Multi-layer GaP wafers with different luminescence efficiency were investigated using microscopical optical methods. According to an interaction between the free carrier plasma and on the longitudinal optical lattice vibrations via their microscopical electric fields in GaP

an mixed plasmon-phonon character is appeared. The carrier distribution has been determined from the fitting of the theoretical line shape to the observed spectra of the plasmon-LO-phonon coupled modes in the cross section of GaP diodes. The optical determination of electrical parameters has some advantages

it is not necessary to contact sample and the spatial resolution amounts to about 1 m. The obtained results are in good agreement with those of electrical measurements. We have performed a synchronistic measurement in the cross section of GaP wafer by microscopical photoluminescence. The experiment demonstrates that there are different deep levels and red emissions in GaP substrate and p layer. The deep level in p local area has an obvious influence on PL intensity-distance curve and on the luminescence efficiency. Therefore

it is important to improve red emission and to control deep level from growth condition. The result showed that the carrier distribution obtained from microscopical Raman spectra is related to the curve of PL intensity-distance using microscopical photoluminescence. They argee reasonably with the distribution of doped impurities and existent defects in GaP substrate and epitaxial layers. It comes to the conclusion that the scanning Raman and photoluminescence technique is a powerful tool for the characterizations of GaP epitaxial layers as well as the diagnostics of LED devices.