Computational Study of O Vacancy and La Dopant in HfO2 and their Effect on the Adsorbed Graphene Layer
Date Issued
2014
Date
2014
Author(s)
Chiu, I-Hsuan
Abstract
In this study, we performed first principles calculations in conjunction with classical force field method and molecular dynamics simulations to investigate the fundamental properties of oxygen vacancy and La dopants in HfO2 as well as their effect on the electronic properties of the adsorbed graphene layer.
In the first part of the thesis, we present our newly-developed atomistic potential model for the cubic and tetragonal phases of HfO2. This new potential model was developed based on the ab initio calculated database, including the structural, mechanical, and dielectric properties of the cubic and tetragonal phases of HfO2, respectively. The reliability and transferability of this newly-developed model for HfO2 has been examined through a series of validations, such as the relative phase stability between c- and t-HfO2, the phase transition temperature, and the migration energy barrier of O vacancy in HfO2. Our calculated results show that all the predictions are in satisfactory agreement with the first-principles predicted properties of the cubic and tetragonal phases of HfO2.
In the second part of the thesis, we performed first principles calculations to investigate the effect of La doping on the structural, electronic and dielectric properties of crystalline HfO2. Our calculated results show that the thermodynamic stability of tetragonal HfO2 relative to its monoclinic phase can be enhanced via doping with the substitutional La atoms. Our calculations also show that the elastic moduli of HfO2 can be lowered by La doping, which could be ascribed to the lowering of the symmetry and the associated formation of O vacancy in the HfO2 matrix. Moreover, this softening of material matrix also indicates that the possibility of the stress-induced stabilization of the tetragonal phase of HfO2 can be enhanced via La doping, providing a physical origin for the appearance of t-HfO2 during high temperature annealing in the experiments. On the other hand, our calculated results also show that the substitutional La doping can increase the dielectric constant of HfO2 while still maintaining sufficient electronic band gap, which was found to be mainly attributed to the reduction of the characteristic force constant of the dielectric material system.
In the third part of the thesis, we performed first principles study on the electronic properties of graphene monolayer on the HfO2 substrate. Our main research interest here is to understand why the HfO2 substrate can significantly degrade the transport properties of the adsorbed graphene layer as revealed in the experiments. Our calculated results show that graphene monolayer is bounded to a perfect HfO2 surface via the van der Waals interaction with a binding energy of around 25~40% larger than that on the SiO2 substrate. The band gap opening at the Dirac point was found to be comparable to that on a silanol SiO2 surface, but the induced charge accumulation at the graphene/HfO2 interface is at least one order of magnitude larger than that between graphene and the silanol surface. Moreover, when graphene monolayer was placed on top of the substrate containing an O vacancy, the adsorbed graphene layer becomes n-type doped primarily due to the charge transfer from the O vacancy site in the HfO2 substrate. Our results further show that the strong interaction between O vacancy and the graphene layer not only can result in a relatively larger band gap opening at the Dirac point but also can degrade the linear band dispersion in the band structure of monolayer graphene. More interestingly, our calculations show that the O vacancy induced n-type doping on graphene can be reduced or eventually turn into p-type doping when there are water molecules adsorbed on the HfO2 surface. In addition, our calculations also suggest that La doping in HfO2 can be an effective approach towards improving the transport properties of the adsorbed graphene layer on the HfO2 substrate.
Subjects
原子力場模型
第一原理計算
二氧化鉿
鑭摻雜二氧化鉿
石墨烯
Type
thesis
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