Defining Properties via Material Microstructure—Defect and Strain Engineering Approaches for Developing Ferroelectric AlScN

AlN has long been used as a low loss piezoelectric material but only recently has it emerged that with the addition of scandium Al_xSc_1-xN also becomes ferroelectric [2] AlScN has a high electromechanical coupling coefficient (k²), low loss tangent, and high piezoelectric coefficient (d₃₃ AlScN ∼ 44 pC/N), along with a large polarization [3]. Unfortunately, the electric fields needed to reorient the polarization, the coercive field, of AlScN is near the breakdown strength of the material, reducing the reliability and viability for memory or electronics applications [3]. Strain engineering approaches can be used to lower the <b><i>coercive field (E_c), </i></b>reducing the energy needed to switch the ferroelectric dipole, because strain favors the layered hexagonal structure. Furthermore, defect engineering can enhance piezoelectric properties and manifest ferroelectric properties.<br/><br/>This talk details the development the impact of strain, defects, and doping on the ferroelectric and piezoelectric response of AlScN thin films. Al_xSc_1-xN thin films are fabricated on a range of different substrates using DC and RF sputtering techniques with varying N2:Ar gas ratios, from 350 - 400 degrees C. X-ray photoelectron spectroscopy (XPS) studies show changes in Sc target power influence not only the concentration of Sc in the microstructure but also the crystalline quality. The impact of Sc on the lattice is further corroborated through X-Ray Diffraction (XRD) studies which show a structural shift towards as a function of increased Sc, up to Sc_0.4. Phase formation beginning at 350 degrees C, but enhanced crystalline orientation begins above 400 degrees C. Ferroelectric, piezoelectric, and dielectric measurements are used to track the impact of microstructure and composition on the ferroelectric and dielectric response. Polarization-electric field loops with defining hysteretic ferroelectric shapes and observations of coercive field shifts up to 30V away from the breakdown voltage are presented. The impact of defects on breakdown strength will be discussed. Piezoresponse force microscopy and First-Order Reversal Curve (FORC) measurements demonstrate the domain kinetics across microstructures; CV and PUND measurements illustrate film quality, and further characterize the contributing and shifting electrical responses. Overall, this work provides insight into the impact of film microstructure on the ferroelectric response as a function of doping, strain, and defect concentration.<br/><br/><b><u>References</u></b><br/>[1] Esteves, Giovanni, et al. "AlN/SiC MEMS for high-temperature applications." Journal ofMicroelectromechanical Systems 28.5 (2019): 859-864<br/>[2] Fichtner, Simon, et al. "AlScN: A III-V semiconductor based ferroelectric." Journal of Applied Physics 125.11 (2019): 114103.<br/>[3] Lu, Yuan. Development and characterization of piezoelectric AlScN-based alloys for electroacoustic applications. Diss. Albert-Ludwigs-Universität Freiburg im Breisgau, 2019.