황철성 교수

The recent progress in ferroelectricity and antiferroelectricity in HfO2-based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as Pb(Zr,Ti)O3, BaTiO3, and SrBi2Ta2O9, which are considered to be feasible candidate materials for non-volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si-compatibility, environmental issues related to Pb, large physical thickness, l

A ferroelectric-resistive random access memory consisting of a conductive BiFeO3 epitaxial thin film with a unipolar diode current modulated by electric polarization orientation is reported. This device has a memory that lasts for months, a sufficiently high on current and on/off ratio to permit ordinary sense amplifiers to measure “1” or “0”, and is fully compatible with complementary metal-oxide semiconductor processing.

In this Review, memristors are examined from the frameworks of both von Neumann and neuromorphic computing architectures. For the former, a new logic computational process based on the material implication is discussed. It consists of several memristors which play roles of combined logic processor and memory, called stateful logic circuit. In this circuit configuration, the logic process flows primarily along a time dimension, whereas in current von Neumann computers it occurs along a spatial di

solid solution thin films of a wide range of film compositions and thicknesses are comprehensively related to the theoretical predictions based on a thermodynamic surface energy model. The theoretical model can semi-quantitatively explain the experimental results on the phase-evolution, but there were non-negligible discrepancies between the two results. To understand these discrepancies, various factors such as the film stress, the role of a TiN capping layer, and the kinetics of crystallizatio

Issues in the circuitry, integration, and material properties of the two‐dimensional (2D) and three‐dimensional (3D) crossbar array (CBA)‐type resistance switching memories are described. Two important quantitative guidelines for the memory integration are provided with respect to the required numbers of signal wires and sneak current paths. The advantage of 3D CBAs over 2D CBAs (i.e., the decrease in effect memory cell size) can be exploited only under certain limited conditions due to the incr

The promising energy storage properties of new lead-free antiferroelectric HfxZr1-xO2 (x = 0.1–0.4) films with high energy storage density are reported. The energy storage density of the Hf0.3Zr0.7O2 capacitor does not decrease with the increase in temperature up to 175 °C, and it decreases by only ≈4.5% after field cycling 109 times.

Al-doped TiO2 thin films to be used in future DRAM capacitors with excellent leakage properties as well as high dielectric constants are fabricated. The next generation stack structured DRAM cell composed of a transistor and a capacitor is shown (see Figure). A large cell capacitance is required for successful operation of DRAMs irrespective of the feature size of the cell. Therefore, as scaling down of the DRAMs proceeds, a higher-k material such as Al-doped TiO2 has to be eventually implemente

(Ba,Sr)TiO3 (BST) thin films with thicknesses ranging from 15 to 50 nm are prepared by a rf magnetron sputtering on Pt/SiO2/Si substrates. The dielectric constants of BST thin films increase with increasing deposition temperature and thicknesses. The leakage current increases with increasing deposition temperature and this prevents the deposition temperature of the 20 nm thick BST thin film from being increased to a value more than 640 °C. The leakage current is also critically dependent upon th

The development of a resistance switching (RS) memory cell that contains rectification functionality in itself, highly reproducible RS performance, and electroforming‐free characteristics is an impending task for the development of resistance switching random access memory. In this work, a two‐layered dielectric structure consisting of HfO 2 and Ta 2 O 5 layers, which are in contact with the TiN and Pt electrode, is presented for achieving these tasks simultaneously in one sample configuration.

The appearance of ferroelectric (FE) and anti-ferroelectric (AFE) properties in HfO2-based thin films is highly intriguing in terms of both the scientific context and practical application in various electronic and energy-related devices. Interestingly, these materials showed a "wake-up effect", which refers to the increase in remanent polarization with increasing electric field cycling number before the occurrence of the fatigue effect. In this work, the wake-up effect from Hf0.5Zr0.5O2 was car

Recent progress in deep learning extends the capability of artificial intelligence to various practical tasks, making the deep neural network (DNN) an extremely versatile hypothesis. While such DNN is virtually built on contemporary data centers of the von Neumann architecture, physical (in part) DNN of non-von Neumann architecture, also known as neuromorphic computing, can remarkably improve learning and inference efficiency. Particularly, resistance-based nonvolatile random access memory (NVRA

A Pt/NbO x /TiO y /NbO x /TiN stack integrated on a 30 nm contact via shows a programming current as low as 10 nA and 1 pA for the set and reset switching, respectively, and a self-rectifying ratio as high as ∼10 5, which are suitable characteristics for low-power memristor applications. It also shows a forming-free characteristic. A charge-trap-associated switching model is proposed to account for this self-rectifying memrisive behavior. In addition, an asymmetric voltage scheme (AVS) to decrea

In this study, the changes in the structural and electrical properties of ferroelectric Hf1-xZrxO2 films with various Zr contents (0.26-0.70) were systematically examined during electric field cycling, resulting in a "wake-up" effect. To quantify the degree of wake-up effect, a "variable" polarization as the difference between remanent and saturation polarization was suggested as a new parameter, which could be calculated by excluding the linear dielectric contribution from the total electric di

) formation has not been clearly elucidated. Several recent experimental and theoretical studies evidently showed that the interface and grain boundary energies of the higher symmetry phases (orthorhombic and tetragonal) contribute to the stabilization of the metastable non-centrosymmetric orthorhombic phase or tetragonal phase. However, there was a clear quantitative discrepancy between the theoretical expectation and experiment results, suggesting that the thermodynamic model may not provide t