Electrical transport properties of Ti-doped Fe2O3(0001) epitaxial films

The electrical transport properties for compositionally and structurally well-defined epitaxial alpha-(TixFe1-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 angstrom/s and 0.22-0.24 angstrom/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 (rho = similar to 1 Omega . cm at 25 degrees 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.