Thickness scaling of (Al_0.8Sc_0.2)N films with remanent polarization beyond 100 μC cm⁻² around 10 nm in thickness

Abstract

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.

High-Temperature Ferroelectric Behavior of Al0.7Sc0.3N

“…of (c) ∆Ec/2 and (d) Pr results from this work with Mizutani et al [21], Zhu et al [20], Gund et al [22], and Wang et al [20]. The results in (a,b) are from this work, while (c,d) are a comparison of our results with previous reports.…”

“…The overall Ec results (apart from the reduced Ec values reported by Wang et al) are consistent with the literature in terms of a lower Ec due to increasing Sc content or decreasing excitation frequency. This may explain both the coincidental values between Gund et al (30% Sc, 3 kHz) and Zhu et al (16% Sc, 333 Hz) and the increased Ec for Mizutani et al (20% Sc, 100 kHz) [19][20][21][22]. Our work shows that the Pr values for Al0.7Sc0.3N remained relatively stable up to 400 °C (Figure 3d).…”

“…The increasing leakage with temperature is important for compensation when possibly measuring changes in polarization (e.g., during retention testing). [19][20][21][22]. Our work shows that the P r values for Al 0.7 Sc 0.3 N remained relatively stable up to 400 • C (Figure 3d).…”

“…Polarization−electric field (P-E) hysteresis loops were collected between probe station temperatures of 23−400 • C using an excitation frequency of 10 kHz. This frequency was chosen to reduce the leakage contribution to the P r values, and 10 kHz is a relatively high frequency when compared to previous reports on ferroelectric AlScN, which range between 333 Hz and 100 kHz [19,21]. Retention testing was used to measure the stability of a polarization state over time to inspect the volatility of that state.…”

“…While this high E c is problematic for integration with low−voltage devices, it can be valuable for retention characteristics as temperatures increase [8]. The E c of AlScN can be tuned through various modifications such as composition [14][15][16], stress [17], epitaxial alignment [18], and temperature [19][20][21][22]. In addition to thickness scaling, these trends are important for integrating AlScN into a high−temperature chip to reduce the operating voltages.…”