The ferroelectricity of (Al1−xScx)N (x = 0–0.34) thin films with various thicknesses was investigated. (Al1−xScx)N films were prepared at 400 °C on (111)Pt/TiOx/SiO2/(001)Si substrates by the radio frequency dual-source reactive magnetron sputtering method using Al and Sc targets under pure N2 gas or a mixture of N2 and Ar gases. The film deposited under N2 gas showed larger remanent polarization than those under N2 + Ar mixture. Ferroelectricity was observed for films with x = 0.10–0.34 for about 140-nm-thick films deposited under N2 gas. The x = 0.22 films showed ferroelectricity down to 48 nm in thickness from the polarization–electric field curves and the positive-up-negative-down measurement. The ferroelectricity of the 9 nm-thick film also was ascertained from scanning nonlinear dielectric microscopy measurement. These results reveal that ferroelectric polarization can switch for films with much broader compositions and thicknesses than those in the previous study.

The temperature dependence of ferroelectric properties was investigated for (Al0.8Sc0.2)N films 9–130 nm thick prepared on (111)Pt/TiO x /SiO2/Si substrates. The coercive fields (E c) of these films decreased with increasing measurement temperature up to 523 K, irrespective of film thickness, thus realizing polarization switching because the applicable maximum electric field is beyond E c. As a resultant, remanent polarization (P r) above 100 μC cm−2 was ascertained for 9 nm thick films at 373 and 423 K, which is more than 5 times larger than those of HfO2-based films of the same thickness. The P r value was almost independent of film thickness when an electric field is applied for switching. In addition, E c showed a smaller thickness dependence than conventional ferroelectrics, including Pb(Zr,Ti)O3. The large P r value beyond 100 μC cm−2 for around 10 nm thick films with small degradation against film thickness, as well as the diminished increase in E c with decreasing film thickness. This showed that (Al0.8Sc0.2)N film is a promising candidate for nonvolatile memory applications requiring high-density and low-voltage operation, including capacitor-type memories and ferroelectric tunnel junction-type memories that consist of metal–ferroelectric–metal structure.

The effect of pure mechanical strain on ferroelectricity was investigated for (001)-one-axis-oriented (Al0.8Sc0.2)N films deposited on (111)­Pt-coated substrates with different thermal expansion coefficients. The mechanical lattice strains were successfully controlled by using substrates with different thermal expansion coefficients, though the composition of the films is the same. The changes in the remanent polarization (P r) and coercive field (E c) values of these films can be understood by the internal parameter u representing crystal anisotropy of a wurtzite structure. These results suggest that the ferroelectric properties of (Al1–x Sc x )N films can be tuned via crystal anisotropy.

It is recently found that wurtzite aluminum nitride exhibits ferroelectricity by alloying with scandium. Because its remanent polarization (Pr) is large, ScxAl1−xN has attracted much attention. Gallium nitride with similar structure and properties is also expected to show ferroelectricity. Herein, ScxGa1−xN was prepared on a silicon substrate at 673 K using the sputtering method, and its ferroelectricity was investigated. Sc0.41Ga0.59N exhibited ferroelectricity before dielectric breakdown. Pr of this film, as evaluated via positive-up-negative-down (PUND) measurement, was around 120 μC/cm2, which can reduce the influence of the leakage current. This was comparable to the value of ScxAl1−xN with the same internal parameter u, which is approximately estimated from the lattice constants. Moreover, in the PUND measurement, the measurement temperature dependence of Ec was observed, which was 4.3 MV/cm at 300 K and reduced to 3.2 MV/cm at 473 K.

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