Electron trapping/detrapping at oxygen vacancies and imprint evolution in La-doped Hf0.5Zr0.5O2 ferroelectric capacitors probed by hard x-ray photoelectron spectroscopy

Abstract

We evaluate the correlation between polarization-dependent oxygen vacancy profile and imprint in technologically relevant TiN/La-doped Hf0.5Zr0.5O2/TiN ferroelectric capacitors using hard x-ray photoelectron spectroscopy (HAXPES) with in situ biasing. The concentration of double positively charged oxygen vacancies (VO..) was inferred from the intensity of reduced hafnium (Hf 3+, (5d1)) in the Hf 3d5/2 spectra, relative to fully oxidized (Hf 4+, (5d0)). HAXPES was performed using two photon energies to discriminate VO.. near the top interface from those averaged over the whole film. The observed time evolution of imprint and the extracted activation energy of ∼ 0.1 eV strongly indicate a fast, low-barrier electronic process such as shallow charge trapping/detrapping, rather than a slower field induced vacancy drift. On the ∼ 30 min timescale following polarization reversal, we propose that VO.. identified via HAXPES reflects a polarization-driven snapshot of the defect charge state (VOx↔VO..), rather than a change in the physical vacancy density previously reported. Following polarization reversal, electrons redistribute between vacancy levels and Hf 5d orbitals. This is correlated with Hf 4+↔ Hf 3+ transitions, leading to 0 ↔ 2+ vacancy charge state modifying the local electronic environment and screening behavior, particularly near interfaces. From the Schottky barrier height change due to polarization reversal, an effective screening length of 0.04 Å was estimated for the top interface. The results reveal a dominant role of polarization-dependent electron trapping/detrapping, partially screening ferroelectric polarization charges, thereby stabilizing one remanent state over the other.