PAMS Seminar: "Epitaxial AlScN Films and Nanowires for Ferroelectric Random-Access Memory Applications" by Dr. Andrew Meng

Dr. Andrew Meng
University of Missouri
Department of Physics and Astronomy

Abstract:
AlN-based alloys are promising candidate ferroelectric materials for neuromorphic computing applications. Compared to traditional complex oxide ferroelectrics, AlN-based materials exhibit relatively higher remanent polarizations and have potential for compatibility with Si processing. We report growth of epitaxial wurtzite AlN and AlScN thin films on Si(111) substrates with a wide range of Sc concentrations using ultra-high vacuum reactive sputtering. We also report growth of single crystal AlScN nanowires on TiN buffered Si(111) substrates. Sc alloying in AlN enhances piezoelectricity and induces ferroelectricity, and epitaxial thin films and nanowires facilitate systematic structure-based investigations of this important and emerging class of materials. Two main effects are observed as a function of increasing Sc concentration in AlScN thin films. First, increasing crystalline disorder is observed together with a structural transition from wurtzite to rocksalt at ~30 at% Sc. Second, nanoscale compositional segregation consistent with spinodal decomposition occurs at intermediate compositions, before the wurtzite to rocksalt phase boundary is reached. Lamellar features arising from composition fluctuations are correlated with polarization domains in AlScN, suggesting that composition segregation can influence ferroelectric properties. The present results provide a route to the creation of single crystal AlScN films on Si(111), as well as a means for self-assembled composition modulation.While increasing Sc incorporation increases mosaic disorder, nanowire growth provides a means to reduce mosaic disorder through finite size effects. We demonstrate ferroelectric properties of AlScN nanowires as measured by piezoforce response microscopy. Finally, we report electrical characteristics of AlScN films with ~12 at% Sc with different growth conditions on TiN buffered Si(111) substrates. The data are consistent with electrical conduction arising from point defects formed during AlScN film growth. The results provide a structure-based approach to minimizing electrical leakage in AlScN devices, which is one of the main challenges in these materials.

Free