Residual thermal strain in thick GaN epifilms revealed by cross-sectional Raman scattering and cathodoluminescence spectra

Strain can significantly alter the physical properties of a solid.We demonstrate that in a thick GaN epilayer there exists a residual thermal strain along the growth direction.This result is clearly revealed by cathodoluminescence spectra, in which the band gap of the GaN film decreases with distance away from the epifilm-substrate interface.This result is further confirmed by Raman scattering spectra in which the phonon modes show a red shift along the growth direction.Our finding is important for the understanding and application of nitride semiconductors.