Masato Uehara

Thin films with wurtzite crystal structure feature some of the best compatibility with the major semiconductor platforms among ferroelectrics, as well as high remanent polarization, excellent stability, and scalability; making them very attractive for microelectronics applications ranging from memories to sensors and actuators. Their intrinsic functionality, which enables these applications directly links device performance to the underlying growth processes. This article gives an overview of the three main deposition methods for the material class (sputtering, molecular beam epitaxy, metal–organic chemical vapor deposition), their individual advantages as well as how they can contribute to solving the main challenges that remain to be overcome in order to bring wurtzite ferroelectrics to large-scale applications. Furthermore, it differentiates the growth of wurtzite ferroelectrics from that of more established thin-film ferroelectrics. Graphical

Alloying wurtzite aluminum nitride and gallium nitride (GaN) with scandium (Sc) enhances the piezoelectric and ferroelectric properties, but increasing the Sc concentration while maintaining the wurtzite phase is a major challenge. We meticulously optimized the sputtering conditions to successfully obtain a high-Sc-concentration GaN alloy (ScxGa1−xN, 0.44 < x ≤ 0.53), which has not yet been achieved. The prepared ScxGa1−xN films exhibit excellent piezoelectricity and ferroelectricity. The maximum piezoelectric constant d33 of 33 pC/N exceeds the values of ScxAl1−xN used in radio frequency filters. The minimum coercive field of ferroelectricity Ec of 1.49 MV/cm is the lowest of any ferroelectric wurtzite to date and is comparable to those of state-of-the-art ferroelectric HfO2-based materials used in compute-in-memory for artificial intelligence. Structural analysis showed that the ScxGa1−xN films have a c-axis-oriented wurtzite texture without a rock salt phase. The lattice constant ratio c/a varies greatly depending on the sputtering conditions. Decreasing c/a causes d33 to increase almost linearly, and minimizing Ec requires a small c/a and high Sc concentration.

Plasma discharge pulse length (tPLength) was investigated for its impact on the crystallinity of GaN films deposited on a sapphire substrate using a high-density convergent plasma sputtering device (CPSD). The study covered tPLength values from 1 to 200 ms, maintaining the substrate temperature at 200 °C. GaN films showed preferential orientation along the (0002) plane for all tPLength settings. X-ray diffraction analysis revealed a heteroepitaxial-like growth pattern with a sixfold symmetric diffraction pattern corresponding to GaN{10−10} planes. At a tPLength of 200 ms, the full width at half maximum of the rocking curve at GaN (0002) diffraction angle decreased to 1.6°. Optimizing the deposition rate per plasma discharge pulse with CPSD indicated the importance of selecting an optimal tPLength for achieving desirable crystalline properties in GaN film sputtering deposition.

In this study, the piezoelectric properties of scandium-alloyed gallium nitride (ScGaN), which is expected to be applied to microelectromechanical systems devices, are evaluated by first-principles calculations. The piezoelectric constant (d33) of GaN is found to increase by up to approximately 30 times upon the addition of 62.5 mol. % of Sc. The piezoelectric stress constant (e33) increases and the elastic constant (C33) decreases with increasing Sc content of ScGaN, driving the rise of d33. The improved piezoelectric properties of ScGaN compared with those of GaN are largely attributed to elastic softening, which is thought to be related to the transition from a wurtzite to hexagonal boron nitride (h-BN) structure driven by the change in bonding states between atoms caused by the addition of Sc to GaN. The crystal orbital Hamilton population analysis suggests that addition of Sc to GaN results in the combination of weaker Sc–N and Ga–N bonding, which makes the crystal structure unstable. This weakened bonding is thought to be the main cause of the destabilization of the wurtzite structure and transition to the h-BN structure of ScGaN. The elastic softening associated with this structural transition leads to the dramatic improvement in piezoelectric properties.

In this study, the switching kinetics in a (001)-out-of-plane-oriented (Al0.8Sc0.2)N thin film were systematically investigated as a function of temperature and frequency. The switching behavior followed the Kolmogorov–Avrami–Ishibashi model within the temperature and write-pulse width ranges of 300–473 K and 10 Hz–600 kHz, respectively. The switching speed increased linearly with the electric field, even at high electric fields. The frequency dependence of the coercive field (Ec) evaluated at room temperature revealed that the scaling regime of the domain-wall propagation did not transition to the flow regime, and the switching velocity of the domain increased linearly with the electric field even at high frequencies up to 600 kHz. The invariant domain-wall motion regime implies the potential of using (Al,Sc)N films for high-frequency applications. In addition, a formula for estimating the coercive field of an (Al,Sc)N film was derived by fitting the parameters obtained from the study of switching behavior. The formula indicated that the switching electric field tends to become insensitive to frequency as temperature increases. Moreover, the frequency dependency of the field was minimal even at room temperature. This is a useful index for estimating the drive voltage at the operating frequency and temperature.

The lattice of wurtzite nitrides such as gallium nitride (GaN) and aluminum nitride (AlN) is distorted by alloying with scandium (Sc). This lattice distortion, which entails a decrease in the lattice constant ratio c/a, leads to a significant improvement in piezoelectricity and the appearance of ferroelectricity, which are attractive properties for material applications. However, systematic studies on the effect of different alloying elements on the lattice distortion of wurtzite nitrides are scarce and restricted to theoretical investigations. Herein, the experimental investigation of the lattice distortion and its influence on the piezoelectricity of cosputtering‐prepared thin films of GaN alloyed with group III and rare‐earth elements other than Sc, that is, yttrium, dysprosium, and ytterbium is described. All the investigated elements greatly reduce the lattice constant ratio of GaN from 1.63 to 1.61 and cause a twofold increase in the piezoelectric charge coefficient. Therefore, it is indicated in the experimental results that alloying with elements other than Sc also results in the distortion of the crystal lattice and the enhancement of the piezoelectric properties of GaN. Furthermore, regardless of the particular alloying element used, a correlation between lattice distortion and piezoelectricity is experimentally demonstrated.

In this study, Sc concentration dependence of Sc _x Al_1−x N/AlN/poly-crystalline diamond/Si surface acoustic wave (SAW) characteristics at high Sc from 23.8% to 44.3% was investigated by fabricating one-port SAW resonator at high frequency. 3.8 GHz one-port resonator fabricated on Sc_0.43Al_0.57N showed an excellent performance of electro-mechanical coupling coefficient (K ²) as high as 6.34% for 2nd mode Sezawa wave, which enables a wide bandwidth in high frequency applications. The temperature coefficient of frequency was approximately −40 to −50 ppm deg⁻¹ for the device fabricated with Sc concentration of 42.9%. This is a smaller value compared to conventional high K² bulk materials such as LiNbO₃. As the result, a high K² 6.34% material system at a higher Sc concentration of ScAlN/AlN/PCD was found to be possible at a high phase velocity of 7000 m s⁻¹. Combined with the extremely high-power durability of diamond based device, high-power durable wideband SAW device at high frequency can be expected.

The ferroelectricity of wurtzite, Sc _x Ga_1−x N (x = 0.35–0.44), was demonstrated in a metal–ferroelectric–metal capacitor. The remanent polarization (P _r) obtained from positive-up negative-down measurements was high, exceeding 120 μC cm⁻². The coercive field (E _c) of Sc_0.44Ga_0.56N was approximately 3.6 MV cm⁻¹ at 300 K, which decreased to 3 MV cm⁻¹ at 473 K. We observed that regardless of the host material, the Sc concentration governed the E _c value in Sc _x Ga_1−x N and Sc _x Al_1−x N, suggesting that the polarization switching started in the wurtzite unit cells containing Sc. Additionally, the E _c of Sc _x Ga_1−x N was lower than that of Sc _x Al_1−x N when P _r was equivalent.