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

Abstract

Currently, there is a lack of nonvolatile memory (NVM) technology that can operate continuously at temperatures >200 °C. While ferroelectric NVM has previously demonstrated long polarization retention and >1013 read/write cycles at room temperature, the largest hurdle comes at higher temperatures for conventional perovskite ferroelectrics. Here, we demonstrate how AlScN can enable high-temperature (>200 °C) nonvolatile memory. The c-axis textured thin films were prepared via reactive radiofrequency magnetron sputtering onto a highly textured Pt (111) surface. Photolithographically defined Pt top electrodes completed the capacitor stack, which was tested in a high temperature vacuum probe station up to 400 °C. Polarization–electric field hysteresis loops between 23 and 400 °C reveal minimal changes in the remanent polarization values, while the coercive field decreased from 4.3 MV/cm to 2.6 MV/cm. Even at 400 °C, the polarization retention exhibited negligible loss for up to 1000 s, demonstrating promise for potential nonvolatile memory capable of high−temperature operation. Fatigue behavior also showed a moderate dependence on operating temperature, but the mechanisms of degradation require additional study.

Thermal Stability of the Ferroelectric Properties in 100 nm-Thick Al0.72Sc0.28N

“…A reduction of more than 40% in the positive coercive field ( E c+ ) is observed when the temperature reaches 673 K, while the negative coercive field ( E c– ) decreases by more than 50% in the same temperature range, as reported in Figure a and Figure b, respectively. This strong reduction of the coercive field with increasing temperature is in agreement with what was already observed in literature for Al 1– x Sc x N layers, even at different Sc-contents. − This result can be explained by the reduction of the energy that must be provided electrically to switch the ferroelectric layer to the other stable polarization state caused by the thermal energy provided to the system by increasing the temperature. Microscopically, this could correspond to an easier domain wall propagation and/or an increase in the rate of nucleation of new domains. − The only domain walls that may exist are the 180° ones that separate domains with antiparallel polarization since no ferroelastic phenomenon has been reported so far in Al 1– x Sc x N. In this regard, the well-known needle-like model proposed by Merz for BaTiO 3 could be applied to Al 1– x Sc x N. In this model, new extremely thin antiparallel domains grow only in the direction of the applied electric field, and their nucleation rate is accelerated by temperature.…”

“…The lack of experimental evidence for the existence of domain walls and their propagation in Al 1– x Sc x N prevents us from concluding whether the microscopic mechanism is dominated by domain wall propagation or nucleation phenomena. The temperature variation can also induce internal stress, which has been demonstrated to impact the switching dynamics in Al 1– x Sc x N and influence its coercive field. , All previous works about the thermal stability of Al 1– x Sc x N ferroelectric properties report the monotonic linear decrease in the coercive field with increasing temperature up to 673 K. − This behavior is also observed in this work during both heating and cooling sweeps, especially for E c– , as reported in Figure b. The non-perfect linear fit for E c+ shown in Figure a could be associated with device-to-device variability.…”

“…The nonhysteretic E c temperature dependence in Al 1−x Sc x N observed by Gund et al 23 up to 466 K can actually be achieved in a much broader temperature range up to 673 K. Figure 2c and Figure 2d report the temperature dependence of both P sw± and FWHM ± for positive and negative applied electric fields, respectively. Drury et al 24 already reported a stable value of P r during heating of a 225 nm-thick Al 0.7 Sc 0.3 N up to 673 K. Figure 2c,d shows that the stability of P sw± is also obtained for 100 nm-thick Al 0.72 Sc 0.28 N in this temperature range. Cooling down the thin film capacitors to room temperature, an initial reduction of both positive and negative switching polarization values is observed.…”

“…The combination of temperature-dependent electrical measurements with in situ temperature-dependent X-ray diffraction analyses, scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDS) is used to carefully examine the temperature-induced modifications of the ferroelectric capacitors. This study represents a step forward in the understanding of the temperature stability of the ferroelectric properties in Al 1– x Sc x N with respect to previous works, which were mainly focused on electrical characterization or material understanding. − Persistent ferroelectric switching with no drop of polarization is confirmed up to 673 K, while irreversible changes start to appear when the ferroelectric capacitors are measured above this temperature. These irreversible changes result in an increase in coercive fields when the capacitors are cooled down, the appearance of wake-up behavior, and a reduction of both measured polarization and relative dielectric constant.…”

“…This explains why Al 1– x Sc x N has gained a lot of attention as one of the main candidates for future high-temperature non-volatile memories. The wurtzite-type crystalline structure of Al 1– x Sc x N deposited on molybdenum (Mo) bottom electrodes was demonstrated to be stable even at 1298 K for thicknesses above 400 nm. , Temperature-dependent electrical measurements of the ferroelectric properties in Al 1– x Sc x N pointed out a strong reduction of the coercive field as well as an impressive P r stability with increasing temperature up to 673 K. − All previous reports on the temperature stability of ferroelectric properties in Al 1– x Sc x N consider films with a thickness above 200 nm. The only exception is the work of Mizutani et al, in which the maximum temperature investigated is 523 K. However, a complete study combining both material characterization and electrical techniques for high temperatures is still missing in films with a thickness below 200 nm.…”