Understanding Fatigue and Wake-Up in Ultra-Thin Ferroelectric Hf_0.5 Zr_0.5O₂ Capacitors Using Cryogenic Measurements

The need for logic and memory devices that are smaller, faster, and more energy-efficient is constantly growing. This has intensified the demand for innovative materials capable of scaling down and aligning with current CMOS processes. A next generation memory solution is non-volatile memories that are energy efficient and can enable new computing architectures. For high performance computing systems such as quantum computing, cryogenic embedded memories are essential that would have an added benefit of eliminating thermally induced performance degradation mechanisms. One material that shows promise for nonvolatile memory applications is HfO₂-based ferroelectric materials and research in this area has surged due to potential memory applications such as 1T1C ferroelectric random-access memory (FeRAM) and ferroelectric field-effect transistors (FeFET).<br/>The HfO₂-based ferroelectric materials exhibit an increase (wake-up) or decrease (fatigue) of the remnant polarization with switching cycles which causes major reliability issues in device applications. Mechanisms such as field-induced phase evolution, domain depinning and defect redistribution with field cycling are reported to cause the observed polarization evolution with field cycling. The thermally activated nature of the above-mentioned mechanisms may produce a significant change in polarization endurance when the measurement temperature is varied. In our present work, we explore the effect of measurement temperature on the functional properties of ferroelectric Hf_0.5Zr_0.5O₂ (HZO) capacitors to further the understanding of mechanisms responsible for the observed endurance characteristics. Measurement are performed in a cryogenic probe station, which has the capability to reach extremely low temperatures as low as 10 Kelvin.<br/>HZO capacitors of varying thickness (ranging from 4 nm to 10 nm) and with both Mo and TiN electrodes are examined. The HZO films have different crystalline phase fractions in the as-deposited (pristine) state. For HZO capacitors that exhibit electrical characteristics expected of films with majority ferroelectric orthorhombic phase (O-phase) in the pristine state, a decrease in remnant polarization and an increase in coercive voltage are observed as the measurement temperature is decreased from room temperature to 10 K. The polarization reduction is recovered by increasing the temperature back to room temperature, suggesting a strong effect of thermally activated domain wall motion at low temperatures. No fatigue during voltage cycling to switch the polarization is observed at low temperatures. On the other hand, pristine HZO capacitors with pinched polarization hysteresis (P-V) loops at room temperature and that, therefore, are expected to have majority non-ferroelectric tetragonal phase (T-phase), show negligible loop pinching and a large increase in switchable remanent polarization at cryogenic temperatures. This suggests that modulating the phase fraction in HZO films can change the temperature dependence of polarization switching and field cycling endurance. The rate of wake-up for these samples is reduced at low temperatures. Synchrotron X-ray diffraction measurements are performed to further understand the temperature dependence of functional properties of HZO capacitors and their relation to crystalline phase fraction.