Cr³⁺ impurities and photoluminescence in LaAlO₃

Photoluminescence (PL) spectra induced in LaAlO₃ by ultraviolet (UV) photons were measured for two types of samples, i.e., single-crystal plates and thin films. The films, which were amorphous when deposited, became polycrystalline after being annealed at 800°C or higher. When the excitation photon energy exceeded the band-gap energy of LaAlO₃, three PL peaks appeared at approximately 1.60 eV in the single crystals and polycrystalline films deposited and annealed on a Si single-crystal substrate. When the films were deposited and annealed on a CaF₂ singlecrystal substrate, the in-plane lattice parameters became smaller and the energies of the three PL peaks shifted to higher values. This indicates that the three PL peaks are susceptible to a crystal field. Furthermore, the intensities of the three PL peaks did not depend on the measurement temperature. Taking these results together with their spectral shapes into consideration, the three PL peaks are due to the R-line luminescence resulting from Cr³⁺ impurities in LaAlO₃. The detection of Cr³⁺ ions by inductively coupled plasma optical emission spectrometry failed, indicating that the Cr³⁺ content of the present samples is below 1 ppm. The fact that the PL peaks appeared in both the single-crystal plates and thin films obtained from different raw chemicals with different processes indicates that there is a strong possibility that the Cr³⁺ ions are present in Al ore. Another PL peak appearing at 2.8eV in the single crystals was assumed to originate from oxygen vacancies.