Temperature-Dependent Switching Behavior and Structural Changes in Ultra-Thin Ferroelectric Hafnia-Zirconia Capacitors

There is a continually increasing demand for logic and memory devices that are smaller, faster, and that conserve energy. This has increased the need for innovative materials that can enable such functionality and be scaled down to align with current CMOS processes. Promising next-generation memory solutions that are non-volatile and can facilitate new computing architectures are greatly desired. Among the materials showing potential for non-volatile memory applications are hafnia-based ferroelectric (FE) materials. Research in this field has surged in part due to the potential of technologies like 1 transistor-1 capacitor (1T1C) FE random-access memory (FeRAM) and FE field-effect transistors (FeFET).
Much of the research on hafnia-based FE devices has concentrated on lowering programming voltage, enhancing reliability at reduced operating voltages, and ensuring compatibility with back-end-of-line (BEOL) processing temperatures. To fully harness their potential, it is crucial to understand the switching dynamics and structural changes that influence reliability in these fluorite-type hafnia-based FE thin films. Here-in, we examine the effects of measurement temperature on the polarization switching dynamics and phase stability of hafnia-zirconia alloy (HZO) based FE capacitors. This is achieved through temperature-dependent electrical measurements correlated with in-situ synchrotron X-ray diffraction (XRD) at different temperatures.
To fabricate the capacitors, 10-nm thick, as-deposited amorphous HZO films are grown at deposition temperatures of 200°C and 250°C using plasma-enhanced atomic layer deposition (PE-ALD). The deposition temperature influences the phase fraction of the polar orthorhombic phase (O-phase) in the films after post metallization annealing. These samples are referred to as ALD 200°C sample (with high fraction of non-polar tetragonal phase (T-phase)) and ALD 250°C sample (with high phase fraction of polar O-phase), respectively. Polarization hysteresis (P-V) loops at various measurement temperatures (T_m) are studied for the two samples. The ALD 200°C sample exhibits pinched anti-ferroelectric-like (AFE) hysteresis loop at room temperature (T_m = 296 K), consistent with a high fraction of T-phase, while the ALD 250°C samples showed a characteristic FE hysteresis loop, indicating a high O-phase fraction. For the ALD 200°C samples, remanent polarization (P_r) increases as T_m is decreased from 423 K to 50 K, while the ALD 250°C samples exhibit a decrease in P_r with decreasing T_m from room temperature.
These observations demonstrate the inherent differences in switching mechanisms of the two samples. The ALD 200°C samples' P-V loops result from a reversible distortion of the T-phase into the O-phase under an applied electric field. The field required for these distortions is temperature-dependent and influences the switching current peak positions. In contrast, for the ALD 250°C samples, the reduction in P_r and pinching of the P-V loops with increasing T_m can be due to a temperature-dependent phase change to T-phase, while the reduction in P_r and increase in coercive voltage (V_c) with decreasing T_m can be due to a change in the crystallographic texture as indicated by in-situ low T_m synchrotron X-ray diffraction (XRD). Overall, the study concludes that HZO thin films with differing initial phase fractions exhibit distinct temperature-dependent switching behaviors due to their structural evolution as T_m varies.
Acknowledgement: “Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.”