Fabrication and Investigation of the Resistive Switching Characteristics of Doped HfO_x Films

Author:Guo Zuo Zuo

Supervisor:liu zheng tang tan zuo zuo


Degree Year:2017





Non-volatile memories have attracted more and more attentions with the rapid development of information technology,and the higher requirements for their properties have been proposed,such as good scalability,high density,and compatibility with complementary-metal-oxide-semiconductor(CMOS)technology.Accordingtothe prediction of Moore’s law,the device size would become smaller and smaller,and the device performance would become better.At present,Flash memory based on floating gate has been widely used in the market.However,with the decrease of device size,the leakage current would become larger,affecting the data retention property.Therefore,novel non-volatile memories are urgently required to replace Flash memory.Recently,much attention has been focused on resistance random access memory(RRAM),especially to binary-metal-oxide based RRAM due to its simple structure,easily controlled composition,fast speed and compatibility with CMOS process,which has been considered one of the most promising candidates for next non-volatile memory.Although RRAM has been studied for over 40 years,the uncertain switching mechanism and the random distribution of switching parameters are still issues that hinder the development and application of RRAM.To solve these problems,the doping technology and the first-principle calculation were performed in this work.The resistive switching characteristics and switching mechanisms of HfOx film and doped HfOx films were investigated.The main research contents and results are listed as follows:HfOx film was fabricated by radio frequency magnetron sputtering,and the effects of film thickness,annealing temperature,substrate temperature and substrate material on the structure,chemical composition and electric properties of HfOx film were investigated.The forming voltage increased with the increase of film thickness which showed weak influence on Set and Reset voltage,however.The film crystallinity enhanced as the annealing temperature or substrate temperature increased.While,the channels for leakage current were easily formed in crystalline film due to the existence of amounts of defects,which led to the poor reliability of HfOx sample.Compared with Si substrate,the switching voltage of the sample on Pt substrate was much smaller due to the lower interface barrier of Pt/HfOx.As a result,a 20-nm-thick HfOx film was prepared on Pt substrate at room temperature and then annealed at 200°C.The resistive switching behavior of HfOx sample was explained based on oxygen vacancy conductive filaments,and the poor uniformity and stability were related to the randomly distributed defects in the film.By modulating the doping concentration of dopants in HfOx film,the resistive switching behaviors and switching mechanisms of Cu/Cu(Ni):HfOx/Si(Pt),Cu/Ti(Al):HfOx/Pt,Cu/Ce(Gd):HfOx/Pt were systematically investigated.Cu and Ni dopants were easier to located on interstitial location of HfO2 structure.The easier migration of Cu and Ni led to the easier formation of conductive filaments and the decreased switching voltage.In addition,the higher doping concentration of Cu or Ni in HfOx film transformed the switching behavior from bipolar to unipolar.The switching mechanisms of bipolar and unipolar switching behavior were dominantly controlled by oxygen vacancy filaments and metallic filaments respectively.Ti and Al acting as the dopants can modulate the concentration and distribution of defects in HfOx film.The doping of Ti or Al in HfOx film can greatly reduce the formation energy of oxygen vacancy,especially oxygen vacancy near dopants according to the first-principle calculation.The increased oxygen vacancies in HfOx film were favored to decrease the switching voltages and the conductive paths formed by oxygen vacancy near dopants led to the improvement in uniformity of switching parameters.However,the higher doping concentration of Ti or Al resulted in excess oxygen vacancies in HfOx film,which increased the randomness of filament formation instead,finally leading to the degeneration of HfOx sample.The switching mechanism for Ti or Al doped HfOx sample can be attributed to the formation of oxygen vacancy filaments,which assisted the hopping transport of electrons.Although Ce or Gd doping decreased the formation energy of oxygen vacancies in HfOx film,the doping of rare element made O harder to escape from the lattice which was adverse to decrease the switching voltage.As a result,only appropriate doping concentration was favor to improve the switching performance of HfOx film.All samples exhibited good retention property,indicating the non-volatile characteristic of HfOx-based RRAM.The multilevel behavior of 6%Cu-doped HfOx film was studied.Cu-doped HfOx samples can show multi-resistance in low resistance state by setting different current compliance,indicating the multilevel behavior which was beneficial to enhancing the storage density.The lower Reset current can be observed under the smaller current compliance,which was conductive to decrease the device consumption.In addition,the samples exhibited good stability over hundreds of switching cycles and the multi-resistances can be clearly distinguished.The metal ions and oxygen vacancies were both responsible for the multilevel behavior.The comparison of resistive switching characteristics of Al buffer layer-inserted and Al-doped HfOx samples was performed.The introduction of Al buffer layer caused the formation of interfacial layer at HfOx/Al interface,which increased oxygen vacancy concentrations near interface and promoted the formation of conductive filaments.The occurrence of interfacial layer affected the switching voltage and resistances.Besides,the enhanced electric field resulted in the abrupt Reset process,leading to the scattered distribution of Reset voltage.By comparison,Al-doped HfOx sample showed smaller switching voltage and good uniform of switching parameters.