Intrinsic exciton transitions in GaN - W. Shan; A.J. Fischer; S.J. Hwang; B.D. Little; R.J. Hauenstein; X.C. Xie; J.J. Song; D.S. Kim; B. Goldenberg; R. Horning; S. Krishnankutty; W.G. Perry; et al. (Journal of Applied Physics, 1998)