Blue photoluminescence activated by surface states in GaN grown by molecular beam epitaxy

We have studied the broad blue band, which emerges in the photoluminescence (PL) spectrum of c-plane GaN layers after etching in hot H3PO4 and subsequent exposure to air. This band exhibited a 100 meV blueshift with increasing excitation intensity and a thermal quenching with activation energies of 12 and 100 meV. These observations led us to suggest that surface states may be formed on etched surfaces and cause bandbending, which leads to a shift in transition energy with excitation. The blue PL is related to transitions from the shallow donors filled with nonequilibrium electrons to the surface states, which capture the photogenerated holes. The observed irreversible bleaching of the blue luminescence may be attributed to the metastable nature of the surface states or to the oxygen desorption.