Realization of Deterministic Multilevel Polarization States in Ferroelectric Hf0.5Zr0.5O2-Based Capacitors

Thin-film materials used as functional layers in ferroelectric random-access memory (FeRAM) devices are polycrystalline, which potentially allows to set intermediate quasi-independent polarization states. Ferroelectric Hf0.5Zr0.5O2 (HZO) is of particular interest and a leading candidate owing to its excellent CMOS compatibility. Here, we describe the methodology to implement multilevel polarization switching in TiN/HZO/TiN metal-ferroelectric-metal (MFM) capacitors as a compact alternative to FeRAM cell scaling and as a model system for further ferroelectric field-effect transistor (FeFET) implementation. Using binary search, we find the voltage amplitudes to write the required intermediate polarization values, and then stabilize them using a closed-loop software verification scheme that adjusts the write amplitude to maintain a reference polarization. Following this approach, we reproducibly set and read 8 well-separated intermediate states (2Pr = 4–28 μC/cm2). When operating in a closed-loop, we observe a slight softening of the intermediate states, while the most saturated state remains relatively stable on the timescale of the measurements; a qualitative explanation can be obtained assuming the energy distribution of charged defects in HZO band gap at the interface. Good retention is found over several hours at both room temperature and 70°C. Only small, level-dependent transients (maximum around 1 s) are observed in case of rewriting the state which subsequently relaxes and does not cause the mixing of the states. The concept of multibit operation in HZO capacitors is discussed based on the obtained results.