PNNL

Abstract

The electrical transport properties for compositionally and structurally well-defined epitaxial a-(TixFe1-x)2O3(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 (? = ~1 O?·?cm at 25?°C). Hall effect measurements reveal carrier concentrations between 1019 and 1020 cm-3 at room temperature and mobilities in the range of 0.1 to 0.6 cm2/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.