Raman spectroscopy of optical phonons as a probe of GaN epitaxial layer structural quality M. M. Bülbül, S. R.P. Smith, B. Obradovic, T. S. Cheng and C. T. Foxon Eur. Phys. J. B, 14 3 (2000) 423-429 Published online: 15 April 2000 DOI: 10.1007/s100510051050