Leakage current through the poly-crystalline HfO2: Trap densities at grains and grain boundaries

We investigate the role of grains and grain boundaries (GBs) in the electron transport through poly-crystalline HfO₂ by means of conductive atomic force microscopy (CAFM) measurements and trap-assisted tunneling simulations. CAFM experiments demonstrate that the leakage current through a thin dielectric film preferentially flows via the GBs. The current I-V characteristics measured on both types of sites, grains, and GBs are successfully simulated by utilizing the multiphonon trap-assisted tunneling model, which accounts for the inelastic charge transport via the electron traps. The extracted density of electrically active traps, whose energy parameters match those of the positively charged oxygen vacancies in hafnia, is ∼3 × 10¹⁹ cm^-3 at the grains, whereas a much higher value of (0.9÷2.1) × 10²¹ cm^-3 is required to reproduce the leakage current through the GBs.