Study of the Metal-Insulator Transition in LaCoO_3-<i>x</i> Epitaxial Films

Abstract

Materials with temperature-dependent metal-to-insulator transitions (MIT) have gained attention for the abrupt collapse of the band gap during the transition. Various novel transistor structures which utilize MITs have been suggested and realized to produce more energy efficient transistors. To achieve practical operation temperatures for device applications, however, MIT materials with transition temperatures below 400 K are unsuitable. This work focuses on one MIT material with a high temperature transition, LaCoO₃. Using oxide molecular-beam epitaxy (MBE) a series of LaCoO_3-<i>x</i> thin films with varied La and Co compositions were grown. All films were grown on LaAlO₃ (001)_<i>p</i> substrates, which like LaCoO₃, has a pseudocubic perovskite structure. The lattice mismatch is less than 0.9%. X-ray diffraction <i>θ</i>-2<i>θ</i> measurements were used to assess the structure of the films; the out-of-plane lattice constant of each film was calculated using a Nelson-Riley analysis. The temperature-dependent resistivity of each film was measured and each shows a change in electrical resistivity of more than two orders of magnitude in the MIT temperature range of 400 - 600 K, which is similar to that of bulk single crystal LaCoO₃. The abruptness and magnitude of the MIT is found to be insensitive to the film composition for samples ranging from La_1.1CoO_3-<i>x</i> to LaCo_1.1O_3-<i>x</i>.