Huajun Sun

Ultrafast Synaptic Events in a Chalcogenide Memristor

Compact and power-efficient plastic electronic synapses are of fundamental importance to overcoming the bottlenecks of developing a neuromorphic chip. Memristor is a strong contender among the various electronic synapses in existence today. However, the speeds of synaptic events are relatively slow in most attempts at emulating synapses due to the material-related mechanism. Here we revealed the intrinsic memristance of stoichiometric crystalline Ge2Sb2Te5 that originates from the charge trapping and releasing by the defects. The device resistance states, representing synaptic weights, were precisely modulated by 30 ns potentiating/depressing electrical pulses. We demonstrated four spike-timing-dependent plasticity (STDP) forms by applying programmed pre- and postsynaptic spiking pulse pairs in different time windows ranging from 50 ms down to 500 ns, the latter of which is 105 times faster than the speed of STDP in human brain. This study provides new opportunities for building ultrafast neuromorphic computing systems and surpassing Von Neumann architecture.

Activity-Dependent Synaptic Plasticity of a Chalcogenide Electronic Synapse for Neuromorphic Systems

Nanoscale inorganic electronic synapses or synaptic devices, which are capable of emulating the functions of biological synapses of brain neuronal systems, are regarded as the basic building blocks for beyond-Von Neumann computing architecture, combining information storage and processing. Here, we demonstrate a Ag/AgInSbTe/Ag structure for chalcogenide memristor-based electronic synapses. The memristive characteristics with reproducible gradual resistance tuning are utilised to mimic the activity-dependent synaptic plasticity that serves as the basis of memory and learning. Bidirectional long-term Hebbian plasticity modulation is implemented by the coactivity of pre- and postsynaptic spikes, and the sign and degree are affected by assorted factors including the temporal difference, spike rate and voltage. Moreover, synaptic saturation is observed to be an adjustment of Hebbian rules to stabilise the growth of synaptic weights. Our results may contribute to the development of highly functional plastic electronic synapses and the further construction of next-generation parallel neuromorphic computing architecture.

16 Boolean logics in three steps with two anti-serially connected memristors

Memristor based logic gates that can execute memory and logic operations are regarded as building blocks for non Von Neumann computation architecture. In this letter, Ta/GeTe/Ag memristors were fabricated and showed reproducible binary switches between high-resistance and low-resistance states. Utilizing a structure with two anti-serially connected memristors, we propose a logic operation methodology, based on which arbitrary Boolean logic can be realized in three steps, and the logic result can be nonvolatilely stored. A functionally complete logic operation: NAND is further verified by HSPICE simulation and experiments. The implementation of logic-in-memory unit may stimulate the development of future massive parallel computing.

Conducting mechanisms of forming-free TiW/Cu2O/Cu memristive devices

P-type Cu2O is a promising CMOS-compatible candidate to fabricate memristive devices for next-generation memory, logic and neuromorphic computing. In this letter, the microscopic switching and conducting mechanisms in TiW/Cu2O/Cu memristive devices have been thoroughly investigated. The bipolar resistive switching behaviors without an electro-forming process are ascribed to the formation and rupture of the conducting filaments composed of copper vacancies. In the low resistive state, the transport of electrons in the filaments follows Motts variable range hopping theory. When the devices switch back to high resistive state, the coexistence of Schottky emission at the Cu/Cu2O interface and electron hopping between the residual filaments is found to dominate the conducting process. Our results will contribute to the further understanding and optimization of p-type memristive materials.

Customized binary and multi-level HfO 2−x -based memristors tuned by oxidation conditions

The memristor is a promising candidate for the next generation non-volatile memory, especially based on HfO2−x, given its compatibility with advanced CMOS technologies. Although various resistive transitions were reported independently, customized binary and multi-level memristors in unified HfO2−x material have not been studied. Here we report Pt/HfO2−x/Ti memristors with double memristive modes, forming-free and low operation voltage, which were tuned by oxidation conditions of HfO2−x films. As O/Hf ratios of HfO2−x films increase, the forming voltages, SET voltages, and Roff/Ron windows increase regularly while their resistive transitions undergo from gradually to sharply in I/V sweep. Two memristors with typical resistive transitions were studied to customize binary and multi-level memristive modes, respectively. For binary mode, high-speed switching with 103 pulses (10 ns) and retention test at 85 °C (>104 s) were achieved. For multi-level mode, the 12-levels stable resistance states were confirmed by ongoing multi-window switching (ranging from 10 ns to 1 μs and completing 10 cycles of each pulse). Our customized binary and multi-level HfO2−x-based memristors show high-speed switching, multi-level storage and excellent stability, which can be separately applied to logic computing and neuromorphic computing, further suitable for in-memory computing chip when deposition atmosphere may be fine-tuned.

Conductance quantization in an AgInSbTe-based memristor at nanosecond scale

Quantized conductance was observed in a cation-migration-based memristor with the structure of Ag/AgInSbTe(AIST)/Ta. The conductance of the memristor exhibits stepwise increases in units of single quantum conductance (77.5 μS), which is attributed to the formation of a metal filament with an atomic contact of different integer multiples. We designed a high speed circuit to conduct the pulse measurement. The quantized conductance can be obtained by applying voltage pulses in intervals as fast as 3 ns with constant amplitude. Considering that the quantized conductance can be modulated by different pulse widths, our results suggest that the AIST-based memristor is a robust candidate for multi-level data storage and neuromorphic computing systems.

Model of dielectric breakdown in hafnia-based ferroelectric capacitors

Ultra-thin ferroelectric hafnia-based thin films are very promising candidates for nanoscale ferroelectric random access memories. However, dielectric breakdown is a main failure mechanism during repeated polarization switching. Generalizing Lou et al.s local phase decomposition model, originally for ferroelectric fatigue, we propose a dielectric breakdown model for ferroelectric hafnia. While charging injection during the polarization reversal is regarded as a key step, eventual phase separation of the Hf cluster accounts for the dielectric breakdown. Using this model, we explain why TaN/HfO2/TaN ferroelectric capacitors are more prone to dielectric breakdown than TiN/HfO2/TiN, and conclude that the lower Schottky barrier for the TaN/Pca21-HfO2 interface stabilizes neutral oxygen vacancies within the dielectric. On the other hand, when TiN electrodes are employed, oxygen vacancies tend to be positively charged. They can further pin the domain walls, resulting in ferroelectric fatigue. The relationship between the conductive filament formation, dielectric breakdown, wake up, and fatigue in ferroelectric HfO2 is discussed within the framework of our model.Ultra-thin ferroelectric hafnia-based thin films are very promising candidates for nanoscale ferroelectric random access memories. However, dielectric breakdown is a main failure mechanism during repeated polarization switching. Generalizing Lou et al.s local phase decomposition model, originally for ferroelectric fatigue, we propose a dielectric breakdown model for ferroelectric hafnia. While charging injection during the polarization reversal is regarded as a key step, eventual phase separation of the Hf cluster accounts for the dielectric breakdown. Using this model, we explain why TaN/HfO2/TaN ferroelectric capacitors are more prone to dielectric breakdown than TiN/HfO2/TiN, and conclude that the lower Schottky barrier for the TaN/Pca21-HfO2 interface stabilizes neutral oxygen vacancies within the dielectric. On the other hand, when TiN electrodes are employed, oxygen vacancies tend to be positively charged. They can further pin the domain walls, resulting in ferroelectric fatigue. The relationship b...

Programmable Frequency Multivibrator Based on Memristor

We proposed a new type of Multivibrator based on memristor. Its mechanism and oscillate conditions have been analyzed systematically.