Preparation And Properties Of ZnO-based Self-rectifying Memristors

Memristor,a typical two terminal device,has received much attention.It shows rapid switching between different resistance states.Moreover,its fabrication process is compatible with CMOS technique.Therefore,it is promising for potential applications in resistance random access memories and artificial neural networks.To achieve an ultra-high density integration,memristors are often fabricated with a crossbar structure.However,the crosstalk problem in the circuit has become increasingly prominent.To prevent the crosstalk,a rectifier such as a diode(or transistor)is always used,which is not beneficial for device scaling and ultr-high density integration.Therefore,a realization of self-rectifying memristors that do not require external rectifiers is extremely important for practical applications.In this thesis,ZnO thin films were used as the memristive material.First,the rectifying and memristive behaviors of a series connection structure of a diode(D)and a memristor(R)were investigated.Second,the memristive behaviors of as-fabricated self-rectifying memristors were studied in detail.The main contents and results are as follows.(1)Preparation and properties of 1D1 R structures.Polycrystalline Zn O thin films and aluminum doped ZnO(AZO)thin films were prepared by magnetron sputtering.The influences of sputtering power,sputtering time,and argon to oxygen ratio(Ar:O2)on the electrical properties of ZnO and AZO films were investigated.It is found that the ZnO films deposited at the argon to oxygen ratio of 8:2 show the best resistive switching behavior.The 1D1 R structure consisting of a Pt/AZO/ZnO/Pt diode(D)and a Pt/ZnO/Pt memristor(R)exhibits unipolar resistive switching behavior and clear inhibition of negative current.However,the preparation process of 1D1 R structures is very complex.Furthermore,the electrical behaviors of such 1D1 R structures are not stable.(2)Preparation and properties of Ti/ZnO/Pt self-rectifying memristors.Due to the difference between work function of Pt and electron affinity of ZnO,a Schottky barrier is always formed at Pt/ZnO interface.In this thesis,Pt/ZnO/Pt structure is rapidly annealed in Ar atmosphere and it is found that quasi-ohmic contact can be achieved between Pt and ZnO due to an increase of the oxygen vacancy concentration in ZnO.Similarly,for the Ti/ZnO/Pt structure,the oxygen vacancy concentration in ZnO is increased due to the formation of a TiOx layer at the Ti/ZnO interface.Therefore,a quasi-ohmic contact between Pt and ZnO is formed.Furthermore,a Schottky barrier is formed at the Ti/Zn O interface due to the reaction between the adsorbed moisture and the naturally formed TiOx layer.However,after the Forming process,the self-rectifying effect of Ti/ZnO/Pt disappears or significantly weakens.Therefore,such a memristor can not solve the crosstalk problem in the circuit.(3)Preparation and properties of Ti/ZnO/AZO/Pt self-rectifying memristors.The self-rectifying memristors based on Ti/ZnO/AZO/Pt structures were fabricated by optimizing the preparation parameters.The self-rectifying memristors exhibit forming-free and nonvolatile bipolar resistive switching characteristics and excellent endurance behavior as well.By analyzing the I-V curves of the memristors and the temperature dependence of electrical properties,it is found that the resistive switching behavior of Ti/ZnO/AZO/Pt devices originates from a purely electronic effect.The conduction is controlled by the space charge limiting current or Schottky emission mechanism.Furthermore,based on Ti/ZnO/AZO/Pt memristors,the long term plasticity and learning-forgetting-relearning processes were implemented.The results show that Ti/Zn O/AZO/Pt self-rectifying memristors can solve the crosstalk current problem in the circuit.Therefore,such memristors are promising for potential applications in resistance random access memories and neural synaptic devices.