Non-ohmic properties of CaCu3Ti4O12 thin films deposited By RF-sputtering

Abstract

undoubtedly has a great potential in the surge protection device industry. The first report [2] on varistor property of CCTO shows that an intrinsic electrostatic barrier built at the CCTO grain boundary is responsible for the nonlinear current voltage behaviour. In the CCTO polycrystalline ceramic, back-to-back potential barriers formed from band bending at the boundary region (like the n-in acceptor interface [3] electron trap, caused by negatively charged intrinsic donor defects) leads to an abrupt drop in potential from grain to grain boundary. This mechanism was further supported by Mei et al. [4] by a thorough study of grain and grain boundary composition. Their study reveals that a Curich secondary phase at the grain boundary with negatively charged vacancies () is responsible for the double Schottky barriers (n-in) with the n-type semiconductor grain boundary and grains [5, 6]. Chung et al. reported that CCTO exhibits nonlinear current-voltage characteristics even in the absence of any dopants [7]. These outstanding electrical properties are quite unusual since CCTO exhibits a body centered cubic perovskite structure with slightly tilted [TiO 6 ] octahedra facing each other [8], i.e. the compound is not ferroelectric. Despite numerous studies including quantum chemical calculations [9], the origin of this high dielectric constant remains still unclear. Impedance spectroscopy has proven that such ceramics consist of semiconducting grains and insulating grain boundaries. Results from Sinclair et al. [3] suggest that the high dielectric constant is actually an internal grain boundary barrier layer capacitance, in other words, it is a grain boundary effect that is not linked to the perovskite crystal structrure. Since the nonlinear electrical properties are not independent from the grain boundary nature, one would expect that they are somehow connected to the grain boundary barrier layer capacitance. It is also known that oxygen adsorbed at the grain boundaries plays Abstract Calcium copper titanate (CaCu 3 Ti 4 O 12 , CCTO), thin films with polycrystalline nature have been deposited by RF sputtering on Pt/Ti/SiO 2 /Si (100) substrates at a room temperature followed by annealing at 600 °C for 2 h in a conventional furnace. The CCTO thin film present a cubic structure with lattice parameter a = 7.379 ± 0.001 Å free of secondary phases. Dielectric spectroscopy was employed to examine the polycrystalline behaviour of CCTO material and the mechanisms responsible for the barrier-layer capacitances associated with Schottky-type barriers and the non-Ohmic properties. The film presents an electric breakdown field (E b = 203 V cm −1) and then nonlinear coefficient (α = 6), which is even lower than that of the ZnO and SnO 2 based varistors The observed electrical features of CCTO thin films are highly dependent on the [CaO 12 ], [CaO 4 ], [CuO 11 ], [CuO 11 V o x ] and [TiO 5 •V • O