Au microcrystal-doped glass films with high optical nonlinearity were prepared by the sol-gel method with dip-coating using NaAuCl4•E2H2O and tetraethyl or thosilicate (TEOS) as starting materials. The glass film with an Au/Si atomic ratio of 0.01 was successful ly prepared. The nonlinear susceptibility ƒÔ(3) of the film was 7.7•~10-9esu. Microcrystals with a radius of about 7A and of about 30A coexisted in the glass film. The nonlinear susceptibility of Au microcrystals itself in the film, ƒÔm(3), was 1.0•~10-7esu and was two times larger than that observed in Au-doped glasses pre pared by the melting method. Subsequent heat treat ment of the films at 1000•Ž changed the absorption spectra and decreased ƒÔ(3) by a factor of two. The atom ic ratio of Au/Si was able to be increased to 0.04.

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.

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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.