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Integrated all-photonic non-volatile multi-level memory

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Abstract

Implementing on-chip non-volatile photonic memories has been a long-term, yet elusive goal. Photonic data storage would dramatically improve performance in existing computing architectures by reducing the latencies associated with electrical memories and potentially eliminating optoelectronic conversions. Furthermore, multi-level photonic memories with random access would allow for leveraging even greater computational capability. However, photonic memories have thus far been volatile. Here, we demonstrate a robust, non-volatile, all-photonic memory based on phase-change materials. By using optical near-field effects, we realize bit storage of up to eight levels in a single device that readily switches between intermediate states. Our on-chip memory cells feature single-shot readout and switching energies as low as 13.4 pJ at speeds approaching 1 GHz. We show that individual memory elements can be addressed using a wavelength multiplexing scheme. Our multi-level, multi-bit devices provide a pathway towards eliminating the von Neumann bottleneck and portend a new paradigm in all-photonic memory and non-conventional computing.

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... This paper offers an overview of various single-bit and multi-bit OPCM cell designs that have been proposed in the literature. As an example, [19] presented an OPCM chip, which utilizes a GST thin film embedded in a silicon nitride ridge waveguide that could store 0 or 1 based on the GST phase. ...
... Several prototypes have been developed in recent years by using GST-based phase change memory cells. For instance, Rios et al [19] demonstrated that PCMs consisting of GST were capable of fast readout and low switching energy consumption. An optically addressing PCM cell was demonstrated by Zhang et al [81] by using a microring resonator and GST material. ...
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... (d) wavelength selective photonic switch based on GST 综 述 Fig. 10 Straight waveguide element device structure [43][44][45][46] . Fig. 11 Hybrid element device structure [47][48][49] . ...
... Phase change materials (PCMs) possess a ′ self-holding ′ quality in which no static power is demanded to sustain the previous state once obtained, hence substantially reducing the power consumption for saving the deployment of a continuous power supply [13,14]. When integrated with an SOI platform, PCMs can provide dynamic control of light with a reduced active volume and correspondingly lower power consumption than is possible with SOI alone [15][16][17][18][19][20][21]. Ge 2 Sb 2 Te 5 (GST) is an outstanding compound semiconductor PCM composed of triple elements: germanium (Ge), antimony (Sb), and tellurium (Te), with an extensive range of optical and electrical properties in distinct phase states. ...
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A novel non-volatile optical filter with a large bandwidth and extinction ratiotunability is firstly experimentally demonstrated by introducing all-optical phase change of Ge 2 Sb 2 Te 5 (GST). The Si-GST hybrid device promises flexible multi-level regulation of essential parameters of filters in an ultra-compact footprint of 30 µm×13 µm. Ultra-low power consumption is realized on account of the saving of external static power that required in other electric-optic or optic-optic driven filters. The GST is loaded onto two triple-waveguides directional couplers located at the coupling regions of an add-drop microring resonator. By initiating the GST phase transition with pump optical pulses, the transmitted optical power to the cross port of the triple-waveguides coupler is adjustable, hence influencing the coupling efficiency states of the microring filter. Consequently, a tunable on-off extinction ratio from 0.7 dB to 18.2 dB and a tunable bandwidth from 0.6 nm to 3.3 nm are experimentally obtained with the aid of optically manipulating crystallization degree of GST. Our device potentially enabling the realization of high-density photonic integrated circuits especially in dense wavelength division multiplexing
... (a) Schematic of LMI material structure [25] ; (b) when the input power density of nanorod and quantum dot systems changes, the extinction crosssection (left yaxis) and imaginary part of the refractive index (right yaxis) of the system also change [25] ; (c) nonlinear function of transmittance versus input power density when quantum components are placed at different locations on the waveguide [15] ; (d) schematic of endselectivity of surfactants to gold nanomaterials; (e) microscopic image of nanorods and nanospheres dimer [28] Fig. 6 Nonlinear activation function caused by PCM. (a) Nonlinear conversion before and after optical synapses is achieved through PCM on the waveguide [29] ; (b) optical nonlinear activation function implemented by PCM [5] ; (c) neural network achieved by nonlinear processing of multiple sets of input pulses utilizing PCM [5] Fig. 7 SOA nonlinear activation function. (a) SOAbased MZI implements a logic AND gate [32] ; (b) (c) nonlinear relationship among SOA gain, input optical power, and bias current [35] ; (d) Sigmoid trigger function neural network combined with XPM and XGM [36] ; (e) onchip SOA array neural network [33] [8] ; (c) four line types that can be achieved by the scheme; (d) diagram of the experimental apparatus [38] ; (e) systemgenerated Sigmoid function; (f)(g) schematic after adding graphene and achievable line patterns [39] 封面文章·特邀综述 第 43 卷 第 16 期/2023 年 8 月/光学学报 ...
... In addition, it is worth noting that a photonic counterpart of an electronic crossbar array has been demonstrated 34 . The passive photonic crossbar array uses waveguide directional couplers and crossings as interconnects and phase-change materials (PCMs) as memories (optical transmissions tuned by the non-volatile crystalline state of the PCM 36 ). ...
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... However, PCMs present another degree of freedom for tunability: the possibility to encode multilevel non-volatile states via partial crystallization. Recently, several works exploited this potential of PCMs for devices in waveguides, thin-films or metasurfaces [12][13][14][15][16][17]. Even though it may appear straightforward to prepare a thin-film to a desired level of partial crystallization -after all, one should just bring it to the correct temperature for a given duration -several issues make it a serious challenge to face. ...
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We propose and demonstrate a simple method to accurately monitor and program arbitrary states of partial crystallization in phase-change materials (PCMs). The method relies both on the optical absorption in PCMs as well as on the physics of crystallization kinetics. Instead of raising temperature incrementally to increase the fraction of crystallized material, we leverage the time evolution of crystallization at constant temperatures and couple this to a real-time optical monitoring to precisely control the change of phase. We experimentally demonstrate this scheme by encoding a dozen of distinct states of crystallization in two different PCMs: GST and Sb 2 S 3 . We further exploit this ’time-crystallization’ for the in-situ analysis of phase change mechanisms and demonstrate that the physics of crystallization in Sb 2 S 3 is fully described by the so-called Johnson-Mehl-Avrami-Kolmogorov formalism. The presented method not only paves the way towards real-time and model-free programming of non-volatile reconfigurable photonic integrated devices, but also provides crucial insights into the physics of crystallization in PCMs.
... 13,14 In general, electronic systems are currently superior in data processing and storage for the flexibility in terms of electronic operation for different purposes. In contrast, photonics is capable of storing and processing data in an optical manner with unprecedented bandwidth and higher speed; [15][16][17] therefore, photonic computing is thought as one of the best candidates for a future computing system. 18,19 Nowadays, various kinds of photonic computing devices by using different materials, for example, phase-change materials (PCMs), 15,16,20 hybrid perovskites materials, 21 and silicon-based material 22 have widely been demonstrated. ...
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Photonic computing has the potential to significantly improve energy efficiency and data processing speed beyond that of von Neumann architecture. Although various optical processing techniques have been developed during recent two decades, the photonic manipulation is still a big challenging due to the bosonic nature of photons. Herein, we propose a ferroelectric field-controlled photonic computing based on the heterostructure of ferroelectric/van der Waals semiconductor. The strong and tunable electrostatic coupling of ferroelectric (PMN-PT) with monolayer WS2 results in a multi-level (24 bits) photoluminescence (PL) output. Furthermore, combining device modeling with experiments, we find that the multi-level PL output is because of the regulation of ferroelectric polarization on the net recombination rate of WS2. The ferroelectric field-controlled multi-level PL output enables us to design an optical arithmetic operation in the PMN-PT/WS2 heterostructure, which provides an attractive solution for photonic information computing.
... Integrated optics is to build up some optical components, such as light-emitting elements, lighttransmitting devices, and receiving elements, on the same substrate/chip in the form of thin films to manufacture a micro-optical system with independent functions. 1 The first problem to be solved in integrated optics is to use thin films to propagate light waves, that is, to utilize thin films as optical waveguides. 2 Relying on the principle of total internal reflection, the waveguide restricts light to propagate in a certain direction. 3 According to the different structures, it can be generally divided into planar and strip waveguides. ...
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In this work, the ridge waveguide was formed by the femtosecond laser ablation with a central wavelength of 0.808 μm and a speed of 50 μm/s on the planar Nd³⁺‐doped fluorophosphate glass waveguide that was fabricated by the C³⁺ ion implantation with an energy of 6.0 MeV and a fluence of 1.0×10¹² C³⁺s/mm². The mechanism of the ion‐implanted waveguide was discussed by the Stopping and Range of Ions in Matter 2013. The cross‐section morphology of the ridge waveguide was captured by an optical microscope. Its mode characteristics were analyzed by the end‐face coupling method. The ridge Nd³⁺‐doped fluorophosphate glass waveguides have potential as fundamental structures for many optoelectronic devices including waveguide amplifiers and lasers.
... In general, the flagship phase change material is ternary Ge 2 Sb 2 Te 5 , which is a pseudo-binary chalcogenide material typically comprising Sb 2 Te 3 and GeTe [17][18][19][20][21][22]. In amorphous Ge -Sb-Te (GST) compounds, the atoms are randomly distributed without long-range order and can be sequentially crystallized into an equilibrium hexagonal structure, as illustrated in Figure 1(a) [23]. The crystalline states generally exhibit low electrical resistivity and low transmittance, whereas the amorphous states exhibit high electrical resistivity and high transmittance. ...
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... The ability to precisely control the V T of a transistor allows it to be used as a memory element, such as flash memory. In recent years, beyond-binary tunability between the "1" and "0" states has become particularly interesting in post-von Neumann applications, such as neuromorphic computation 1,2 , multi-state memory 3,4 and multiplexed sensing 5 . The capacity to finely tune V T over a wide range is crucial for these applications, as it directly impacts the storage capacity and weight precision of associated devices. ...
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... Applications of GST films, 52 such as THz and IR modulators and filters, 33 spatial light modulators, 53 memory and logic devices, 54 or even neuromorphic photonic devices, 55 require precise knowledge of their dielectric and optical constants in the THz and IR ranges. The retrieved broadband complex dielectric permittivity of the three GST phases [Fig. ...
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In recent years, with the further ministration of the semiconductor device in integrated circuits, power consumption and data transmission bandwidth have become insurmountable obstacles. As an integrated technology, photonic integrated circuits (PICs) have a promising potential in the post‐Moore era with more advantages in data processing, communication, and diversified sensing applications for their ultra‐high process speed and low power consumption. Silicon photonics is believed to be an encouraging solution to realize PICs because of the mature CMOS process. The past decades have witnessed a huge growth in silicon PICs. However, there is still a demand for the development of silicon PICs to enable powerful chip‐scale systems and new functionalities. In this paper, a review of the photonic components, functional blocks, and emerging applications for PICs is offered. The common photonic components are classified into several sections, including on‐chip light sources, fiber‐to‐chip couplers, photonic resonators, waveguide‐based sensors, on‐chip photodetectors, and modulators. The functional blocks of the PICs mentioned in this review are photonic memories and photonic neural networks. Finally, the paper concludes with emerging applications for further study.
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Elemental antimony (Sb) is regarded as a promising candidate to improve the programming consistency and cycling endurance of phase-change memory and neuro-inspired computing devices. Although bulk amorphous Sb crystallizes spontaneously, the stability of the amorphous form can be greatly increased by reducing the thickness of thin films down to several nanometers, either with or without capping layers. Computational and experimental studies have explained the depressed crystallization kinetics caused by capping and interfacial confinement; however, it is unclear why amorphous Sb thin films remain stable even in the absence of capping layers. In this work, we carry out thorough ab initio molecular dynamics (AIMD) simulations to investigate the effects of free surfaces on the crystallization kinetics of amorphous Sb. We reveal a stark contrast in the crystallization behavior between bulk and surface models at 450 K, which stems from deviations from the bulk structural features in the regions approaching the surfaces. The presence of free surfaces intrinsically tends to create a sub-nanometer region where crystallization is suppressed, which impedes the incubation process and thus constrains the nucleation in two dimensions, stabilizing the amorphous phase in thin-film Sb-based memory devices.
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The demand for information processing at ultrahigh speed with large data transmission capacity is continuously rising. Necessary building blocks for on-chip photonic integrated circuits (PICs) are reconfigurable integrated low-loss high-speed modulators and switches. Phase change materials (PCMs) provide unique opportunities for integration into PICs. Here, the investigation of layered gallium monosulfide (GaS) as a novel low-loss PCM from infrared to optical frequencies is pioneered, with high index contrast (n ≈ .) at the optical telecommunication band. The GaS bandgap switches from. ±. eV for the amorphous state to. ±. eV for the crystalline state. It is demonstrated that the reversible GaS amorphous-to-crystalline phase transition can be operated thermally and by picosecond green (nm) laser irradiation. The design of a reconfigurable integrated optical modulator on-chip based on Mach-Zehnder Interferometers (MZI) with the GaS PCM cell deposited on one of the arms for application is presented at the telecommunication wavelength of = nm, where the standard single mode optical fiber exhibits zero chromatic dispersion, and at = nm, where a minimum optical loss of. dB km − is obtained. This opens the route to applications such as reconfigurable modulators, beam steering using phase modulation, and photonic neural networks. The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/. /adom.
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An optical switch operating at a wavelength of 1.55 μm and showing a 12 dB modulation depth is introduced. The device is implemented in a silicon racetrack resonator using an overcladding layer of the phase change data storage material Ge2Sb2Te5, which exhibits high contrast in its optical properties upon transitions between its crystalline and amorphous structural phases. These transitions are triggered using a pulsed laser diode at λ = 975 nm and used to tune the resonant frequency of the resonator and the resultant modulation depth of the 1.55 μm transmitted light.
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The phase transformation dynamics induced in Ge2Sb2Te5 films by picosecond laser pulses were studied using real-time reflectivity measurements. With subnanosecond resolution. Evidence was found that the thermal diffusivity of the substrate plays a crucial role in, determining the ability of. the films to crystallize and amorphize. A film/substrate configuration with optimized heat flow. conditions for ultrafast phase cycling with picosecond laser pulses was designed and produced. In this system; we achieved reversible phase transformations with large optical contrast (>20%) using single laser pulses with a duration of 30 ps within well-defined fluence windows. The amorphization (writing) process is completed within less than 1 ns, whereas, crystallization (erasing) needs approximately 13 ns to be completed. (C) 2004 American Institute of Physics.
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Light is intrinsically very difficult to store in a small space. The ability to trap photons for a long time (photon lifetime, τph) and to slow the propagation of light plays a significant role in quantum information1, 2, 3 and optical processing4, 5, 6. Photonic-crystal cavities with an ultrahigh quality factor (Q) are attracting attention7, 8 because of their extremely small volume; however, high-Q demonstrations have been accomplished only with spectral measurements9, 10, 11. Here we describe time-domain measurements on photonic-crystal cavities with the highest Q among wavelength-scale cavities, and show directly that photons are trapped for one nanosecond. These techniques constitute clear and accurate ways of investigating ultrasmall and long τph systems. We also show that optical pulses are delayed for ~1.45 ns, corresponding to light propagation at ~2×10−5 c the speed of light in a vacuum, which is the slowest for any dielectric slow-light medium. Furthermore, we succeeded in dynamically changing the Q within the τph, which is key to realizing the dynamic control of light12, 13 and photon-trapping memory14.
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We propose an all-photonic, non-volatile memory and processing element based on phase-change thin-films deposited onto nanophotonic waveguides. Using photonic microring resonators partially covered with Ge2Sb2Te5 (GST) multi-level memory operation in integrated photonic circuits can be achieved. GST provides a dramatic change in refractive index upon transition from the amorphous to crystalline state, which is exploited to reversibly control both the extinction ratio and resonance wavelength of the microcavity with an additional gating port in analogy to optical transistors. Our analysis shows excellent sensitivity to the degree of crystallization inside the GST, thus providing the basis for non-von Neuman neuromorphic computing.
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A comprehensive and thorough review of PCM technologies, including a discussion of material and device issues, is provided in this paper. ABSTRACT | In this paper, recent progress of phase change memory (PCM) is reviewed. The electrical and thermal proper-ties of phase change materials are surveyed with a focus on the scalability of the materials and their impact on device design. Innovations in the device structure, memory cell selector, and strategies for achieving multibit operation and 3-D, multilayer high-density memory arrays are described. The scaling prop-erties of PCM are illustrated with recent experimental results using special device test structures and novel material synthe-sis. Factors affecting the reliability of PCM are discussed.
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Phase-change memory (PCM) has emerged as one among the most promising technologies for next-generation non-volatile solid-state memory. Multilevel storage, namely storage of non-binary information in a memory cell, is a key factor for reducing the total cost-per-bit and thus increasing the competiveness of PCM technology in the nonvolatile memory market. In this paper, we present a family of advanced programming schemes for multilevel storage in PCM. The proposed schemes are based on iterative write-and-verify algorithms that exploit the unique programming characteristics of PCM in order to achieve significant improvements in resistance-level packing density, robustness to cell variability, programming latency, energy- per-bit and cell storage capacity. Experimental results from PCM test-arrays are presented to validate the proposed programming schemes. In addition, the reliability issues of multilevel PCM in terms of resistance drift and read noise are discussed.
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Ultra-small, low-power, all-optical switching and memory elements, such as all-optical flip-flops, as well as photonic integrated circuits of many such elements, are in great demand for all-optical signal buffering, switching and processing. Silicon-on-insulator is considered to be a promising platform to accommodate such photonic circuits in large-scale configurations. Through heterogeneous integration of InP membranes onto silicon-on-insulator, a single microdisk laser with a diameter of 7.5 µm, coupled to a silicon-on-insulator wire waveguide, is demonstrated here as an all-optical flip-flop working in a continuous-wave regime with an electrical power consumption of a few milliwatts, allowing switching in 60 ps with 1.8 fJ optical energy. The total power consumption and the device size are, to the best of our knowledge, the smallest reported to date at telecom wavelengths. This is also the only electrically pumped, all-optical flip-flop on silicon built upon complementary metal-oxide semiconductor technology.
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The search for a universal memory storage device that combines rapid read and write speeds, high storage density and non-volatility is driving the exploration of new materials in nanostructured form. Phase-change materials, which can be reversibly switched between amorphous and crystalline states, are promising in this respect, but top-down processing of these materials into nanostructures often damages their useful properties. Self-assembled nanowire-based phase-change material memory devices offer an attractive solution owing to their sub-lithographic sizes and unique geometry, coupled with the facile etch-free processes with which they can be fabricated. Here, we explore the effects of nanoscaling on the memory-storage capability of self-assembled Ge2Sb2Te5 nanowires, an important phase-change material. Our measurements of write-current amplitude, switching speed, endurance and data retention time in these devices show that such nanowires are promising building blocks for non-volatile scalable memory and may represent the ultimate size limit in exploring current-induced phase transition in nanoscale systems.
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Phase-change materials offer a promising route for the practical realisation of new forms of general-purpose and 'brain-like' computers. An experimental proof-of-principle of such remakable capabilities is presented that includes (i) the reliable execution by a phase-change 'processor' of the four basic arithmetic functions of addition, subtraction, multiplication and division, (ii) the demonstration of an 'integrate and fire' hardware neuron using a single phase-change cell and (iii) the expostion of synaptic-like functionality via the 'memflector', an optical analogue of the memristor.
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We survey the current state of phase change memory (PCM), a non-volatile solid-state memory technology built around the large electrical contrast between the highly-resistive amorphous and highly-conductive crystalline states in so-called phase change materials. PCM technology has made rapid progress in a short time, having passed older technologies in terms of both sophisticated demonstrations of scaling to small device dimensions, as well as integrated large-array demonstrators with impressive retention, endurance, performance and yield characteristics. We introduce the physics behind PCM technology, assess how its characteristics match up with various potential applications across the memory-storage hierarchy, and discuss its strengths including scalability and rapid switching speed. We then address challenges for the technology, including the design of PCM cells for low RESET current, the need to control device-to-device variability, and undesirable changes in the phase change material that can be induced by the fabrication procedure. We then turn to issues related to operation of PCM devices, including retention, device-to-device thermal crosstalk, endurance, and bias-polarity effects. Several factors that can be expected to enhance PCM in the future are addressed, including Multi-Level Cell technology for PCM (which offers higher density through the use of intermediate resistance states), the role of coding, and possible routes to an ultra-high density PCM technology. Comment: Review article
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Phase-change materials are some of the most promising materials for data-storage applications. They are already used in rewriteable optical data storage and offer great potential as an emerging non-volatile electronic memory. This review looks at the unique property combination that characterizes phase-change materials. The crystalline state often shows an octahedral-like atomic arrangement, frequently accompanied by pronounced lattice distortions and huge vacancy concentrations. This can be attributed to the chemical bonding in phase-change alloys, which is promoted by p-orbitals. From this insight, phase-change alloys with desired properties can be designed. This is demonstrated for the optical properties of phase-change alloys, in particular the contrast between the amorphous and crystalline states. The origin of the fast crystallization kinetics is also discussed.
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The development of materials whose refractive index can be optically transformed as desired, such as chalcogenide-based phase-change materials, has revolutionized the media and data storage industries by providing inexpensive, high-speed, portable and reliable platforms able to store vast quantities of data. Phase-change materials switch between two solid states--amorphous and crystalline--in response to a stimulus, such as heat, with an associated change in the physical properties of the material, including optical absorption, electrical conductance and Young's modulus. The initial applications of these materials (particularly the germanium antimony tellurium alloy Ge2Sb2Te5) exploited the reversible change in their optical properties in rewritable optical data storage technologies. More recently, the change in their electrical conductivity has also been extensively studied in the development of non-volatile phase-change memories. Here we show that by combining the optical and electronic property modulation of such materials, display and data visualization applications that go beyond data storage can be created. Using extremely thin phase-change materials and transparent conductors, we demonstrate electrically induced stable colour changes in both reflective and semi-transparent modes. Further, we show how a pixelated approach can be used in displays on both rigid and flexible films. This optoelectronic framework using low-dimensional phase-change materials has many likely applications, such as ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent 'smart' glasses, 'smart' contact lenses and artificial retina devices.
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Phase change materials are widely considered for application in non-volatile memories due to their ability to achieve phase transformation in nano-second time scale. However the knowledge of fast crystallization dynamics in these materials are limited due to the lack of fast and accurate temperature control methods. In this work we have developed an experimental methodology that enables ultra-fast characterization of phase-change dynamics on a more technologically relevant melt-quenched amorphous phase using practical PCM device structures. We have extracted the crystallization growth velocity (U) in a functional capped PCM device over eight orders of magnitude (10(-10) m/s < U < 10(-1) m/s) spanning a wide temperature range (415 K < T < 580 K). We also observed direct evidence of non-Arrhenius crystallization behavior in programmed PCM devices at very high heating rates (>10(8) K/s), which reveals the extreme fragility of Ge2Sb2Te5 in its super-cooled liquid phase. Furthermore, these crystallization properties were studied as a function of device programming cycles and the results show degradation in the cell retention properties due to elemental segregation. The above experiments are enabled by the use of an on-chip fast heater and thermometer called as Micro Thermal Stage (MTS) integrated with a vertical phase change memory (PCM) cell. The temperature at the PCM layer can be controlled up to 600 K using MTS and with a thermal time constant of 800 ns leading to heating rates ~ 10(8) K/s that are close to the typical device operating conditions during PCM programming. The MTS allows us to independently control the electrical and thermal aspects of phase transformation (inseparable in a conventional PCM cell) and extract the temperature dependence of key material properties in real PCM devices.
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Phase-change materials integrated into nanophotonic circuits provide a flexible way to realize tunable optical components. Relying on the enormous refractive-index contrast between the amorphous and crystalline states, such materials are promising candidates for on-chip photonic memories. Non-volatile memory operation employing arrays of microring resonators is demonstrated as a route toward all-photonic chipscale information processing.
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A class of two-terminal passive circuit elements that can also act as memories could be the building blocks of a form of massively parallel computation known as memcomputing.
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Optical computers will be more interesting if they take advantage of phenomena that are unique to optics. In this respect, telecommunications hardware might have something to offer.
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Historically, the application of phase-change materials and devices has been limited to the provision of non-volatile memories. Recently, however, the potential has been demonstrated for using phase-change devices as the basis for new forms of brain-like computing, by exploiting their multilevel resistance capability to provide electronic mimics of biological synapses. Here, a different and previously under-explored property that is also intrinsic to phase-change materials and devices, namely accumulation, is exploited to demonstrate that nanometer-scale electronic phase-change devices can also provide a powerful form of arithmetic computing. Complicated arithmetic operations are carried out, including parallel factorization and fractional division, using simple nanoscale phase-change cells that process and store data simultaneously and at the same physical location, promising a most efficient and effective means for implementing beyond von-Neumann computing. This same accumulation property can be used to provide a particularly simple form phase-change integrate-and-fire “neuron”, which, by combining both phase-change synapse and neuron electronic mimics, potentially opens up a route to the realization of all-phase-change neuromorphic processing.
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Ultra-compact four-channel wavelength division multiplexing (WDM) devices in add/drop filter and multi-mode interferometer configurations were demonstrated on silicon-on-insulator for future on-chip optical interconnects. Both devices show a crosstalk level ≤-13dB and a footprint ≤0.006mm2.
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A rapid and reversible transition between a highly resistive and conductive state effected by an electric field, which we have observed in various types of disordered semiconducting material, is described in detail. The switching parameters and chemical composition of a typical material are presented, and microscopic mechanisms for the conduction phenomena are suggested.
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Silicon photonics is currently a very active and progressive area of research, as silicon optical circuits have emerged as the replacement technology for copper-based circuits in communication and broadband networks. The demand for ever improving communications and computing performance continues, and this in turn means that photonic circuits are finding ever increasing application areas. This text provides an important and timely overview of the hot topics in the field, covering the various aspects of the technology that form the research area of silicon photonics. With contributions from some of the worlds leading researchers in silicon photonics, this book collates the latest advances in the technology. Silicon Photonics: the State of the Art opens with a highly informative foreword, and continues to feature: the integrated photonic circuit; silicon photonic waveguides; photonic bandgap waveguides; mechanisms for optical modulation in silicon; silicon based light sources; optical detection technologies for silicon photonics; passive silicon photonic devices; photonic and electronic integration approaches; applications in communications and sensors. Silicon Photonics: the State of the Art covers the essential elements of the entire field that is silicon photonics and is therefore an invaluable text for photonics engineers and professionals working in the fields of optical networks, optical communications, and semiconductor electronics. It is also an informative reference for graduate students studying for PhD in fibre optics, integrated optics, optical networking, microelectronics, or telecommunications.
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Phase-change random-access memory (PCRAM) is one of the leading candidates for next-generation data-storage devices, but the trade-off between crystallization (writing) speed and amorphous-phase stability (data retention) presents a key challenge. We control the crystallization kinetics of a phase-change material by applying a constant low voltage via prestructural ordering (incubation) effects. A crystallization speed of 500 picoseconds was achieved, as well as high-speed reversible switching using 500-picosecond pulses. Ab initio molecular dynamics simulations reveal the phase-change kinetics in PCRAM devices and the structural origin of the incubation-assisted increase in crystallization speed. This paves the way for achieving a broadly applicable memory device, capable of nonvolatile operations beyond gigahertz data-transfer rates.
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The structure of laser-crystallized thin films of Ge <sub> 2 </sub> Sb <sub>2+x</sub> Te <sub> 5 </sub> (0.0≪x≤1.0) formed by the sputtering method were identified by x-ray diffraction studies to be composed of two phases: one phase is chiefly NaCl type crystal with a lattice constant of about 6 Å and a composition corresponding to Ge <sub> 2 </sub> Sb <sub> 2 </sub> Te <sub> 5 </sub>; the other phase comprises a small amount of an amorphous component such as Sb metal. Results of the Rietveld and the whole-powder-pattern fitting analyses show good agreement when assuming that (i) the 4(a) site is wholly occupied by only Te, (ii) the 4(b) site is randomly occupied by Ge or Sb atoms, and (iii) a little less than 20% of the 4(b) site is always vacant independent of the x value. The above results and the fact that halo noise rises with x increasing from 0.0 to 1.0 indicate a more precise model of crystal structure as follows. That is, Sb atoms added beyond the stoichiometric ratio, Ge <sub> 2 </sub> Sb <sub> 2 </sub> Te <sub> 5 </sub>, never fill up the vacancies of the 4(b) site in the NaCl type structure; the excess Sb atoms will remain in the amorphous state and concentrate, for example, at the grain boundary. The authors conclude that the amounts of the amorphous component produced through the crystallization process predominantly determine the crystallization rates and the critical temperatures of Ge–Sb–Te amorphous films, reportedly that they show a gentle and continuous dependence on the compositional deviation from the GeTe–Sb <sub> 2 </sub> Te <sub> 3 </sub> pse- udobinary line. © 2000 American Institute of Physics.
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An optical gate switch using Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> phase-change material integrated with a silicon waveguide is reported. The switch is very small (~2 ¿m) owing to the large difference in absorption coefficient between the crystalline state and the amorphous state. The prototype switch has been fabricated and successfully switched from the transparent on-state to the opaque off-state by laser pulse irradiation. An extinction ratio of more than 12.5 dB was achieved over a wavelength range of 75 nm.
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The increasing speed of fibre-optic-based telecommunications has focused attention on high-speed optical processing of digital information. Complex optical processing requires a high-density, high-speed, low-power optical memory that can be integrated with planar semiconductor technology for buffering of decisions and telecommunication data. Recently, ring lasers with extremely small size and low operating power have been made, and we demonstrate here a memory element constructed by interconnecting these microscopic lasers. Our device occupies an area of 18 x 40 microm2 on an InP/InGaAsP photonic integrated circuit, and switches within 20 ps with 5.5 fJ optical switching energy. Simulations show that the element has the potential for much smaller dimensions and switching times. Large numbers of such memory elements can be densely integrated and interconnected on a photonic integrated circuit: fast digital optical information processing systems employing large-scale integration should now be viable.
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Non-volatile 'flash' memories are key components of integrated circuits because they retain their data when power is interrupted. Despite their great commercial success, the semiconductor industry is searching for alternative non-volatile memories with improved performance and better opportunities for scaling down the size of memory cells. Here we demonstrate the feasibility of a new semiconductor memory concept. The individual memory cell is based on a narrow line of phase-change material. By sending low-power current pulses through the line, the phase-change material can be programmed reversibly between two distinguishable resistive states on a timescale of nanoseconds. Reducing the dimensions of the phase-change line to the nanometre scale improves the performance in terms of speed and power consumption. These advantages are achieved by the use of a doped-SbTe phase-change material. The simplicity of the concept promises that integration into a logic complementary metal oxide semiconductor (CMOS) process flow might be possible with only a few additional lithographic steps.
The authors also acknowledge support from the DFG and the State of Baden-Württemberg through the DFG-Center for Functional Nanostructures (CFN) within subproject A6.4
  • Wirtschaft
Wirtschaft (sdw). H.B. acknowledges support from the John Fell Fund and the EPSRC (EP/J00541X/2 and EP/J018694/1).The authors also acknowledge support from the DFG and the State of Baden-Württemberg through the DFG-Center for Functional Nanostructures (CFN) within subproject A6.4. This work was partly carried out with the support of the Karlsruhe Nano Micro Facility (KNMF, http://www.knmf.kit.edu), a Helmholtz Research Infrastructure at Karlsruhe Institute of Technology (KIT, http://www. kit.edu). The authors thank S. Diewald for assistance with device fabrication and M. Blaicher for technical assistance with device design.