The electrical transport properties for compositionally and structurally well-defined epitaxial α-(Ti${}_x$Fe${}_{1-x}$)${}_2$O${}_3$(0001) films have been investigated for $x$ ≤ 0.09. All films were grown by oxygen plasma-assisted molecular beam epitaxy using two different growth rates: 0.05–0.06 Å/s and 0.22–0.24 Å/s. Despite no detectable difference in cation valence and structural properties, films grown at the lower rate were highly resistive whereas those grown at the higher rate were semiconducting (ρ $=$ ∼1 Ω · cm at 25 °C). Hall effect measurements reveal carrier concentrations between 10${}^{19}$ and 10${}^{20}$ cm${}^{-3}$ at room temperature and mobilities in the range of 0.1 to 0.6 cm${}^2$/V · s for films grown at the higher rate. The conduction mechanism transitions from small-polaron hopping at higher temperatures to variable-range hopping at a transition temperature between 180 and 140 K. The absence of conductivity in the slow-grown films is attributed to donor electron compensation by cation vacancies, which may form to a greater extent at the lower rate because of higher oxygen fugacity at the growth front.

• Received 27 October 2011

DOI:https://doi.org/10.1103/PhysRevB.84.245325