High-Q adiabatic micro-resonators on a wafer-scale ion-sliced 4H-silicon carbide-on-insulator platform.

A 4H-silicon carbide-on-insulator (4H-SiCOI) has emerged as a prominent material contender for integrated photonics owing to its outstanding material properties such as CMOS compatibility, high refractive index, and high second- and third-order nonlinearities. Although various micro-resonators have been realized on the 4H-SiCOI platform, enabling numerous applications including frequency conversion and electro-optical modulators, they may suffer from a challenge associated with spatial mode interactions, primarily due to the widespread use of multimode waveguides. We study the suppression of spatial mode interaction with Euler bends, and demonstrate micro-resonators with improved Q values above 1 × 105 on ion-sliced 4H-SiCOI platform with a SiC thickness nonuniformity less than 1%. The spatial-mode-interaction-free micro-resonators reported on the CMOS-compatible wafer-scale 4H-SiCOI platform would constitute an important ingredient for the envisaged large-scale integrated nonlinear photonic circuits.

Simultaneously achieving high performance of thermal stability and power consumption via doping yttrium in Sn15Sb85thin film.

The effects of yttrium dopants on the phase change behavior and microstructure of Sn15Sb85films have been systematically investigated. The yttrium-doped Sn15Sb85film has the higher phase transition temperature, ten year data retention ability and crystallization activation energy, which represent a great improvement in thermal stability and data retention. X-ray diffraction, transmission electron microscopy and x-ray photoelectron spectroscopy reveal that the amorphous Sn and Y components restrict the grain growth and decrease the grain size. Raman mode typically associated with Sb is altered when the substance crystallized. Atomic force microscopy results show that the surface morphology of the doped films becomes smoother. T-shaped phase change storage cells based on yttrium-doped Sn15Sb85films exhibit the lower power consumption. The results demonstrate that the crystallization characteristics of Sn15Sb85film can be tuned and optimized through the yttrium dopant for the excellent performances of phase change memory.

Visible and near-infrared microdisk resonators on a 4H-silicon-carbide-on-insulator platform.

Wavelength-sized microdisk resonators were fabricated on a single crystalline 4H-silicon-carbide-on-insulator (4H-SiCOI) platform. By carrying out micro-photoluminescence measurements at room temperature, we show that the microdisk resonators support whispering-gallery modes (WGMs) with quality factors up to 5.25×103 and mode volumes down to 2.61×(λ/n)3 at the visible and near-infrared wavelengths. Moreover, the demonstrated wavelength-sized microdisk resonators exhibit WGMs whose resonant wavelengths are compatible with the zero-phonon lines of silicon related spin defects in 4H-SiCOI, making them a promising candidate for applications in cavity quantum electrodynamics and integrated quantum photonic circuits.

Non-volatile multi-level cell storage via sequential phase transition in Sb7Te3/GeSb6Te multilayer thin film.

For high-performance data centers, huge data transfer, reliable data storage and emerging in-memory computing require memory technology with the combination of accelerated access, large capacity and persistence. As for phase-change memory, the Sb-rich compounds Sb7Te3and GeSb6Te have demonstrated fast switching speed and considerable difference of phase transition temperature. A multilayer structure is built up with the two compounds to reach three non-volatile resistance states. Sequential phase transition in a relationship with the temperature is confirmed to contribute to different resistance states with sufficient thermal stability. With the verification of nanoscale confinement for the integration of Sb7Te3/GeSb6Te multilayer thin film, T-shape PCM cells are fabricated and two SET operations are executed with 40 ns-width pulses, exhibiting good potential for the multi-level PCM candidate.

Reliable resistive switching of epitaxial single crystalline cubic Y-HfO2 RRAMs with Si as bottom electrodes.

Previous studies have mainly focused on the resistive switching (RS) of amorphous or polycrystalline HfO2-RRAM. The RS of single crystalline HfO2 films has been rarely reported. Yttrium doped HfO2 (YDH) thin films were fabricated and successful Y incorporation into HfO2 was confirmed by x-ray photoemission spectroscopy. A pure cubic phase of YDH and an abrupt YDH/Si interface were obtained and verified by x-ray diffraction, Raman spectroscopy and transmission electron microscopy. A Pt/YDH/n++-Si heterostructure using Si as the bottom electrode was fabricated, which shows stable RS with an ON/OFF ratio of 100 and a reliable data retention (104 s). The electron transport mechanism was investigated in detail. It indicates that hopping conduction is dominating when the device is at a high resistance state, while space charge limited conduction acts as the dominant factor at a low resistance state. Such behavior, which is different from devices using TiN or Ti as electrodes, was attributed to the Y doping and specific YDH/Si interface. Our results demonstrate a proof of concept study to use highly doped Si as bottom electrodes along with single crystalline YDH as insulator layer for such RRAM applications as wireless sensors and synaptic simulation.

Excellent thermal stability owing to Ge and C doping in Sb2Te-based high-speed phase-change memory.

The contradictory nature between transition speed and thermal stability of phase-change materials has always been the key limitation to the achievement of wide applications under harsh conditions. Ge2.3Sb2.0Te phase-change alloy is proposed here to feature high thermal stability (10 year data retention above 220 °C) and fast switching speed (SET programming speed up to 5 ns) for electronic storage. In mushroom-shaped device cells, the nanocomposite materials implement an endurance life of nearly 1 × 105 cycles. Such operation speed among high-temperature alloys is the best ever reported. And the moderate incorporation of C offers intriguing benefits that include enhanced thermal stability and reduced RESET voltage in the above-mentioned Ge-rich Sb2Te-based memory cells. Through microscopic analysis, the local segregation of C dopants can further refine the crystalline grains and thus induce a lower volume change and roughness upon heating. These properties are crucial with regard to the application potential in high-performance and high-density embedded memories.

Covalent Functionalization of Black Phosphorus with Conjugated Polymer for Information Storage.

Major disadvantages of black phosphorus (BP) are its poor air-stability and poor solubility in common organic solvents. The best way to solve this problem is to incorporate BP into a polymer backbone or a polymer matrix to form novel functional materials that can provide both challenges and opportunities for new innovation in optoelectronic and photonic applications. As a proof-of concept application, we synthesized in situ the first highly soluble conjugated polymer-covalently functionalized BP derivative (PDDF-g-BP) which was used to fabricate a resistive random access memory (RRAM) device with a configuration of Au/PDDF-g-BP/ITO. In contrast to PDDF without memory effect, PDDF-g-BP-based device exhibits a nonvolatile rewritable memory performance, with a turn-on and turn-off voltages of +1.95 V and -2.34 V, and an ON/OFF current ratio of 104 . The current through the device in both the ON and OFF states is still kept unchanged even at 200th switching cycle. The PDDF/BP blends show a very unstable memory performance with a very small ON/OFF current ratio.

Multi-level storage and ultra-high speed of superlattice-like Ge50Te50/Ge8Sb92 thin film for phase-change memory application.

Superlattice-like Ge50Te50/Ge8Sb92 (SLL GT/GS) thin film was systematically investigated for multi-level storage and ultra-fast switching phase-change memory application. In situ resistance measurement indicates that SLL GT/GS thin film exhibits two distinct resistance steps with elevated temperature. The thermal stability of the amorphous state and intermediate state were evaluated with the Kissinger and Arrhenius plots. The phase-structure evolution revealed that the amorphous SLL GT/GS thin film crystallized into rhombohedral Sb phase first, then the rhombohedral GeTe phase. The microstructure, layered structure, and interface stability of SLL GT/GS thin film was confirmed by using transmission electron microscopy. The transition speed of crystallization and amorphization was measured by the picosecond laser pump-probe system. The volume variation during the crystallization was obtained from x-ray reflectivity. Phase-change memory (PCM) cells based on SLL GT/GS thin film were fabricated to verify the multi-level switching under an electrical pulse as short as 30 ns. These results illustrate that the SLL GT/GS thin film has great potentiality in high-density and high-speed PCM applications.

In Situ Synthesis and Characterization of Poly(aryleneethynylene)-Grafted Reduced Graphene Oxide.

Using highly soluble bromo-functionalized reduced graphene oxide (RGBr) as a key graphene template for surface-directing Sonogashira-Hagihara polymerization, a novel soluble poly(arylene-ethynylene)-grafted reduced graphene oxide, hereafter abbreviated as PAE-g-RGO, was prepared in situ. The entirely different electron distribution of LUMO and HOMO of PAE-g-RGO suggested the existence of a charge-transfer (CT) state (PAE(.-) -RGO(.+) ). The negative ΔGCS value (-2.57 eV) indicates that the occurrence of the charge separation via (1) RGO* in o-DCB is exothermic and favorable. Upon irradiation with 365 nm light, the light-induced electron paramagnetic resonance (LEPR) spectrum of PAE-g-RGO showed a decrease in the spin-state density owing to photoinduced intramolecular electron transfer events in this system. A sandwich-type Al/PAE-g-RGO/ITO device showed representative bistable electrical switching behavior. The nonvolatile memory performance was attributed to the CT-induced conductance changes, which was supported by molecular computation results and conductive atomic force microscopy (C-AFM) images.

Improvement of Multilevel Memory Performance of MnTe Thin Films by Ta Doping.

The pressing need for data storage in the era of big data has driven the development of new storage technologies. As a prominent contender for next-generation memory, phase-change memory can effectively increase storage density through multilevel cell operation and can be applied to neuromorphic and in-memory computing. Herein, the structure and properties of Ta-doped MnTe thin films and their inherent correlations are systematically investigated. Amorphous MnTe thin films sequentially precipitated cubic MnTe2 and hexagonal Te phases with increasing temperature, causing resistance changes. Ta doping inhibited phase segregation in the films and improved their thermal stability in the amorphous state. A phase-change memory cell based on a Ta2.8%-MnTe thin film exhibited three stable resistive states with low resistive drift coefficients. The study findings reveal the possibility of regulating the two-step phase-change process in Ta-MnTe thin films, providing insight into the design of multilevel phase-change memory.

A Complicated Route from Disorder to Order in Antimony-Tellurium Binary Phase Change Materials.

The disorder-to-order (crystallization) process in phase-change materials determines the speed and storage polymorphism of phase-change memory devices. Only by clarifying the fine-structure variation can the devices be insightfully designed, and encode and store information. As essential phase-change parent materials, the crystallized Sb-Te binary system is generally considered to have the cationic/anionic site occupied by Sb/Te atoms. Here, direct atomic identification and simulation demonstrate that the ultrafast crystallization speed of Sb-Te materials is due to the random nature of lattice site occupation by different classes of atoms with the resulting octahedral motifs having high similarity to the amorphous state. It is further proved that after atomic ordering with disordered chemical occupation, chemical ordering takes place, which results in different storage states with different resistance values. These new insights into the complicated route from disorder to order will play an essential role in designing neuromorphic devices with varying polymorphisms.

Study on the crystallization process of GaSb-Sb2Te3 pseudobinary films for phase-change random access memory.

Non-isothermal change in electrical resistance was used to investigate the crystallization process of GaSb-Sb2Te3 pseudobinary films prepared by co-sputtering using GaSb and Sb2Te3 targets. The crystallization parameters were determined directly by in-situ electrical resistance-temperature measurements. The activation energy of crystallization and rate factor were deduced from the Kissinger's plot. The kinetics exponent was calculated using the Ozawa's method. The crystallization temperature (185-228 degrees C) and activation energy (2.01-5.65 eV) increased monotonically with increasing Ga concentration from 5 to 34 mol%, while the average kinetics exponent decreased from 1.63 to 1.02. The crystallization mechanism of the compositions with Ga concentration more than 10 mol% was one-dimensional growth from the nuclei due to the average kinetics exponent smaller than 1.5. Crystallization time of the studied compositions was estimated theoretically by the Johnson-Mehl-Avrami equation and measured experimentally by the reflectivity change induced by the laser pulse. It is shown that Ga27Sb47Te26 film exhibited the shortest crystallization time, suggesting a potential candidate for phase-change random access memory application.

Reactive ion etching of Si(x)Sb2Te in CF4/Ar plasma for nonvolatile phase-change memory device.

Si(x)Sb2Te material system is novel for phase-change random access memory applications. Its properties are more outstanding than the widely used material Ge2Sb2Te5. Etching process is one of the critical steps in the device fabrication. The etching characteristics of phase-change material Si(x)Sb2Te were studied with CF4/Ar gas mixture by a reactive ion etching system. The changes of etching rate, etching profile and surface root-mean-square roughness resulted from variation of the gas-mixing ratio were investigated under constant pressure (50 mTorr) and applying power (200 W). Si0.34Sb2Te is with the highest phase-change speed and the lowest power consumption in the PCRAM memory among these compositions, which means it is the most promising candidate for the PCRAM applications. So the most optimized CF4/Ar gas ratio for Si0.34Sb2Te was studied, the value is 25/25. The etching rate is 155 nm/min, and the selectivity of Si0.34Sb2Te to SiO2 is as high as 3.4 times. Furthermore, the smooth surface was achieved with this optimized gas ratio.

Advantages of Ta-Doped Sb3Te1 Materials for Phase Change Memory Applications.

Phase change memory (PCM), a typical representative of new storage technologies, offers significant advantages in terms of capacity and endurance. However, among the research on phase change materials, thermal stability and switching speed performance have always been the direction where breakthroughs are needed. In this research, as a high-speed and good thermal stability material, Ta was proposed to be doped in Sb3Te1 alloy to improve the phase transition performance and electrical properties. The characterization shows that Ta-doped Sb3Te1 can crystallize at temperatures up to 232 °C and devices can operate at speeds of 6 ns and 8 × 104 operation cycles. The reduction of grain size and the density change rate (3.39%) show excellent performances, which are both smaller than that of Ge2Sb2Te5 (GST) and Sb3Te1. These properties conclusively demonstrate that Ta incorporation of Sb3Te1 alloy is a material with better thermal stability and faster crystallization rates for PCM applications.

A High-Precision Current-Mode Bandgap Reference with Nonlinear Temperature Compensation.

A high-precision current-mode bandgap reference (BGR) circuit with a high-order temperature compensation is presented in this paper. In order to achieve a high-precision BGR circuit, the equation of the nonlinear current has been modified and the high-order term of the current flowing into the nonlinear compensation bipolar junction transistor (NLCBJT) is compensated further. According to the modified equation, two solutions are designed to improve the output accuracy of BGR circuits. The first solution is to divide the NLCBJT branch into two branches to reduce the coefficient of the nonlinear temperature compensation current. The second solution is to inject the nonlinear current into the two branches based on the first one to further eliminate the temperature coefficient (TC) of the current flowing into the NLCBJT. The proposed BGR circuit has been designed using the Semiconductor Manufacturing International Corporation (SMIC) 55 nm CMOS process. The simulation results show that the variations in currents flowing into NLCBJTs improved from 148.41 nA to 69.35 nA and 7.4 nA, respectively, the TC of the output reference current of the proposed circuit is approximately 3.78 ppm/°C at a temperature range of -50 °C to 120 °C with a supply voltage of 3.3 V, the quiescent current consumption of the entire BGR circuit is 42.13 µA, and the size of the BGR layout is 0.044 mm2, leading to the development of a high-precision BGR circuit.

Neuromorphic Photonic Memory Devices Using Ultrafast, Non-Volatile Phase-Change Materials.

The search for ultrafast photonic memory devices is inspired by the ever-increasing number of cloud-computing, supercomputing, and artificial-intelligence applications, together with the unique advantages of signal processing in the optical domain such as high speed, large bandwidth, and low energy consumption. By embracing silicon photonics with chalcogenide phase-change materials (PCMs), non-volatile integrated photonic memory is developed with promising potential in photonic integrated circuits and nanophotonic applications. While conventional PCMs suffer from slow crystallization speed, scandium-doped antimony telluride (SST) has been recently developed for ultrafast phase-change random-access memory applications. An ultrafast non-volatile photonic memory based on an SST thin film with a 2 ns write/erase speed is demonstrated, which is the fastest write/erase speed ever reported in integrated phase-change photonic devices. SST-based photonic memories exhibit multilevel capabilities and good stability at room temperature. By mapping the memory level to the biological synapse weight, an artificial neural network based on photonic memory devices is successfully established for image classification. Additionally, a reflective nanodisplay application using SST with optoelectronic modulation capabilities is demonstrated. Both the optical and electrical changes in SST during the phase transition and the fast-switching speed demonstrate their potential for use in photonic computing, neuromorphic computing, nanophotonics, and optoelectronic applications.

The Effect of Carbon Doping on the Crystal Structure and Electrical Properties of Sb2Te3.

As a new generation of non-volatile memory, phase change random access memory (PCRAM) has the potential to fill the hierarchical gap between DRAM and NAND FLASH in computer storage. Sb2Te3, one of the candidate materials for high-speed PCRAM, has high crystallization speed and poor thermal stability. In this work, we investigated the effect of carbon doping on Sb2Te3. It was found that the FCC phase of C-doped Sb2Te3 appeared at 200 °C and began to transform into the HEX phase at 25 °C, which is different from the previous reports where no FCC phase was observed in C-Sb2Te3. Based on the experimental observation and first-principles density functional theory calculation, it is found that the formation energy of FCC-Sb2Te3 structure decreases gradually with the increase in C doping concentration. Moreover, doped C atoms tend to form C molecular clusters in sp2 hybridization at the grain boundary of Sb2Te3, which is similar to the layered structure of graphite. And after doping C atoms, the thermal stability of Sb2Te3 is improved. We have fabricated the PCRAM device cell array of a C-Sb2Te3 alloy, which has an operating speed of 5 ns, a high thermal stability (10-year data retention temperature 138.1 °C), a low device power consumption (0.57 pJ), a continuously adjustable resistance value, and a very low resistance drift coefficient.

Nanoscale Phase Change Material Array by Sub-Resolution Assist Feature for Storage Class Memory Application.

High density phase change memory array requires both minimized critical dimension (CD) and maximized process window for the phase change material layer. High in-wafer uniformity of the nanoscale patterning of chalcogenides material is challenging given the optical proximity effect (OPE) in the lithography process and the micro-loading effect in the etching process. In this study, we demonstrate an approach to fabricate high density phase change material arrays with half-pitch down to around 70 nm by the co-optimization of lithography and plasma etching process. The focused-energy matrix was performed to improve the pattern process window of phase change material on a 12-inch wafer. A variety of patternings from an isolated line to a dense pitch line were investigated using immersion lithography system. The collapse of the edge line is observed due to the OPE induced shrinkage in linewidth, which is deteriorative as the patterning density increases. The sub-resolution assist feature (SRAF) was placed to increase the width of the lines at both edges of each patterning by taking advantage of the optical interference between the main features and the assistant features. The survival of the line at the edges is confirmed with around a 70 nm half-pitch feature in various arrays. A uniform etching profile across the pitch line pattern of phase change material was demonstrated in which the micro-loading effect and the plasma etching damage were significantly suppressed by co-optimizing the etching parameters. The results pave the way to achieve high density device arrays with improved uniformity and reliability for mass storage applications.

Pt Modified Sb2Te3 Alloy Ensuring High-Performance Phase Change Memory.

Phase change memory (PCM), due to the advantages in capacity and endurance, has the opportunity to become the next generation of general-purpose memory. However, operation speed and data retention are still bottlenecks for PCM development. The most direct way to solve this problem is to find a material with high speed and good thermal stability. In this paper, platinum doping is proposed to improve performance. The 10-year data retention temperature of the doped material is up to 104 °C; the device achieves an operation speed of 6 ns and more than 3 × 105 operation cycles. An excellent performance was derived from the reduced grain size (10 nm) and the smaller density change rate (4.76%), which are less than those of Ge2Sb2Te5 (GST) and Sb2Te3. Hence, platinum doping is an effective approach to improve the performance of PCM and provide both good thermal stability and high operation speed.

An Output-Capacitorless Low-Dropout Regulator with Slew-Rate Enhancement.

A novel output-capacitorless low-dropout regulator (OCL-LDO) with an embedded slew-rate-enhancement (SRE) circuit is presented in this paper. The SRE circuit adopts a transient current-boost strategy to improve the slew rate at the gate of the power transistor when a large voltage spike at the output is detected. In addition, a feed-forward transconductance cell is introduced to form a push−pull output structure with the power transistor. The simulation results show that the maximum transient output voltage variation is 23.5 mV when the load current ILOAD is stepped from 0 to 100 mA in 100 ns with a load capacitance of 100 pF, and the settling time is 1.2 µs. The proposed OCL-LDO consumes a quiescent current of 30 µA and has a dropout voltage of 200 mV for the maximum output current of 100 mA.