Raman-active modes of porous gallium phosphide at high pressures and low temperatures

Porous gallium phosphide (GaP) with a honeycomb-like morphology and a skeleton relative volume concentration c = 0.7 was investigated by Raman spectroscopy under pressure up to 10 GPa at T = 5 K. The porous samples were prepared by electrochemical etching. The transverse optical (TO) and longitudinal optical (LO) mode frequencies were found to shift with pressure similarly to those of bulk GaP. As in bulk GaP, the TO feature of the porous GaP exhibits a pressure-induced narrowing which is interpreted in terms of a Fermi resonance. The scattering intensity observed on the low-frequency side of the LO mode is attributed to surface-related Frohlich mode scattering. The latter results are interpreted on the basis of an effective medium expression for the dielectric function. The Raman spectra indicate that both the morphology and degree of porosity are unaffected by pressure in the range investigated.