Preparation And Electrical Properties Of HfO₂/TiO₂ Bilayer Structure Resistive Random Access Memory

The widely used floating gate FLASH memory is facing challenge of low switching speed and high operating voltage.The thickness of tunneling oxide layer continue to decrease duo to CMOS scaling,resulting in charge storage failure in conventional charge based Flash.Recently,Resistive Random Access Memory?RRAM?has been extensively investigated due to its simple structure,low operating consumption,high density and fast switching speed.RRAM is considered as a promising candidate for the next generation of nonvolatile memory.Therefore,RRAM has attracted much interest of scientists and industrial companies.Resistive switching behavior has been reported in various materials.Transition metal oxides of high dielectric materials?TiO₂ and HfO₂?,which have the advantage of good thermal stability and wide band gap,demonstrated excellent performance in RRAM application and attracted the interest of many researchers.In this work,we focus on the preparation and resisitive switching properties of HfO₂/TiO₂ bi-layer RRAM device.Annealing treatment under N₂ and O₂ ambient were investigated in this work.It was found that the improved RS properties were obtained in N₂ annealing atmosphere.Repeatable resistive switching behaviors were obtained in both N₂ annealed and as prepared devices.Switching voltages became smaller and the distribution of switching voltages were more concentrated for N₂ annealed devices,compared with the as prepared devices.To clarify the effect of annealing treatment in various atmospheres on the devices,conduction mechanism,which is related to the RS properties was analyzed.The results showed that ohm conduction was the main mechanism in low resistance state?LRS?.While the space charge limited current?SCLC?was the dominant conduction mechanism in high resistance state?HRS?for the as prepared device;for the device annealed in N₂,the conduction mechanism was dominated by Pool-Frenkel emission.As Pool-Frenkel emission is a oxygen vacancies dominated conduction,so it can be induced that the conduction mechanism variation in N₂ was attributed to the increase of oxygen vacancies in the switching layer,which was the main reason for the improvement of RS characteristics.As a result,we confirmed that N₂ annealing treatment is an effective way to improve RS performance.Further more,we fabricated Pt/HfO₂/TiO₂/ITO RRAM device on PEN flexible substrate.The device array showed a light transmittance of about 80%at the wavelength of 500 nm and exhibited repeatable resistive switching behavior under bending state.Compared with the Pt/HfO₂/ITO memory device,the switching voltages?VSET and VRESET?became smaller for Pt/HfO₂/TiO₂/ITO device.Besides,the highly improved uniformity was achieved,which showed the coefficient of variation of VSET and VRESET to be 9.90%and 6.35%for bilayer structure RRAM.Multilevel storage capability can be successfully obtained by varying the stop voltage in the RESET process for bilayer HfO₂/TiO₂ RRAM.By the analysis of current conduction mechanism,we demonstrated that the multilevel high resistance state?HRS?was attributed to the increased barrier height with increasing the stop voltage.We prepared Pt/HfO₂/TiO₂/ITO RRAM device and studied the resistive switching properties in this paper.Excellent resistive switching performance was achieved and the resistive switching mechanism was analyzed.This work can provide reliable experimental evidence for designing and application of RRAM device.On the other hand,the results in this work also have some scientific value for understanding the resistive switching mechanism.