Effect Of Bending On Resistive Switching Of Flexible HfO₂-based Thin Films

With the proposal of concept of flexible electronic,people are then paying more attention to research of future flexible electronic products,such as wearable device,flexible solar cell and e-skins.As for memory devices,the development of flash memory has been limited because of the physical size limit about the 22nm process.Therefore,researching about nano-size resistive random access memory(RRAM)has become the hot topic in the international.However,how to deposited RRAM on flexible substrate so that it not only remain the high-performance of inorganic materials but also has the good ductility.Thus,in this work,the HfO2 nanoparticles,Li-induced HfO2 and laminated structure were prepared using hydrothermal-annealed method,combining with the Finite element method(FEM)and First-principles Calculations to research the bending effect on the resistive switching of flexible HfO2-based thin films.The specific contents of the dissertation are as follows:First,the HfO2 nanoparticles were prepared by hydrothermal-annealed method correspond to monoclinic phase with good crystallinity and particle size of 7-18 nm.The nanoparticles obtained by hydrothermal-annealed growth at 500℃ have the relatively highest crystallinity and best resistive switching character.The switching mechanism of HfO2 can be ascribed to the formation/rupture of filamentary paths due to the field-induced migration of oxygen vacancies and oxygen ions.Subsequently,the optimum annealed temperature was selected to prepare Li-doped HfO2 nanoparticles.It is found that low-cost Li element doping in the form of Hf could effectively improve the oxygen vacancy concentration in the device and enhance the formation of oxygen vacancy filaments.At the same time,the optimal HfO2 nanoparticles were selected for bending tests with different bending radius(4-10mm).It was found that the ON/OFF ratio of the film decreased significantly with the decrease of bending radius,and the threshold voltage was increased.The bending-induced micro-crack is responsible for the incomplete rupture of the conducting filaments,which made the cooperating with filament and ohm contact.Secondly,the effect of film sequence on the resistance characteristics of devices is studied.It is found that the HfO2/NiO structure has good electronic characteristics because of the low barrier height.Compared with single layer devices,the rupture of oxygen vacancy filaments are concentrated near the heterojunction due to the addition of NiO functional layer,which improving the stability of devices.The overall barrier height of TiO2/HfO2 structure is slightly increased,and the device still has good electrical characteristics.NiO/HfO2/TiO2 structure is dominated by TCLC process due to the introduction of too many interfaces.The interfacial barrier increases and the switching character become worse.At the same time,Li-HfO2/NiO structure was constructed,which exhibits higher ON/OFF ratio than the laminated structure.It was found that Li-doped HfO2 has n-type semiconductor characteristics and constructs p-n interface with p-type NiO.Finally,the HfO2/NiO and Li-HfO2/NiO structures were selected for bending tests under 10mm bending radius.It was found that the electronic character of NiO/HfO2 devices decreased significantly after 3,000 bending times,while the NiO/Li-HfO2 devices could be maintained until 5,000 times.At the same time,with the increase of bending times,the transport characteristics of the device change from SCLC to co-operate with SCLC and ohmic conduction.Finite element analysis shows that when the bending radius decreases to 10 mm,the cracks begin to sprout in the middle of the functional layer,and with the reduction of the bending radius,the cracks were gradually expand and cut off the branches of oxygen vacancy filaments.When the bending radius decreases to 5 mm,the crack grows into a penetrating crack in the functional layer,which seriously degrades the resistance characteristics of the device.In conclusion,the HfO2-based devices mediated by doping and film sequence were still have repeatable resistance switching characteristics after bending test,although the micro-cracks caused by bending can degrade the resistance characteristics of the devices.This work has reference value for inorganic flexible resistive memory devices.