Investigation of longitudinal optical phonon-plasmon coupled modes in epitaxial GaN films on sapphire substrate

We have studied the longitudinal optical phonon-plasmon coupled (LPP) modes in device quality GaN expitaxy thin films deposited on sapphire substrate using infrared reflection spectroscopy. By theoretical calculation and fitting with the experimental LR reflection spectra for a series of Si doped GaN epilayers we obtain the phonon vibration parameters and the plasmon frequency and damping constant in GaN. The LPP modes have been deduced by the imaginary parts of the reciprocal dielectric function in IR reflection measurement. Both high and low branches of the LO phonon-plasmon coupling mode (LPP+ and LPP-) are well resolved. With the increase of doping level, the frequency of the LPP+ mode shifts to higher while that of the LPP- mode trends toward TO phonon of GaN. The carrier concentration and mobility in GaN have also been deduced. The IR measured concentration of carriers coincides with the Hall measurement. The mobility ratio of Hall to IR is about 1.75 similar to1.96, implying a dominating mechanism of ionized impurity scattering. Raman measurement has also been performed on the same series of samples. The spectra reveal the similar behavior of the LPP modes to that in IR measurement. Our results show that both IR reflection and Raman scattering are powerful and nondestructive tools to investigate the carrier properties in GaN thin films.