The interaction of dopants and native point defects in functional complex oxides

Oxide semiconductors are often doped with heterovalent substitutional impurities, which modify the Fermi level and lead to the formation of charge-compensating native point defects. Using first-principles calculations, we demonstrate the interplay between doping and native defect formation in complex oxides, highlighting the potential for both positive and detrimental effects. As a first example, we study the proton-conducting complex oxide SrZrO$_3$ (SZO). In undoped SZO, the defect chemistry is dominated by oxygen vacancies ($V_\mathrm{O}$) and strontium vacancies ($V_\mathrm{Sr}$) [1], whose concentrations are constrained by charge neutrality. Upon acceptor doping with Sc or Y at the Zr site, the concentration of $V_\mathrm{O}$ can be increased, and the concentration of $V_\mathrm{Sr}$ can be reduced; we discuss how this promotes both proton solubility and diffusion. However, under certain growth conditions, Sc and Y will substitute at the Sr site and act as donors, with detrimental consequences for proton conductivity. The second example is the alkaline-earth stannates (ASnO$_3$; A = Ba, Sr, Ca), which are promising transparent conducting oxides. The stannates can be doped with La donors. High levels of $n$-type doping can be achieved in BaSnO$_3$; however, the achievable carrier concentrations in (Ba,Sr)SnO3 alloys are much lower [J. Vac. Sci. Technol. A 34, 050601 (2016)]. We investigate the origin of this $n$-type doping difficulty, finding that the formation energy of A-site cation vacancies becomes very low under $n$-type conditions; these vacancies act as charge-compensating acceptors, reducing the $n$-type conductivity. This effect is discussed in light of recent experimental results, and we provide guidelines for engineering the growth environment to achieve higher $n$-type doping.