Yellow luminescence band in undoped GaN revealed by two-wavelength excited photoluminescence

The below-gap emission components including yellow luminescence (YL) band of an MOCVD grown undoped GaN have been studied by the two-wavelength-excited photoluminescence (TWEPL). The nature of each emission line has been investigated by using an intermittent below-gap excitation (BGE) light of 1.17 eV on an above-gap excitation (AGE) light of 3.49 eV. The intensity of DAP and the YL decreased while it increased for IOX after irradiation of the BGE. The intensity change in PL after addition of the BGE implies the presence of defect levels in the energy position corresponding to the photon energy of the BGE. Possible recombination models are listed and examined. Only the recombination model in which the YL corresponds to the transition from a shallow donor to a deep state at about 1 eV above the valence band maximum satisfies our experimental result. The possible origin of this defect state is discussed.