Effects of heat treatment and UV irradiation on the transition metal impurities in LaAlO3

Impurities present in LaAlO₃, were identified by analyzing electron spin resonance spectra and optical absorption spectra. Especially, the effects of ultraviolet photon irradiation and thermal annealing on these spectra were studied in detail. The experimental results confirm that LaAlO₃ contains Cr and Fe as impurities. They also indicate that electrons and holes are released by the irradiation of UV photons with energies higher than 4.5 eV or by the annealing in an oxidizing atmosphere at temperatures above 500 °C. The released electrons are captured by trivalent impurity ions (Fe³⁺ or Cr³⁺), while the holes are captured by trivalent vacancies of La³⁺ or Al³⁺. It is assumed that the opposite reaction is induced by the irradiation of visible light or by the annealing in a reducing atmosphere at temperatures above 500 °C.