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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.

Perfect absorption ranging from visible, infrared, terahertz, to microwave is desirable for solar cell, photodetection, telecommunications, and molecular sensing. Recently, the air/dielectric–metal stacks/substrate‐based asymmetric Fabry–Pérot (FP) cavity has attracted much attention owing to lithography‐free design which is scalable and low cost. Herein, a reversibly tunable asymmetric FP cavity high absorber in the near‐infrared (NIR) region is experimentally demonstrated, relying on chalcogenide (Ge2Sb2Te5)–metal (Au) stacked layers. It exhibits an extremely high absorptance of 0.99 at a resonant wavelength of 1180 nm for amorphous (AM) state; yet, the peak absorptance redshifts to 1680 nm for crystalline (CR) state. Importantly, it takes about 5 ns to reversibly transit the peak absorptance by reamorphizing the GST225. It is also experimentally shown the Brewster modes can be excited in the cavity absorber, and a wide tuning of Brewster modes (from 820 to 1500 nm) is realized as switching the state between AM and CR. The Goos–Hänchen (GH) shift can be observed at the Brewster angle for each state. For the proof of concept multibiosensing application, the cavity to detect two different solutions of the copper sulfate and glucose in both AM and CR states is numerically functionalized, respectively.

Metamaterial (MM) perfect absorbers are realised over various spectra from visible to microwave. Recently, different approaches have been explored to integrate tunability into MM absorbers. Particularly, tuning has been illustrated through electrical-, thermal-, and photo-induced changes to the permittivity of the active medium within MM absorbers. However, the intricate design, expensive nanofabrication process, and the volatile nature of the active medium limit the widespread applications of MM absorbers. Metal–dielectric stack layered hyperbolic metamaterials (HMMs) have recently attracted much attention due to their extraordinary optical properties and rather simple design. Herein, we experimentally realised a reconfigurable HMM perfect absorber based on alternating gold (Au) and Ge2Sb2Te5 (GST225) layers for the near-infrared (N-IR) region. It shows that a red-shift of 500 nm of the absorptance peak can be obtained by changing the GST225 state from amorphous to crystalline. The nearly perfect absorptance is omnidirectional and polarisation-independent. Additionally, the absorptance peak can be reversibly switched in just five nanoseconds by re-amorphising the GST225, enabling a dynamically reconfigurable HMM absorber. Experimental data are validated numerically using the finite-difference time-domain method. The absorber fabricated using our strategy has advantages of being reconfigurable, uncomplicated, and lithography-free over conventional MM absorbers, which may open up a new path for applications in energy harvesting, photodetectors, biochemical sensing, and thermal camouflage techniques.

To date, the second harmonic generation (SHG) has a great effect on photonic devices. However, it is a formidable challenge to achieve reconfigurable SHG. Hereby, we experimentally demonstrate the SHG response from the oriented Ge2Sb2Te5 (GST) grains induced by polarized laser pulses for the first time. The orientation of GST grains is found to be perpendicular to the polarization direction of the pump laser. Such unique ordered structures result in a periodic change of SHG intensity with the input polarization angle of the pump laser rotating every 180°. These findings may pave avenues for generating nonlinear optical sources with a simple process, scalability, and switchable functionality. National Research Foundation (NRF) Accepted version This work was partially supported by the National Natural Science Foundation of China (51472258, 51672292, and 61627826), International Science and Technology Cooperation Program of China (2016YFE0110600) and National Key Research and Development Program of China(2017YFB0701703), National Research Foundation, Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

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.

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.

Thermal lithography is an alternative technique that can fabricate patterns beyond the diffraction limit. In thermal lithography, it is hard to obtain morphology on demand due to the difficulty in controlling thermal diffusion caused by the long laser pulses. To address this issue, enhanced thermal lithography with multiple femtosecond laser pulse exposure is proposed in this paper. C-Sb2Te3 (CST), a fine-tunable phase change material, is used as a negative heat-mode resist for the first time. The resolution and steepness of the microstructure prepared by this method are obviously enhanced compared with the conventional way. The height of the wet-etched microstructures can be preciously adjusted by tuning the exposure dose of femtosecond laser pulses. Microstructure and thermal field analysis results reveal the origin of the high resolution and fine-tuning. CST-based phase-change memory cell arrays were fabricated with this method to confirm the feasibility and advantage. This work provides an effective way to achieve enhanced thermal lithography in the manufacturing of micro/nanostructure-based electronic and photonic devices.

The objective of this work is to design and demonstrate multilevel optical switches by combining different phase change materials. Ge2Sb2Te5 and VO2 nanolayer structures were designed to maximize the optical contrast between four different reflective states. These different optical states arise due to the independent structural phase transitions of VO2 and Ge2Sb2Te5 at different temperatures. The transfer matrix method was used to model Fresnel reflection for each structural phase combination and then to optimize the VO2 and Ge2Sb2Te5 layer thicknesses, which were found to be 70 nm and 50 nm. These multilevel optical switching results provide further possibilities to design composite materials for applications in active and programmable photonics. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Published version

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.

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.