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.