On the role of thermal strain for micro-Raman determination of carrier concentrations in MOVPE-n-GaN

Doped and nominally undoped epitaxial GaN films grown on sapphire substrates by MOVPE are characterized by micro-Raman, Hall and X-ray diffraction measurements. The role of residual strain in the films for the Raman determination of carrier concentrations and mobilities from LO-phonon-plasmon coupled modes is investigated with respect of superposed phonon frequency shifts. The relaxation of the stress at the cleaved edge was explored and utilized to seperate both origins of frequency shifts. Furthermore, Raman results reveal a free carrier dependence even concerning non-polar and transverse optical phonons.