Low temperature growth of piezoelectric AlN films by plasma enhanced atomic layer deposition and magnetoelectric coupling with nickel for energy harvesting applications

Synthesis of a piezoelectric aluminum nitride (AlN) films is often challenged by high temperature processes in order to obtain highly crystalline (002)-oriented films with low impurity levels. In this study, by means of plasma-enhanced atomic layer deposition (PEALD), high quality AlN films were grown at low deposition temperatures of $180^{\circ}C$ and $250^{\circ}C$. The deposition temperatures and the purging times in the PEALD sequence have been carefully investigated to obtain preferential (002) plane-normal orientation of the crystallites with piezoelectric properties. Interestingly, low temperature processes allow growing AlN films directly on nickel foils as magnetoelectric layer-by-layer composites in which the degradation of magnetic properties is negligible in this PEALD temperature range. The highest piezoelectric coefficient $e_{31,f}$ was evaluated to be about 0.37 C$.m^{-2}$ for the (002)-oriented AlN film of 500 nm-thick. The magnetoelectric coefficient was measured to be about 3.5 V.cm$^{-1}$.Oe-1 with a DC bias magnetic field of 30 Oe superimposed by a weak AC magnetic field of 12.5 Oe modulated at 46 Hz.