First-principles study of Ti-doped sapphire. II. Formation and reduction of complex defects

Eliminating the effect of residual infrared absorption in titanium sapphire remains a crucial task to fulfill, despite that this kind of laser crystal has been developed for decades. The Ti3+-Ti4+ (Ti-Al(0) -Ti-Al(1+)) pair model is the most widely accepted explanation to this residual absorption, but theoretical analyses based on first-principles calculation have not yet depicted a clear picture for the variation of the Ti-Al(0)-Ti-Al(1+) pair and a variety of other potentially important defects in titanium sapphire when fabricating conditions change. Here, we extend the work in [W. Jing, M. Liu, J. Wen, L. Ning, M. Yin, and C.-K. Duan, Phys. Rev. B 104, 165103 (2021)] about binding tendencies of TiAl-TiAl pairs and optical transition properties to providing a comprehensive understanding of the formation and processing condition dependence of various potentially significant defects and complexes besides the (Ti-Al-Ti-Al)(0,1+) pairs. Apart from the complexes composed of V-Al(3) and Ti-0,Ti-1+ Al defects, two new significant negatively charged defects, (V-Al-Al-i-V-Al)(3-)and (V-Al-Ti-i-V-Al)(2-) are revealed. These two complexes are correspondingly an interstitial Al3+ and Ti4+ surrounded by two neighboring aluminium vacancies V-Al(3-). We show that V-Al(3-) plays the key role to stabilize Ti-Al(0)-Ti-Al(1+) and form various stable complexes with Ti-Al(1+) and Ti-Al(0)-Ti-Al(1+) Al in weak and moderate reductive atmospheres. Thus, besides annealing at strong reductive atmosphere at elevated temperatures, Al ion injection or annealing in Al vapor is a potential method to eliminate the harmful residual infrared absorption, which is pinpointed at the reduction of the concentration of V-Al(3-) and its variant (V-Al-Al-i-V-Al)(3-). While in extremely strong reductive atmosphere, isolated Ti substitution dominates over those complexes containing V-Al(3-), but there still remains a tiny amount of Ti-Al(0)-Ti-Al(1+) and Ti-Al(1+), which are charge compensators to balance the charge of trace amount Ti-Al(1-) further reduced from Ti-Al(0) . And this effect can be further eliminated by lower temperature reductive-atmosphere annealing. In addition, we obtain a simple numerical expression to predict the achievable figure of merit when the concentration of Ti-Al(1+) is given. Formation energies for simple defects and binding energies for complexes obtained in this work may serve as the basis for simulations and design various quenching and annealing processes to further reduce harmful defect species in titanium sapphire.

Automated summary

Eliminating residual infrared absorption in titanium sapphire remains a crucial task. Our research provides a comprehensive understanding of the formation of various potentially significant defects and complexes, as well as their dependence on processing conditions.