In Fig. 6(a), the PLE spectra of films show a broad band from 230 to 300 nm which is ascribed to the charge transfer band (CTB), originating from charge-transfer excitation from O 2p to Ga 4s4p orbitals [24]. The other peaks centered at about 410 nm and 550 nm are associated with 4A2→4T1 and 4A2→4T2 transitions of the 3d–3d configuration of Cr3+ [25], respectively. It is observed that the intensity of the excitation band is enhanced as the oxygen pressure is changed from 0 to 1 Pa.