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1. 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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2. Colossal Electromechanical Response in Antiferroelectric‐based Nanoscale Multilayers

   https://doi.org/10.1002/adma.202419690  

   Acharya, Megha; Alaerts, Louis; Banyas, Ella; Kang, Deokyoung; Ricci, Francesco; Pan, Hao; Hanrahan, Brendan; Spanier, Jonathan E; Neaton, Jeffery B; Hautier, Geoffroy; et al (April 2025, Advanced Materials)

   Abstract The pursuit of smaller, energy‐efficient devices drives the exploration of electromechanically active thin films (<1 µm) to enable micro‐ and nano‐electromechanical systems. While the electromechanical response of such films is limited by substrate‐induced mechanical clamping, large electromechanical responses in antiferroelectric and multilayer thin‐film heterostructures have garnered interest. Here, multilayer thin‐film heterostructures based on antiferroelectric PbHfO₃and ferroelectric PbHf_1‐xTi_xO₃overcome substrate clamping to produce electromechanical strains >4.5%. By varying the chemistry of the PbHf_1‐xTi_xO₃layer (x = 0.3‐0.6) it is possible to alter the threshold field for the antiferroelectric‐to‐ferroelectric phase transition, reducing the field required to induce the onset of large electromechanical response. Furthermore, varying the interface density (from 0.008 to 3.1 nm⁻¹) enhances the electrical‐breakdown field by >450%. Attaining the electromechanical strains does not necessitate creating a new material with unprecedented piezoelectric coefficients, but developing heterostructures capable of withstanding large fields, thus addressing traditional limitations of thin‐film piezoelectrics. 

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3. Silicon-doped β -Ga ₂ O ₃ films grown at 1 µ m/h by suboxide molecular-beam epitaxy

   https://doi.org/10.1063/5.0139622  

   Azizie, Kathy; Hensling, Felix V.; Gorsak, Cameron A.; Kim, Yunjo; Pieczulewski, Naomi A.; Dryden, Daniel M.; Senevirathna, M. K.; Coye, Selena; Shang, Shun-Li; Steele, Jacob; et al (April 2023, APL Materials)

   We report the use of suboxide molecular-beam epitaxy ( S-MBE) to grow β-Ga 2 O 3 at a growth rate of ∼1 µm/h with control of the silicon doping concentration from 5 × 10 16 to 10 19  cm −3 . In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga 2 O, i.e., gallium suboxide, is supplied. Directly supplying Ga 2 O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of β-Ga 2 O 3 by conventional MBE. As a result, a growth rate of ∼1 µm/h is readily achieved at a relatively low growth temperature ( T sub ≈ 525 °C), resulting in films with high structural perfection and smooth surfaces (rms roughness of <2 nm on ∼1 µm thick films). Silicon-containing oxide sources (SiO and SiO 2 ) producing an SiO suboxide molecular beam are used to dope the β-Ga 2 O 3 layers. Temperature-dependent Hall effect measurements on a 1 µm thick film with a mobile carrier concentration of 2.7 × 10 17  cm −3 reveal a room-temperature mobility of 124 cm 2  V −1  s −1 that increases to 627 cm 2  V −1  s −1 at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working metal–semiconductor field-effect transistors made from these silicon-doped β-Ga 2 O 3 films grown by S-MBE at growth rates of ∼1 µm/h. 

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4. A comparative consideration of the link between poling procedure, induced damage, and piezoelectric response in perovskite ferroelectrics

   https://doi.org/10.1063/5.0270025  

   Mervosh, Michael W; Jones, Haley; Argüelles, Andrea P; Trolier-McKinstry, Susan; Randall, Clive A (June 2025, Journal of Applied Physics)

   There has been much interest in recent years in improving Direct Current Poling (DCP) for piezoelectric materials. Some of the more promising substitutes include Alternating Current Poling (ACP), Water Quench Poling (WQP), and ACP with Field Cooling (ACP-FC). This paper summarizes the merits of these poling strategies and compares them to pulse poling. The results show that pulse poling outpaces both DCP and ACP in terms of the magnitude of piezoelectric response across a range of materials. Hard and soft piezoelectric samples in both single crystal and textured form were poled using all these techniques. For the single crystal samples (with compositions of Mn: Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT) and Sm: PIN–PMN–PT), pulse poling generated the greatest increase in d33 and keff relative to DCP, with both piezoelectrics seeing increases above 65%. In the case of the {001} textured Mn: PMN–PZT–PT material, pulse poling and ACP-FC reduced the loss of the system and improved its mechanical quality factor (Qm) by 20% and 4%, respectively. These phenomena were further investigated via Rayleigh analysis to quantify each poling strategy’s impact on domain wall dynamics. The textured ceramic samples showed lower overall values of α (which is related to the mobility and concentration of domain walls) when compared to the single crystals. It was found that α decreased for the unconventionally poled textured samples relative to DCP, whereas the single crystals’ α values increased. Samples that underwent WQP experienced significant microcracking, limiting possible applications. 

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5. MnO(001) thin films on MgO(001) grown by reactive MBE using supersonic molecular beams

   https://doi.org/10.1063/5.0198832  

   Pedersen, Andrew J; Liu, Junchen; Li, Fanxing; Lamb, H Henry (April 2024, The Journal of Chemical Physics)

   MnO(001) thin films were grown on commercial MgO(001) substrates at 520 °C by reactive molecular beam epitaxy (MBE) using Mn vapor and O2-seeded supersonic molecular beams (SMBs) both with and without radio frequency (RF) plasma excitation. For comparison, MnO(001) films were grown by reactive MBE using O2 from a leak valve. X-ray photoelectron spectroscopy confirmed the Mn2+ oxidation state and 10%–15% excess oxygen near the growth surface. Reflection high-energy electron diffraction and x-ray diffraction evidenced that the films were rock salt cubic MnO with very strong (001) orientation. High-angle annular dark field scanning transmission electron microscopy with energy-dispersive x-ray spectroscopy demonstrated abrupt MnO/MgO interfaces and indicated [(001)MnO||(001)MgO] epitaxial growth. Ex situ atomic force microscopy of films deposited without RF excitation revealed smooth growth surfaces. An SMB-grown MnO(001) film was converted to Mn3O4 with strong (110) orientation by post-growth exposure to an RF-discharge (RFD) SMB source providing O atoms; the surface of the resultant film contained elongated pits aligned with the MgO110 directions. In contrast, using the RFD-SMB source for growth resulted in MnO(001) films with elongated growth pits and square pyramidal hillocks aligned along the MgO110 and 100 directions, respectively. 

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