Chalcogenide Phase-change Materials Research Articles

Monolithic back-end-of-line integration of phase change materials into foundry-manufactured silicon photonics

Monolithic integration of novel materials without modifying the existing photonic component library is crucial to advancing heterogeneous silicon photonic integrated circuits. Here we show the introduction of a silicon nitride etch stop layer at select areas, coupled with low-loss oxide trench, enabling incorporation of functional materials without compromising foundry-verified device reliability. As an illustration, two distinct chalcogenide phase change materials (PCMs) with remarkable nonvolatile modulation capabilities, namely Sb2Se3 and Ge2Sb2Se4Te1, were monolithic back-end-of-line integrated, offering compact phase and intensity tuning units with zero-static power consumption. By employing these building blocks, the phase error of a push-pull Mach–Zehnder interferometer optical switch could be reduced with a 48% peak power consumption reduction. Mirco-ring filters with >5-bit wavelength selective intensity modulation and waveguide-based >7-bit intensity-modulation broadband attenuators could also be achieved. This foundry-compatible platform could open up the possibility of integrating other excellent optoelectronic materials into future silicon photonic process design kits.

A Review on Material Selection Benchmarking in GeTe-Based RF Phase-Change Switches for Each Layer.

The global demand for radio frequency (RF) modules and components has grown exponentially in recent decades. RF switches are the essential unit in RF front-end and reconfigurable systems leading to the rapid development of novel and advanced switch technology. Germanium telluride (GeTe), as one of the Chalcogenide phase-change materials, has been applied as an RF switch due to its low insertion loss, high isolation, fast switching speed, and low power consumption in recent years. In this review, an in-depth exploration of GeTe film characterization is presented, followed by a comparison of the device structure of directly heated and indirectly heated RF phase-change switches (RFPCSs). Focusing on the prototypical structure of indirectly heated RFPCSs as the reference, the intrinsic properties of each material layer and the rationale behind the material selection is analyzed. Furthermore, the design size of each material layer of the device and its subsequent RF performance are summarized. Finally, we cast our gaze toward the promising future prospects of RFPCS technology.

Optical nanoimaging of laser-switched phase-change plasmonic infrared antennas.

We investigate the plasmonic properties of laser-printed chalcogenide phase-change material In3SeTb2 (IST) antennas through near-field nanoimaging. Antennas of varying lengths were fabricated by laser switching an amorphous IST film into its crystalline metallic state. Near-field imaging elucidates the pronounced field confinement and enhancement at the antenna extremities along with the emergence of different ordered plasmonic modes with increasing length. Compared to gold antennas, the PCM antennas exhibit slightly lower but still substantial near-field enhancement with greater compactness. The interplay between antenna length, illumination angle, and excitation frequency enables versatile control over the resonant near-field distribution. Our work provides deeper understanding and tunable functionalities of laser-printed PCM nanoantennas for potential applications in compact, dynamically reconfigurable nanophotonic devices.

Understanding the link between fragile-to-strong kinetics and β-relaxation in chalcogenide glasses

β-relaxation has been a universal behavior in various glasses with bond kinds of covalent and metallic, as well as the metavalent, which is the typical bonding of chalcogenide phase-change materials (PCMs). Recently, the fragile-to-strong (F–S) transition kinetics has been a significant feature in supercooled liquids of PCMs, to alleviate the contradiction between good thermal stability nearby glass transition temperature and fast crystallization speed around melting temperature. By analyzing the results from the measurements of powder mechanical spectroscopy and flash differential scanning calorimetry, in this work, we revealed the link between β-relaxation and F–S crystallization kinetics in GeTe-GeSe chalcogenide glass system. It was found a positive correlation between β-relaxation and F–S transition, which is accompanied by the results of resistance drift, crystallization temperature, as well as the maximum crystal growth rate. However, the GeTe component, which has large β-relaxation behavior but no F–S transition, does not obey the positive correlation because of the large activation energy of α-relaxation. These findings are expected to build a foundation and illustrate the microscopic mechanism between F–S crystallization kinetics and β-relaxation behavior in chalcogenide glasses.

Optical switching beyond a million cycles of low-loss phase change material Sb2Se3

The development of the next generation of optical phase change technologies for integrated photonic and free-space platforms relies on the availability of materials that can be switched repeatedly over large volumes and with low optical losses. In recent years, the antimony-based chalcogenide phase-change material Sb2Se3 has been identified as particularly promising for a number of applications owing to good optical transparency in the near-infrared part of the spectrum and a high refractive index close to silicon. The crystallization temperature of Sb2Se3 of around 460 K allows switching to be achieved at moderate energies using optical or electrical control signals while providing sufficient data retention time for non-volatile storage. Here, we investigate the parameter space for optical switching of films of Sb2Se3 for a range of film thicknesses relevant to optical applications. By identifying optimal switching conditions, we demonstrate endurance of up to 107 cycles at reversible switching rates of 20 kHz. Our work demonstrates that the combination of intrinsic film parameters with pumping conditions is particularly critical for achieving high endurance in optical phase change applications.

An Open‐Source Multifunctional Testing Platform for Optical Phase Change Materials

Owing to their unique tunable optical properties, chalcogenide phase change materials are increasingly being investigated for optics and photonics applications. However, in situ characterization of their phase transition characteristics is a capability that remains inaccessible to many researchers. Herein, a multifunctional silicon microheater platform capable of in situ measurement of structural, kinetic, optical, and thermal properties of these materials is introduced. The platform can be fabricated leveraging industry‐standard silicon foundry manufacturing processes. This platform is fully open‐sourced, including complete hardware design and associated software codes.

Tunable optical power splitter based on directional coupler structure with phase change material Sb2Se3

Traditional optical power splitters (OPSs) have fixed power split ratios, and although some can be tuned with an electro-optic polymer, continuous energy supply increases power consumption. Combining OPS with chalcogenide phase-change materials (PCMs) allows us to tune the power split ratio with a low power owing to the nonvolatility and contrasting optical properties of the PCM. In this paper, a compact 1 × 2 arbitrary ratio OPS with hybrid directional coupler structure with PCM Sb2Se3 is proposed for lower-power and reconfigurable PICs. It demonstrates a continuous change in the power split ratios by changing the crystalline fraction of the Sb2Se3 material covered on coupling waveguide. The non-volatile states of Sb2Se3 enables zero static power of the device. The splitting ratio can be tuned in a broad range from 1 % to 97 % with the excess loss of 0.11 dB at 1550 nm. Such device can be extended to perform a handwritten digit recognition task with the simulated properties to determine the performance of the OPS device in the neuromorphic network. Its recognition accuracy achieves as high as 95 % for the MNIST (Modified National Institute of Standard and Technology) handwritten digits, which is close to that of device with ideal characteristics based on the software computing. The results indicate that the proposed OPS can not only achieve large range tunable power allocation, but also be further applied to large-scale PICs and optical neuromorphic networks.

High optical contrast and multi-level storage of the ultracompact plasmonic device based on phase change materials

The combination of photonic devices and chalcogenide phase change materials (PCMs) provides a potential solution for photonic storage and computing. However, there are still some issues that need to be addressed for currently integrated phase-change photonic memory such as low speed, high energy consumption, and large footprint. To enable stronger interaction between light and PCMs, we utilize the plasmonic enhancement effect to amplify the local electric field, resulting in high energy efficiency in photonic storage and computation. This work proposes a nanoscale elliptical plasmonic nanoantenna that dramatically improves switching speed and energy by placing an Ag elliptical ring wrapped with Ge2Sb2Te5 (GST) on a silicon waveguide. The device exhibits crystalline and amorphous states with insertion losses of 0.3 dB and 1.7 dB, respectively, at a wavelength of 1550 nm. Write and erase operations can be completed by optical pulses of 1.5 ns–1 mW and 3 ns–3 mW, respectively. An ultra-small device volume of only 4.465 × 10−4 (µm3) is capable of achieving a switching contrast ratio of 28.5 %. Moreover, the non-volatile storage of multi-level intermediate states facilitates a wide weight-variation range and allows for precise weight updates, enhancing the overall performance and accuracy of the system. In the handwritten digit recognition task of the three-layer perceptron, the device demonstrates a recognition accuracy rate of 95 %, indicating that our approach can enable a more efficient neuromorphic photonic computing network.

Flexible germanium monotelluride phase change films with ultra-high bending stability for wearable piezoresistive sensors

Although non-stoichiometric chalcogenide phase change materials (e.g., Ge2Sb2Te5, Sb2Te3) have been used in wearable electronics, their non-stoichiometric vacancies will inevitably degrade their bending stability and electrical contrast. In this study, we demonstrated that stoichiometric germanium monotelluride films deposited on flexible polyimide substrates exhibited excellent phase transition characteristics and ultra-high bending stability under different bending states. The GeTe films showed improved bending stability under compressive strain than under tensile strain, and the GeTe films still exhibited high conductivity contrast (6 orders of magnitude) after 2000 bending cycles, with potential use in flexible memory devices. As a proof-of-concept, the bending stability of the stoichiometric GeTe films was shown to be significantly better than the non-stoichiometric Ge2Sb2Te5 films. Notably, we fabricated a flexible piezoresistive sensor based on the GeTe film, which exhibited good sensitivity (gauge factor is 38), low detection limit (0.1%), short response time (83.3 ms), good reliability and repeatability (more than 1200 cycles) and high heat resistance (200 ℃), and was used to track human pulse, as well as finger and face movements. The results showed that the piezoresistive sensor had good sensitivity and a stable time-resolved electrical response, and could be used to detect the movement of different human body parts and the small signals of different pressure points. These findings indicated that GeTe-based wearable piezoresistive sensors hold significant promise for medical health monitoring and biosensing applications.

Programmable nanophotonic planar resonator filter-absorber based on phase-change InSbTe

Reconfigurable plasmonic-photonic electromagnetic devices have been incessantly investigated for their great ability to optically modulate through external stimuli to meet today's emerging needs, with chalcogenide phase-change materials being promising candidates due to their remarkably unique electrical and optics, enabling new perspectives in recent photonic applications. In this work, we propose a reconfigurable resonator using planar layers of stacked ultrathin films based on Metal-dielectric-PCM, which we designed and analyzed numerically by the Finite Element Method (FEM). The structure is based on thin films of Gold (Au), aluminum oxide (Al2O3), and PCM (In3SbTe2) used as substrate. The modulation between the PCM phases (amorphous and crystalline) allows the alternation from the filter to the absorber structure in the infrared (IR) spectrum (1000–2500 nm), with an efficiency greater than 70% in both cases. The influence of the thickness of the material is also analyzed to verify tolerances for manufacturing errors and dynamically control the efficiency of transmittance and absorptance peaks. The physical mechanisms of field coupling and transmitted/absorbed power density are investigated. We also analyzed the effects on polarization angles for Transversal Electric (TE) and Transversal Magnetic (TM) polarized waves for both cases.

An optical scheme of on-chip matrixing by phase-change based tunable weighting of photonic tensor unit

In this paper, we propose an optical scheme of on-chip matrixing for matrix-vector multiplications (MVMs) by configuring each matrix element into a photonic tensor processing unit (TPU) with wavelength division multiplexing and the actively tunable weighting for scalar multiplication. The low loss chalcogenide phase change material of Ge2Sb2Se4Te (GSST) is employed and modeled with intermediate states for multilevel tunable weighting of each TPU. The dynamic electro-thermal process of GSST phase transition using ITO for electrical heating is simulated and well confirms the switchable weighting of TPUs. Simulation results reveal that a 7 V voltage pulse of 500 ns duration followed by a 9 V voltage pulse of 1500 ns duration and another pulse of 14 V voltage and 500 ns duration can set data value of ‘0’ and ‘1’ for each TPU. Taking a set of incident light with varied wavelengths and powers as input vectors, the quantized MVM outputs of photocurrents with varied amplitude are obtained after photoelectric conversion. Finally, the photonic integrated circuit level simulations by Lumerical INTERCONNECT perfectly confirm our scheme of on-chip matrixing for optical MVMs and computing.

Reversibly reconfigurable GSST metasurface for broadband beam steering and achromatic focusing in the long-wave infrared.

Active optical metasurfaces promise compact, lightweight, and energy-efficient optical systems with unprecedented performance. Chalcogenide phase-change material Ge2Sb2Se4Te1 (GSST) has shown tremendous advantages in the design of mid-infrared active metasurfaces. However, most of the GSST-based active metasurfaces can only work efficiently within a narrow frequency range. Furthermore, their design flexibility and reversible switching capability are severely restricted by the melting of GSST during re-amorphization. Here, we propose broadband, reversibly tunable, GSST-based transmissive metasurfaces operating in the long-wave infrared spectrum, where the GSST micro-rods are cladded by refractory materials. To accurately evaluate the performance of the proposed metasurfaces, two figures of merits are defined: FOMΦ for the evaluation of wavefront matching, and FOMop for the assessment of the overall performance incorporating both wavefront modulation efficiency and switching contrast ratio. For the proof of concept, two meta-devices are numerically presented: a multifunctional deflector that offers continuous beam steering and long-wave pass filtering simultaneously, and a large-area (1 cm × 1 cm) broadband (11-14 µm) varifocal metalens with the ability of achromatic imaging (12.5-13.5 µm). In particular, the metalens features high FOMop values over 16 dB in the achromatic band, with the average focusing efficiency approximating 70% (60%) in amorphous (crystalline) state and a spectral switching contrast ratio surpassing 25 dB. Our design scheme provides an additional degree of freedom for dynamic modulation and offers a novel approach for achieving high-efficiency mid-infrared compact optical devices.

Electrically Tunable Steganographic Nano-Optical Coatings.

Thin film coatings with tunable colors have a broad range of applications, from solid-state reflective displays to steganography. Here, we propose a novel approach to chalcogenide phase change material (PCM)-incorporated steganographic nano-optical coatings (SNOC) as thin film color reflectors for optical steganography. The proposed SNOC design combines a broad-band and a narrow-band absorber made up of PCMs to achieve tunable optical Fano resonance in the visible wavelength, which is a scalable platform for accessing the full-color range. We demonstrate that the line width of the Fano resonance can be dynamically tuned by switching the structural phase of PCM from amorphous to crystalline, which is crucial for obtaining high-purity colors. For steganography applications, the cavity layer of SNOC is divided into an ultralow loss PCM and a high index dielectric material with identical optical thickness. We show that electrically tunable color pixels can be fabricated using the SNOC on a microheater device.

Tunable structural transmissive color in fano-resonant optical coatings employing phase-change materials

Reversible, nonvolatile, and pronounced refractive index modulation is an unprecedented combination of properties enabled by chalcogenide phase-change materials (PCMs). This combination of properties makes PCMs a fast-growing platform for active, low-energy nanophotonics, including tunability to otherwise passive thin-film optical coatings. Here, we integrate the PCM Sb2Se3 into a novel four-layer thin-film optical coating that exploits photonic Fano resonances to achieve tunable structural colors in both reflection and transmission. We show, contrary to traditional coatings, that Fano-resonant optical coatings (FROCs) allow for achieving transmissive and reflective structures with narrowband peaks at the same resonant wavelength. Moreover, we demonstrate asymmetric optical response in reflection, where Fano resonance and narrow-band filtering are observed depending upon the light incidence side. Finally, we use a multi-objective inverse design via machine learning (ML) to provide a wide range of solution sets with optimized structures while providing information on the performance limitations of the PCM-based FROCs. Adding tunability to the newly introduced Fano-resonant optical coatings opens various applications in spectral and beam splitting, and simultaneous reflective and transmissive displays, diffractive objects, and holograms.

Recent Advancements in Reconfigurable mmWave Devices Based on Phase-Change and Metal Insulator Transition Materials

Chalcogenide Phase Change Materials (PCM) and metal insulator transition (MIT) materials are a group of materials that are capable of switching between low resistance and high resistance states. These emerging materials have been widely used in optical storage media and memory devices. Over the past recent years, there have been interests in exploiting the PCM and MIT materials, especially germanium antimony telluride (GST) alloys and vanadium dioxide (VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), for radio frequency (RF) applications. The PCM and MIT-based RF devices are expected to bridge the gap between semiconductor switches and microelectromechanical system (MEMS) switches as they combine the low insertion loss performance of MEMS technology and the small size and reliability performance of semiconductor technology. This article presents an overview of the PCM and MIT materials for RF circuits and discusses the recent advancements in reconfigurable millimeter-wave (mmWave) devices based on PCM and MIT materials in depth.

Preface

International Conference on Current Trend in Physics and Photonics (ICCTPP-2022), a virtual conference was organized by School of Physics, Dr. Vishwanath Karad MIT World Peace University- Pune, India in association with IOP Science Publishing from 9th June to 11th June 2022. The conference was Chaired by Dr. Sachin Kulkarni, Head, School of Physics, MITWPU Pune and Co-Chaired by Prof. Dr. Narendra Mathakari and Dr. Ajit Deore from School of Physics.The purpose of the conference was to bring together leading academic scientists, researchers, industry professionals to exchange and share their experiences and research results on all aspects of general Physics and in particular the emerging field of photonics. In General Physics, the focus was mainly on material science, nanoscience, computational physics and theoretical physics. In Photonics, the focus was primarily on Lasers, Nanophotonics, Green photonics, Solar Energy, Plasmonics and Nonlinear Optics.The three-day conference program was carefully structured to encourage mutual inspiration and fruitful debate among researchers. Well known experts across India and the world were associated with the conference for keynote addresses, plenary sessions and invited talks. In addition, high quality research was presented by academicians, industry professionals, and research scholars from leading institutions across India, UAE, South Korea, Republic of Korea, Thailand, Tunisia and Algeria through oral and poster sessions. Total Ten oral and poster sessions was scheduled with topics ranging from Photonics, Optics, Nano Science and Material Science.On the first day the conference lamp was ignited with the Keynote address of Prof. Jagadish Chennupati, Distinguished Professor and Head of Semiconductor Optoelectronics and Nanotechnology Group in the Research School of Physics, Australian National University, Australia and the Plenary Talk of Prof. Venu Gopal Achanta, Director of CSIR-National Physical Laboratory, India. Prof. Chennupati enlighten the attendees on ‘Semiconductor Nanostructures for Optoelectronics Applications’ and Prof. Achanta on ‘Nanophotonics’. During afternoon session Dr. Nayana Vaval, Scientist, NCL, Pune, India delivered her talk on ‘Study of temporary bound states’ and Dr. Almantas Pivrikas, Senior Lecturer, Murdoch University, Australia expressed his views on ‘opto-electronic devices, such as photovoltaic cells, light emitting diodes, field-effect transistors and sensors’.The second day started with the Plenary talk of Prof. Prashant Sonar, ARC Future Fellow and Professor in School of Chemistry and Physics and Centre for Material Science at Queensland University of Technology, (QUT), Australia on the topic ‘Novel Conjugated Functional Materials for Photonics, Electronics, and Sensing’. Afternoon session was two invited talks, one by Dr. K. V. Sreekanth, Scientist-II, IMRE, A-Star, Singapore on ‘Reconfigurable nanophotonics with chalcogenide phase change materials’ and another by Dr. Dmitry Vasilyev, Dept of Physics, Johannes Gutenberg-Universität Mainz, Germany on ‘Imaging Spin Detection in Microscopy and Spectroscopy’.List of ICCTPP-2022, Committee Members are available in this Pdf.

Broadband and ultrafast terahertz modulation with GeTe thin films

Chalcogenide phase change materials (PCMs) are gaining much interest for optoelectronic applications because of their capability to perform nonvolatile phase transition between the amorphous and crystalline states under Joule heat generated by applied electrical bias or optical excitation. Utilizing the high dielectric tunability of PCMs, we experimentally demonstrated the terahertz (THz) modulation with germanium telluride (GeTe) thin films. With THz time-domain spectroscopy (THz-TDS), we showed that the transmission of the 250-nm GeTe film can be tuned from 95% (amorphous) to 35% (crystallized) in the range of 0.1–1.2 THz. The high optical contrast of the GeTe film was also confirmed by Fourier transform infrared spectrometer and scattering-type scanning near-field optical microscope at higher frequencies. Ultrafast time-resolved optical pump–THz probe experiments were performed with 400-nm laser pulses. The amorphous and crystalline phases of GeTe exhibit different volatile THz modulation capabilities on the picosecond time scale and their decay times are 1.39 ps and 0.73 ps, respectively.

Varifocal Metalens Using Tunable and Ultralow-loss Dielectrics.

The field of flat optics that uses nanostructured, so-called metasurfaces, has seen remarkable progress over the last decade. Chalcogenide phase-change materials (PCMs) offer a promising platform for realizing reconfigurable metasurfaces, as their optical properties can be reversibly tuned. Yet, demonstrations of phase-change metalenses to date have employed material compositions such as Ge2 Sb2 Te5 , which show high absorption in the visible to near-IR wavelengths particularly in their crystalline state, limiting the applicability. Here, by using a low-loss PCM Sb2 Se3 , for the first time, active polarization-insensitive phase-change metalenses at near-IR wavelengths with comparable efficiencies in both material states are shown. An active metalens with a tunable focusing intensity of 95% and a focusing efficiency of 23% is demonstrated. A varifocal metalens is then demonstrated with a tunable focal length from 41 to 123µm with comparable focusing efficiency (5.7% and 3%). The ultralow-loss nature of the material introduces exciting new possibilities for optical communications, multi-depth imaging, beam steering, optical routing, and holography.

Nanocomposites of chalcogenide phase-change materials: from C-doping of thin films to advanced multilayers

Engineering of chalcogenide phase-change materials at the nanoscale is required to improve the performances of ultimate size memory devices and reduce their power consumption.

Enhanced stretchability towards a flexible and wearable reflective display coating using chalcogenide phase change materials.

Reflective color switching coatings based on chalcogenide phase change material (PCM) is becoming one of the most promising display technologies in the future. However, it is still a challenge to kindly control the stress and enhance the stretchability for flexible display coatings. Here, we report crack-reduced reflective color coatings on a flexible substrate by using buckling structure to regulate the distribution of vacancies in PCM. It significantly suppresses the formation of cracks and improves the robustness of optical and electrical properties during stretching of the display device, which opens the doors of opportunity for phase change display applications in a wide range of flexible and wearable display fields.