Memristors are a leading candidate for future storage and neuromorphic computing technologies 1-10 due to characteristics such as device scalability, multi-state switching, fast switching speed, high switching endurance and CMOS compatibility 6,[11][12][13][14][15][16] . Most research and development efforts have been focused on improving device switching performance in optimal conditions, and the reliability of memristors in harsh environments such as at high temperature and on bending substrates has so far received much less attention. Since the programming processes in memristors based on traditional oxide materials mostly rely on ion moving and ionic valence changing 16,17 , the thermal instability at elevated temperatures could result in device failure 18 . Thus, to the best of our knowledge, there has been no reliable switching behaviours observed in memristors at temperatures above 200 °C 18,19 , which limits their potential application in harsh electronics such as those demanded in aerospace, military, automobile, geothermal, oil and gas industries. Common high temperature electronic materials, such as SiC and III-nitride 20,21 , are not adoptable in fabricating memristors, and therefore searching for new materials and structures for robust memristors with good performance is desirable.By stacking two-dimensional (2D) layered materials together 22-30 , van der Waals (vdW) heterostructures can combine the superior properties of each 2D component. 2D materials have shown excellent structural stability 31,32 and electrical properties, which could provide significant improvements in the robustness of electronic devices. For example, graphene possesses unparalleled breaking strength, and ultra-high thermal and chemical stabilities 33 ; molybdenum disulfide (MoS 2 ) has shown good flexibility, large Young's modulus (comparable to stainless steel), 34 and excellent thermal stability up to 1,100 °C 32 ; and various functionalized 2D material layers, or certain grain boundaries within 2D materials, have shown switching behaviours [35][36][37][38][39][40][41][42][43][44] . Since both the thickness and roughness of 2D layered materials can be controlled accurately at the atomic scale, the reliability and uniformity of the electronic devices based on such materials and their vdW heterostructures could also be optimized.In this Article, we report robust memristors based on a vdW heterostructure made of fully layered 2D materials (graphene/MoS 2−x O x / graphene), which exhibit repeatable bipolar resistive switching with endurance up to 10 7 and high thermal stability with an operating temperature of up to 340 °C. The MoS 2−x O x layer was found to be responsible for the high thermal stability of the devices by performing high temperature in situ high-resolution transmission electron microscopy (HRTEM) studies. Further in situ scanning transmission electron microscopy (STEM) investigations on the cross section of a functional device revealed a well-defined conduction channel and a switching mechanism based on the migr...
2 Impact ionization, which supports carrier multiplication, is promising for applications in single photon detection 1 and sharp threshold swing field effect devices 2 . However, initiating impact ionization of avalanche breakdown requires a high applied electric field in a long active region, hampering carrier-multiplication with high gain, low bias and superior noise performance 3, 4 . Here we report the observation of ballistic avalanche phenomena in sub-mean free path (MFP) scaled vertical InSe 5 /black phosphorus (BP) 6-9 heterostructures 10 . We use these heterojunctions to fabricate avalanche photodetectors (APD) and impact ionization transistors with sensitive mid-IR light detection (4 μm wavelength) and steep subthreshold swing (SS) (<0.25 mV/dec). The devices show a low avalanche threshold (<1 volt), low noise figure and distinctive density spectral shape. Our transport measurements suggest that the breakdown originates from a ballistic avalanche phenomenon, where the sub-MFP BP channel support the lattice impact-ionization by electrons and holes and the abrupt current amplification without scattering from the obstacles in a deterministic nature. Our results provide new strategies for the development of advanced photodetectors 1, 11, 12 via efficient carrier manipulation at the nanoscale.The upper panel of Fig. 1a shows the schematic of our heterostructure device with electrical connections. It consists of a thin γ-rhombohedral InSe/BP heterostructure connected to bottom and top metal electrodes on substrate. The detailed fabrication processes are presented in the Methods Section. During our measurements, we define the biased electrode connected to BP (~10 nm) as the drain and the grounded electrode connected to InSe (also ~10 nm) as the source. Considering that the lateral resistance of unstacked InSe is much smaller than that of the junction (Supplementary Section 1a), carriers also mainly transport vertically along the nanoscale InSe/BP channel. As a result, n-type InSe and p-type BP 13-15 form a vertical vdW heterojunction. The bottom panel of Fig. 1a 3 schematically shows the lattice structure at the junction interface. Notably, we assembled the InSe/BP junction in a glove box, resulting in a nearly perfect interface. In Fig. 1b, the high-resolution transmission electron microscope image verifies that the atomic stack is clean without the presence of any contamination or amorphous oxide even after all the device fabrication processes.We first characterized the transport properties of our heterostructure devices. With proper gate voltage 16 (10 V here, to ensure a necessary low doping level of BP and InSe, see below for details), the vertical vdW junction presents a standard rectification behaviour as a regular pn diode under moderate bias. In contrast, the reverse-biased current abruptly increases approximately 5 orders above a certain threshold voltage (~-4.8 V here), as shown in Fig. 1c. This "hard-knee" rapid change in current signals a typical avalanche breakdown 16, 17 resulting from impact ...
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