Identification of defects responsible for optically stimulated luminescence (OSL) from copper-diffused LiAlO2 crystals

Abstract

A large optically stimulated luminescence (CW-OSL) response has been observed from bulk lithium aluminate (LiAlO2) crystals that have been copper-diffused. After introducing the copper, an optical absorption band peaking at 277nm is present and is assigned to Cu+ (3d10) ions at Li+ sites. Photoluminescence (PL and PLE) from copper-diffused samples shows an emission band near 359nm and an excitation band near 273nm. These PL bands are also assigned to the Cu+ ions. After an irradiation at room temperature with x-rays, the copper-diffused crystals give an intense OSL signal when stimulated with light in the 425–500nm region. The OSL emitted light has a peak near 357nm and thus is directly associated with recombination at a Cu ion. Electron paramagnetic resonance (EPR) shows that the x-rays produce two distinct Cu2+ (3d9) centers, both at Li+ sites. One of the trapped-hole centers is a Cu2+ with no nearby defects and the other is a Cu2+ with a neighboring Li+ vacancy. Monitoring the EPR spectra before and after OSL shows that only the isolated Cu2+ ions (i.e., those with no nearby defect) participate in the OSL process. The electron traps participating in the OSL are shown with EPR to be transition-metal ions that are unintentionally present. Specifically, Fe2+ ions replacing Li+ ions, possibly with a nearby lithium vacancy, convert to Fe+ (3d7) ions when they trap an electron during irradiation. These electrons are optically released by the OSL exciting light and migrate to the holes trapped as isolated Cu2+ ions, where electron–hole recombination gives the characteristic Cu+ emission. By combining OSL, PL, and EPR, we establish the general OSL mechanism in copper-diffused LiAlO2 crystals.