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 Gamma (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-mod ulated transmission. At 6 K, excitonic signatures due to phonon emissi on are observed at E-gx + (h) over bar omega(ph) for TA(X), LA(X), and TO(X) phonons (E-gx = free exciton band gap), whereas at 120 K signat ures for both absorption and emission of LA(X) and TA(X) phonons appea r. These observations yield E-gx = 2.3301(4) eV at 6 K. In several GaP specimens, signatures A and/or C for excitons bound to sulfur (S) and /or nitrogen (N) impurities, respectively, are observed in the piezo-m odulated transmission. A parallel investigation of the spectra of reco mbination 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 ob served in the first order Raman spectrum (LOGamma and TOGamma). In add ition, broader replicas are observed for the A line in combination wit h acoustic and optical phonon branches (A-LA, A-TA, A-X). The phonon e nergies obtained from both piezo-modulation and photoluminescence expe riments are compared with those reported in the literature. Finally, t he suppression of S diffusion from a GaP substrate into a GaP epilayer achieved with an intervening GaP/AlGaP superlattice is demonstrated i n both modulation and photoluminescence experiments. (C) 1997 American Institute of Physics. [S0021-8979(97)04421-6].