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Phase transitions can occur in certain materials such as transition metal oxides (TMOs) and chalcogenides when there is a change in external conditions such as temperature and pressure. Along with phase transitions in these phase change materials (PCMs) come dramatic contrasts in various physical properties, which can be engineered to manipulate electrons, photons, polaritons, and phonons at the nanoscale, offering new opportunities for reconfigurable, active nanodevices. In this review, we particularly discuss phase-transition-enabled active nanotechnologies in nonvolatile electrical memory, tunable metamaterials, and metasurfaces for manipulation of both free-space photons and in-plane polaritons, and multifunctional emissivity control in the infrared (IR) spectrum. The fundamentals of PCMs are first introduced to explain the origins and principles of phase transitions. Thereafter, we discuss multiphysical nanodevices for electronic, photonic, and thermal management, attesting to the broad applications and exciting promises of PCMs. Emerging trends and valuable applications in all-optical neuromorphic devices, thermal data storage, and encryption are outlined in the end.

Dielectric optical nanoantennas are promising as fundamental building blocks in next generation color displays, metasurface holograms, and wavefront shaping optical devices. Due to the high refractive index of the nanoantenna material, they support geometry-dependent Mie resonances in the visible spectrum. Although phase change materials, such as the germanium-antimony-tellurium alloys, and post-transition metal oxides, such as ITO, have been used to tune antennas in the near-infrared spectrum, reversibly tuning the response of dielectric antennas in the visible spectrum remains challenging. In this paper, we designed and experimentally demonstrated dielectric nanodisc arrays exhibiting reversible tunability of Mie resonances in the visible spectrum. We achieved tunability by exploiting phase transitions in Sb

All-dielectric metasurfaces provide unique solutions for advanced wavefront manipulation of light with complete control of amplitude and phase at sub-wavelength scales. One limitation, however, for most of these devices is the lack of any post-fabrication tunability of their response. To break this limit, a promising approach is employing phase-change materials (PCMs), which provide fast, low energy, and non-volatile means to endow metasurfaces with a switching mechanism. In this regard, great advancements have been done in the mid-infrared and near-infrared spectrum using different chalcogenides. In the visible spectral range, however, very few devices have demonstrated full phase manipulation, high efficiencies, and reversible optical modulation. In this work, a programmable all-dielectric Huygens' metasurface made of antimony sulfide (Sb

The switchable optical and electrical properties of phase change materials (PCMs) are finding new applications beyond data storage in reconfigurable photonic devices. However, high power heat pulses are needed to melt-quench the material from crystalline to amorphous. This is especially true in silicon photonics, where the high thermal conductivity of the waveguide material makes heating the PCM energy inefficient. Here, we improve the energy efficiency of the laser-induced phase transitions by inserting a layer of two-dimensional (2D) material, either MoS2 or WS2, between the silica or silicon and the PCM. The 2D material reduces the required laser power by at least 40% during the amorphization (RESET) process, depending on the substrate. Thermal simulations confirm that both MoS2 and WS2 2D layers act as a thermal barrier, which efficiently confines energy within the PCM layer. Remarkably, the thermal insulation effect of the 2D layer is equivalent to a ~100 nm layer of SiO2. The high thermal boundary resistance induced by the van der Waals (vdW)-bonded layers limits the thermal diffusion through the layer interfaces. Hence, 2D materials with stable vdW interfaces can be used to improve the thermal efficiency of PCM-tuned Si photonic devices. Furthermore, our waveguide simulations show that the 2D layer does not affect the propagating mode in the Si waveguide, thus this simple additional thin film produces a substantial energy efficiency improvement without degrading the optical performance of the waveguide. Our findings pave the way for energy-efficient laser-induced structural phase transitions in PCM-based reconfigurable photonic devices., 34 pages, 12 figures

Chalcogenide based phase change random access memory (PCRAM) holds great promise for high speed and large data storage applications. This memory is scalable, requires a low switching energy, has a high endurance, has fast switching speed, and is nonvolatile. However, decreasing the switching time whilst increasing the cycle endurance is a key challenge for this technology to replace dynamic random access memory. Here we demonstrate high speed and high endurance ultrafast transient switching in the SET state of a prototypical phase change memory cell. Volatile switching is modeled by electron-phonon and lattice scattering on short timescales and charge carrier excitation on long timescales. This volatile switching in phase change materials enables the design of hybrid memory modulators and ultrafast logic circuits.

Alloys of sulfur, selenium and tellurium, often referred to as chalcogenide semiconductors, offer a highly versatile, compositionally-controllable material platform for a variety of passive and active photonic applications. They are optically nonlinear, photoconductive materials with wide transmission windows that present various high- and low-index dielectric, low-epsilon and plasmonic properties across ultra-violet, visible and infrared frequencies, in addition to an, non-volatile, electrically/optically induced switching capability between phase states with markedly different electromagnetic properties. This roadmap collection presents an in-depth account of the critical role that chalcogenide semiconductors play within various traditional and emerging photonic technology platforms. The potential of this field going forward is demonstrated by presenting context and outlook on selected socio-economically important research streams utilizing chalcogenide semiconductors. To this end, this roadmap encompasses selected topics that range from systematic design of material properties and switching kinetics to device-level nanostructuring and integration within various photonic system architectures.

Chalcogenide phase change materials (PCMs) have been extensively applied in data storage, and they are now being proposed for high resolution displays, holographic displays, reprogrammable photonics, and all-optical neural networks. These wide-ranging applications all exploit the radical property contrast between the PCMs different structural phases, extremely fast switching speed, long-term stability, and low energy consumption. Designing PCM photonic devices requires an accurate model to predict the response of the device during phase transitions. Here, we describe an approach that accurately predicts the microstructure and optical response of phase change materials during laser induced heating. The framework couples the Gillespie Cellular Automata approach for modelling phase transitions with effective medium theory and Fresnel equations. The accuracy of the approach is verified by comparing the PCM optical response and microstructure evolution with the results of nanosecond laser switching experiments. We anticipate that this approach to simulating the switching response of PCMs will become an important component for designing and simulating programmable photonics devices. The method is particularly important for predicting the multi-level optical response of PCMs, which is important for all-optical neural networks and PCM-programmable perceptrons., Comment: 26 pages, 6 figures

Chalcogenide phase change materials have many fascinating characteristics, i.e. extraordinarily large optical and electrical changes and non-volatility, making them appealing for information storage and reprogrammable photonics. Designing programmable photonics devices requires an accurate model to describe the switching behavior. Here, we present a multi-physics cellular automata-based framework which combines laser-induced heating, Gillespie’s Cellular Automata approach, effective medium theory and Fresnel’s Laws to model the microstructural evolution and concomitant optical response of phase change photonics devices. From the framework, we can simulate the change in optical constants during the phase switching process as well as the transient change in crystallite distribution, reflection, and transmission. The accuracy of the multi-physics model is also verified by nanosecond-pulsed laser switching experiments and transmission electron microscopy.

Recently, tunable high absorptance from various nanophotonic structures has been demonstrated. However, most of these structures require nano-lithography, which is expensive and slow. Lithography-free tuneable absorbers are rarely explored for tuneable visible and near-infrared photonics. Herein, we demonstrate a gold (Au)/chalcogenide dual-layer that is resonantly coupled to Au nanoparticles (NPs). The structure exhibits angle and polarisation-independent high absorptance. At resonance, waveguide cavity-like modes are excited between the film and NPs whilst gap plasmon modes are excited between the NPs. Coalescence of the waveguide cavity-like modes, the gap plasmon modes, and the highly absorbing chalcogenide semiconductor not only leads to perfect absorptance but also a reconfigurable response via reversible structural phase transitions in the chalcogenide film. In the amorphous state, the design provides nearly perfect absorptance for both p- and s-polarisation states at an incident angle of 20°. However, after switching to the crystalline state, the peak absorptance spectrally broadens and redshifts from 980 to 1520 nm. This experimental observation was theoretically validated by the finite element method. Thermal-electric modeling was performed to show that the structural transition from crystalline to amorphous states is possible in just 5 ns, thus allowing high-speed reconfigurable perfect absorbers.

In the last decade phase change materials (PCM) research has switched from practical application in optical data storage toward electrical phase change random access memory technologies (PCRAM). As these devices are commercialised, we expect the research direction to switch once again toward electrical-photonic devices. The objective of this review is to introduce the concepts in PCM-tuned photonics. We will start by highlighting the key works in the field, before concentrating on PCM-tuned Metal-Dielectric-Metal (MDM) structures. We will discuss how to design tuneable-MDM photonics devices, their advantages, and their limitations. Finally we will discuss new materials for phase change photonics.

The objective of this Opinion is to stimulate new research into materials that can meet the needs of tomorrow’s programmable photonics components. Herein, we argue that the inherent property portfolios of the common telluride phase change materials, which have been successfully applied in data storage technologies, are unsuitable for most emerging programmable photonics applications. We believe that newer PCMs with wider bandgaps, such as Sb2S3, Sb2Se3, and Ge2Sb2Se4Te (GSST), can be optimized to meet the demands of holographic displays, optical neural network memories, and beam steering devices.

Large refractive index changes in Sb2S3 phase-change material promise nonvolatile ultrathin displays., Materials that exhibit large and rapid switching of their optical properties in the visible spectrum hold the key to color-changing devices. Antimony trisulfide (Sb2S3) is a chalcogenide material that exhibits large refractive index changes of ~1 between crystalline and amorphous states. However, little is known about its ability to endure multiple switching cycles, its capacity for recording high-resolution patterns, nor the optical properties of the crystallized state. Unexpectedly, we show that crystalline Sb2S3 films that are just 20 nm thick can produce substantial birefringent phase retardation. We also report a high-speed rewritable patterning approach at subdiffraction resolutions (>40,000 dpi) using 780-nm femtosecond laser pulses. Partial reamorphization is demonstrated and then used to write and erase multiple microscale color images with a wide range of colors over a ~120-nm band in the visible spectrum. These solid-state, rapid-switching, and ultrahigh-resolution color-changing devices could find applications in nonvolatile ultrathin displays.

If electro-optic conversion of current photonic NNs could be postponed until the very end of the network, then the execution time is simply the photon time-of-flight delay. Here we discuss a first design and performance of an all-optical perceptron and feed-forward NN. Key is the dual-purpose foundry-approved heterogeneous integration of phase-change-materials resulting in a) volatile nonlinear activation function (threshold) realized with ps-short optical pulses resulting in a non-equilibrium variation of the materials permittivity, and b) thermo-optically writing a non-volatile optical multi-cell (5-bit) memory for the NN weights after being (offline) trained. Once trained, the weights only required a rare update, thus saving power. Performance wise, such an integrated all-optical NN is capable of < fJ/MAC using experimental demonstrated pump-probe [Waldecker et al, Nat. Mat. 2015] with a delay per perceptron being ~ps [Miscuglio et al. Opt.Mat.Exp. 2018] has a high cascadability.

High quality van der Waals chalcogenides are important for phase change data storage, thermoelectrics, and spintronics. Using a combination of statistical design of experiments and density functional theory, we clarify how the out-of-equilibrium van der Waals epitaxial deposition methods can improve the crystal quality of Sb2Te3 films. We compare films grown by radio frequency sputtering and pulsed laser deposition (PLD). The growth factors that influence the crystal quality for each method are different. For PLD grown films a thin amorphous Sb2Te3 seed layer most significantly influences the crystal quality. In contrast, the crystalline quality of films grown by sputtering is rather sensitive to the deposition temperature and less affected by the presence of a seed layer. This difference is somewhat surprising as both methods are out-of-thermal-equilibrium plasma-based methods. Non-adiabatic quantum molecular dynamics simulations show that this difference originates from the density of excited atoms in the plasma. The PLD plasma is more intense and with higher energy than that used in sputtering, and this increases the electronic temperature of the deposited atoms, which concomitantly increases the adatom diffusion lengths in PLD. In contrast, the adatom diffusivity is dominated by the thermal temperature for sputter grown films. These results explain the wide range of Sb2Te3 and superlattice crystal qualities observed in the literature. These results indicate that, contrary to popular belief, plasma-based deposition methods are suitable for growing high quality crystalline chalcogenides.

We demonstrate a facile method to fabricate Ag-Sb2S3 nanoporous plasmonic absorbers on plastic, crystalline, and glassy substrates. Our lithography-free method exploits the immiscibility of Ag and Sb2S3 to control the surface morphology of the absorbing structure. We experimentally and theoretically show that the nanoporous structure exhibits a high absorption across the visible and near infrared spectrum, which is achieved by the coalescence of surface/bulk plasmons and the Sb2S3 semiconductor bandgap absorption. The absorptance is independent of polarisation over a broad range of incident angles. We found that the Ag-Sb2S3 nanoporous structure is an effective absorber on both flexible and non-flexible substrates, and the simple deposition of the Ag-Sb2S3 nanoporous structure facilitates fabrication over a 100 mm diameter wafer. Our work shows that plasmonic nanostructures can be fabricated at room temperature on an industrial scale using sputtering, which is already used to commercially produce photovoltaic cells. We foresee these inexpensive absorbers being applied to wearable light-harvesting systems, bio-sensing, and display technologies.

This work investigates the problematic diffusion of metal atoms into phase change chalcogenides, which can destroy resonances in photonic devices. Interfaces between Ge2Sb2Te5 and metal layers were studied using X-ray reflectivity (XRR) and reflectometry of metal-Ge2Sb2Te5 layered stacks. The diffusion of metal atoms influences the crystallisation temperature and optical properties of phase change materials. When Au, Ag, Al, W structures are directly deposited on Ge2Sb2Te5 inter-diffusion occurs. Indeed, Au forms AuTe2 layers at the interface. Diffusion barrier layers, such as Si3N4 or stable diffusionless plasmonic materials, such as TiN, can prevent the interfacial damage. This work shows that the interfacial diffusion must be considered when designing phase change material tuned photonic devices, and that TiN is the most suitable plasmonic material to interface directly with Ge2Sb2Te5., Comment: 23 pages, 8 figures, article

The objective of this paper is to demonstrate that Ag readily diffuses into Sb2S3 and that electric fields can control the diffusion. Ag diffusion influences the crystallization temperature and electrical properties of Sb2S3. We studied the interface between Ag and Sb2S3 using X-ray reflectivity and show that the Ag cations can be controlled by applying an electric field. We believe this effect has technological applications in data storage devices.

The objective of this work is to demonstrate the usefulness of fractional factorial design for optimizing the crystal quality of chalcogenide van der Waals (vdW) crystals. We statistically analyze the growth parameters of highly c axis oriented Sb2Te3 crystals and Sb2Te3-GeTe phase change vdW heterostructured superlattices. The statistical significance of the growth parameters of temperature, pressure, power, buffer materials, and buffer layer thickness was found by fractional factorial design and response surface analysis. Temperature, pressure, power, and their second-order interactions are the major factors that significantly influence the quality of the crystals. Additionally, using tungsten rather than molybdenum as a buffer layer significantly enhances the crystal quality. Fractional factorial design minimizes the number of experiments that are necessary to find the optimal growth conditions, resulting in an order of magnitude improvement in the crystal quality. We highlight that statistical design of experiment methods, which is more commonly used in product design, should be considered more broadly by those designing and optimizing materials.

We designed and demonstrated a four-state programmable mid-infrared band-stop absorber that exploits two different phase-change materials. This programmability is possible by exploiting Fabry–Perot resonances in a Ge2Sb2Te5 film and vanadium dioxide nanoparticles' (VO2 NPs) dual layer. The reflectivity trough can be tuned to four different infrared (IR) wavelengths from 1906 to 2960 nm by heating the structure to different temperatures. The near-perfect absorber is reconfigurable, lithography-free, industrially scalable, polarization-insensitive, and omnidirectional. Our strategy opens a path for programmable infrared photonics.

The combination of ferroelectrics and phase-change materials provides a route towards phase-change data storage at room temperature, without heating.

Perfect absorbers that can efficiently absorb electromagnetic waves over a broad spectral range are crucial for energy harvesting, light detection, and optical camouflage. Recently, perfect absorbers based on a metasurface have attracted intensive attention. However, high-performance metasurface absorbers in the visible spectra require strict fabrication tolerances, and this is a formidable challenge. Moreover, fabricating subwavelength meta-atoms requires a top-down approach, thus limiting their scalability and spectral applicability. Here, we introduce a plasmonic nearly perfect absorber that exhibits a measured polarization-insensitive absorptance of ∼92% across the spectral region from 400 to 1000 nm. The absorber is realized via a one-step self-assembly deposition of 50 nm gold (Au) nanoparticle (NP) clusters onto a 35 nm-thick Ge2Sb2Te5 (GST225) chalcogenide film. An excellent agreement between the measured and theoretically simulated absorptance was found. The coalescence of the lossy GST225 dielectric layer and high density of localized surface plasmon resonance modes induced by the randomly distributed Au NPs play a vital role in obtaining the nearly perfect absorptance. The exceptionally high absorptance together with large-area high-throughput self-assembly fabrication demonstrates their potential for industrial-scale manufacturability and consequential widespread applications in thermophotovoltaics, photodetection, and sensing.

This work reveals that doping Ag into Sb 2 S 3 substantially decreases its crystallisation temperature. We show that applying an electric field to Sb 2 S 3 through Ag electrodes provides control of the crystallisation temperature and crystallisation rate. The crystal nuclei incubation time decreases substantially when the applied electric field is set to 200 kV/m. The applied electric field appears to force the Ag cations through the amorphous chalcogenide film resulting in Ag doped Sb 2 S 3 filaments that extend from the cathode to the anode. This was confirmed by X-ray fluorescence composition mapping. Density functional theory molecular dynamics modelling of Ag doped Sb 2 S 3 reveals that the diffusion constant of Ag is twice that of Sb or S over a wide temperature range, which implies that the Ag atoms are mobile in the amorphous Sb 2 S 3 structure. The applied electric field provides a mechanism to enhance the crystallisation kinetics of Ag-doped Sb 2 S 3 .

The effect of oxygen on the local structure of Ge atoms in GeTe-O materials has been investigated. Oxygen leads to a significant modification to the vibrational modes of Ge octahedra, which results from a decrease in its coordination. We find that a defective octahedral Ge network is the crucial fingerprint for rapid and reversible structural transitions in GeTe-based phase change materials. The appearance of oxide Raman modes confirms phase separation into GeO and TeO at high level O doping. Counterintuitively, despite the increase in crystallization temperature of oxygen doped GeTe-O phase change materials, when GeTe-O materials are used in electrical phase change memory cells, the electrical switching energy is lower than the pure GeTe material. This switching energy reduction is ascribed to the smaller change in volume, and therefore smaller enthalpy change, for the oxygen doped GeTe materials.

The narrowband surface plasmon resonance of metallic nanostructures was once thought to limit the bandwidth of absorptance, yet recent demonstrations show that it can be harnessed using mechanisms such as multiple resonances, impedance matching, and slow-light modes to create broadband absorptance. However, in the visible spectrum, realization of absorbers based on patterned plasmonic nanostructures is challenging due to strict fabrication tolerances. Here we experimentally compare two different candidates for visible light broadband high absorptance. The first candidate is planar thin film dual layers of Ge2Sb2Te5 and aluminum (Al), while the second structure employs ultrathin Al grating/Ge2Sb2Te5 dual layers. In both cases, the absorbers yield a measured absorptance greater than 78% in the visible. A remarkably high-absorptance bandwidth of 120 nm was measured and associated with the large imaginary part of Ge2Sb2Te5 dielectric function. We find that the simple dual-layer planar structure is an effectiv...

The ultrafast, reversible, nonvolatile and multistimuli responsive phase change of Ge-Sb-Te (GST) alloy makes it an interesting “smart” material. The optical features of GST undergo significant variation when its state changes between amorphous and crystalline, meaning that they are useful for tuning photonic components. A GST phase change material (PCM) can be efficiently triggered by stimuli such as short optical or electrical pulses, providing versatility in high-performance photonic applications and excellent capability to control light. In this review, we study the fundamentals of GST-tuned photonics and systematically summarise the progress in this area. We then introduce current developments in both GST-metal hybrid metamaterials and GST-based dielectric metamaterials, and investigate the strategy of designing reversibly switchable GST-based photonic devices and their advantages. These devices may have a vast array of potential applications in optical memories, switches, data storage, cloaking, photodetectors, modulators, antennas etc. Finally, the prospect of implementing GST PCM in emerging fields within photonics is considered.

The optical contrast in a phase change material is concomitant with its structural transition. We connect these two by first recognizing that Friedel oscillations couple electrons propagating in opposite directions and supply an additional Coulomb energy. As the crystal switches phase, this energy acquires time dependence and the Landau-Zener mechanism operates, steering population transfer from the valence to the conduction band and vice versa. Spectroscopy suggests that the oscillator energy dominates the optical properties and a calculation involving the crystalline field and spin-orbit interaction yields good estimates for of both structural phases. Further analysis relates the optical gap with the crystalline-field energy as well as activation energy for electrical conduction. This last property characterizes the amorphous phase, thereby furnishing a link between the crystalline field and the activation energy and ultimately with the resistance drift exponent. Providing optical means to quantify resistance drift in PCMs could circumvent the need for fabricating expensive devices and performing time consuming measurements.

The development of various plasmonic nanoporous materials has attracted much interest in different areas of research including bioengineering and biosensing because of their large surface area and versatile porous structure. Here, we introduce a novel technique for fabricating silver-stibnite nanoporous plasmonic films. Unlike conventional techniques that are usually used to fabricate nanoporous plasmonic films, we use a room-temperature growth method that is wet-chemistry free, which enables wafer-scale fabrication of nanoporous films on flexible substrates. We show the existence of propagating surface plasmon polaritons in nanoporous films and demonstrate the extreme bulk refractive index sensitivity of the films using the Goos-Hänchen shift interrogation scheme. In the proof-of-concept biosensing experiments, we functionalize the nanoporous films with biotin-thiol using a modified functionalization technique, to capture streptavidin. The fractal nature of the films increases the overlap between the local field and the immobilized biomolecules. The extreme sensitivity of the Goos-Hänchen shift allows femtomolar concentrations of streptavidin to be detected in real time, which is unprecedented using surface plasmons excited via the Kretschmann configuration.

The objective of this work is to demonstrate the usefulness of fractional factorial design for optimizing the crystal quality of chalcogenide van der Waals (vdW) crystals. We statistically analyze the growth parameters of highly c axis oriented Sb

The ability to manipulate light propagation at the nanoscale is of vital importance for future integrated photonic circuits. In this work we exploit the high contrast in the optical properties of the phase change material Ge2Sb2Te5 to control the propagation of surface plasmon polaritons at a Au/SiO2 interface. Using grating couplers, normally incident light at λ = 1.55 μm is converted into propagating surface plasmons on a Au waveguide. Single laser pulses (λ = 975 nm) are applied to a thin film of Ge2Sb2Te5 placed on top of the device, which, upon transition from its amorphous to crystalline structural phase, dramatically increases both its refractive index and absorption coefficient, thus inhibiting propagation of the plasmonic mode. This effect is investigated for different interaction lengths between the phase change material and the Au waveguide, and contrast values in the transmitted intensity up to several tens of percents are demonstrated.

Chalcogenides are materials that substantially consist of sulphur, selenium, and tellurium. Their dielectric properties can be tuned by thermally induced structural phase transitions, photostructural transitions, and dissolution of metal dopants. We have designed active photonic structures using a range of `tuneable' chalcogenides. The resonant frequency of plasmonic structures was tuned over a 100 nm band in the visible, metal-chalcogenidemetal structures provide tuning of over a band of 0.5 μm in the mid-infrared, and hyperbolic metamaterials incorporating chalcogenides provide a means to alter the radiative decay rate of uorescent photons.

A glass-based substrate technology that fills the gap between a truly flexible extended length distributed sensor medium and the multifunctionality of optical chips is demonstrated. Flat fiber chips will open further degrees of freedom to control the behavior of light via mechanical manipulation. A flexible flat format will also allow straightforward incorporation into smart structures. Coupled with low manufacturing costs, these flexichips can also be a key enabler to disposable high-end sensing devices or fully distributed point sensors. In this study, Bragg gratings were used to demonstrate the optical flatness of the flat fiber core layer. Furthermore, the effective index values obtained from the grating experiment were input into a dynamic model, subsequently proving the influence of the dumbbell-shaped flat fiber cross section on the resultant UV written waveguides. Evanescent field sensing was also demonstrated by adopting a stepped Bragg approach.

The role of stress on the crystallization process of the phase change data storage material, Ge2Sb2Te5, is studied. When thin Ge2Sb2Te5 films are capped with Si3N4, stress is generated in the Ge2Sb2Te5 layer which causes the crystallization temperature to increase. Si3N4 films of 25 nm thickness increase the crystallization temperature from 446 K to 464 K. We show that stress predominantly destabilizes voids and increases the number of Ge-Sb and homopolar bonds in the vicinity of Ge atoms, and this makes the crystallization less probable, thus resulting in the increase in the measured temperature.The role of stress on the crystallization process of the phase change data storage material, Ge2Sb2Te5, is studied. When thin Ge2Sb2Te5 films are capped with Si3N4, stress is generated in the Ge2Sb2Te5 layer which causes the crystallization temperature to increase. Si3N4 films of 25 nm thickness increase the crystallization temperature from 446 K to 464 K. We show that stress predominantly destabilizes voids and increases the number of Ge-Sb and homopolar bonds in the vicinity of Ge atoms, and this makes the crystallization less probable, thus resulting in the increase in the measured temperature.

The limit to which the phase change memory material Ge(2)Sb(2)Te(5) can be scaled toward the smallest possible memory cell is investigated using structural and optical methodologies. The encapsulation material surrounding the Ge(2)Sb(2)Te(5) has an increasingly dominant effect on the material's ability to change phase, and a profound increase in the crystallization temperature is observed when the Ge(2)Sb(2)Te(5) layer is less than 6 nm thick. We have found that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress, we have maintained the bulk crystallization temperature in Ge(2)Sb(2)Te(5) films just 2 nm thick.

Van der Waals heterostructure superlattices of Sb2 Te1 and GeTe are strain-engineered to promote switchable atomic disordering, which is confined to the GeTe layer. Careful control of the strain in the structures presents a new degree of freedom to design the properties of functional superlattice structures for data storage and photonics applications.

Oxygen-doped germanium telluride phase change materials are proposed for high temperature applications. Up to 8 at.% oxygen is readily incorporated into GeTe, causing an increased crystallisation temperature and activation energy. The rhombohedral structure of the GeTe crystal is preserved in the oxygen doped films. For higher oxygen concentrations the material is found to phase separate into GeO2 and TeO2, which inhibits the technologically useful abrupt change in properties. Increasing the oxygen content in GeTe-O reduces the difference in film thickness and mass density between the amorphous and crystalline states. For oxygen concentrations between 5 and 6 at.%, the amorphous material and the crystalline material have the same density. Above 6 at.% O doping, crystallisation exhibits an anomalous density change, where the volume of the crystalline state is larger than that of the amorphous. The high thermal stability and zero-density change characteristic of Oxygen-incorporated GeTe, is recommended for efficient and low stress phase change memory devices that may operate at elevated temperatures.

The objective of this paper is to review the characterisation methods and procedures used to laser switch phase change materials, and then assess their applicability for characterising phase change materials for active photonics devices. Specifically we characterise the performance of our pump-probe laser system and compare it with other ‘static’ and ‘dynamic’ testers. Our pump-probe system was developed to measure the phase transformation kinetics of chalcogenide films by simultaneously measuring the transmission and reflection of a probe laser with a temporal resolution of 1 ns. We also use the system to measure the second order nonlinear refractive index of chalcogenide thin films. Laser switching of chalcogenides are efficient methods to screen new materials but the switching time seems to have a strong dependence on the measurement method and procedure. Therefore in this article we recommend bespoke methods and procedures for assessing the performance of new chalcogenide compositions for specific photonic devices.

By confining phase transitions to the nanoscale interface between two different crystals, interfacial phase change memory heterostructures represent the state of the art for energy efficient data storage. We present the effect of strain engineering on the electrical switching performance of the –GeTe superlattice van der Waals devices. Multiple Ge atoms switching through a two-dimensional Te layer reduces the activation barrier for further atoms to switch; an effect that can be enhanced by biaxial strain. The out-of-plane phonon mode of the GeTe crystal remains active in the superlattice heterostructures. The large in-plane biaxial strain imposed by the layers on the GeTe layers substantially improves the switching speed, reset energy, and cyclability of the superlattice memory devices. Moreover, carefully controlling residual stress in the layers of –GeTe interfacial phase change memories provides a new degree of freedom to design the properties of functional superlattice structures for memory and photonics applications.

Coherent phonons (CPs) generated by laser pulses on the femtosecond scale have been proposed as a means to achieve ultrafast, nonthermal switching in phase-change materials such as ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ (GST). Here we use ultrafast optical pump pulses to induce coherent acoustic phonons and stroboscopically measure the corresponding lattice distortions in GST using 100-ps x-ray pulses from the European Synchrotron Radiation Facility (ESRF) storage ring. A linear-chain model provides a good description of the observed changes in the diffraction signal; however, the magnitudes of the measured shifts are too large to be explained by thermal effects alone, implying the presence of excited-state effects in addition to temperature-driven expansion. The information on the movement of atoms during the excitation process can lead to greater insight into the possibilities of using CP-induced phase transitions in GST.