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1. Defects and oxygen impurities in ferroelectric wurtzite Al1− x Sc x N alloys

   https://doi.org/10.1063/5.0211892  

   Lee, Cheng-Wei; Din, Naseem Ud; Brennecka, Geoff L; Gorai, Prashun (July 2024, Applied Physics Letters)

   III-nitrides and related alloys are widely used for optoelectronics and as acoustic resonators. Ferroelectric wurtzite nitrides are of particular interest because of their potential for direct integration with Si and wide bandgap semiconductors and unique polarization switching characteristics; such interest has taken off since the first report of ferroelectric Al1−xScxN alloys. However, the coercive fields needed to switch polarization are on the order of MV/cm, which are 1–2 orders of magnitude larger than oxide perovskite ferroelectrics. Atomic-scale point defects are known to impact the dielectric properties, including breakdown fields and leakage currents, as well as ferroelectric switching. However, very little is known about the native defects and impurities in Al1−xScxN and their effect on the dielectric and ferroelectric properties. In this study, we use first-principles calculations to determine the formation energetics of native defects and unintentional oxygen incorporation and their effects on the polarization switching barriers in Al1−xScxN alloys. We find that nitrogen vacancies are the dominant native defects, and unintentional oxygen incorporation on the nitrogen site is present in high concentrations. They introduce multiple mid-gap states that can lead to premature dielectric breakdown and increased temperature-activated leakage currents in ferroelectrics. We also find that nitrogen vacancy and substitutional oxygen reduce the switching barrier in Al1−xScxN at low Sc compositions. The effect is minimal or even negative (increases barrier) at higher Sc compositions. Unintentional defects are generally considered to adversely affect ferroelectric properties, but our findings reveal that controlled introduction of point defects by tuning synthesis conditions can instead benefit polarization switching in ferroelectric Al1−xScxN at certain compositions. 

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2. Compensation of the Stress Gradient in Physical Vapor Deposited Al1−xScxN Films for Microelectromechanical Systems with Low Out-of-Plane Bending

   https://doi.org/10.3390/mi13081169  

   Beaucejour, Rossiny; D’Agati, Michael; Kalyan, Kritank; Olsson, Roy H. (August 2022, Micromachines)

   Thin film through-thickness stress gradients produce out-of-plane bending in released microelectromechanical systems (MEMS) structures. We study the stress and stress gradient of Al0.68Sc0.32N thin films deposited directly on Si. We show that Al0.68Sc0.32N cantilever structures realized in films with low average film stress have significant out-of-plane bending when the Al1−xScxN material is deposited under constant sputtering conditions. We demonstrate a method where the total process gas flow is varied during the deposition to compensate for the native through-thickness stress gradient in sputtered Al1−xScxN thin films. This method is utilized to reduce the out-of-plane bending of 200 µm long, 500 nm thick Al0.68Sc0.32N MEMS cantilevers from greater than 128 µm to less than 3 µm. 

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3. Enabling ultra-low-voltage switching in BaTiO3

   https://doi.org/10.1038/s41563-022-01266-6  

   Jiang, Y.; Parsonnet, E.; Qualls, A.; Zhao, W.; Susarla, S.; Pesquera, D.; Dasgupta, A.; Acharya, M.; Zhang, H.; Gosavi, T.; et al (May 2022, Nature Materials)

   Single crystals of BaTiO3 exhibit small switching fields and energies, but thin-film performance is considerably worse, thus precluding their use in next-generation devices. Here, we demonstrate high-quality BaTiO3 thin films with nearly bulk-like properties. Thickness scaling provides access to the coercive voltages (<100 mV) and fields (<10 kV cm−1) required for future applications and results in a switching energy of <2 J cm−3 (corresponding to <2 aJ per bit in a 10 × 10 × 10 nm3 device). While reduction in film thickness reduces coercive voltage, it does so at the expense of remanent polarization. Depolarization fields impact polar state stability in thicker films but fortunately suppress the coercive field, thus driving a deviation from Janovec–Kay–Dunn scaling and enabling a constant coercive field for films <150 nm in thickness. Switching studies reveal fast speeds (switching times of ~2 ns for 25-nm-thick films with 5-µm-diameter capacitors) and a pathway to subnanosecond switching. Finally, integration of BaTiO3 thin films onto silicon substrates is shown. We also discuss what remains to be demonstrated to enable the use of these materials for next-generation devices. 

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4. Strongly enhanced second-order optical nonlinearity in CMOS-compatible Al1− x Sc x N thin films

   https://doi.org/10.1063/5.0061787  

   Yoshioka, Valerie; Lu, Jian; Tang, Zichen; Jin, Jicheng; Olsson, III, Roy_H; Zhen, Bo (October 2021, APL Materials)

   Silicon photonics has enabled large-scale production of integrated optical devices for a vast array of applications. However, extending its use to nonlinear devices is difficult since silicon does not exhibit an intrinsic second-order nonlinearity. While heterogeneous integration of strongly nonlinear materials is possible, it often requires additional procedures since these materials cannot be directly grown on silicon. On the other hand, CMOS-compatible materials often suffer from weaker nonlinearities, compromising efficiency. A promising alternative to current material platforms is scandium-doped aluminum nitride (Al1−xScxN), which maintains the CMOS compatibility of aluminum nitride (AlN) and has been used in electrical devices for its enhanced piezoelectricity. Here, we observe enhancement in optical second-order susceptibility (χ(2)) in CMOS-compatible Al1−xScxN thin films with varying Sc concentrations. For Al0.64Sc0.36N, the χ(2) component d33 is enhanced to 62.3 ± 5.6 pm/V, which is 12 times stronger than intrinsic AlN and twice as strong as lithium niobate. Increasing the Sc concentration enhances both χ(2) components, but loss increases with a higher Sc concentration as well, with Al0.64Sc0.36N exhibiting 17.2 dB/cm propagation loss at 1550 nm and Al0.80Sc0.20N exhibiting 8.2 dB/cm at 1550 nm. Since other material properties of this alloy are also affected by Sc, tuning the Sc concentration can balance strong nonlinearity, loss, and other factors depending on the needs of specific applications. As such, Al1−xScxN could facilitate low cost development of nonlinear integrated photonic devices. 

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5. Characterization of ferroelectric switching in 43 nm Y-36 lithium niobate films

   https://doi.org/10.1063/5.0285779  

   Hurtado, L; Piazza, G (August 2025, Applied Physics Letters)

   Lithium niobate (LN) is a promising ferroelectric material used for emerging memories and radio frequency (RF) micromechanical resonators. The ferroelectric behavior of the bulk properties of LN has been well studied, and investigations on thin films have shown promising performance. However, the macroscopic ferroelectric properties of LN films that are sub-100 nm thick, which are desired to truly harness the advantages and scalability of the material, have not been explored. Here, we report the ferroelectric properties of 43 nm ultra-thin films of Y-36 LN sandwiched between two metal electrodes. Y-36 is a particularly promising cut for RF microacoustics, but can also be employed for integrated memories and photonics. Switching occurred with an average positive coercive field (+Ec) of 0.92 MV cm−1 and an average −Ec of 0.39 MV cm−1, resulting in the ability to switch the film polarization with < 2 V. The 43 nm film maintains a large positive remanent polarization (+PR) of 58 μC·cm−2 and a -PR of 55 μC·cm−2. The thin film shows excellent endurance, maintaining a stable PR value after 1 billion polarization switching cycles. Retention characteristics also show stable PR value after 100 s. Additionally, findings indicate a power series relationship of Ec∝ Fβ with β value at 0.253 and 0.053 for +Ec and −Ec, respectively. Overall, the characterization of these ultra-thin films of LN showcase its potential for miniaturization and application to tunable RF acoustics or emerging memories. 

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