Ferroelectric properties of anisotropically strained epitaxial NaNbO3 films grown on NdGaO3

Abstract

Due to the lattice mismatch between the film and substrate, an anisotropically in-plain strain can be imposed to an epitaxially grown film, which (i) depends on a number of parameters and (ii) can strongly affect the electronic properties of this thin layer. NaNbO3 films are epitaxially deposited on (110) NdGaO3 via Metal Organic Chemical Vapor Deposition (MOCVD). X-ray analysis shows that up to a thickness of approximately 15nm, the film is completely strained, while above this thickness, relaxation of the compressive strain takes place. The analysis of the complex permittivity of three films of different thickness (27 – 80nm) as a function of temperature, electric field direction, AC and DC electric field reveals that (i) the compressive strain shifts the temperature of maximum (Tmax) from about 628K of a bulk NaNbO3 to close to room temperature of thin films, (ii) the room temperature permittivity of these strained films is enhanced by a factor of 1.06 – 2.74 compared to that of bulk material, (iii) there is a strong anisotropy in all ferroelectric characteristics for electric field orientation along the [110] and [001] orientations of NdGaO3, and (iv) a strong dependence of the permittivity on the ac amplitude of the electric field as well as the dc component of the electric field. The experimental results are discussed in terms of theories on domain wall mobility and pinning as well as effect of relaxor ferroelectrics. These strained films represent ideal candidates for the applications of sensors, such as surface acoustic wave sensor