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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The aim of this study is to obtain a systematic insight into the scandium (Sc) concentration dependence of the spontaneous polarization (Psp) in ScxAl1−xN by using first-principles calculations. From the calculated results, we observe that a structural change from a c-axis polarized wurtzite structure to a nonpolarized layered hexagonal structure takes place at x = 0.67, and the spontaneous polarization decreases nonlinearly with a change in the Sc concentration. Furthermore, we calculate the spontaneous polarizations from each cation individually and find that the spontaneous polarization from aluminum–nitrogen (Al–N) (P sp AlN ) is higher than that from scandium–nitrogen (Sc–N) (P sp ScN ). This difference in spontaneous polarization arising from the cationic origin is attributed to the difference in the internal parameter along the c axis (Δ u 3 k) based on the layered hexagonal structure. [ABSTRACT FROM AUTHOR]

The ferroelectricity of (Al1−xScx)N (x = 0–0.34) thin films with various thicknesses was investigated. (Al1−xScx)N films were prepared at 400 °C on (111)Pt/TiOx/SiO2/(001)Si substrates by the radio frequency dual-source reactive magnetron sputtering method using Al and Sc targets under pure N2 gas or a mixture of N2 and Ar gases. The film deposited under N2 gas showed larger remanent polarization than those under N2 + Ar mixture. Ferroelectricity was observed for films with x = 0.10–0.34 for about 140-nm-thick films deposited under N2 gas. The x = 0.22 films showed ferroelectricity down to 48 nm in thickness from the polarization–electric field curves and the positive-up-negative-down measurement. The ferroelectricity of the 9 nm-thick film also was ascertained from scanning nonlinear dielectric microscopy measurement. These results reveal that ferroelectric polarization can switch for films with much broader compositions and thicknesses than those in the previous study. [ABSTRACT FROM AUTHOR]