Qiong Bai

Graduation Semester and Year

2012

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

First Advisor

Qiming Zhang

Abstract

We have performed studies on n-type and p-type cuprous oxide with and without doping by first principles methods. Generalized Gradient Approximation (GGA) is used for the geometry optimization. The total energy and the final electronic structure calculations are conducted using the nonlocal screened Heyd-Scuseria-Ernzerhof (HSE) hybrid density functionals approach for cuprous oxide. The HSE hybrid density functionals overcome the shortcoming of GGA, which gives a much reliable value of the band gap. The formation energies of native point defects in undoped cuprous oxide have been studied by HSE hybrid functional theory in two conditions, which are vacuum based condition and solution based condition. Copper vacancy always has the lowest formation energy and provides p-type conductivity in both Cu-rich and O-rich conditions in vacuum based conditions. In the solution with low pH value, n-type cuprous oxide is achievable, where the antisite defect CuO is the dominant defect and responsible for the n-type conduction. Various elements are used to dope in cuprous oxide. In n-type doped cuprous oxide, certain halogen atoms, i.e., F, Cl and Br, are used to substitute the O atom. The metallic atoms, i.e., Ca, Mg, and Zn, are used to substitute Cu atom. The dopant in substitutional site has lower formation energy. Cl has the shallowest defect level among halogen atoms, which is favorable for electron to go to conduction band. In metallic atoms, Ca is the best candidate dopant with low formation energy and shallow transition level. The extra electron can be excited into bottom of conduction band. Also, the bottom of conduction band is dominated by 3s orbital of O atom, which is non-localized. The excited electron can move fast in 3s orbital and further improve the conductivity. For p-type doping to improve the properties of the p-type material, certain group V elements, such as N, P and As, are used to substitute an O atom. In a solution with pH=9, the formation energy of nitrogen substitutional defect is lower than that of copper vacancy, which means that nitrogen substitutional defect is a dominant defect and provides more free carriers, and further improves the conductivity of cuprous oxide. From the consideration of both the formation energy and the donor level, N is the best dopant among these three dopants. Finally, n-type doping in ZnO by Y has been studied. Oxygen vacancies, zinc interstitials, and zinc substitution by Y are considered here. We considered the formation energy of defects from two limit conditions, which are Zn-rich and O-rich. In Zn-rich condition, copper vacancy has the lowest formation energy. In O-rich condition, yttrium substitutional has the lowest formation energy and is a dominant defect, which will decrease the resistivity of ZnO.

Disciplines

Engineering | Materials Science and Engineering

Comments

Degree granted by The University of Texas at Arlington

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