Effect of Oxygen Vacancy on the Band Gap and Nanosecond Laser-Induced Damage Threshold of Ta₂O₅ Films

Ta₂O₅ films are deposited on fused silica substrates by electron beam evaporation method. The optical property, x-ray photoelectron spectroscopy, band gap and nanosecond laser-induced damage threshold (LIDT) of the films before and after annealing are studied. It is found that the existence of an oxygen vacancy results in the decrease of the transmittance, refractive index, both macroscopic band gap and microscopic band gap, and the LIDT of Ta₂O₅ films. If the oxygen vacancy forms, the macroscopic band gap decreases 2%. However, when the oxygen vacancy forms the microscopic band gap decreases 73% for crystalline Ta₂O₅ and 77% for amorphous Ta₂O₅. The serious decrease of microscopic band gap may significantly increase the absorbance of the micro-area in Ta₂O₅ films when irradiated by laser, thus the damage probability increases. It is consistent with our experimental results that the LIDT of the as-deposited Ta₂O₅ films is 7.3 J/cm², which increases 26% to 9.2 J/cm² when the oxygen vacancy is eliminated after annealing.