Abnormal Raman spectra of PbTe crystalline thin films grown by molecular beam epitaxy

Abstract

High quality single crystalline PbTe thin films have been grown on BaF2(111) by using molecular beam epitaxy(MBE).Raman scattering measurements have revealed the vibration modes of oxide on the surface，longitudinal optical(LO) p honon at Brillouin center q≈0，and coupled plasmon-phonon.With the variation of the focusing-depth(FD) of the pumping laser beam，the integrated intensities，p eak positions and line widths of the observed Raman peaks showed different behav iors.When FD changed from 3μm above the top-surface to 3μm below the bottom-su rface, the PbTe LO phonon frequency shifted from 119 to 124cm-1.Diffe rent Raman scattering results were also observed by comparison of the freshly su rface-etched and the as grown PbTe samples. These phenomena originated from the different vibration modes. The frequency shift of LO phonon is explained by the fact that the strain relaxation caused by the lattice mismatch between PbTe and BaF2 is as high as 4.2％.