Yellow luminescence and related deep states in undoped GaN

Photocapacitance spectra in undoped, metal-organic vapor-phase-epitaxy-grown GaN layers, in a range of photon energies from 0.6 to 3.5 eV, reveal two main persistent features: a broad increase of the capacitance from 2.0 to 2.5 eV, and a steep decrease at 1 eV, only observed after a previous light exposure to photon energies above 2.5 eV. A deep trap (${\mathrm{E}}_{\mathrm{v}}$ +1 eV) that captures photoelectrons from the valence band, after being emptied with photons above 2.5 eV, is proposed as the origin of these features. Optical-current deep-level transient spectroscopy results also show the presence of a trap at 0.94 eV above the valence band, only detected after light excitation with photon energies above 2.5 eV. A correlation is found between the 'yellow band' luminescence intensity at 2.2 eV and the amplitude of the photocapacitance decrease at 1 eV, pointing to a deep trap at 1 eV above the valence band as the recombination path for the yellow band. The detection of the yellow band with below-the-gap photoluminescence excitation supports the proposed model.