OH centers and the conductivity of hydrogen-doped In$_{2}$O$_{3}$ single crystals

Mechanisms for the n-type conductivity of In$_{2}$O$_{3}$ have been controversial. Recent experiments suggest that O vacancies are the cause of conductivity.\footnote{S. Lee and D.C. Paine, Appl. Phys. Lett. \textbf{102}, 052101 (2013).} However, other recent experiments find that the H-doping of thin films gives rise to shallow donors.\footnote{T. Koida \textit{et al.}, Jpn. J. Appl. Phys. \textbf{46}, L685 (2007).} Theory also finds that interstitial H and H at an O vacancy are shallow donors in In$_{2}$O$_{3}$.\footnote{S. Limpijumnong \textit{et al.}, Phys. Rev. B \textbf{80}, 193202 (2009).} We have performed a series of IR absorption experiments to determine the properties of OH and OD centers in In$_{2}$O$_{3}$ single crystals. Annealing In$_{2}$O$_{3}$ samples in H$_{2}$ or D$_{2}$ at temperatures near 450$^{\circ}$C (30 min) produces an n-type layer $\approx $0.05 mm thick with an n-type doping of 2x10$^{19}$ cm$^{-3}$. The resulting free-carrier absorption is correlated with an OH center with a vibrational frequency of 3306 cm$^{-3}$ that we associate with interstitial H.\footnote{M. Stavola, J. Appl. Phys., to be published.}