Enhanced bulk oxygen outdiffusion from LiNbO3 induced by preferential sputtering of oxygen at elevated temperatures

The optical absorption spectra of 5 keV Ar+ and Ti+ irradiated LiNbO3 Z-cut single crystals at various temperatures have been investigated. After Ar+ irradiation to a dose of 1017 ions/cm2, an optical absorption band develops in the range 326–800 nm at elevated temperatures. This band is ascribed to the formation of bulk oxygen vacancy related polarons, its intensity increasing with temperature. By using various sequences of irradiation and annealing, a synergetic effect between ion irradiation and substrate annealing on the oxygen vacancy generation in LiNbO3 is revealed. Irradiation with Ti+, whose ballistic behavior is similar to Ar, produced an absorption band at room temperature, while for Ar+ no such absorption was observed. This band is ascribed to Ti3+ transitions. However, at 660 °C, the Ti+ irradiation induced absorption band had a significantly lower intensity than that induced by Ar+. It is proposed that this “chemical effect” of subplanted Ti in the surface region is a result of Ti reaction with oxygen, thereby trapping some of the outdiffusing oxygen as TiOx and reducing the oxygen deficiency in the surface region. A bulk oxygen outdiffusion mechanism is developed based on these observations, which involves preferential sputtering of oxygen during irradiation, rendering an oxygen deficiency in the surface region. This deficiency of surface oxygen enhances the outdiffusion of highly mobile oxygen in the bulk towards the surface at elevated temperatures.