Structural characterization of Al_0.55Ga_0.45N epitaxial layer determined by high resolution x-ray diffraction and transmission electron microscopy

Structural characteristics of Al_0.55Ga_0.45N epilayer were investigated by high resolution x-ray diffraction (HRXRD) and transmission electron microscopy (TEM); the epilayer was grown on GaN/sapphire substrates using a high-temperature AlN interlayer by metal organic chemical vapor deposition technique. The mosaic characteristics including tilt, twist, heterogeneous strain, and correlation lengths were extracted by symmetric and asymmetric XRD rocking curves as well as reciprocal space map (RSM). According to Williamson-Hall plots, the vertical coherence length of AlGaN epilayer was calculated, which is consistent with the thickness of AlGaN layer measured by cross section TEM. Besides, the lateral coherence length was determined from RSM as well. Deducing from the tilt and twist results, the screw-type and edge-type dislocation densities are 1.0×10⁸ cm^-2 and 1.8×10¹⁰ cm^-2, which agree with the results observed from TEM.