Relationship of the longer wavelength threshold and the narrower surface band gap: For GaN and GaAlN photocathodes

In order to study the cause of the longer-wavelength threshold of GaN and GaAlN photocathodes, three GaN photocathodes and two GaAlN photocathodes were designed and prepared by Cs/O activation following MOCVD, and first-principles calculations about the GaN(0001), Ga0.75Al0.25N(0001), and Ga0.625Al0.375N(0001) surfaces were performed. Results show that the calculated band gap values of the bulk GaN, Ga0.75Al0.25N, and Ga0.625Al0.375N are 1.630 eV, 2.261 eV, and 2.272 eV respectively. Meanwhile the band gap of GaN(0001), Ga0.75Al0.25N(0001) and Ga0.625Al0.375N(0001) surfaces are much smaller than 1 eV. The relation of the smaller surface band gap and the longer-wavelength threshold was discussed. We found that the lattice parameters of the surfaces are larger than those of bulk GaN and GaAlN, which results in the decrease of the band gap. So the phenomenon about the longer wavelength threshold is theoretically explained.