Theoretical Investigation Of Doping Effects On HfO₂-Based Resistive Random Access Memory

Resistive random access memory(RRAM)has been considered to be the most promising candidate for next generation non-volatile memory due to its non-volatile character,low consumption,high density and high switching speed.Hafnium oxide,which is a high-k material with perfect compatibility with CMOS,stands out to be one of the best candidates among the resistive switching materials.To optimize the storage parameters of HfO2-based RRAM,doping is generally considered as an effective and convenient method.Although many doping elements have been adopted to improve the performance of HfO2-based RRAM,the doping rules still remain ambiguous,which may lead to blind experiments.The first-principles calculations can not only predict the doping effects,but also unravel the resistive switching mechanism from microscopic view and provide valuable clues for experiments.Firstly,the electron transport mechanism was studied in doped HfO2-based RRAM.I-V characteristics can be deduced to compare with experiments by solving the simultaneous equations of the Poisson equation and Fowler-Northeim emission equations after obtaining the activation energies of defects from first-principles calculations.Moreover,one can extract the carrier transport path in a conductive filament(CF)with dopant based on variable-range hopping theory that clarifies the different resistance between every two defects.It is explained that the carrier transport mechanism in conductive filaments is based on the trap-assisted tunneling model.The thesis main studied the doping effects on the band structure of HfO2 and the defect energy level of oxygen vacancy.Firstly,the band structure and density of states were calculated for 96 atoms HfO2 supercell that contains 3-fold(VO3)and 4-fold(VO4)oxygen vacancy,respectively.The V03 and V04 defect energy level can be located and the partial density of states shows that both VO3 and VO4 energy level consist of O 2p and Hf 5d electrons.Then six doping elements that have different numbers of valence electrons were chosen(Hf-like and Hf-unlike dopants).The doping effects on VO3 and VO4 energy level were investigated individually for the six dopants and the contributions of orbital electrons were analyzed for defect state.It is predicted that the kindred dopants may have the same effects on HfO2.Herein,we obtained comprehensive doping guidelines by studying the doping effects on the activation energy and formation energy of oxygen vacancy,combined with experiments of doped HfO2-RRAM in literatures.The results show that,Hf-like dopants can achieve low activation energy and high formation energy of oxygen vacancy,while Hf-unlike dopants may increase activation energy and reduce formation energy of oxygen vacancy.As compared with the experimental data from literatures,it is concluded that Hf-like dopants will improve the uniformity of switching parameters,decrease the SET/RESET voltage,deteriorate the ON/OFF ratio and increase forming voltage.For Hf-unlike dopants,it will enhance ON/OFF ratio,lower forming voltage and SET/RESET voltage,while the uniformity of parameters becomes worse.The thesis furnished optimum doping guide lines for HfO2-RRAM that may achieve high ON/OFF ratio,high uniformity and low switching voltages.The model provided a theoretical basis for studying the doping effects on metal-oxide based RRAM.