Cathodoluminescence and Al composition of biaxially stressed AlGaN/GaN epitaxial layers

We report on the cathodoluminescence (CL) of Al_0.25Ga_0.75N (x = 0.25-) and underlying GaN films and on the Al composition of the AlGaN with the biaxial residual stress taken into account. Crack-free thick AlGaN layers were fabricated by introducing a thin AlN interlayer on a high quality GaN epitaxial layer. The CL band-edge emission of AlGaN from a non-cracked region appeared at a lower energy than that from the cracked region. This is the direct indication, that crack relaxes biaxial tensile stress in AlGaN layers. When the Al composition x was lower than 0.5, the Al compositions measured by X-ray diffraction fell between the values estimated by Vegard's law and the values of pseudomorphic assumption due to partially relaxed residual stress in the AlGaN films. When x > 0.5, they are in agreement with Vegard's law, indicating that the AlGaN layer is fully relaxed.