Investigation of Imprint in FE-HfO₂ and Its Recovery

Abstract

Ferroelectric (FE)-HfO2 based FETs (FEFETs) are one of the most promising candidates for emerging memories. However, the FE material suffers from a unique reliability phenomenon known as imprint: the coercive voltage shifts during data retention, which has been regarded as a major issue for memory operation, while the mechanism causing it is still under research. In this paper, imprint and its recovery in FE-HfO2 are investigated in detail by comprehensive electrical measurements to reveal its underlying mechanism including the cause of asymmetric coercive voltage shifts. The recovery measurements clarify that domain switching is indispensable for the recovery from imprint. The sub-loop imprint effect shows that imprint and its recovery must be independent for each domain. In addition, switching time measurements and corresponding fitting results with the nucleation-limited-switching (NLS) model strongly indicate that imprint is caused by domainseeds-pinning. Based on these results, we conclude that charge trapping and de-trapping affecting activation barriers for domain switching, accompanied by domain switching is responsible for imprint and its recovery.

Defect profiling in FEFET Si:HfO2 layers

“…This is believed to result from the commencement of the positive (stress and relaxation) voltage-induced polarisation of the FE layer to the low Vt state, and hence the lower Vt value. That this process can take place at these low voltages (for Vstress ~ 1.5V, Vrelax ~0.5V), after long (stress and relaxation) times is consistent with results on the time and bias dependence of polarisation [4]. This effect is more pronounced at lower temperatures, also in agreement with a recent report suggesting increased orthorhombic phase in Si-doped HfO2 on reducing temperature [6].…”

“…At such physical thickness, the presence (and absolute number) of defects give rise to capture and emission of electrons injected in the FE-layer, which also modulate the device performance. The combination of this defectdominated behaviour together with the ferroelectric polarisation presents quite a challenge for device operation and reliability engineering [4] and is the objective of this work. We elucidate the energy distributions of oxide defect states in Si doped-HfO2 for FE-RAM applications using multiple defect modelling techniques.…”

“…Trapping and noise measurements were performed on 9.5nm Si-doped HfO2/TiN [4] devices, where a thin oxide interfacial layer was present between the silicon substrate and ferroelectric layers, whereas a thicker (7.5 nm) oxide layer was used in the photo-depopulation study. Results are benchmarked to undoped HfO2/TiN stacks subjected to similar thermal budget on planar transistor structures, which display ferroelectric and paraelectric behaviour, respectively.…”

Time‐Dependent Imprint in Hf0.5Zr0.5O2 Ferroelectric Thin Films

“…[ 5–11 ] However, there are still reliability issues for practical use, including insufficient switching endurance [ 12,13 ] and fatigue properties. [ 14–16 ] In the case of HfO 2 ‐based ferroelectric thin films, their large time‐dependent imprint [ 17–22 ] and wake‐up effect [ 16,17,23–27 ] —which distinguish them from conventional ferroelectric materials—play important roles in understanding these critical issues.…”

“…Recently, it was reported that the imprint effect in a HfO 2 ‐based ferroelectric capacitor occurs during polarization retention, even at room temperature. [ 17–19,21,22 ] The time‐dependent imprint also occurs in ferroelectric‐gate field effect transistors. [ 20 ] Zacharaki et al.…”