Enhancement of crystal anisotropy and ferroelectricity by decreasing thickness in (Al,Sc)N films

Abstract

The thickness dependences of crystal structures and ferroelectric properties were investigated for (Al_0.8Sc_0.2)N films with thicknesses of 12 to 130 nm deposited on (111)Pt/TiO_x/SiO₂/(100)Si substrates. The internal structural parameter u, representing the crystal anisotropy of the wurtzite structure, decreased with decreasing film thickness. This was attributable mainly to the in-plane compressive strain originating from the larger atomic distance of (Al_0.8Sc_0.2)N film compared to that of the underlying Pt layer. Well-saturated P_r values were obtained at room temperature for the films down to 20 nm in thickness. The P_r value of the 12-nm-thick film tended to saturate against the electric field when the measurement temperature increased to 150 °C. These P_r values are considerably higher than those of conventional ferroelectric materials such as Pb(Zr,Ti)O₃ and HfO₂-based films in the thickness region below 20 nm. The P_r value tended to increase for film thicknesses below 50 nm. This originated from the increase in crystal anisotropy with decreasing film thickness due to the strain from the underlying Pt layers. Moreover, the significantly large P_r values of these strained films were larger than the expected values of pure AlN and were in good agreement with the theoretically calculated predictions based on the crystal anisotropy, u parameter. This suggests that the P_r values of (Al,Sc)N can be controlled mainly by the u parameter.