Fabrication And Functionalization Of HfO_x-based Memristor

Over the past few decades,the continuous shrinkage of complementary metal oxide semiconductor（complementary metal oxide semiconductor,CMOS）transistors has contributed to exponential advances in computing and information technology.But today,as the working speed gap between processing units and memory units has increased dramatically,the movement of data between these units has become the most important factor affecting energy consumption and system throughput in traditional von Neumann architectures.So people began to study the new artificial neural network architecture and neuromorphic computation,memristor is an important part of them.In this paper,two kinds of memristors based on HfO_x and HfO_x:Ag single resistance variable layer were prepared by magnetron sputtering and lithography technology,which realized the"analog switch"and Forming-free characteristics.Two kinds of memristor synapses based on HfO_x:Ag/HfO_y and HfO_x:Ag/AlO_x double resistive layers have been fabricated to realize the synaptic behavior of memristors under continuous excitation and pulse excitation.The main research results are as follows:（1）Study the basic I-V,switch ratio,high and low resistance state current of Ag/HfO_x/p⁺⁺-Si device under different voltage stimulation,different thickness and different oxygen content in medium layer,results show that the Ag/HfO_x/p⁺⁺-Si device has gradually change characteristic in i-v curve,is a"nonvolatile analog switch".But the switch ratio is about 40,not large enough and accompanied with obvious Forming processes,such character is not very suitable for today’s synapses.By analyzing the test data of the Ag/HfO_x/p⁺⁺-Si memristor,the optimal working range is obtained.After that a improved method of doping Ag in the medium layer of memristor to eliminate Forming process is proposed.（2）Ag/HfO_x:Ag/p⁺⁺-Si memristor was fabricated to eliminate Forming process.According to the experimental results,the method of doping Ag enabled us to obtain Forming-free devices.However,due to the doping of Ag,the resistance of the dielectric layer decreased,and then the current of high resistance state increased,resulting in a significant reduction in the switching ratio of the device to about 3.Then an improved method of keeping Ag doped but adding the second resistance layer is proposed.（3）Keep the HfO_x:Ag as the first layer,add a second layer of HfO_y with high oxygen conten,we fabricate the Ag/HfO_x:Ag/HfO_y/p⁺⁺-Si device.Then the device basic I-V,switching ratio,high and low resistance state current are tested.It is found that the device can work stably with the addition of the second HfO_y resistance layer,and the switching ratio increases at about 100 due to the decrease of the high resistance current.Therefore,we run ests on the synaptic performance of the device,and the results show that the device proform good in unidirectional regulation,short-term plasticity and multistage memory behavior under continuous voltage stimulation,but the device has a large proportion of long-term memory decay to 42%,which is not very ideal.（4）In order to further optimize the device performance in switching ratio and memory ability,we use AlO_x as the second layer,fabricate the Ag/HfO_x:Ag/AlO_x/p⁺⁺-Si device and test the basic I-V,switch ratio,high and low resistance current of it.Found that after the change of the second layer,the switch ratio of the device is obviously raised to about 300-3000 and able to work in relative stable state.Therefore,we conducted a large number of tests on the synaptic performance of the device,and the results show that the device could have good unidirectional regulation,short-term plasticity and multilevel memory behavior,and the long-term memory decline of the device was only 10%.Furthermore,we also investigate the pulse amplitude plasticity,pulse width plasticity and pulse frequency plasticity of the device in pulse mode.