  Yellow luminescence (YL) is probably the longest and most studied
defect-related luminescence band in GaN, yet its electronic structure or
chemical identity remain unclear. Most of the theoretical work so far has
attributed the feature to bulk defects, whereas spectroscopic studies have
suggested a surface origin. Here, we apply deep level spectroscopy using
sub-bandgap surface photovoltage that provides the energy distribution of the
surface charge density. Comparison of surface charge spectra obtained under
identical conditions before and after various surface treatments reveals the
dynamics of the surface charge density. Further comparison with spectra of the
entire state obtained using photoluminescence shows how the charge density
stored in YL-related defects is eliminated upon a mild anneal in vacuum. This
suggests that the YL-related defect involves a certain molecule adsorbed on the
GaN surface, possibly in a complex with an intrinsic surface defect. The
observed interaction with air strongly indicates that the YL-related deep level
is a surface state.
