Temperature dependence of Raman scattering in porous gallium phosphide

Porous layers fabricated by electrochemical anodization of (111)A-oriented n-GaP:Te substrates were studied by Raman scattering spectroscopy in the temperature interval from 10 to 300 K. Along with the transverse-optical (TO) and longitudinal-optical (LO) modes, the RS spectra of porous layers show Frohlich-type vibrations located in the frequency gap between the bulk optical phonons, A longitudinal-transverse splitting of these surface-related vibrations was evidenced at low temperatures. Apart from that, the porous layers prepared on highly doped substrates were found to show LO-phonon-plasmon coupled (LOPC) modes in the whole temperature interval studied. Observation of LOPC modes at low temperatures is explained taking into account that the GaP skeleton consists of both depleted surface layers surrounding the pores and conductive regions. The free electrons in these regions, originating from the impurities actually located in the depletion layers, are shown to be subject to spatial confinement increasing with decreasing temperature.