Nitrogen Vacancies Induce Fatigue in Ferroelectric $\mathrm{Al_{0.93}B_{0.07}N}$

Abstract: Wurtzite ferroelectrics (e.g., $\mathrm{Al_{0.93}B_{0.07}N}$) are being explored for high-temperature and emerging near-, or in-compute, memory architectures due to the material advantages offered by their large remanent polarization and robust chemical stability. Despite these advantages, current $\mathrm{Al_{0.93}B_{0.07}N}$ devices do not have sufficient endurance lifetime to meet roadmap targets. To identify the defects responsible for this limited endurance, a combination of electronic measurements and optical spectroscopies characterized the evolution of defect states within $\mathrm{Al_{0.93}B_{0.07}N}$ with cycling. Ultrathin ($\sim$10 nm) metal contacts were used to optically probe regions subject to ferroelectric switching; photoluminescence spectroscopy identified the emergence of a transition near 2.1 eV whose intensity scaled with the non-switching polarization quantified via positive-up negative-down (PUND) measurements. Accompanying thermally stimulated depolarization current (TSDC) and modulus spectroscopy measurements also observed the strengthening of a state near 2.1 eV. The origin of this feature is ascribed to transitions between a nitrogen vacancy and another defect deeper in the bandgap. Recognizing that the impurity concentration is largely fixed, strengthening of this transition indicates an increase in the number of nitrogen vacancies. Switching, therefore, creates vacancies in $\mathrm{Al_{0.93}B_{0.07}N}$ likely due to hot-atom damage induced by the aggressive fields necessary to switch wurtzite materials that ultimately limits endurance.