Quantification of oxygen vacancies in ferroelectric hafnia

Scaling ferroelectric hafnia requires an understanding of how oxygen vacancies (VO) couple with the strain and phase composition and how they evolve with cycling. A VO distribution can produce an internal field, modifying key performance indicators such as imprint, wake-up, fatigue and leakage current.

X-ray photoelectron spectroscopy (XPS) combined with Ar+ ion sputtering has been the predominantly used approach for quantifying VO concentration. However, sputtering affects film chemistry and can overestimate VO by an order of magnitude. As a result, this method should be approached with more caution. We demonstrate that non-destructive, hard x-ray photoemission (HAXPES) using synchrotron radiation should be preferred. In addition, we show that a widespread assignation of one of the O 1s core-level peak components to the presence of VO is erroneous and leads to further errors in the VO concentration. The conclusions are supported by careful comparison between XPS and HAXPES experimental results and first-principles calculations. We provide clear indications for reliable analysis and interpretation of the photoemission data, guiding progress in materials engineering of ferroelectric devices.