AgInSbTe Research Articles

High switching uniformity and 50 fJ/bit energy consumption achieved in amorphous silicon-based memristive device with an AgInSbTe buffer layer

In this work, we demonstrated the high switching uniformity and 50 fJ/bit energy consumption in an amorphous silicon-based resistive switching (RS) device by inserting the AgInSbTe (AIST) layer between the silicon insulating layer and Ag top electrodes. The improved RS performance is attributed to the introduction of an Ag ion reservoir layer, which helps to suppress conducting filament overgrowth. After insertion of the AIST layer, the cumulative probability of low/high resistance states decreased from 176.8%/46.2% to 3.1%/11.9%, respectively. The advantages of promoting Ag dissolution enable the realization of fast switching speed (<50 ns) and low set voltage (∼70 mV), which gives our device low energy consumption (∼50 fJ/bit). Moreover, the multi-step of set/reset analytical model of our dual-layer RS device was developed based on the formation and dissolution of the Ag-ion-based conductive filaments. Our work presents an effective method for obtaining high-performance Si-based memory for practical applications.

Laser heat-mode patterning with improved aspect-ratio

Heat-mode lithography has received much attention owing to its capacity of breaking through the diffraction limit and realizing nano-patterning. However, the relatively lower aspect ratio of patterns seriously limits its scope of applications. In the currently available literature, the aspect ratio of submicron structures prepared by this method is less than 0.3:1. In this work, a new two-step development process was proposed to considerably improve the aspect-ratio of micro/nanostructures in the AgInSbTe (AIST) heat-mode resist. An aspect ratio of 1:1 can be kept even for the grating structures with a linewidth/period of 200nm/400 nm. The developed patterns can be transferred to silicon substrates with high fidelity. This simple and effective technique addresses key issues of heat-mode lithography in the manufacture of functional nanostructures requiring high aspect ratio, such as photomasks, templets of nanoimprint, and so on.

Dual Buffer Layers for Developing Electrochemical Metallization Memory With Low Current and High Endurance

In this letter, the dual buffer layers of AgInSbTe (AIST) and monolayer defective graphene (DG) are introduced into amorphous carbon (a-C)-based electrochemical metallization (ECM) type memristor. The advantages of promoting Ag dissolution and localizing cation diffusion region of AIST and DG layers can effectively suppress the well-known current overshoot issue and break the current-retention dilemma. Compared with its counterpart without buffer layer (Ag/a-C/Pt), the device with dual buffer layers (Ag/AIST/DG/a-C/Pt) presents excellent resistive switching (RS) performance, such as lower forming voltage with reduced overshoot current, smaller parameter fluctuations, and better cycling endurance. More importantly, the nonvolatile RS with low operation current of 10 μA and remarkable endurance (> 3.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cycles) is realized, which is among the best of reported a-C-based ECM memories. Benefit from the low operation current, the device also shows the capacity of multilevel memory with six nonvolatile resistance states through regulating the compliance current.

Analytical modeling of electrochemical metallization memory device with dual-layer structure of Ag/AgInSbTe/amorphous C/Pt

An analytical model was proposed to simulate the resistive switching (RS) process of electrochemical metallization memory (ECM) device with a dual-layer structure, in which the AgInSbTe (AIST) buffer layer was introduced to combine with the amorphous carbon (a-C) dielectric layer. This dual-layer ECM device presented good memory performance, such as high switching uniformity, however, a suitable model was absent to understand its RS mechanism. The analytical model of Ag/AIST/a-C/Pt device was developed based on the formation and dissolution of the Ag-ion-based CF. Comparing to the common RS model considering one-step set/reset process, the multi-step of set/reset was utilized to fit with the dual-layer structure. The set/reset always occurs in the AIST layer at first step and in the a-C layer subsequently due to the lower active energy and thermal conductivity of AIST layer. Random disturbances were also introduced to simulate the device variation during the cycling process. Eventually, this model can well fit with the experimental data, providing an analytical tool for describing the RS process of ECM device.

Effects of blocking oxide layer types on the performance of nonvolatile floating gate memory containing AgInSbTe-SiO2 nanocomposite thin films

Influence of blocking oxide layer types on the performance of nonvolatile floating gate memory (NFGM) containing AgInSbTe (AIST)-SiO2 nanocomposite as the charge-trapping layer was investigated. In the NFGM containing a 7-nm thick SiO2 blocking oxide layer deposited by plasma-enhanced chemical vapor deposition (PECVD), a fairly large memory window (ΔVFB) shift of 20.9V with the charge density of 1.3×1013cm−2 at ±15V gate voltage sweep was achieved. As to the NFGM containing HfO2 blocking oxide layer, the device performance was inferior to that of PECVD-SiO2 device due to the diffusion of HfO2 into the nanocomposite layer. Interdiffusion resulted in the formation of HfSiOx phase and oxygen defects which might act as the leakage paths, consequently deteriorating the charge trapping efficiency of AIST nanocrystals (NCs) embedded in nanocomposite layer. Analytical results illustrated that the film quality and barrier height feature of blocking oxide layer are essential to form the deep trap sites in the programming layer to achieve a satisfactory NFGM performance with long-term reliability.

Viscosity of liquid Ag-In-Sb-Te: Evidence of a fragile-to-strong crossover.

The temperature-dependent viscosity η(T) is measured for the equilibrium liquid of the chalcogenide Ag-In-Sb-Te (AIST), the first time this has been reported for a material of actual interest for phase-change memory. The measurements, in the range 829-1254 K, are made using an oscillating-crucible viscometer, and show a liquid with high fragility and low viscosity, similar to liquid pure metals. Combining the high-temperature viscosity measurements with values inferred from crystal growth rates in the supercooled liquid allows the form of η(T) to be estimated over the entire temperature range from above the melting point down to the glass transition. It is then clear that η(T) for liquid AIST cannot be described with a single fragility value, unlike other phase-change chalcogenides such as liquid Ge-Sb-Te. There is clear evidence for a fragile-to-strong crossover on cooling liquid AIST, similar to that analyzed in Te85Ge15. The change in fragility associated with the crossover in both these cases is rather weak, giving a broad temperature range over which η(T) is near-Arrhenius. We discuss how such behavior may be beneficial for the performance of phase-change memory. Consideration of the fragile-to-strong crossover in liquid chalcogenides may be important in tuning compositions to optimize the device performance.

Nonvolatile floating gate memory containing AgInSbTe–SiO2 nanocomposite layer and capping the HfO2/SiO2 composite blocking oxide layer

An extremely large memory window shift of about 30.7 V and high charge storage density = 2.3 × 1013 cm−2 at ±23 V gate voltage sweep were achieved in the nonvolatile floating gate memory (NFGM) device containing the AgInSbTe (AIST)–SiO2 nanocomposite as the charge trap layer and HfO2/SiO2 as the blocking oxide layer. Due to the deep trap sites formed by high-density AIST nanocrystals (NCs) in the nanocomposite matrix and the high-barrier-height feature of the composite blocking oxide layer, a good retention property of the device with a charge loss of about 16.1% at ±15 V gate voltage stress for 104 s at the test temperature of 85 °C was observed. In addition to inhibiting the Hf diffusion into the programming layer, incorporation of the SiO2 layer prepared by plasma-enhanced chemical vapor deposition in the sample provided a good Coulomb blockade effect and allowed significant charge storage in AIST NCs. Analytical results demonstrated the feasibility of an AIST-SiO2 nanocomposite layer in memory device fabrication with a simplified processing method and post-annealing at a comparatively low temperature of 400 °C in comparison with previous NC-based NFGM studies.

Characterization of AgInSbTe-SiO2 Nanocomposite Thin Film for Hydrogen Gas Sensor Applications

ABSTRACTHydrogen (H2) sensing property of AgInSbTe (AIST)-SiO2 nanocomposite thin film prepared by target-attachment sputtering method was investigated in this work. The sample subjected to a 400°C-annealing for 90 sec exhibits a significant sensitivity (58.9 %) and short response time (75 sec) upon the exposure to an ambient containing 200 ppm H2 at 75°C. The gas sensing capability is ascribed to the presence of antimony oxides, e.g., Sb2O3 and Sb2O5, in nanocomposite layer which provide the charge carriers for sensing reactions. Moreover, the high specific-surface-area (SSA) feature of AIST nanocrystals in nanocomoposite layer provided numerous sites for reduction/oxidation reactions and thus a good H2 gas sensing property can be achieved.

Enhancement of Nonvolatile Floating Gate Memory Devices Containing AgInSbTe-SiO2 Nanocomposite by Inserting HfO2/SiO2 Blocking Oxide Layer

ABSTRACTThis work presents an enhancement of nonvolatile floating gate memory (NFGM) devices comprised of AgInSbTe (AIST) nanocomposite as the charge-storage trap layer and HfO2 or HfO2/SiO2 as the blocking oxide layer. A significantly large memory window (ΔVFB) shift = 30.7 V and storage charge density = 2.3×1013 cm−2 at ±23V gate voltage sweep were achieved in HfO2/SiO2/AIST sample. Retention time analysis observed a ΔVFB shift about 19.3 V and the charge loss about 13.4% in such a sample under the ±15V gate voltage stress after 104 sec retention time test. Regardless of applied bias direction, the sample containing HfO2/SiO2 layer exhibited the leakage current density as low as 150 nA/cm2 as revealed by the current-voltage (I-V) measurement. This effectively suppresses the electron injection between gate electrode and charge trapping layer and leads to a substantial enhancement of NFGM characteristics.

Laser direct writing pattern structures on AgInSbTe phase change thin film

Different pattern structures are obtained on the AgInSbTe (AIST) phase change film as induced by laser beam. Atomic force microscopy (AFM) was used to observe and analyze the different pattern structures. The AFM photos clearly show the gradually changing process of pattern structures induced by different threshold effects, such as crystallization threshold, microbump threshold, melting threshold, and ablation threshold. The analysis indicates that the AIST material is very effective in the fabrication of pattern structures and can offer relevant guidance for application of the material in the future.

Characterizations of AgInSbTe and Its Nanocomposite Thin Films for Phase-Change Memory Applications

Phase transition kinetics and microstructures of AgInSbTe (AIST) and AIST- nanocomposite applied to phase-change memories (PCMs) are investigated. In situ electrical property measurement found that the incorporation of escalates the recrystallization temperature of nanocomposite. Both X-ray diffraction and transmission electron microscopy showed grain refinement in the nanocomposite which, in turn, results in an increase of the activation energy of phase transition, as indicated by subsequent Kissinger’s analysis. Increase of and in the nanocomposite was ascribed to AIST grain refinement and hindrance to grain growth due to dispersed particles in the sample matrix. Johnson–Mehl–Avrami analysis revealed the decrease of Avrami exponent in nanocomposite, implying that the dispersed particles promote the heterogeneous phase transition. Static characteristics and reversible binary switching behavior of PCM devices not only confirmed the results of microstructure characterizations but also illustrated the feasibility of the AIST and its nanocomposite layer for PCM fabrication.

Characteristics of AgInSbTe–SiO2 nanocomposite thin film applied to nonvolatile floating gate memory devices

Nanocomposite thin films containing AgInSbTe (AIST) particles embedded in anSiO2 matrix was prepared by sputtering deposition and its feasibility for nonvolatilefloating gate memory (NFGM) was investigated. The sample subjected to a400 °C annealing exhibited a distinct hysteresis memory window(ΔVFB) shift = 6.6 V andcharge density = 5.2 × 1012 cm − 2 after ± 8 V gate voltage sweep. Electrical measurement revealed the current transport is via the Schottkyemission in low applied field and the space-charge-limited conduction mechanism in high appliedfield in the samples, regardless of their thermal history. Transmission electron microscopy andx-ray photoelectron spectroscopy indicated that the metallicSb2Te nanocrystals (NCs) with diameters about 5–7 nm dispersed in a nanocomposite layer may serve as thediscrete charge-storage traps for nonvolatile memory. Analytical results illustrate the utilization of anAIST–SiO2 nanocomposite layer as the core structure of NFGM devices is able to simplify the devicestructure and fabrication process.

Switching Casimir forces with phase-change materials

We demonstrate here a controllable variation in the Casimir force. Changes in the force of up to 20% at separations of ~100 nm between Au and AgInSbTe (AIST) surfaces were achieved upon crystallization of an amorphous sample of AIST. This material is well known for its structural transformation, which produces a significant change in the optical properties and is exploited in optical data storage systems. The finding paves the way to the control of forces in nanosystems, such as micro- or nanoswitches by stimulating the phase change transition via localized heat sources.

Optically High Spatial Resolution Obtained Using Super-Resolution Near-Field Structure Disk System

Super-resolution near-field structure (Super-RENS) disks with AgInSbTe (AIST) mask layers with varying thicknesses were prepared and their spatial resolutions were experimentally evaluated. The thickness of each AIST mask layer was changed from 6.5 to 18.6 nm. A Super-RENS disk with a mask layer thickness of 18.6 nm was recorded and retrieved at a small mark length of 23 nm, which is far beyond the diffraction limit. By the third-harmonic spectrum analysis of read signals, a readout spatial resolution of 19.3 nm was obtained. This spatial resolution can be used for optical disks with capacities of more than 1 TB.

Phase Transition Kinetics and Recording Characteristics of Nanocomposite Layers Prepared by Sputtering Process Utilizing AgInSbTe–SiO2 Composite Target

AgInSbTe (AIST)–SiO2 nanocomposite thin films were successfully prepared by sputtering deposition utilizing an AIST–SiO2 composite target and their phase-transition behaviors were investigated. Transmission electron microscopy (TEM) revealed that the as-deposited composite layer contained nano-scale quaternary alloy particles about 5 nm in size randomly embedded in the SiO2 matrix. In situ reflectivity–temperature measurements showed that the AIST–SiO2 nanocomposite and pristine AIST layers exhibit similar abrupt reflectivity changes at about 200 °C. However, the presence of the SiO2 phase hindered particle growth in the nanocomposite layers which, in turn, caused a mild increase in the phase-transition temperatures. Johnson–Mehl–Avrami (JMA) analysis indicated that, regardless of the film thickness, the amorphous-to-crystalline transition in nanocomposite layers proceeds in a bulk-like manner. Dynamic tests revealed that modulations higher than 0.8, 0.6, and 0.5 were achieved when 11T, 4T, and 3T signals, respectively were, written in the one-layer AIST–SiO2 disk sample. The unique phase transition behavior of nanocomposite layers and signal property analysis of disk samples demonstrated that a nanocomposite layer can be a new alternative for write-once optical data storage.

Variation of Microstructure and Electrical Conductivity of Amorphous AgInSbTe and SbTe Films during Crystallization

Two kinds of chalcogenide films with similar Sb/Te atomic ratios, AgInSbTe (AIST) and SbTe (ST) were deposited on alkali-free glass using the RF sputtering method. The microstructure characteristics of both ST and AIST films were demonstrated to correlate with the electrical properties. TEM observation and GI-XRD profiles revealed that the structures of as-deposited AIST and ST films were amorphous characteristics. The sheet resistance of the as-deposited AIST films was twice as high as that of ST films and the effect of Ag and In additives was proposed. After annealing at 523 K for 1 h, the sheet resistance of both chalcogenide films decreased by four orders of magnitude due to crystallizations and the sheet resistance of crystallized AIST film was still twice as large as that of crystallized ST film. DSC measurement was used to determine the crystallization temperature of AIST films as 476 K and that of ST films as 445 K. The activation energies of crystallization were determined using Kissinger’s method, and 0.94 eV and 0.84 eV were obtained for AIST and ST films, respectively. TEM observations showed that AgSbTe2 phase exists in the AIST film after heat treatment in addition to � -Sb phase. No indium compounds were discovered in the AIST film by the energy dispersive spectroscopy (EDS) and XRD measurements. [doi:10.2320/matertrans.48.610]

Activation Energy of AgInSbTe Film through Isothermal Sheet Resistance Measurements

RF sputtering was used to deposit AgInSbTe (AIST) films on silicon substrates. The as-deposited amorphous films were crystallized at temperatures above 413 K and the sheet resistance change was used to characterize the degree of crystallization and the associate activation energy in this study. The sheet resistance of amorphous films quickly decreased when the crystallization was initiated and reached a steady lower value with the IOC index of around 0.7, which means 70% crystallization. The kinetics of sheet resistance change was researched isothermally in an argon atmosphere and compared to the results of DSC measurement with constant heating rates. It was found that activation energy, 0.82 eV, obtained from isothermal sheet resistance measurement was rather close to that, 0.92 eV, obtained from DSC measurement. The Avrami exponent was determined to be 1.1∼1.4 in isothermal sheet resistance measurement. A lower Avrami exponent implied that impingement effects possibly resulted in the sheet resistance decreasing with about 78% crystallization of amorphous films.

On a thermally induced readout mechanism in super-resolution optical disks

We have simultaneously measured the carrier-to-noise ratio (CNR) as well as the transmitted and reflected light intensities from platinum oxide based super-resolution near-field structure (PtOx super-RENS) disks. The the reflected and transmitted light intensities were found to decrease and increase, respectively, as the CNR value increased. The phase-change material AgInSbTe (AIST) used in PtOx super-RENS disks was found to exhibit a strong optical nonlinearity with respect to readout laser power. AIST becomes transparent at higher laser powers. To ascertain whether the presence of Pt nanoparticles is important to the readout mechanism, a super-RENS disk was fabricated in which the PtOx layer was replaced with a metal-free phthalocyanine (H2Pc) layer and the CNR of the H2Pc disk was measured. From the observation that the CNR value was equivalent to that of a disk made using PtOx, we conclude that the presence of nanoparticles does not play an important role in the super-RENS readout mechanism. Finally, we also investigated the use of Si and the alloy Ge2Sb2Te5 in lieu of AIST in a super-RENS disk and simple three layer structure disks. The super-resolution effect was observed for all disk types. Based upon these observations, we discuss the possibility of a thermal origin for the super-resolution effect in all super-resolution disks.

Simulation of Recrystallization in Phase-Change Recording Materials

Simulations of the movement of the crystalline/amorphous boundary during the writing process were performed. The movement of the boundary was traced by drawing arrows representing the crystallization direction and speed. The lines traced by these arrows made a good match with streamlines of crystal growth shown in a transmission electron microscope (TEM) image. We have also introduced the idea of a “recrystallization ring” to explain the mark-formation mechanism by investigating the temperature dependency of the crystallization growth rate. The simulations for GeSbTe and Ag–InSbTe recording materials illustrate the differences in the crystallization mechanisms of these materials.