Hyunchul Sohn

The severe degradation of ferroelectricity in Hf0.5Zr0.5O2 (HZO) films thinner than 5 nm poses a significant challenge for device applications. In this study, we demonstrate that the crystallization of ultrathin HZO films can be improved simply by optimizing the annealing sequence, without introducing additional dopants or structural modifications. Specifically, postdeposition annealing (PDA) and postmetallization annealing (PMA), conducted before and after the deposition of the top electrode, respectively, were applied to HZO films of varying thicknesses to investigate their ferroelectric properties. PMA led to a significant reduction in remnant polarization (Pr), primarily due to insufficient crystallization in 3 nm-thick HZO films. By contrast, PDA in an O2 atmosphere resulted in enhanced crystallization and a smaller decrease in Pr, achieving a maximum 2Pr value of 12 μC/cm2. Additionally, this annealing condition resulted in reduced leakage current and improved endurance, which could be attributed to the suppression of oxygen-vacancy formation in the HZO films. These findings highlight that a simple modification of the annealing scheme enables ultrathin HZO films to retain sufficient ferroelectricity for practical implementation in low-power, high-density next-generation devices.

In this study, the effect of supercycle scheme for La‐doping in HfO₂ films by atomic layer deposition (ALD) on their ferroelectric properties is investigated along with the variation of La distribution in HfO₂ films. La is doped into HfO₂ films by two ALD supercycle methods: metal‐oxidant‐dopant‐oxide (MODO) and metal‐dopant‐oxidant (MDO). Secondary ion mass spectrometry analys is shows that the MDO supercycle method results in a uniform distribution of La in the depth direction, while the MODO supercycle method results in depth‐dependent segregation of La in the HfO₂ film. For similar La concentrations, HfO₂ films doped with La by the MDO supercycle show a higher percentage of orthorhombic phase with higher oxygen vacancy concentrations than films by the MODO supercycle. For 3% La, the HfO₂ film by MDO shows a remanent polarization (2P_r) of ≈31 μC cm⁻² at ± 4 MV cm⁻¹, which is twice as large as the remanent polarization of 15 μC cm⁻² for the HfO₂ film by MODO. In addition, the La‐doped HfO₂ film by MDO with a larger fraction of orthorhombic phase shows a smaller wake‐up effect than the La‐doped HfO₂ film by MODO.

The resistive random-access memory (RRAM) with multi-level storage capability has been considered one of the most promising emerging devices to mimic synaptic behavior and accelerate analog computations. In this study, we investigated the reset-first bipolar resistive switching (RS) and multi-level characteristics of a LaNiO_3−x thin film deposited using a reactive magnetron co-sputtering method. Polycrystalline phases of LaNiO₃ (LNO), without La₂O₃ and NiO phases, were observed at similar fractions of Ni and La at a constant partial pressure of oxygen. The relative chemical proportions of Ni³⁺ and Ni²⁺ ions in LaNiO_3−x indicated that it was an oxygen-deficient LaNiO_3−x thin film, exhibiting RS behavior, compared to LNO without Ni²⁺ ions. The TiN/LaNiO_3−x/Pt devices exhibited gradual resistance changes under various DC/AC voltage sweeps and consecutive pulse modes. The nonlinearity values of the conductance, measured via constant-pulse programming, were 0.15 for potentiation and 0.35 for depression, indicating the potential of the as-fabricated devices as analog computing devices. The LaNiO_3−x-based device could reach multi-level states without an electroforming step and is a promising candidate for state-of-the-art RS memory and synaptic devices for neuromorphic computing.

Ovonic threshold switching (OTS) selector devices based on chalcogenide materials are promising candidates for addressing the sneak current in high-density cross-point array structures owing to their high selectivity, high endurance, and fast switching speed. However, the OTS mechanism remains controversial and needs to be clarified. In this study, the formation of local conductive filaments (CFs) during threshold switching in an amorphous Ga₂Te₃ OTS selector device was investigated by electrical measurements and conductive-atomic force microscopy (C-AFM). The amorphous Ga₂Te₃ OTS selector device requires a forming process before the threshold switching processes. In addition, the off-current density (J_OFF) was dependent on the area of the bottom electrode. The difference between the threshold voltage (V_TH) and the hold voltage (V_H) increased as the applied higher electric field increased. The drift of V_TH (V_TH drift) depended on the relaxation time and measurement temperature. The requirements of the forming process, area dependence of the J_OFF, the difference between the V_TH−V_H with the applied electric field, and V_TH drift are expected to depend on locally formed CFs. In addition, the analysis of the C-AFM results strongly supports the formation of local CFs during threshold switching in an amorphous Ga₂Te₃ OTS selector device. The understanding of OTS behavior uncovered in this study may provide guidance for improving the characteristics of and designing materials for future applications of OTS selector devices.

Multi-component chalcogenide thin films of GeTe_x and GeTe_xS_1−x are prepared by atomic layer deposition technique. With the conformal deposition characteristics, its electrical properties of threshold switching are demonstrated for emerging computing.

Reset-first resistive random access memory (RRAM) devices were demonstrated for off-stoichiometric Ni_1−xO thin films deposited using reactive sputtering with a high oxygen partial pressure. The Ni_1−xO based RRAM devices exhibited both unipolar and bipolar resistive switching characteristics without an electroforming step. Auger electron spectroscopy showed nickel deficiency in the Ni_1−xO films, and X-ray photoemission spectroscopy showed that the Ni³⁺ valence state in the Ni_1−xO films increased with increasing oxygen partial pressure. Conductive atomic force microscopy showed that the conductivity of the Ni_1−xO films increased with increasing oxygen partial pressure during deposition, possibly contributing to the reset-first switching of the Ni_1−xO films.