Interfacial Coupling And Memristive Behavior Modulating Of Homogeneous Hafnium Oxide Bilayer

The next-generation processor,namely neuromorphic computing chip,is required to be high parallel processing capability,high error-tolerant rate and high memory density,like human brain,for meeting the demand of artificial intelligence deveploment.Using novel materials and devices to emulate synapses and neurons is a bottom-up and fundamental strategy.For this reason,we put forward a structure of homogenous hafnium oxide bilayer,constructed synapse-like memristors,investigated the effect of interfacial coupling on formation and development of conductive filaments,and explored their memristive behaviors.The main achievements are as follows:The memristor with a structure of W/Hf O_y/Hf O_x/Pt?x<y?was fabricated for mimicking chemical synapse morphologically.The conductance of the device shows a negative potentiation and positive inhabitation during the period of both voltage sweeping and voltage pulse testing.The memristor then can emulate diverse synaptic plasticity,such as paired-pulse facilitation?PPF?,paired-pulse depression?PPD?,excitatory postsynaptic current?EPSC?,inhibitory postsynaptic currents?IPSC?,spike-rate dependent plasticity?SRDP?,experience dependent plasticity,spike-time dependent plasticity?STDP?and metaplasticity,when programed voltage pulses was applied.Moreover,the energy consumption of the memristor is ultra-low?5-7.5 f J?under each voltage pulse echo,resembling that of synapse.The microstructural characteristic results of such a device shows that after the electrical forming process the amorphous hafnium oxide in bilayer electrolyte transformed into crystalline,and the distribution of oxygen vacancy?V_O?along the bilayer structure is hump shape which duplicates the distribution of Ca²⁺in the chemical synapse.During the process of increasing the conductance?synaptic weight?of the device by applying consecutive electrical stimulations,the crystalline hafnium oxide appeared a phase transformation from orthorhombic phase to monoclinic phase,and grains in hafnium bilayer tended to align orderly.When the device was in high resistive state,the orientation of the grain became disordered again.Therefore,the process of conductivity increase is the process of stress-induced adaptive phase transformation and grain alignment caused by migration of V_O in the bilayer.Such a process is similar to the operation of synaptic Ca²⁺gate protein,and thus making our device more like the real synapse from four perspectives of microstructure,kinetics of micro-ionic transportation,synaptic plasticity obtained from output responses and energy consumption of each pulse event.Our device is an extremely potential and feasible unit of neuromorphic computing.Later,we fabricated not only a unidirectional threshold switching memristor Ag/Hf O_y/Hf O_x/Pt with low set voltage??0.28 V?,the sharpest switching slop??0.6m V/dec?and wide range of operation current?1 n A to 300?A?,found that the connection strength of Ag conductive filaments can be disturbed by stress-induced grain rotation,constructed the coupling model of grain rotation and evolution of Ag conductive filements in the electrolyte,which provided a supplementary of microstructural change for the mechanism of threshold switching,and achieved functions of nano-electronic logic gate circuit based on the device.Moreover,we also obtained forming-free Ag/Hf O_y/Hf O_x/Ag memristor with gradual threshold switching behavior,emulated short-term synaptic plasticity,and preliminarily analyzed the resemblance among short range of Ag migration,phase transformation and synaptic micro-dynamics in the hafnium oxide bilayer.