The morphology and dimension of the conductive filament formed in a memristive device are strongly influenced by the thickness of its switching medium layer. Aggressive scaling of this active layer thickness is critical toward reducing the operating current, voltage, and energy consumption in filamentary-type memristors. Previously, the thickness of this filament layer has been limited to above a few nanometers due to processing constraints, making it challenging to further suppress the on-state current and the switching voltage. Here, the formation of conductive filaments in a material medium with sub-nanometer thickness formed through the oxidation of atomically thin two-dimensional boron nitride is studied. The resulting memristive device exhibits sub-nanometer filamentary switching with sub-pA operation current and femtojoule per bit energy consumption. Furthermore, by confining the filament to the atomic scale, current switching characteristics are observed that are distinct from that in thicker medium due to the profoundly different atomic kinetics. The filament morphology in such an aggressively scaled memristive device is also theoretically explored. These ultralow energy devices are promising for realizing femtojoule and sub-femtojoule electronic computation, which can be attractive for applications in a wide range of electronics systems that desire ultralow power operation.

Recently, non-volatile resistance switching or memristor (equivalently, atomristor in atomic layers) effect was discovered in transitional metal dichalcogenides (TMD) vertical devices. Owing to the monolayer-thin transport and high crystalline quality, ON-state resistances below 10 Ω are achievable, making MoS2 atomristors suitable as energy-efficient radio-frequency (RF) switches. MoS2 RF switches afford zero-hold voltage, hence, zero-static power dissipation, overcoming the limitation of transistor and mechanical switches. Furthermore, MoS2 switches are fully electronic and can be integrated on arbitrary substrates unlike phase-change RF switches. High-frequency results reveal that a key figure of merit, the cutoff frequency (fc), is about 10 THz for sub-μm2 switches with favorable scaling that can afford fc above 100 THz for nanoscale devices, exceeding the performance of contemporary switches that suffer from an area-invariant scaling. These results indicate a new electronic application of TMDs as non-volatile switches for communication platforms, including mobile systems, low-power internet-of-things, and THz beam steering.

Excitatory and inhibitory postsynaptic potentials are the two fundamental categories of synaptic responses underlying the diverse functionalities of the mammalian nervous system. Recent advances in neuroscience have revealed the co-release of both glutamate and GABA neurotransmitters from a single axon terminal in neurons at the ventral tegmental area that can result in the reconfiguration of the postsynaptic potentials between excitatory and inhibitory effects. The ability to mimic such features of the biological synapses in semiconductor devices, which is lacking in the conventional field effect transistor-type and memristor-type artificial synaptic devices, can enhance the functionalities and versatility of neuromorphic electronic systems in performing tasks such as image recognition, learning, and cognition. Here, we demonstrate an artificial synaptic device concept, an ambipolar junction synaptic devices, which utilizes the tunable electronic properties of the heterojunction between two layered semiconductor materials black phosphorus and tin selenide to mimic the different states of the synaptic connection and, hence, realize the dynamic reconfigurability between excitatory and inhibitory postsynaptic effects. The resulting device relies only on the electrical biases at either the presynaptic or the postsynaptic terminal to facilitate such dynamic reconfigurability. It is distinctively different from the conventional heterosynaptic device in terms of both its operational characteristics and biological equivalence. Key properties of the synapses such as potentiation and depression and spike-timing-dependent plasticity are mimicked in the device for both the excitatory and inhibitory response modes. The device offers reconfiguration properties with the potential to enable useful functionalities in hardware-based artificial neural network.

IntroductionIn the past decade, the research community has seen intense research efforts in the field of two-dimensional (2D) materials. Many of the common 2D materials, such as graphene, hexagonal boron nitride (hBN) and molybdenum disulfide (MoS2), have relatively symmetrical 2D crystal lattices, resulting in mostly isotropic in-plane physical properties. The electrical, optical and phonon properties of these materials are similar along the different in-plane crystal directions of their 2D lattice. Having similar properties along different crystal orientations are favorable for many conventional electronic and photonic applications. For example, transistors built along different crystal directions on the wafer will have the same characteristics, which is beneficial for controlling the variability and uniformity of the device performance across a large wafer. On the other hand, there is a selected group of emerging 2D layered materials that possess highly asymmetrical in-plane crystal structure. Such layered materials include black phosphorus (BP) and its arsenic alloys, compounds with isoelectronic structure as BP including the monochalcogenides of group IV elements like Ge and Sn, as well as a class of low-symmetry transition metal dichalcogenide (LS-TMDC) materials such as rhenium disulfide (ReS2) and rhenium diselenide (ReSe2) (Figure 1, upper panel). Due to their reduced crystal symmetry, they have distinct electrical, optical, thermal and mechanical characteristics along different in-plane crystal directions. This new degree of freedom in their physical properties, together with other unique features arising from their low symmetry crystal lattice, can provide previously unexplored tunability on the characteristics of electrical, optical, thermal and mechanical devices (lower panels of Figure 1), hence opening the door to a wide range of opportunities for developing conceptually new semiconductor device applications (Figure 1, lower panel).In this review, we will discuss the recent progresses in studying the fundamental properties and novel device applications of low-symmetry 2D crystals.We will also offer our perspectives on the promising future research directions related to this emerging class of materials.Black Phosphorus is a low-symmetry layered material that has recently received much attention. Similar to graphite, bulk black phosphorus is an elemental layered material and is the most stable allotrope of phosphorus due to its unique puckered orthorhombic crystal structure. Similar to the carbon atoms in graphene, each phosphorus atom in BP is connected to three adjacent phosphorus atoms to form a stable, linked ring structure, with each ring consisting of six phosphorus atoms (Figure 1, upper panel). The adjacent layer spacing is around 0.53 nm and the lattice constant in this orthorhombic system along the z-direction is 1.05 nm. The crystal structure of BP belongs to space group Cmca (a=3.31, b=10.54, c=4.36). Within each layer of BP, the phosphorus atoms stay on two separate planes with shorter i...

In this work, we study the high critical breakdown field in β-Ga2O3 perpendicular to its (100) crystal plane using a β-Ga2O3/graphene vertical heterostructure. Measurements indicate a record breakdown field of 5.2 MV/cm perpendicular to the (100) plane that is significantly larger than the previously reported values on lateral β-Ga2O3 field-effect-transistors (FETs). This result is compared with the critical field typically measured within the (100) crystal plane, and the observed anisotropy is explained through a combined theoretical and experimental analysis.