Growth of Atomically Smooth Epitaxial Nickel Ferrite Films by Direct Liquid Injection CVD

摘要

Magnetoelectric heterostructures are being actively investigated for utilization in next generation microwave devices such as tunable filters and phase shifters. For efficient microwave absorption and magnetoelectric coupling, relatively thick (>1mm) epitaxial spinel ferrite films with smooth topographies are required for the magnetic/ferroelectric heterostructures. Towards this goal, direct liquid injection (DLI)-CVD has been utilized for epitaxial growth of nickel ferrite (NiFe_2O_4) films on MgAl_2O_4 (100) and MgO (100) substrates with high deposition rates. Anhydrous Ni(acac)_2 and Fe(acac)_3 (acac=acetylacetonate) are used as precursor sources dissolved in N,N-dimethyl formamide for the DLI vaporizer system. The influence of deposition temperature on the film properties has been investigated using optimized process conditions for flow of the injected precursors and oxygen. Epitaxial nickel ferrite films of stoichiometric composition are obtained in the temperature range 500-800 °C on both substrates with growth rates in the range 0.6-1.1 mm h~(-1). Because of changes in the surface diffusion behavior, the film morphology is found to be dependent on the deposition temperature with atomically smooth films being obtained for deposition in the temperature range 600-700 °C. Magnetic measurements reveal an increase in the saturation magnetization for the films with increasing growth temperature, which correlates well with the trend for improved epitaxial growth as indicated by X-ray and Raman spectroscopy measurements. Nickel ferrite films deposited on MgAl_2O_4 (100) at 800 °C exhibit saturation magnetization very close to the bulk value of 300 emu cm~(-3).