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Optoelectronic modulation spectroscopy (OEMS) has been used to explore the sub band-gap responses of deep energy states in GaN prepared by MOVPE. Modulation of the channel current of an AlGaN HEMT revealed a replica structure centred on 2.16 eV which cannot be attributed to Fabry-Perot oscillations. This was superimposed on a broad background with an anti-phase response, suggesting a continuous spectrum of states to which electrons were excited from the valence band.The phase of the replica responses confirmed that each peak was due to a transition to the conduction band. The average peak separation was 89 meV, close to the LO phonon energy in GaN, 92 meV, suggesting that charge in the state at 2.16 eV was strongly coupled to the lattice. This appears to be the first report of the phonon replica structure in an absorption spectrum. The observations are compared with a previously published luminescence replica spectrum. It is demonstrated using configuration coordinate diagrams that there should be a coincidence of lines, and this was confirmed experimentally. These observations provide confirmation that the electron state concerned is located 2.16 eV below the conduction band.The geometry of the configuration coordinate diagram leads to the conclusion that the Frank-Condon energy of this defect was between 0.40 and 0.55 eV with a corresponding Stokes shift between 0.80 and 1.10 eV.

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