Shuaizhi Zheng

HfO2-based three-dimensional (3D) ferroelectric field-effect transistors (FeFETs) demonstrate great potential for ultra-high density memory applications. However, the device-to-device variation in threshold voltage (Vth) remains a challenge for their practical implementation. In this work, we systematically investigate the device-to-device variation induced by the variation of ferroelectric properties with the substrate orientation and potential mitigation approaches, an aspect that remains unexplored in HfO2-based 3D FeFETs. First, a device model incorporating randomly distributed nanoscale ferroelectric regions with distinct polarization magnitudes is developed. We reveal that the polarization disparities both among ferroelectric regions and between ferroelectric and non-ferroelectric regions have distinct effects on the device-to-device variation, and that these effects depend critically on the proportion of ferroelectric regions. Building on the inevitable presence of non-ferroelectric regions in the devices, we further explore variation mitigation strategies aimed at alleviating microscopic polarization non-uniformity through adjustments in device structures and operating conditions, such as employing a thicker oxide insulator or lowering the operating voltage. These results provide a theoretical foundation for optimizing the reliability of HfO2-based 3D FeFETs.

Ce-doped HfO2 (HCO) ferroelectric thin films have attracted considerable attention for next-generation integrated ferroelectric devices owing to their excellent compatibility with complementary metal-oxide-semiconductor technology and promising ferroelectric properties. However, previously reported HCO films generally require relatively large thicknesses and high processing temperatures, limiting their application in highly scaled devices. Here, we demonstrate HCO ferroelectric thin films grown by atomic layer deposition (ALD) and systematically investigate the effects of Ce concentration and film thickness on their structural and ferroelectric properties. The HCO film with 5.0% Ce doping exhibits the highest ferroelectric orthorhombic phase fraction, which is attributed to the formation of coupled defects involving Ce-substitution on Hf sites and oxygen vacancies, as revealed by first-principles calculations. Moreover, the HCO films annealed at 700 °C exhibit excellent ferroelectric properties at the thickness ranging from 12 to 24 nm. In particular, the 18 nm-thick HCO film with 5.0% Ce achieves a remanent polarization of 19.4 μC/cm2 and an excellent switching endurance exceeding 1 × 109 cycles. These results demonstrate the potential of ALD-grown HCO as a scalable material platform for highly integrated and multifunctional ferroelectric devices.

Laminated HfO2-based ferroelectric thin films show great promise for applications in high-density ferroelectric field-effect transistor memories, nanoelectromechanical system resonators, and energy storage devices. However, the polarization fatigue performance of widely investigated Hf0.5Zr0.5O2 (HZO)/Al2O3/HZO laminated films remains poor. In this work, we propose a novel laminated ferroelectric film, HZO/HfN/HZO, to enhance the fatigue performance. Compared with HZO/Al2O3/HZO thin films, the HZO/HfN/HZO films demonstrate significantly enhanced fatigue resistance, enduring up to 1 × 108 cycles at a measurement frequency of 100 kHz and exhibiting slower polarization degradation. This improvement is revealed to stem from a reduced interface trap charge density. Moreover, the lower trap density is attributed to the smaller electric field across the HfN layer that suppresses its degradation and a bandgap comparable to HZO that minimizes carrier energy release at the HfN/HZO interface. This work presents a key step for developing highly reliable laminated HfO2 ferroelectric films and devices.

The low-temperature processed ( $\le 450^{\circ }$ C) Hf ${}_{{0}.{5}}$ Zr ${}_{{0}.{5}}$ O₂ (HZO) ferroelectric memcapacitors are promising for ultra-low-power neuromorphic computing. However, they still suffer from small memory window, poor endurance, and relatively high operating voltages. Here, we propose the electrode stress engineering to enhance ferroelectricity in ultrathin HZO films and thereby modulate their memcapacitive characteristics. Remarkably, the W/HZO (6 nm)/Mo memcapacitors fabricated at 400°C show a record-large non-volatile relative permittivity memory window ( $\varepsilon _{\text {MW}}\text {)}$ of 11.15 under an ultra-low operating voltage of 1.5 V, along with robust write/read endurance, and 10-year extrapolated retention at 85°C. Furthermore, these devices successfully emulate synaptic potentiation and depression behaviors with excellent linearity. Leveraging these characteristics, the memcapacitors demonstrate remarkable neuromorphic computing capabilities, achieving a digit-recognition accuracy as high as 97.48%. This work highlights the strong potential of HZO ferroelectric memcapacitors for energy-efficient neuromorphic computing.

Ferroelectric field effect transistor (FeFET) memory, integrated with the recently discovered HfO2-based ferroelectric thin films, has attracted significant interest as a promising technology for next-generation non-volatile memory applications. In this work, we present a fully depleted silicon-on-insulator (FD-SOI) independently double-gated FeFET (Fe-FlexFET) based on Hf0.5Zr0.5O2 (HZO) ferroelectric thin film. The Fe-FlexFET provides flexible dynamic threshold voltage (VTH) control, which can not only eliminate the reliability issue of VTH shift but also be utilized to construct reconfigurable integrated circuits. Based on a technology computer-aided design numerical model, we demonstrate that the large VTH tuning capability could be efficiently achieved via increasing the HZO thickness and reducing the silicon thickness. Moreover, the Fe-FlexFET exhibits significantly improved robustness against single-event transients compared to the FD-SOI FeFET, attributed to the collection of holes and electrons at the bottom gate junction. This study paves the way for highly reliable HfO2-based FeFET memories and underscores their great potential for application in reconfigurable integrated circuits.

Atomic-layer-deposited hafnium zirconium oxide (HZO) ferroelectric films have emerged as highly promising candidates for next-generation ferroelectric devices. However, the stabilization of the ferroelectric phase in HZO films thicker than 20 nm remains a persistent challenge, fundamentally limiting their applications in piezoelectric transducers, pyroelectric detectors, and energy storage devices. Here, we propose a tetragonal-phase nucleation strategy guided by phase-field simulation, breaking through the thickness barrier for high-performance ferroelectric HZO films. Through a newly developed phase-field crystallization model, we first identify that a higher initial density of tetragonal-phase nuclei concurrently enhances low-temperature crystallinity and promotes the formation of the ferroelectric orthorhombic phase. Furthermore, an innovative HZO (3.5 nm)/ZrO2 (2.5 nm) multilayer film is designed to increase tetragonal-phase nucleation density. The 54-nm-thick film annealed at an unprecedentedly low temperature of 400 °C exhibits a high fraction of ferroelectric phase and excellent ferroelectric properties. Notably, it achieves a remanent polarization significantly exceeding that of sub-20-nm-thick HZO films prepared by conventional processes. Our study advances the understanding of ferroelectric phase formation in HZO films while simultaneously extending their technological applicability.

With advancements in artificial neural networks and information processing technology, a variety of neuromorphic synaptic devices have been proposed to emulate human sensory systems, with vision being a crucial information source. Moreover, as practical applications become increasingly complex, the need for multifunctional visual synapses to expand the application range becomes urgent. This study introduces a MoS₂/WSe₂ van der Waals (vdW) heterojunction and utilizes it to replicate artificial visual synapses by harnessing the cooperative effect of photoconductivity and pyroconductivity mechanisms. By adjusting the optical power, pulse width, and pulse number of the optical stimulus, the heterojunction effectively simulates synaptic properties. Under the combined action of an external electric field and the built-in electric field (E_bi), the heterojunction exhibits broadband synaptic properties in the visible to near-infrared spectrum (405-1550 nm) while consuming low power of 0.3-1.1 pJ per spike. The heterojunction can detect ultraweak optical signals at 660 nm with an optical power intensity of 14 μW/cm², displaying a high specific detectivity (D*) of 3.98 × 10¹¹ Jones. Furthermore, at 405, 808, 1064, and 1550 nm, the D* of the heterojunction is 4.16 × 10¹¹, 3.61 × 10⁹, 4.96 × 10⁷, and 1.64 × 10⁷ Jones, respectively. Visual synaptic devices based on the MoS₂/WSe₂ vdW heterojunction hold significant promise for the future development of integrated sensing and memory processing devices.

Inspired by human visual synapses, high performance and versatile infrared vision synapses can be achieved in the p-WSe₂/n-Ta₂NiS₅ van der Waals heterojunction by introducing narrow band gap materials and bias-induced band bending.

HfO2-based ferroelectric field-effect transistors (FeFETs) are regarded as one of the most promising non-volatile memory technologies in the future. However, the charge trapping phenomenon during the program/erase operation is still a challenge. In this work, we comprehensively investigate the behaviors of semiconductor/insulator interface charge trapping in HfO2-based FeFETs. Through analyzing the effects of the spatial distribution of interface traps and the polarization switching speed, the coupling effects of semiconductor/insulator interface charge trapping and polarization switching are recognized. We also find that the band tail state traps have much less influence on the electrical characteristics of the FeFETs than the deep level state traps. Through engineering the devices with band tail state traps with concentrations as small as possible, the influences of charge trapping could be effectively suppressed. Moreover, the gate voltage (VG) scanning rate has a significant influence on the interface charge trapping process due to the time dependent change of ferroelectric polarization. The largest memory window could be obtained by carefully choosing the VG scanning rate of the FeFETs based on the polarization switching speed. This work represents a key step for realizing highly reliable HfO2-based FeFETs.

The ferroelectric field-effect transistors (FeFETs) with HfO₂-based ferroelectric layers in the gate stacks are emerging as one of the most promising candidates for the next-generation nonvolatile memory devices due to their scalability and compatibility with conventional Si technology. Moreover, owing to the high radiation hardness of the HfO₂-based ferroelectric thin films, HfO₂-based FeFETs have attracted great interest in the fields of radiation-hard (rad-hard) memory. However, the reliability of their memory states under irradiation, which represents the validity of the stored information, has not been investigated. Here, we focus on the impact of the total ionizing dose (TID) on erased and programmed states of HfO₂-based FeFETs. The TID radiation (X-ray) characteristics of erased and programmed HfO₂-based FeFETs are characterized using an on-site read operation. Both the erased and programmed states show robust stability under irradiation at a dose rate of 90 rad(Si)/s, and even at 230 rad(Si)/s, only the erased state shows a slight variation. The possible factors contributing to memory state degradation are discussed. Through the analysis of the threshold voltage shift and subthreshold swing evolution, as well as studies of ferroelectric polarization stability under radiation, it is revealed that the erased state degradation is caused by oxide-trapped charges rather than interface degradation or polarization switching. The physical mechanism of the difference in radiation-induced oxide-trapped charges buildup in programmed and erased FeFETs is analyzed to explain different TID radiation characteristics between them. Our work suggests that the HfO₂-based FeFETs have great potential in radiation environment applications.