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.

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.

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.

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.

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.

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.

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.

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.

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.

Minimizing the Programming Power of Phase Change Memory by Using Graphene Nanoribbon Edge-Contact.

Nonvolatile phase-change random access memory (PCRAM) is regarded as one of the promising candidates for emerging mass storage in the era of Big Data. However, relatively high programming energy hurdles the further reduction of power consumption in PCRAM. Utilizing narrow edge-contact of graphene can effectively reduce the active volume of phase change material in each cell, and therefore realize low-power operation. Here, it demonstrates that the power consumption can be reduced to ≈53.7 fJ in a cell with ≈3 nm-wide graphene nanoribbon (GNR) as edge-contact, whose cross-sectional area is only ≈1 nm2 . It is found that the polarity of the bias pulse determines its cycle endurance in the asymmetric structure. If a positive bias is applied to the graphene electrode, the endurance can be extended at least one order longer than the case with a reversal of polarity. In addition, the introduction of the hexagonal boron nitride (h-BN) multilayer leads to a low resistance drift and a high programming speed in a memory cell. The work represents a great technological advance for the low-power PCRAM and can benefit in-memory computing in the future.

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.

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.

Improvement of Resistive Switching Performance in Sulfur-Doped HfOx-Based RRAM.

In order to improve the electrical performance of resistive random access memory (RRAM), sulfur (S)-doping technology for HfOx-based RRAM is systematically investigated in this paper. HfOx films with different S-doping contents are achieved by atmospheric pressure chemical vapor deposition (APCVD) under a series of preparation temperatures. The effect of S on crystallinity, surface topography, element composition of HfOx thin films and resistive switching (RS) performance of HfOx-based devices are discussed. Compared with an undoped device, the VSET/VRESET of the S-doped device with optimal S content (~1.66 At.%) is reduced, and the compliance current (Icc) is limited from 1 mA to 100 µA. Moreover, it also has high uniformity of resistance and voltage, stable endurance, good retention characteristics, fast response speed (SET 6.25 µs/RESET 7.50 µs) and low energy consumption (SET 9.08 nJ/RESET 6.72 nJ). Based on X-ray photoelectron spectroscopy (XPS) data and fitting of the high/low resistance state (HRS/LRS) conduction behavior, a switching mechanism is considered to explain the formation and rupture of conductive filaments (CFs) composed of oxygen vacancies in undoped and S-doped HfOx-based devices. Doping by sulfur is proposed to introduce the appropriate concentration oxygen vacancies into HfOx film and suppress the random formation of CFs in HfOx-based device, and thus improve the performance of the TiN/HfOx/ITO device.

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 °C. 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.

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.

Toward a Reliable Synaptic Simulation Using Al-Doped HfO2 RRAM.

The potential in a synaptic simulation for neuromorphic computation has revived the research interest of resistive random access memory (RRAM). However, novel applications require reliable multilevel resistive switching (RS), which still represents a challenge. We demonstrate in this work the achievement of reliable HfO2-based RRAM devices for synaptic simulation by performing the Al doping and the postdeposition annealing (PDA). Transmission electron microscopy and operando hard X-ray photoelectron spectroscopy results reveal the positive impact of Al doping on the formation of oxygen vacancies. Detailed I-V characterizations demonstrate that the 16.5% Al doping concentration leads to better RS properties of the device. In comparison with the other reported results based on HfO2 RRAM, our devices with 16.5% Al-doping and PDA at 450 °C show better reliable multilevel RS (∼20 levels) performance and an increased on/off ratio. The 16.5% Al:HfO2 sample with PDA at 450 °C shows good potentiation/depression characteristics with low pulse width (10 µs) along with a good On/Off ratio (>1000), good data retention at room temperature, and high temperature and good program/erase endurance characteristics with a pulse width of 50 ns. The synapse features including potentiation, depression, and spike time-dependent plasticity were successfully achieved using optimized Al-HfO2 RRAM devices. Our results demonstrate the beneficial effects of Al doping and PDA on the enhancement of the performances of RRAM devices for the synaptic simulation in neuromorphic computing applications.

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.

Designing Multiple Crystallization in Superlattice-like Phase-Change Materials for Multilevel Phase-Change Memory.

A multilevel phase-change memory device was successfully designed, which was fabricated using a Ge40Te60/Cr superlattice-like (SLL) structure. In the SLL films, a two-step phase change process is observed at elevated temperatures, which reveals the crystallization of Ge40Te60 (GT) and an interface-dominated formation of Cr2Ge2Te6 (CrGT). The bonding of Cr-Te and Ge-Ge is accompanied by the breaking of a Ge-Te bond, which is mainly in the Ge-rich GeTe4-nGen units. The formation of CrGT is related to the breaking apart of the edge-sharing octahedron in GT and Cr replacement at Ge sites. The crystalline GT acts as the crystallization precursors in the formation of the CrGT phase. The stable reversible two-step phase change can guarantee the reliability of the multilevel storage. The present work may shed light on the possible mechanism of the CrGT phase transition-based interfacial dynamic process. The designed multiple crystallization system demonstrates a potential for multilevel storage.

Sc-Centered Octahedron Enables High-Speed Phase Change Memory with Improved Data Retention and Reduced Power Consumption.

Phase change memory (PCM) with advantages of high operation speed, multilevel storage capability, spiking-time-dependent plasticity, etc., has wide application scenarios in both Von Neumann systems and neuromorphic systems. In the automotive application, intelligent system not only needs high efficiency to handle massive data processing but also good robustness to retain the existing data against high working temperature. In this work, Sc-doped GeTe is developed for PCM, which has achieved 120 °C data retention for 10 years, 6 ns operation speed, and 7 nJ low power consumption. The high data retention is attributed to the high coordination number of Sc and its strong bonds with Te atoms in the amorphous phase, which enhances the robustness of the atomic matrices. Sc-centered octahedrons in amorphous state provide a nucleation center, leading to fast crystallization. In the crystalline phase, Sc atoms occupy Ge vacancies to form a homogenous GeTe-like rhombohedral phase. The strong covalent-like Sc-Te bonds weaken the neighboring Ge-Te bonds, lowering energy for melting. Together with the increased energy efficiency originated from confined grain size, the reduced power consumption has been achieved. The improvements in data retention, speed, and power efficiency have made Sc-doped GeTe a promising candidate for high-performance automobile electronics application.