A correspondence between the E2 level (Ec-0.55eV) in n-type samples undergoing thermo-ionization and photo-ionization was established. The optical cross-section in the vicinity of the threshold for photo-ionization of this level was measured by means of capacitance-transient spectroscopy. Analysis yielded an optical activation energy of 0.85eV, and a Franck-Condon parameter of 0.30eV, at 90K.

Optical characterization of the 'E2' deep level in GaN P.Hacke, P.Ramvall, S.Tanaka, Y.Aoyagi, A.Kuramata, K.Horino, H.Munekata: Applied Physics Letters, 1999, 74[4], 543-5