**1. Introduction**

Ferroelectrics are materials that possess spontaneous polarization in the absence of an applied electric field, and the direction of its polarization vector can be flipped by the applied electric field [1,2]. It is an essential component in a wide range of applications, such as non-volatile memories and radio frequency (RF) devices [3–5]. Components based on ferroelectric thin films are also being developed for a variety of sensor and actuator applications, as well as tunable microwave circuits [6,7]. Many ferroelectric materials are perovskites with drawbacks, such as low quasi-electric transition temperatures, nonlinear shifts or limited compatibility with complementary metal oxide semiconductors (CMOSs) or III-nitride technologies. These issues have so far prevented the popularization of ferroelectric functionality in microelectronics [8].

Aluminum nitride (AlN) thin films have a relatively high acoustic phase velocity, low acoustic wave loss, considerable piezoelectric coupling constant, and a coefficient of thermal expansion similar to that of Si and GaAs. These unique properties of AlN films make them widely used in mechanical, microelectronic, optical, MEMS transducers, surface wave devices (SAWs) and high-frequency broadband RF filters in the communication frontend [9,10]. AlN thin film is a III-V group semiconductor with a Wurtzite-type structure, and possesses polarization (N-polar and metal-polar) along the c-axis due to the separation of aluminum and nitrogen atoms in each plane under certain stress conditions [8]. However, AlN does not possess ferroelectricity because its polarization direction cannot be switched

**Citation:** Nie, R.; Shao, S.; Luo, Z.; Kang, X.; Wu, T. Characterization of Ferroelectric *Al*0.7*Sc*0.3*N* Thin Film on Pt and Mo Bottom Electrodes. *Micromachines* **2022**, *13*, 1629. https://doi.org/10.3390/ mi13101629

Academic Editor: Agne Žukauskait ˙ e˙

Received: 20 June 2022 Accepted: 19 July 2022 Published: 28 September 2022

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in an electric field below its own dielectric breakdown limit. In recent years, aluminum scandium nitride (AlScN) has become a hot research topic [11,12]. The significantly higher piezoelectric coefficient of AlScN compared to AlN has led to piezoelectric devices based on AlScN with high electromechanical coupling coefficients [13]. More interestingly, AlScN has been approved to be ferroelectric with a high Sc ratio, and its ferroelectric switching voltage can be flexibly adjusted depending on the Remanent stress and Sc content to meet the needs of ferroelectric thin films in a wide range of application scenarios [14,15]. The two polarization states of AlScN are shown in Figure 1.

**Figure 1.** The two polarization states formed when the scandium atom occupies the position of the aluminum atom: (**a**) N-polar and (**b**) metal-polar.

In this work, the ferroelectric properties of AlScN with a 30% Sc content on Pt and Mo bottom electrodes were studied. It was found that the ferroelectric properties of the films grown on Mo and Pt are significantly different. The leakage current of the Mo sample is quite high and the polarization value obtained from the hysteresis curve is almost double that of the Pt sample. In the PUND test, this gap was maintained and it was observed that the remanent polarizations in the positive and negative directions were not equal. Given that the top and bottom electrodes of the device are not of the same material, it can be concluded that different metal electrodes play a role in the ferroelectricity of the film [16].
