Fabrication and integration of photonic devices for phase-change memory and neuromorphic computing

In the past decade,there has been tremendous progress in integrating chalcogenide phase-change materials(PCMs)on the silicon photonic platform for non-volatile memory to neuromorphic in-memory computing applications.In particular,these non von Neumann computational elements and systems benefit from mass manufacturing of silicon photonic integrated circuits(PICs)on 8-inch wafers using a 130 nm complementary metal-oxide semiconductor line.Chip manufacturing based on deep-ultraviolet lithography and electron-beam lithography enables rapid prototyping of PICs,which can be integrated with high-quality PCMs based on the wafer-scale sputtering technique as a back-end-of-line process.In this article,we present an overview of recent advances in waveguide integrated PCM memory cells,functional devices,and neuromorphic systems,with an emphasis on fabrication and integration processes to attain state-of-the-art device performance.After a short overview of PCM based photonic devices,we discuss the materials properties of the functional layer as well as the progress on the light guiding layer,namely,the silicon and germanium waveguide platforms.Next,we discuss the cleanroom fabrication flow of waveguide devices integrated with thin films and nanowires,silicon waveguides and plasmonic microheaters for the electrothermal switching of PCMs and mixed-mode operation.Finally,the fabrication of photonic and photonic–electronic neuromorphic computing systems is reviewed.These systems consist of arrays of PCM memory elements for associative learning,matrix-vector multiplication,and pattern recognition.With large-scale integration,the neuromorphic photonic computing paradigm holds the promise to outperform digital electronic accelerators by taking the advantages of ultra-high bandwidth,high speed,and energy-efficient operation in running machine learning algorithms.

Ta-Doped Sb2Te Allows Ultrafast Phase-Change Memory with Excellent High-Temperature Operation Characteristics

Phase-change memory(PCM)has considerable promise for new applications based on von Neumann and emerging neuromorphic computing systems.However,a key challenge in harnessing the advantages of PCM devices is achieving high-speed operation of these devices at elevated temperatures,which is critical for the efficient processing and reliable storage of data at full capacity.Herein,we report a novel PCM device based on Ta-doped antimony telluride(Sb2Te),which exhibits both high-speed characteristics and excellent high-temperature characteristics,with an operation speed of 2 ns,endurance of >106 cycles,and reversible switching at 140℃.The high coordination number of Ta and the strong bonds between Ta and Sb/Te atoms contribute to the robustness of the amorphous structure,which improves the thermal stability.Furthermore,the small grains in the three-dimensional limit lead to an increased energy efficiency and a reduced risk of layer segregation,reducing the power consumption and improving the long-term endurance.Our findings for this new Ta-Sb2Te material system can facilitate the development of PCMs with improved performance and novel applications.

Universal memory based on phase-change materials:From phase-change random access memory to optoelectronic hybrid storage

The era of information explosion is coming and information need to be continuously stored and randomly accessed over long-term periods,which constitute an insurmountable challenge for existing data centers.At present,computing devices use the von Neumann architecture with separate computing and memory units,which exposes the shortcomings of“memory bottleneck”.Nonvolatile memristor can realize data storage and in-memory computing at the same time and promises to overcome this bottleneck.Phase-change random access memory(PCRAM)is called one of the best solutions for next generation non-volatile memory.Due to its high speed,good data retention,high density,low power consumption,PCRAM has the broad commercial prospects in the in-memory computing application.In this review,the research progress of phase-change materials and device structures for PCRAM,as well as the most critical performances for a universal memory,such as speed,capacity,and power consumption,are reviewed.By comparing the advantages and disadvantages of phase-change optical disk and PCRAM,a new concept of optoelectronic hybrid storage based on phase-change material is proposed.Furthermore,its feasibility to replace existing memory technologies as a universal memory is also discussed as well.

Unconventional phase transition of phase-change-memory materials for optical data storage

Recent years, optically controlled phase-change memory draws intensive attention owing to some advanced applications including integrated all-optical nonvolatile memory, in-memory computing, and neuromorphic computing. The light-induced phase transition is the key for this technology. Traditional understanding on the role of light is the heating effect. Generally, the RESET operation of phase-change memory is believed to be a melt-quenching-amorphization process. However, some recent experimental and theoretical investigations have revealed that ultrafast laser can manipulate the structures of phase-change materials by non-thermal effects and induces unconventional phase transitions including solid-to-solid amorphization and order-to-order phase transitions. Compared with the conventional thermal amorphization,these transitions have potential superiors such as faster speed, better endurance, and low power consumption. This article summarizes some recent progress of experimental observations and theoretical analyses on these unconventional phase transitions. The discussions mainly focus on the physical mechanism at atomic scale to provide guidance to control the phase transitions for optical storage. Outlook on some possible applications of the non-thermal phase transition is also presented to develop new types of devices.

Paraffin/SiC as a Novel Composite Phase-Change Material for a Lithium-Ion Battery Thermal Management System

A lithium-ion battery thermal management system has always been a hot spot in the battery industry. In this study, a novel high-thermal-conductivity composite phase-change material(CPCM) made by paraffin wax and silicon was adopted to facilitate heat transfer. Moreover, high resistance or even insulation of CPCM is capable of preventing short circuits between the cells. The heat transfer mechanism of CPCMs was determined under a scanning electron microscope. A thermogravimetric analyzer was employed to determine the thermal stability. A diff erential scanning calorimeter was used to explore the thermophysical properties of the composite samples. By comparing the results of the experiment, it was reported that under the silicon carbide content of 5%, the parameters were better than others. The phase-change enthalpy of CPCM was 199.4 J/g, the leakage rate of liquid was 4.6%, and the melting point was 53.6℃. To verify the practicality of CPCM, a three-dimensional layered battery pack model was built in the COMSOL Multiphysics software. By simulating the thermal runaway inside the battery packs of various materials, it was reported that the addition of CPCM significantly narrowed the temperature range of the battery pack from 300–370 to 303–304 K. Therefore, CPCM can eff ectively increase the rate of heat transfer to prevent the chain of thermal runaway reactions. It also enables the battery pack to run at a stable temperature.

Design of broadband achromatic metasurface device based on phase-change material Ge_(2)Sb_(2)Te_(5)

Based on the phase-change material Ge_(2)Sb_(2)Te_(5)(GST),achromatic metasurface optical device in the longer-infrared wavelength is designed.With the combination of the linear phase gradient GST nanopillar and the adjustment of the crystalline fraction m value of GST,the polarization insensitive achromic metalenses and beam deflector metasurface within the longer-infrared wavelength 9.5μm to 13μm are realized.The design results show that the achromatic metalenses can be focused on the same focal plane within the working waveband.The simulation calculation results show that the fullwidth at half-maximum(FWHM)of the focusing spot reaches the diffraction limit at each wavelength.In addition,the same method is also used to design a broadband achromatic beam deflector metasurface with the same deflection angle of 19°.The method proposed in this article not only provides new ideas for the design of achromatic metasurfaces,but also provides new possibilities for the integration of optical imaging,optical coding and other related optical systems.

An artificial synapse by superlattice-like phase-change material for low-power brain-inspired computing

Phase-change material(PCM)is generating widespread interest as a new candidate for artificial synapses in bioinspired computer systems.However,the amorphization process of PCM devices tends to be abrupt,unlike continuous synaptic depression.The relatively large power consumption and poor analog behavior of PCM devices greatly limit their applications.Here,we fabricate a GeTe/Sb2Te3 superlattice-like PCM device which allows a progressive RESET process.Our devices feature low-power consumption operation and potential high-density integration,which can effectively simulate biological synaptic characteristics.The programming energy can be further reduced by properly selecting the resistance range and operating method.The fabricated devices are implemented in both artificial neural networks(ANN)and convolutional neural network(CNN)simulations,demonstrating high accuracy in brain-like pattern recognition.

Forming technology of boiling structure on evaporation surface of phase-change heat sink for high-power light emitting diode

Boiling structures on evaporation surface of red copper sheet with a diameter （D） of 10 mm and a wall thickness （h） of 1 mm were processed by the ploughing-extrusion （P-E） processing method, which is one part of the phase-change heat sink for high power （HP） light emitting diode （LED）. The experimental results show that two different structures of rectangular- and triangular-shaped micro-grooves are formed in P-E process. When P-E depth （ap）, interval of helical grooves （dp） and rotation speed （n） are 0.12 ram, 0.2 mm and 100 r/min, respectively, the boiling structures of triangular-shaped grooves with the fin height of 0.15 mm that has good evaporation performance are obtained. The shapes of the boiling structures are restricted by dp and ap, and dp is determined by n and amount of feed （f）. The ploughing speed has an important influence on the formation of groove structure in P-E process.

Three-Dimensional Simulations of RESET Operation in Phase-Change Random Access Memory with Blade-Type Like Phase Change Layer by Finite Element Modeling

An optimized device structure for reducing the RESET current of phase-change random access memory （PCRAM） with blade-type like （BTL） phase change layer is proposed. The electrical thermal analysis of the BTL cell and the blade heater contactor structure by three-dimensional finite element modeling are compared with each other during RESET operation. The simulation results show that the programming region of the phase change layer in the BTL cell is much smaller, and thermal electrical distributions of the BTL cell are more concentrated on the TiN/GST interface. The results indicate that the BTL cell has the superiorities of increasing the heating efficiency, decreasing the power consumption and reducing the RESET current from 0.67mA to 0.32mA. Therefore, the BTL cell will be appropriate for high performance PCRAM device with lower power consumption and lower RESET current.

Scaling properties of phase-change line memory

Phase-change line memory cells with different line widths are fabricated using focused-ion-beam deposited C-Pt as a hard mask. The electrical performance of these memory devices was characterized. The current^oltage （I-V） and resistance-voltage （RV） characteristics demonstrate that the power consumption decreases with the width of the phase-change line. A three-dimensional simulation is carried out to further study the scaling properties of the phase- change line memory. The results show that the resistive amorphous （RESET） power consumption is proportional to the cross-sectional area of the phase-change line, but increases as the line length decreases.

Theoretical analysis of nanosecond crystallization kinetics of phase-change optical recording films

A theoretical investigation of nanosecond crystallization kinetics of the phase-change optical recording films is presented. An extended Kissinger equation for the square-root heating is derived, which properly describes the temperature evolution of the films by nanosecond laser heating. The extended Kissinger equation was used to explain our previous experimental results.

Study of the phase-change coefficients of the cavitation model in cavitation flow fields generated from cone cavitator

The influence of phase-change coefficients variations in the Singhal cavitation model on the calculation results has been numerically studied. By comparing the numerical results and experimental data, the relationship between the coefficients and cavitation numbers is obtained. The calculation results of 2d axisymmetrical cylinder with 45-degree cone cavitator show that under different cavitation numbers, there are three typical kind of cavities, which are respectively main cavity, secondary cavity and rear cavity. The coefficients variations have a great influence respectively on the three type cavities in shape, collapse position, collapse strength, etc, and different cavitation numbers are corresponding to different phase-change coefficients. The cavitation flow field can be divided into three typical zones according to the cavitation number: weak-cavitation zone, secondary-cavitation zone and supercavitation zone. For 45-degree cone cavitator cylinder, the evaporation coefficients will firstly decrease and then increase with the decrease of cavitation numbers in secondary-cavitation zone, while the condensation coefficients keep relatively lower and almost unchanged. In weak-cavitation zone, there only exists the smaller main cavity attached to the model head or there is no obvious cavity. In supercavitation zone, the secondary cavity attached to the model will fall off and merge into the new rear cavity.

Melting and Solidification Heat Transfer Characteristics of a Phase-Change Material in a Latent Heat Storage Vessel: Effects of a Perforated Partition Plate and Metal Fiber

Today, latent heat storage technology has advanced to allow reuse of waste heat in the middle-temperature range. This paper describes an approach to develop a latent heat storage system using middle-temperature waste heat (~100oC - 200oC) from factories. Direct contact melting and solidification behavior between a heat-transfer fluid (oil) and a latent heat storage material mixture were observed. The mixture consisted of mannitol and erythritol (Cm = 70 mass %, Ce = 30 mass %) as a phase-change material (PCM). The weight of the PCM was 3.0 kg and the flow rate of the oil, foil, was 1.0, 1.5, or 2.0 kg/min. To decrease the solidified height of the PCM mixture during the solidification process, a perforated partition plate was installed in the PCM region in the heat storage vessel. PCM coated oil droplets were broken by the perforated partition plate, preventing the solidified height of the PCM from increasing. The solidification and melting processes were repeated using metal fiber. It was found that installing the metal fiber was more effective than installing the perforated partition plate to prevent the flow out problem of the PCM.

Special kinetics features of scandium antimonide thin films conducive to swiftly embedded phase-change memory applications

Embedded phase-change random-access memory(ePCRAM)applications demand superior data retention in amorphous phase-change materials(PCMs).Traditional PCM design strategies have focused on enhancing the thermal stability of the amorphous phase,often at the expense of the crystallization speed.While this approach supports reliable microchip operations,it compromises the ability to achieve rapid responses.To address this limitation,we modified ultrafast-crystallizing Sb thin films by incorporating Sc dopants,achieving the highest 10-year retention temperature(~175℃)among binary antimonide PCMs while maintaining a sub-10-ns SET operation speed.This reconciliation of two seemingly contradictory properties arises from the unique kinetic features of the 5-nm-thick Sc12Sb88 films,which exhibit an enlarged fragile-to-strong crossover in viscosity at medium supercooled temperature zones and an incompatible sublattice ordering behavior between the Sc and Sb atoms.By tailoring the crystallization kinetics of PCMs through strategic doping and nanoscale confinement,we provide new opportunities for developing robust yet swift ePCRAMs.

In situ characterization of vacancy ordering in Ge-Sb-Te phase-change memory alloys

Tailoring the degree of structural disorder in Ge-Sb-Te alloys is important for the development of non-volatile phase-change memory and neuro-inspired computing.Upon crystallization from the amorphous phase,these alloys form a cubic rocksalt-like structure with a high content of intrinsic vacancies.Further thermal annealing results in a gradual structural transition towards a layered structure and an insulator-to-metal transition.In this work,we elucidate the atomic-level details of the structural transition in crystalline GeSb_(2)Te_(4) by in situ high-resolution transmission electron microscopy experiments and ab initio density functional theory calculations,providing a comprehensive real-time and real-space view of the vacancy ordering process.We also discuss the impact of vacancy ordering on altering the electronic and optical properties of GeSb_(2)Te_(4),which is relevant to multilevel storage applications.The phase evolution paths in Ge-Sb-Te alloys and Sb_(2)Te_(3)are illustrated using a summary diagram,which serves as a guide for designing phase-change memory devices.

Experimental Investigation of a Phase-ChangeMaterial’s Stabilizing Role in a Pilot of Smart Salt-Gradient Solar Ponds

Faced with the world’s environmental and energy-related challenges,researchers are turning to innovative,sustainable and intelligent solutions to produce,store,and distribute energy.This work explores the trend of using a smart sensor to monitor the stability and efficiency of a salt-gradient solar pond.Several studies have been conducted to improve the thermal efficiency of salt-gradient solar ponds by introducing other materials.This study investigates the thermal and salinity behaviors of a pilot of smart salt-gradient solar ponds with(SGSP)and without(SGSPP)paraffin wax(PW)as a phase-change material(PCM).Temperature and salinity were measured experimentally using a smart sensor,with the measurements being used to investigate the stabilizing effects of placing the PCM in the solar pond’s lower convective zone.The experimental results show that the pond with the PCM(SGSPP)achieved greater thermal and salinity stability,with there being a lesser temperature and salinity gradient between the different layers when compared to a solar pond without thePCM(SGSP).The use of the PCM,therefore,helped control the maximum and minimum temperature of the pond’s storage zone.The UCZ has been found to operate approximately 4 degrees above the average ambient temperature of the day in the SGSPP and 7 degrees in SGSP.Moreover,an unstable situation is generated after 5 days from starting the operation and at 1.9 m from the bottom,and certain points have the tendency to be neutral from the upper depths in 1,3 m of the bottom.

Electrically programmable phase-change photonic memory for optical neural networks with nanoseconds in situ training capability

Optical neural networks (ONNs), enabling low latency and high parallel data processing withoutelectromagnetic interference, have become a viable player for fast and energy-efficient processing andcalculation to meet the increasing demand for hash rate. Photonic memories employing nonvolatile phase-change materials could achieve zero static power consumption, low thermal cross talk, large-scale, andhigh-energy-efficient photonic neural networks. Nevertheless, the switching speed and dynamic energyconsumption of phase-change material-based photonic memories make them inapplicable for in situ training.Here, by integrating a patch of phase change thin film with a PIN-diode-embedded microring resonator,a bifunctional photonic memory enabling both 5-bit storage and nanoseconds volatile modulation wasdemonstrated. For the first time, a concept is presented for electrically programmable phase-changematerial-driven photonic memory integrated with nanosecond modulation to allow fast in situ training and zerostatic power consumption data processing in ONNs. ONNs with an optical convolution kernel constructedby our photonic memory theoretically achieved an accuracy of predictions higher than 95% when testedby the MNIST handwritten digit database. This provides a feasible solution to constructing large-scalenonvolatile ONNs with high-speed in situ training capability.

Phase-change composite filled natural nanotubes in hydrogel promote wound healing under photothermally triggered drug release

It is of great importance to treat a bacterial-infected wound by a smart dressing capable of delivering antibiotics in a smart manner without causing drug resistance.The construction of smart release nanocontainers responsive to near-infrared(NIR)laser irradiation in an on-demand and stepwise way is a promising strategy for avoiding the emergence of multidrug-resistant bacteria.Here,we develop a hydrogel composite made of alginate and nanotubes with an efficient NIR-triggered release of rifampicin and outstanding antibacterial ability.This composite hydrogel is prepared through co-encapsulating antibacterial drug(rifampicin),NIR-absorbing dye(indocyanine green),and phase-change materials(a eutectic mixture of fatty acids)into halloysite nanotubes,followed by incorporation into alginate hydrogels,allowing the in-situ gelation at room temperature and maintaining the integrity of drug-loaded nanotubes.Among them,the eutectic mixture with a melting point of 39℃ serves as the biocompatible phase-change material to facilitate the NIR-triggered drug release.The resultant phase-change material gated-nanotubes exhibit a prominent photothermal efficiency with multistep drug release under laser irradiation.In an in vitro assay,composite hydrogel provides good antibacterial potency against Staphylococcus aureus,one of the most prevalent microorganisms of dangerous gas gangrene.A bacterial-infected rat full-thickness wound model demonstrates that the NIR-responsive composite hydrogel inhibits the bacteria colonization and suppresses the inflammatory response caused by bacteria,promoting angiogenesis and collagen deposition to accelerate wound regeneration.The NIR-responsive composite hydrogel has a great po-tential as an antibacterial wound dressing functionalized with controlled multistep treatment of the infected sites.

Heat Absorbing and Releasing Experiments with Improved Phase-change Thermal Storage Canisters

This article,based on authors＇ long-term study,proposes an improved foamed-Ni-packed phase-change thermal storage canister,which takes advantage of the foamed-Ni characteristic of instinctive porous structure and excellent properties to ameliorate its void distribution and thermal conductivity. The improved canister and the unimproved one without foamed-Ni package,are put to heat absorbing and releasing tests to investigate the effects of heat absorbing temperature upon the phase-change materials （PCM） melting time under three temperature schemes by using platinum resistance thermometers （PT100） and data acquisition modules （ADAM-4000） to gather the data of varying temperature. Afterwards,the computerized tomography （CT） is employed to scan the void distribution in both canisters. Compared to the unimproved canister,the experimental results evidence the superiority of the improved one in higher uniformity in void and temperature distribution as well as faster thermal responses.

氨吸附制冷特性的实验研究

对以NH3为制冷剂 ,MgCl2 、CaCl2 、SrCl2 、BaCl2 和活性炭为吸附剂所组成的吸附式制冷工质对的基础吸附性能进行了实验研究 ,得到了吸附等温线。实验结果表明 ,CaCl2 —NH3、SrCl2 —NH3、工质对性能较佳 ,具有商业开发价值。本文还研究了活性炭浸渍SrCl2