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Ever since the ferroelectricity of complementary metal-oxide semiconductor (CMOS) compatible HfO2-based materials was discovered, numerous studies have been conducted on their ferroelectric (FE) properties and device applications. In particular, pure-HfO2 FE materials without external doping have attracted considerable attention owing to their excellent robustness against variation because variations that appear in conventional doped-HfO2 FEs are not observed in electrical characteristics induced by dopant fluctuations in pure-HfO2 FEs. Studies on metal/FE/insulator/semiconductor (MFIS) stack are required to apply the ferroelectricity of pure-HfO2 to memory devices that are completely compatible with Si-based CMOS processes. In pure-HfO2 based MFIS stacks, the polarization tends to reduce with increasing thickness of the HfO2, although the leakage current diminishes. To overcome the tradeoff between the polarization and leakage current with respect to the thickness of the HfO2, an Al2O3 layer was inserted between the HfO2 layers to form a laminated FE structure. By employing the laminated FE, leakage current was effectively suppressed by the Al2O3 and lower HfO2 layers, and polarization was enhanced by the FE sum of the upper and lower HfO2 layers. Therefore, an MFIS structure with maximized polarization and minimized leakage current was successfully demonstrated using laminated FE. In addition, the feasibility of the proposed MFIS with laminated FE for nonvolatile memory device applications was confirmed by verifying the multistate operations of a FE tunnel junction. [ABSTRACT FROM AUTHOR]

It has been reported that the HfOx-based metal–ferroelectric–insulator–semiconductor (MFIS) stack is one of the most feasible structures for memory applications in ferroelectric devices. In addition, in MFIS stacks, the formation of a high-quality thin insulator is considered to be a crucial process, because it determines the device performance and reliability. In this study, the effects of different SiO2 thin interfacial layers (ILs) formed by chemical oxidation and atomic layer deposition (ALD) on the ferroelectricity of HfOx were investigated in MFIS stacks. Notably, the IL formation method affected the crystallization temperature of HfOx ferroelectricity. In particular, the chemical oxide IL sample was sufficiently crystallized only at an RTA temperature greater than 800 °C, whereas the ALD oxide IL sample began to undergo a complete ferroelectric phase transition from a low temperature (600 °C). The x-ray analysis verified that the IL formation method affected the initial film state of the HfOx layer by modulating the Hf-O bonding states, resulting in a different temperature dependence for ferroelectricity formation during the subsequent crystallization annealing stage. Consequently, it was confirmed that stable polarization (2Pr > 30 μC/cm2) could be obtained over a wide temperature range (600–800 °C) through IL material engineering; this could facilitate the integration of HfOx-based ferroelectric devices in terms of the heat budget and the application of ferroelectric devices as commercial devices. [ABSTRACT FROM AUTHOR]

Ferroelectric tunnel junction (FTJ) has been considered as a promising candidate for next-generation memory devices due to its non-destructive and low power operations. In this article, we demonstrate the interlayer (IL) engineering in the FTJs to boost device performances. Through the analysis on the material and electrical characteristics of the fabricated FTJs with engineered IL stacks, it is clearly found that the insertion of an Al2O3 layer between the SiO2 insulator and the pure-HfOx FE improves the read disturbance (2Vc = 2.2 V increased), the endurance characteristics (tenfold improvement), and the cell-to-cell TER variation simultaneously without the degradation of the ferroelectricity (less than 5%) and the polarization switching speeds through grain size modulation. Based on these investigations, the guidelines of IL engineering for low power ferroelectric devices were provided to obtain stable and fast memory operations. [ABSTRACT FROM AUTHOR]

As the computing paradigm has shifted toward edge computing, improving the security of edge devices is attracting significant attention. However, because edge devices have limited resources in terms of power and area, it is difficult to apply a conventional cryptography system to protect them. On the other hand, as a simple security application, a physical unclonable function (PUF) can be implemented without power and area problems because it provides a security key by utilizing process variations without additional external circuits. Ferroelectric tunnel junctions (FTJs) are 2-terminal devices that store information by changing the resistance of a ferroelectric material, where the resistance is determined by the polarization states of the ferroelectric domains. Because polycrystalline ferroelectric materials have a multi-domain nature, domain variation can also be used as a randomness source to induce cell-to-cell variations along with process variations. In this paper, we demonstrate PUF operations of a low-power, small area 16 × 16 hafnium oxide (pure-HfOx)-based FTJ array using certain metrics. It is clear that the proposed array consisting of scaled FTJs has adequate randomness for security applications such that the array-level PUF operations are robust against model-based machine learning attacks. [ABSTRACT FROM AUTHOR]

In this article, we report a method for recovering a resistive-switching random access memory (RRAM) from the reset breakdown and enhancing the endurance characteristics. A SiNx-based RRAM device has been fabricated and its switching characteristics are analyzed with a particular interest in reset breakdown. It has been found that the SiNx RRAM in permanent reset breakdown can be revived to high-resistance state (HRS) by thermal recovery, a low-temperature annealing method. The memory window could be widened even with enhanced reset stability and conductance distribution. Temperature-dependent conductance change has been measured in order to figure out the substantial component of conductive filament and the mechanism of thermal recovery. The experimental evidences show that the reset breakdown process is the result of unwanted Si dangling bond (Si-DB) formation. By thermal recovery, Ni filament could be ruptured without applying a high electric field which induces negative set. [ABSTRACT FROM AUTHOR]

In this article, we propose a novel cell transistor structure to facilitate the mass production of 4F2 dynamic random access memory (DRAM). 3-D TCAD simulation results show that the proposed structure exhibits a better DRAM operation margin than the conventional vertical transistors. In particular, we confirmed that the failure mode due to the secondary effect of the floating body, the most pressing issue in relation to 4F2 DRAM, is dramatically improved by adopting this structure. [ABSTRACT FROM AUTHOR]

We propose an accurate and effective method, low-frequency noise (LFN) spectroscopy, to examine the resistive switching mechanism in ferroelectric tunnel junctions (FTJs) based on pure hafnium oxide (HfOx). Contrary to previous studies that primarily focused on the ferroelectric (FE) resistive switching (RS) in HfOx-based FTJs, the results of this study demonstrate that non-FE RS affected by the redistribution of oxygen vacancies also plays a significant role in determining the performance of FTJs. LFN spectroscopy is conducted in different conditions by changing the operating temperature and inducing DC cycling stress. The results reveal that the RS mechanism changes from FE to non-FE RS with increased program bias in all conditions. This change is facilitated by the rise in temperature and the number of DC cycling stress. [ABSTRACT FROM AUTHOR]

[Display omitted] • The ferroelectric gate stacks with variation-robust and thermally stable ferroelectric pure-HfOx is demonstrated. • The comparable or superior ferroelectricity of the pure-HfOx to doped-HfOx is demonstrated even in nano device-friendly fabrication processes. • The origin of the pure-HfOx ferroelectricity is comprehensively investigated by electrical and material analysis. • The feasibility of the ferroelectric pure-HfOx to memory applications is verified through the FTJ operations. Ferroelectricity in hafnium oxide (HfO x) and its various applications have been rigorously investigated in recent years. In particular, most studies have focused on a non-centrosymmetric ferroelectric phase that is developed via the injection of external ions into HfO x. However, the drawbacks of doped HfO x , such as serious dopant variation and thermal instability, have hindered its commercialization in low-power logic and memory applications. In this study, we demonstrated the ferroelectricity of pure HfO x without additional ion doping in metal-ferroelectric-insulator-semiconductor (MFIS) stacks. We experimentally confirmed that variation-resistant and thermally stable ferroelectric pure HfO x could be achieved with remarkable polarization by controlling the oxygen vacancies inside pure HfO x and the thermal quenching process for orthorhombic phase transition. Furthermore, to verify the feasibility of the ferroelectric pure HfO x in memory devices, the modulation of tunneling resistance was measured using MFIS ferroelectric tunnel junction (FTJ) devices, revealing that the tunneling current was adjustable over a 2-order current range via the polarization of the pure HfO x , implying that multiple current states could be obtained for multilevel memory operations. [ABSTRACT FROM AUTHOR]

We propose self-rectifying resistive random access memory (RRAM) synapse to prevent reverse leakage current problem which occurs when RRAM is integrated with integrate and fire (IF) circuit in spiking neural network (SNN). Ni/W/SiNx/n-Si RRAM was fabricated by varying the bottom electrode (BE) doping concentration and their rectifying characteristics were analyzed. Low BE doping concentration device showed self-rectifying characteristics without any additional selector or diode device. Furthermore, hardware-based system-level simulation was conducted to evaluate the effect of self-rectifying RRAM synapse on MNIST classification accuracy. About 93.34% accuracy was obtained using the proposed RRAM. [ABSTRACT FROM AUTHOR]

In this Letter, we present reset-voltage-dependent precise tuning operation of TiOx/Al2O3-based memristive devices. For the high resistance state (HRS) with high reset voltage, abrupt set operations are observed with a large variation, while the HRS obtained by low reset voltage provides gradual and uniform switching behaviors. The improvement of gradual switching and the programming accuracy are analyzed regarding cycle-to-cycle as well as device-to-device variations. We believe that these results can be applied to operate TiOx/Al2O3-based memristors in areas requiring highly accurate tuning characteristics. [ABSTRACT FROM AUTHOR]