Longitudinal Excitons in GaN

Many of the previous investigations of longitudinal excitons have involved reflection and absorption measurements rather than emission. In these measurements it is more difficult to resolve the longitudinal exciton from the ${\Gamma }_5$ and ${\Gamma }_6$ free excitons in wurtzite material. The longitudinal excitons have energies and oscillator strengths that depend on the direction of propagation and they are not observable along the principal axis of the crystal. In the wurtzite structure, such as GaN, the ${\Gamma }_5$ exciton is the pure transverse mode, whereas the longitudinal is a mixed mode going from pure longitudinal, for the propogation direction K perpendicular to C, to pure transverse for K parallel to C. If more than one orientation is present in the sample, it is clear that more than one longitudinal exciton may be seen since it is a mixed mode. In the current experiment we observe more than one mode, which we associate with more than one crystal orientation. This may result from the columnar growth often observed in GaN.