Correct quantification of oxygen vacancies in ferroelectric hafnia

Abstract

Determination of the oxygen vacancies (VO) distribution within ultra-thin film ferroelectric hafnia (HfO2)-based devices is crucial to engineering optimal properties for non-volatile memory and logic devices. So far, x-ray photoelectron spectroscopy (XPS) combined with Ar+ ion sputtering has been the predominantly used approach for quantifying the VO concentration. Here, we show that using Ar+ ion sputtering to depth-profile the hafnia-based film affects film chemistry and can introduce errors in VO estimation by up to an order of magnitude. As a result, this method should be approached with more caution. This paper demonstrates that non-destructive, hard x-ray photoemission (HAXPES) using synchrotron radiation ought to be favored. In addition, we show that reliable, quantitative evaluation of the physical chemistry is compromised by surprising and commonplace mistakes in parameters used for Hf 4f core-level spectral analysis. Third, 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 measurement of 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, which should allow progress in materials engineering of ferroelectric devices.