Deep UV Photo Emission Electron Microscopy to Spatially Probe Work Function in Oxygen Vacancy Reduced Hf_xZr_(1-x)O₂

Oxygen vacancies are known to both stabilize the ferroelectric orthorhombic phase of Hf_xZr_(1-x)O₂ (HZO) and promote leakage pathways limiting endurance of devices based on this material. For this reason, the energy states of oxygen vacancies were investigated using photoemission electron microscopy (PEEM) as their concentration was varied via laser exposure. Following a controlled oxygen vacancy (V<i>_O</i>) reduction via incremental and areal laser dosing on HZO, 213nm deep-ultraviolet (DUV) photoemission electron microscopy (PEEM) was used to spatially probe the resulting modification in work function. Work function was found to increase monotonically with the laser-induced reduction in oxygen vacancy concentration culminating in a total increase near 70 meV. The change implies a Fermi-level shift toward the valence band as the total available charge states are reduced as oxygen vacancies are removed. A reduction in charge states is also supported by the observed reduction in photo emission yield after laser dosing. Beyond the quantitative change, the smooth monotonic change in work-function and photoemission yield suggest the presence of a “defect band” that forms in HZO due to the presence of vacancies rather than a single discrete state.