Cycling Induced Imprint Phenomenon at Intermediate State used for Multi-Level Operation in HfO₂-FeFET

We present a detailed understanding of the imprint behavior during cycling at intermediate states, consisting of mixed programmed and erased domains, used for multi-level operation in Hf0<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>-FeFET devices. Analysis of the transient current, corresponding to spontaneous polarization reversal, clarified that domains, kept at the same state, are independently affected by imprint even if the neighboring domains are reversed every time during cycling, which is a possible scenario during multi-level operation. Furthermore, imprint phenomenon during cycling is more severe compared to imprint during retention, indicating that electrical stress without spontaneous polarization reversal accelerates the imprint effect. We demonstrate two strategies for reliable multi-level operation: (I) full program/erase before programming of intermediate states and (II) addition of full program/erase (refresh) after cycling.