, free and impurity-bound excitons in gallium phosphide: A revisit with modulation and photoluminescence spectroscopy

The momentum conserving indirect excitonic transitions, from the ⌫ 15 valence band maximum to the conduction band minima close to the X 1 point in the Brillouin zone have been measured for GaP in piezo-modulated transmission. At 6 K, excitonic signatures due to phonon emission are observed at E gx ϩប␻ ph for TA͑X͒, LA͑X͒, and TO͑X͒ phonons ͑E gx ϭfree exciton band gap͒, whereas at 120 K signatures for both absorption and emission of LA͑X͒ and TA͑X͒ phonons appear. These observations yield E gx ϭ2.3301(4) eV at 6 K. In several GaP specimens, signatures A and/or C for excitons bound to sulfur ͑S͒ and/or nitrogen ͑N͒ impurities, respectively, are observed in the piezo-modulated transmission. A parallel investigation of the spectra of recombination radiation reveals emission lines for excitons bound to S and N as well as their phonon sidebands. The phonon replicas of N consist of sharp lines in combination with the zone center optical phonons observed in the first order Raman spectrum ͑LO ⌫ and TO ⌫). In addition, broader replicas are observed for the A line in combination with acoustic and optical phonon branches ͑A-LA, A-TA, A-X͒. The phonon energies obtained from both piezo-modulation and photoluminescence experiments are compared with those reported in the literature. Finally, the suppression of S diffusion from a GaP substrate into a GaP epilayer achieved with an intervening GaP/AlGaP superlattice is demonstrated in both modulation and photoluminescence experiments.