Residual stress in AlN films grown on sapphire substrates by molecular beam epitaxy

Residual stress in AlN films grown by molecular beam epitaxy (MBE) has been studied by Raman scattering spectroscopy. A strain-free Raman frequency and a biaxial stress coefficient for E-2(high) mode are experimentally determined to be 657.8 +/- 0.3 cm(-1) and 2.4 +/- 0.2 cm(-1) /GPa, respectively. By using these parameters, the residual stress of a series of AlN layers grown under different buffer layer conditions has been investigated. The residual compressive stress is found to be obviously decreased by increasing the Al/N beam flux ratio of the buffer layer, indicating the generation of tensile stress due to stronger coalescence of AlN grains, as also confirmed by the in-situ reflection high energy electron diffraction (RHEED) monitoring observation. The stronger coalescence does lead to improved quality of AlN films as expected. (C) 2016 Elsevier Ltd. All rights reserved.