CRITICAL THICKNESS OF GAN THIN-FILMS ON SAPPHIRE(0001)

Synchrotron x-ray diffraction was employed to measure the lattice cons tants a and c of GaN films grown with an AlN buffer layer on sapphire (0001) over a thickness range of 50 Angstrom to 1 mu m. We used multip le reflections and a least-squares fit method for high reliability. As the thickness increased, the lattice constant a increased from 3.133 Angstrom to 3.196 Angstrom and c decreased from 5.226 Angstrom to 5.18 3 Angstrom. The expected trend was fitted to an equilibrium theory, al lowing the critical thickness of GaN on AlN to be estimated at 29 Angs trom +/- 4 Angstrom in good agreement with a theoretical prediction. ( C) 1996 American Institute of Physics.