Materials For Nonvolatile Memory Devices Research Articles

A perspective on semiconductor devices based on fluorite-structured ferroelectrics from the materials–device integration perspective

Ferroelectric materials are known to be ideal materials for nonvolatile memory devices, owing to their two electrically switchable spontaneous polarization states. However, difficulties in scaling down devices with ferroelectric materials have hindered their practical applications and research. The discovery of ferroelectricity in fluorite-structured ferroelectrics has revived research on semiconductor devices based on ferroelectrics. With their scalability and established fabrication techniques, the performance of nanoscale electronic devices with fluorite-structured ferroelectrics is being rapidly developed. However, the fundamental physics behind the superior ferroelectricity is yet to be elucidated. From this Perspective, the status of research on fluorite-structured ferroelectrics and state-of-the-art semiconductor devices based on them are comprehensively reviewed. In particular, the fundamental physics of fluorite-structured oxides is critically reviewed based on a newly developed theory as well as on the classical theory on ferroelectrics. A perspective on the establishment of emerging semiconductor devices based on fluorite-structured ferroelectrics is provided from the viewpoint of materials science and engineering.

Hierarchical Short- and Medium-Range Order Structures in Amorphous GexSe1–x for Selectors Applications

In the upcoming process to overcome the limitations of the standard von Neumann architecture, synaptic electronics is gaining a primary role for the development of in-memory computing. In this field, Ge-based compounds have been proposed as switching materials for nonvolatile memory devices and for selectors. By employing the classical molecular dynamics, we study the structural features of both the liquid states at 1500K and the amorphous phase at 300K of Ge-rich and Se-rich chalcogenides binary GexSe1-x systems in the range x 0.4-0.6. The simulations rely on a model of interatomic potentials where ions interact through steric repulsion, as well as Coulomb and charge-dipole interactions given by the large electronic polarizability of Se ions. Our results indicate the formation of temperature-dependent hierarchical structures with short-range local orders and medium-range structures, which vary with the Ge content. Our work demonstrates that nanosecond-long simulations, not accessible via ab initio techniques, are required to obtain a realistic amorphous phase from the melt. Our classical molecular dynamics simulations are able to describe the profound structural differences between the melt and the glassy structures of GeSe chalcogenides. These results open to the understanding of the interplay between chemical composition, atomic structure, and electrical properties in switching materials.

Bromine Vacancy Redistribution and Metallic‐Ion‐Migration‐Induced Air‐Stable Resistive Switching Behavior in All‐Inorganic Perovskite CsPbBr3 Film‐Based Memory Device

AbstractAll‐inorganic halide perovskites have attracted a great deal of attention for applications in resistive switching (RS) memory devices due to their superior stability compared to organic–inorganic hybrid halide perovskites. RS memory devices utilizing air‐stable all‐inorganic halide perovskite cesium lead bromide (CsPbBr3) film as the switching layer, which are successfully prepared by spin coating at low temperature, are demonstrated. Memory devices based on CsPbBr3 film exhibit typical reproducible bipolar RS behavior and superior switching characteristics, including the high ON/OFF ratio (≈104), long data retention (>5 × 104 s), and environmental stability. In addition, multilevel storage capability can be achieved through controlling the different compliance currents. The formation and rupture of bromine (Br) vacancy conducting filaments (CFs) is proposed to explain the switching behavior in the Pt‐anode‐based memory devices, which is verified by XPS depth‐profiling analysis. Moreover, the coexistence of Br vacancies and Ag metallic CFs is suggested to be responsible for the switching behavior in Ag‐anode based device. These results demonstrate that the all‐inorganic halide perovskite CsPbBr3 film will be the promising switching material for nonvolatile memory devices.

Fabrication of resistive switching memory based on solution processed PMMA-HfOx blended thin films

In this study, we developed PMMA-HfOx blended resistive random access memory (ReRAM) devices using solution processing to overcome the drawbacks of the individual organic and inorganic materials. Resistive switching behaviors of solution-processed PMMA, PMMA-HfOx, and HfOx film-based ReRAM devices were investigated. The poor electrical characteristic of PMMA and brittle mechanical properties of HfOx can be improved by blending PMMA and HfOx together. The PMMA-HfOx blended ReRAM device exhibited a larger memory window, stable endurance and retention, a lower operation power, and better set/reset voltage distributions. Furthermore, these new systems featured multilevel conduction states at different reset bias for non-volatile multilevel memory applications. Therefore, solution-processed PMMA-HfOx blended films are a promising material for non-volatile memory devices on flexible or wearable electronic systems.

Resistive switching memory properties of layer-by-layer assembled enzyme multilayers

The properties of enzymes, which can cause reversible changes in currents through redox reactions in solution, are of fundamental and practical importance in bio-electrochemical applications. These redox properties of enzymes are often associated with their charge-trap sites. Here, we demonstrate that reversible changes in resistance in dried lysozyme (LYS) films can be generated by an externally applied voltage as a result of charge trap/release. Based on such changes, LYS can be used as resistive switching active material for nonvolatile memory devices. In this study, cationic LYS and anionic poly(styrene sulfonate) (PSS) layers were alternately deposited onto Pt-coated silicon substrates using a layer-by-layer assembly method. Then, top electrodes were deposited onto the top of LYS/PSS multilayers to complete the fabrication of the memory-like device. The LYS/PSS multilayer devices exhibited typical resistive switching characteristics with an ON/OFF current ratio above 102, a fast switching speed of 100 ns and stable performance. Furthermore, the insertion of insulating polyelectrolytes (PEs) between the respective LYS layers significantly enhanced the memory performance of the devices showing a high ON/OFF current ratio of ∼106 and low levels of power consumption.

Reliable Organic Nonvolatile Memory Device Using a Polyfluorene-Derivative Single-Layer Film

This letter describes the reversible switching performance of metal-organic-semiconductor (MOS) memory devices containing a polyfluorene-derivative single-layer film. The space-charge-limited current contributes to the switching behavior of WPF-oxy-F memory devices. The polyfluorene derivative reported here provides a significant advance to the field of organic semiconductors because it provides a type of organic memory material for nonvolatile memory devices. The following properties are responsible for its memory capabilities: its use of a single-layer film, a large on/off ratio (Ion/Ioff ~ 104), a long retention time (more than 10 000 s), acceptable thermal stability up to 120 degC, and an excellent device-to-device switching uniformity.

Non-Volatile Solid-Electrolyte Memory Devices: Electronic versus Optical Latent Image Formation in Silver and Copper Halides

We explore the possibility of using solid-electrolytes as active materials for non-volatile memory devices. We show that large ionic concentrations can be reversibly induced and then depleted using field effect in Cu2S thin films. Conditions needed to grow/etch nanowires in solid-electrolytes are also discussed. The field-induced ionic conductivities does not involve oxidation/reduction of Cu++ while nanowire growth/etch does. We discuss retention issues, write/erase speed and other related parameters in Cu2S and other SE such as Ag2S and CuI. Interestingly, it is easier to grow/etch nanowires in Ag2S than in Cu2S. Similarities and differences between electronic and optical reduction/oxidation in SE and silver halides are also discussed.

Stress-induced leakage current in p/sup +/ poly MOS capacitors with poly-Si and poly-Si/sub 0.7/Ge/sub 0.3/ gate material

The gate bias polarity dependence of stress-induced leakage current (SILC) of PMOS capacitors with a p/sup +/ polycrystalline silicon (poly-Si) and polycrystalline Silicon-Germanium (poly-Si/sub 0.7/Ge/sub 0.3/) gate on 5.6-nm thick gate oxides has been investigated. It is shown that the SILC characteristics are highly asymmetric with gate bias polarity. This asymmetric behavior is explained by the occurrence of a different injection mechanism for negative bias, compared to positive bias where Fowler-Nordheim (FN) tunneling is the main conduction mechanism. For gate injection, a larger oxide field is required to obtain the same tunneling current, which leads to reduced SILC at low fields. Moreover, at negative gate bias, the higher valence band position of poly-SiGe compared to poly-Si reduces the barrier height for tunneling to traps and hence leads to increased SILC. At positive gate bias, reduced SILC is observed for poly-SiGe gates compared to poly-Si gates. This is most likely due to a lower concentration of Boron in the dielectric in the case of poly-SiGe compared to poly-Si. This makes Boron-doped poly-SiGe a very interesting gate material for nonvolatile memory devices.