Oxygen Tuned Local Structure and Phase-Change Performance of Germanium Telluride

Ultrafast Temporal-Spatial Dynamics of Phase Transition in N-Doped Ge2Sb2Te5 Film Induced by Femtosecond Laser Pulse Irradiation

Element-doped phase change material (PCM) could improve the performances, e.g., better thermal stability, higher electrical resistance, and faster crystallization speed; thus, the influence of the doping element needs to be further investigated. In this paper, a femtosecond laser, which could realize the ultrafast phase transition rate of PCM between amorphization and crystallization, was used to explore the properties of nitrogen-doped Ge₂Sb₂Te₅ (GST), and a bond effect was proposed. The pure GST and different nitrogen contents of doped GST films were investigated by femtosecond laser pulse excitation through a pump-probe shadowgraph imaging technique. The results showed that the element-doped films could change photon absorption because of the increase in free carriers. This caused the faster rate of reflectivity to change in the irradiated area by the laser beam as the more nitrogen doped. When the nitrogen content increased, the crystallization evolution became harder because it enhanced the bond effect, which suppressed crystalline grain growth and improved the thermal stability. Based on the analysis in the paper, the desired performances of PCMs, e.g., ultrafast dynamics, crystallization evolution, and thermal stability, could be controlled according to the demands by modifying the bond effect.

Low Energy Switching of Phase Change Materials Using a 2D Thermal Boundary Layer

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 substrate 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 interface. 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.

Enabling Active Nanotechnologies by Phase Transition: From Electronics, Photonics to Thermotics

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.

Scaling and Confinement in Ultrathin Chalcogenide Films as Exemplified by GeTe

Chalcogenides such as GeTe, PbTe, Sb₂ Te₃ , and Bi₂ Se₃ are characterized by an unconventional combination of properties enabling a plethora of applications ranging from thermo-electrics to phase change materials, topological insulators, and photonic switches. Chalcogenides possess pronounced optical absorption, relatively low effective masses, reasonably high electron mobilities, soft bonds, large bond polarizabilities, and low thermal conductivities. These remarkable characteristics are linked to an unconventional bonding mechanism characterized by a competition between electron delocalization and electron localization. Confinement, that is, the reduction of the sample dimension as realized in thin films should alter this competition and modify chemical bonds and the resulting properties. Here, pronounced changes of optical and vibrational properties are demonstrated for crystalline films of GeTe, while amorphous films of GeTe show no similar thickness dependence. For crystalline films, this thickness dependence persists up to remarkably large thicknesses above 15 nm. X-ray diffraction and accompanying simulations employing density functional theory relate these changes to thickness dependent structural (Peierls) distortions, due to an increased electron localization between adjacent atoms upon reducing the film thickness. A thickness dependence and hence potential to modify film properties for all chalcogenide films with a similar bonding mechanism is expected.

Evaluating Ovonic Threshold Switching Materials with Topological Constraint Theory

The physical properties of ovonic threshold switching (OTS) materials are of great interest due to the use of OTS materials as selectors in cross-point array nonvolatile memory systems. Here, we show that the topological constraint theory (TCT) of chalcogenide glasses provides a robust framework to describe the physical properties of sputtered thin film OTS materials and electronic devices. Using the mean coordination number (MCN) of an OTS alloy as a comparative metric, we show that changes in data trends from several measurements are signatures of the transition from a floppy to a rigid glass network as described by TCT. This approach provides a means to optimize OTS selector materials for device applications using film-level measurements.

Two-Dimensional GeTe: Air Stability and Photocatalytic Performance for Hydrogen Evolution

As a key method to convert solar into chemical energy, photocatalytic water decomposition has garnered attention. Moreover, the development of graphene and graphene-like two-dimensional (2D) materials has brought fresh vitality in the field of photocatalysis. Here, we prepared two to four layers of GeTe nanosheets by ultrasonic-assisted liquid-phase exfoliation in argon and air, which we referred to as Ar-GeTe and O-GeTe, respectively. The photocatalytic hydrogen production potential of 2D GeTe was experimentally investigated for the first time. The results indicated that minimally layered GeTe samples are indirect-gap semiconductors with the GeTe band gap increasing after oxidation. All samples have suitable band positions that can drive photocatalytic water splitting into H₂ under mild conditions, providing maximum hydrogen evolution rates of 1.13 mmol g^-1 h^-1 (Ar-GeTe) and 0.54 mmol g^-1 h^-1 (O-GeTe). With density functional theory computations, the structural stability of GeTe in air was discussed, revealing that oxygen atoms could easily combine with Ge to form a more stable structure, thus impacting the photocatalytic performance of 2D GeTe. Therefore, the light requirement and oxygen deficiency of the material give an advantage in the field of energy supply in space.

Pressure-Induced Phase Transitions in Germanium Telluride: Raman Signatures of Anharmonicity and Oxidation

Pressure-induced phase transitions in GeTe, a prototype phase change material, have been studied to date with diffraction which is not sensitive to anharmonicity-induced dynamical effects. GeTe is also prone to surface oxidation which may compromise surface sensitive measurements. These factors could be responsible for the lack of clarity about the phases and transitions intervening in the phase diagram of GeTe. We have used high-pressure Raman scattering and ab initio pseudopotential density functional calculations to unambiguously establish the high-pressure phase diagram and identify three phases up to 57 GPa, a low-pressure rhombohedral phase, an intermediate pressure cubic phase, and a high-pressure orthorhombic phase. We detect substantial broadening and softening of Raman modes at low pressure and identify the transition regions and possible intermediate phases.

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

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

Shortening Nucleation Time to Enable Ultrafast Phase Transition in Zn1Sb7Te12 Pseudo-Binary Alloy

Zn₁Sb₇Te₁₂ thin films have been deposited by magnetron co-sputtering of ZnTe and Sb₂Te₃ targets. The microstructure, phase-change speed, optical cycling stability, and crystallization kinetics have been investigated during thermal annealing and laser irradiation. The thermal-annealed and laser-irradiated films give a clear evidence of the coexistence of trigonal Sb₂Te₃ and cubic ZnTe phases, which are homogeneously distributed in a single alloy as confirmed by advanced scanning transmission electron microscopy. The formation of both phases increases the initial nucleation sites, leading to the rapid phase-change speed in the Zn₁Sb₇Te₁₂ film. The film has a minimum crystallization time of ∼3 ns at 70 mW with almost no incubation period for the formation of critical nuclei compared to Ge₂Sb₂Te₅ and other Zn-based films. Moreover, the complete crystallization of Zn₁Sb₇Te₁₂ thin films is achieved within 10 ns. The ultrafast two-dimensional nucleation and crystal growth speed in Zn₁Sb₇Te₁₂ obtained from the laser-irradiated system is almost 7 times faster compared to that in Ge₂Sb₂Te₅ film. Controlling the crystallization process through doping ZnTe into Sb₂Te₃ is thus promising for the development of high-speed optical switching technology.

Presetting conductive pathway induced the switching uniformity evolution of a-SiNx:H resistive switching memory

Si-based resistive random access memory (RRAM) devices at the nanoscale with high uniformity have great potential applications in the future. We demonstrate that the uniformity evolution of the a-SiN_x:H RRAM at the low resistance state (LRS) and the high resistance state (HRS) can be clearly monitored by presetting a Si dangling bond (Si-DB) conductive pathway through thermal energy. It is found that the increased magnitude of uniformity for the LRS and the HRS are determined by the number of preset Si-DBs, which can be controlled by tuning thermal energy. As for LRS, the Si-DBs produced under the electric field along with the preset Si-DB conductive pathways form the main conductive pathway. Theoretical calculation of current-voltage (I-V) curves indicates that the Si-DB conductive pathways obey the trap-assisted tunneling model. In the HRS, the preset Si-DBs induced by thermal energy are the unique source of the conductive pathway. The transmission mechanism involves a trap-to-trap process by the hopping of electrons under a low electric field, Poole-Frenkel emission in the main region under the medium electric field and Fowler-Nordheim tunneling under the high electric field. Our discovery of the uniformity evolution for a-SiN_x:H RRAM device through presetting the Si-DB conductive pathway provides new insight into the resistive switching mechanism of next generation Si-based RRAM devices.

Sonication-assisted liquid-phase exfoliated α-GeTe: a two-dimensional material with high Fe3+ sensitivity

We describe sonication-assisted liquid-phase exfoliation of rhombohedral germanium telluride (α-GeTe) to obtain a good dispersion of α-GeTe nanosheets in ethanol. The thickness of the α-GeTe nanosheets is dependent on the exfoliation conditions, and few-layer α-GeTe nanosheets of 2-4 layers and even monolayer α-GeTe were obtained. We use first-principles calculations to investigate the structural, electronic, and optical properties of monolayer and bulk α-GeTe and compare the optical band gap of centrifugally fractionated α-GeTe nanosheet dispersions with the computational predictions. We demonstrate that few layer α-GeTe nanosheets are purified selectively through centrifugation, and they exhibit high sensitivity to Fe3+. The scalable production of two-dimensional α-GeTe nanosheets can be used in the future optoelectronic industry.

Forming-free performance of a-SiN x :H-based resistive switching memory obtained by oxygen plasma treatment

An a-SiN _x -based resistive random access memory (RRAM) device with a forming-free characteristic has significant potentials for the industrialization of the next-generation memories. We demonstrate that a forming-free a-SiN _x O _y RRAM device can be achieved by an oxygen plasma treatment of ultra-thin a-SiN _x :H films. Electron spin resonance spectroscopy reveals that Si dangling bonds with a high density (10¹⁹ cm^-3) are distributed in the initial state, which exist in the forms of Si₂N≡Si·, SiO₂≡Si·, O₃≡Si·, and N₃≡Si·. X-ray photoelectron spectroscopy and temperature-dependent current analyses reveal that the silicon dangling bonds induced by the oxygen plasma treatment and external electric field contribute to the low resistance state (LRS). For the high resistance state (HRS), the rupture of the silicon dangling bond pathway is attributed to the partial passivation of Si dangling bonds by H⁺ and O^2-. Both LRS and HRS transmissions obey the hopping conduction model. The proposed oxygen plasma treatment, introduced to generate a high density of Si dangling bonds in the SiN _x O _y :H films, provides a new approach to forming-free RRAM devices.

Sb2Te3 and Its Superlattices: Optimization by Statistical Design

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.

Realization of Functional Complete Stateful Boolean Logic in Memristive Crossbar

Nonvolatile stateful logic computing in memristors is a promising paradigm with which to realize the unity of information storage and processing in the same physical location that has shown great feasibility for breaking the von Neumann bottleneck in traditional computing architecture. How to reduce the computational complexity of memristor-based logic functions is a matter of concern. Here, based on a general logic expression, we proposed a method to implement the arbitrary logic of complete 16 Boolean logic in two steps with one memristor in the crossbar architecture. A representative functional complete NAND logic is successfully experimentally demonstrated in the filamentary Ag-AgGeTe-Ta memristors to prove the validity of our method. We believe our work may promote the development of the revolutionary logic in memory architectures.