Planar peristrophic multiplexing metasurfaces

As a promising counterpart of two-dimensional metamaterials, metasurfaces enable to arbitrarily control the wavefront of light at subwavelength scale and hold promise for planar holography and applicable multiplexing devices. Nevertheless, the degrees of freedom (DoF) to orthogonally multiplex data have been almost exhausted. Compared with state-of-the-art methods that extensively employ the orthogonal basis such as wavelength, polarization or orbital angular momentum, we propose an unprecedented method of peristrophic multiplexing by combining the spatial frequency orthogonality with the subwavelength detour phase principle. The orthogonal relationship between the spatial frequency of incident light and the locally shifted building blocks of metasurfaces can be regarded as an additional DoF. We experimentally demonstrate the viability of the multiplexed holograms. Moreover, this newly-explored orthogonality is compatible with conventional DoFs. Our findings will contribute to the development of multiplexing metasurfaces and provide a novel solution to nanophotonics, such as large-capacity chip-scale devices and highly integrated communication

Low Thermal Conductivity Phase Change Memory Superlattices

Phase change memory devices are typically reset by melt-quenching a material to radically lower its electrical conductance. The high power and concomitantly high current density required to reset phase change materials is the major issue that limits the access times of 3D phase change memory architectures. Phase change superlattices were developed to lower the reset energy by confining the phase transition to the interface between two different phase change materials. However, the high thermal conductivity of the superlattices means that heat is poorly confined within the phase change material, and most of the thermal energy is wasted to the surrounding materials. Here, we identified Ti as a useful dopant for substantially lowering the thermal conductivity of Sb2Te3-GeTe superlattices whilst also stabilising the layered structure from unwanted disordering. We demonstrate via laser heating that lowering the thermal conductivity by doping the Sb2Te3 layers with Ti halves the switching energy compared to superlattices that only use interfacial phase change transitions and strain engineering. The thermally optimized superlattice has (0 0 l) crystallographic orientation yet a thermal conductivity of just 0.25 W/m.K in the "on" (set) state. Prototype phase change memory devices that incorporate this Ti-doped superlattice switch faster and and at a substantially lower voltage than the undoped superlattice. During switching the Ti-doped Sb2Te3 layers remain stable within the superlattice and only the Ge atoms are active and undergo interfacial phase transitions. In conclusion, we show the potential of thermally optimised Sb2Te3-GeTe superlattices for a new generation of energy-efficient electrical and optical phase change memory.Comment: 4 Figures, 7 Supplementary Figures, 27 pages including a supplemen

Synaptic modulation of conductivity and magnetism in a CoPt-based electrochemical transistor

Among various neuromorphic devices for artificial intelligence, the electrochemical transistor, in which the channel conductance can be modulated by the insertion of ions according to the history of gate voltage across the electrolyte, emerges as an efficient one. Despite the striking progress in exploring novel channel materials, few studies report the ferromagnetic metal-based synaptic transistors, limiting their application in synaptic spintronics. Here, we present synaptic modulation of both conductivity as well as magnetism based on the three-terminal electrochemical transistor with a channel of ferromagnetic CoPt alloy. The CoPt metal channel exhibits perpendicular magnetization and anomalous Hall effect. Then, we demonstrated its essential synaptic functionalities, including depression and potentiation of synaptic weight as well as paired-pulse facilitation. Additionally, we are also able to switch the short-term to long-term plasticity transition using different gate parameters, such as amplitude, duration, and frequency. Last, the device presents multilevel reversible and nonvolatile states of conductivity and magnetic coercivity (HC), both of which exhibit satisfying retention behaviours. The results provide a platform to construct future spin-based synaptic devices.Submitted/Accepted versionThe authors acknowledge funding from the National Research Foundation (NRF), Singapore under its 21st Competitive Research Programs (CRP grant no. NRF-CRP21-2018-0003). X.R.W. acknowledges support from Academic Research Fund Tier 2 (grant no. MOE-T2EP50120-0006) from Singapore Ministry of Education and the Agency for Science, Technology and Research (A*STAR) under its AME IRG grant (project no. A20E5c0094). S. L acknowledges research scholarship from CRP grant

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

Gold-free contacts on Al x Ga 1-x N/GaN high electron mobility transistor structure grown on a 200-mm diameter Si(111) substrate

The authors report on the fabrication and characterization of low-temperature processed gold-free Ohmic contacts for AlxGa1−xN/GaN high electron mobility transistors (HEMTs). The HEMT structure grown on a 200-mm diameter Si(111) substrate is used in this study. Using the Ti/Al/NiV metal stack scheme, the source/drain Ohmic contact optimization is accomplished through the variation of Ti/Al thickness ratio and thermal annealing conditions. For an optimized Ti/Al stack thickness (20/200 nm) annealed at 500 °C for 30 s with smooth contact surface morphology, a specific contact resistivity of ∼6.3 × 10−6 Ω cm2 is achieved. Furthermore, with gold-free Ni/Al gates, the fabricated HEMTs exhibit ION/IOFF ratio of ∼109 and a subthreshold swing of ∼71 mV/dec. The demonstrated gold-free contact schemes thus provide a solution toward the implementation of GaN-based HEMT process on a Si foundry platform.ASTAR (Agency for Sci., Tech. and Research, S’pore)Published versio

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

Aqueous synthesis, doping, and processing of n-type Ag₂Se for high thermoelectric performance at near-room-temperature

Herein, we have successfully synthesized binary Ag2Se, composite Ag0:Ag2Se, and ternary Cu+:Ag2Se through an ambient aqueous-solution-based approach in a one-pot reaction at room temperature and atmospheric pressure without involving high-temperature heating, multiple-processes treatment, and organic solvents/surfactants. Effective controllability over phases and compositions/components are demonstrated with feasibility for large-scale production through an exquisite alteration in reaction parameters especially pH for enhancing and understanding thermoelectric properties. Thermoelectric ZT reaches 0.8-1.1 at near-room-temperature for n-type Ag2Se and Cu+ doping further improves to 0.9-1.2 over a temperature range of 300-393 K, which is the largest compared to that reported by wet chemistry methods. This improvement is related to the enhanced electrical conductivity and the suppressed thermal conductivity due to the incorporation of Cu+ into the lattice of Ag2Se at very low concentrations (x%Cu+:Ag2Se, x = 1.0, 1.5, and 2.0).Agency for Science, Technology and Research (A*STAR)The authors acknowledge financial support from the A*STAR SERC PHAROS program under grant number 1527200023

High Aspect Subdiffraction-Limit Photolithography via a Silver Superlens

Photolithography is the technology of choice for mass patterning in semiconductor and data storage industries. Superlenses have demonstrated the capability of subdiffraction-limit imaging and been envisioned as a promising technology for potential nanophotolithography. Unfortunately, subdiffraction-limit patterns generated by current superlenses exhibited poor profile depth far below the requirement for photolithography. Here, we report an experimental demonstration of sub-50 nm resolution nanophotolithography via a smooth silver superlens with a high aspect profile of ∼45 nm, as well as grayscale subdiffraction-limit three-dimensional nanopatterning. Theoretical analysis and simulation show that smooth interfaces play a critical role. Superlens-based lithography can be integrated with conventional UV photolithography systems to endow them with the capability of nanophotolithography, which could provide a cost-effective approach for large scale and rapid nanopatterning

Strong (110) Texturing and Heteroepitaxial Growth of Thin Mo Films on MoS₂ Monolayer

Growth of textured and low-resistivity metallic seed layers for AlN-based piezoelectric films is of high importance for bulk acoustic wave resonator applications. Through optimization of Mo physical vapor deposition parameters, namely, the Ar flow rate, strong (110) texturing and low electrical resistivities (∼3 × 10–7 Ω m) were observed for 43 ± 3 nm thick Mo films on a CVD-grown MoS2 monolayer on c-Al2O3(0001) substrates. The strong texturing was attributed to the growth template effect of the monolayer MoS2 due to the presence of a local epitaxial relationship between (110)-Mo and (0001)-MoS2 (i.e., through MoS2(0001)[112̅0]||Mo(110)[1̅11] and/or MoS2(0001)­[112̅0]||Mo(110)[001]), coupled with an atomic-scale flatness of the MoS2 surface, which promotes layer-by-layer growth of the Mo film. The deposited Mo/MoS2 monolayer stack can also be easily peeled-off from the growth Al2O3(0001) substrate for possible subsequent transfers onto arbitrary substrates (e.g., SiO2/Si(001)) due to a weak van der Waals coupling at the MoS2 and Al2O3(0001) interface, facilitating vertical stacking strategies for monolithic integration of high quality and therefore high-performance, AlN-based piezoelectric devices and sensors on the Si platform