Principle Studies On Properties Of Oxygen Vacancy Of HfO₂in Different Phases

RRAM, as its low power, fast operation high endurance andsome other advantages, has become one of the most important candidatesof next generation non-volatile memory. HfO2is not only an importantbinary transition metal oxides material in semiconductor industry, but alsoused as resistive switching material of RRAM.In this thesis, the first principle calculations were performedon these three phases HfO2with CASTEP code in Material Studio to studythe oxygen vacancy’s behaviors in HfO2, and predict their influence toRRAM. Mono-crystalline HfO2has three phases: monoclinic(c-), cubic(c-),and tetragonal(t-). We have calculated the band structure and DOS of bothpure and VO-contained HfO2in different phases. In all these three phases,the VOwill induce trap states in forbidden band, and electron can hopbetween the trap states. When a great deal of VOconcentrated, these trap states provide a path for electron to pass, and make the HfO2become lowresistive states. We have calculated the formation energy of VO, and foundthat the formation energy is drastically lower in c-, t-HfO2(7.56eV and6.47eV respectively) than that in m-HfO2(12.44eV for V3,12.24eV for V4),which means the electroforming will be easier in these two phases. Wehave calculated the migration barrier of VO, and found that the migrationbarrier is much lower in c-(1.20eV) and t-HfO2(0.93eV and1.35eV) thanthat in m-HfO2(2.53eV,2.67eV,3.98eV). The result means that in thesetwo phases, the resistive switching is faster, but the data retention isweaker.We also studied the Y-doping effect on resistive switchingproperties of HfO2in different phases. When doped Y in-HfO2, the defectlevel will move to conduction band, in c-HfO2, it even touches theconduction band,, which indicates metal-like conduction mechanism.Meanwhile, Y-doping will drastically decrease the formation energy andmigration barrier of VOin all three phases HfO2. Y-doping will also makeHfO2prefer the c-, t-Phases, which has lower VOformation energy andmigration barrier than m-phase. This means that Y-doping have doubleeffects to device performance of RRAM.