Effects of Fe doping on the strain and optical properties of GaN epilayers grown on sapphire substrates

The effects of Fe doping on a series of Fe-doped GaN epilayers with different doping concentrations grown on sapphire substrates were investigated in detail by confocal micro-Raman spectroscopy under the back-scattering geometric configuration. Careful investigation of the Ehigh2 and A1(LO) modes of the Fe- and Si-doped epilayers as well as the intentionally undoped free-standing GaN reveals that the compressive residual strain in the Fe-doped GaN epilayers tends to relax as the Fe concentration increases. This finding is further supported by X-ray diffraction measurements. The relaxation of compressive residual strain is most likely due to compensation by the tensile strain induced by incorporation of iron atoms in the GaN epilayers. The influence of Fe doping on the background electron concentration was also discussed by analyzing the upper branch of the A1(LO)–plasmon coupled mode.