Date of Completion

12-13-2019

Embargo Period

1-14-2020

Keywords

ZnO, ZnMgO, Nanowires, MOCVD, Hydrothermal, Characterization, Growth, Thin Film

Major Advisor

Dr. Mehdi Anwar

Associate Advisor

Dr, Necmi Biyiki

Associate Advisor

Dr Helena Silva

Associate Advisor

Dr Faquir Jain

Field of Study

Electrical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Zinc oxide is a wide bandgap semiconductor with relatively large exciton energy of 60 meV, and longitudinal optical phonon energy of 72 meV. ZnO can be grown in any nanostructure (nanoribbons, nanowires, nanorods, films, core-shell, among others), under any growth condition (metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxial (MBE), hydrothermal, radio-frequency sputtering, sonochemical, etc) to be implemented in a variety of applications. For instance, by adding Mg to ZnO, the energy band separation increases and shift to a lower wavelength. Using ZnMgO, in the form of vertical nanowires, can improve the resolution of solar-blind UV-detectors. Nevertheless, the sol-solubility of ZnO – MgO phase diagram, exemplify the challenges to increase the Mg mole fraction without resulting in MgO mix phase.

In this study, the Mg mole fraction under MOCVD synthesis is pushed from 19% to 30%, without MgO phase, by adjusting the growth parameters. The highest Mg mole fraction yet, for nanowires, is validated using EDS on SEM and TEM, XRD and PL. During the characterization of the ZnMgO nanowires, it is observed that the lattice volume does not remain constant with the increment of Mg mole fraction as it has been, until now, assumed. Based on these observations and using experimental data, new stiffness coefficients are proposed.

Taking advantage of the very well developed ZnMgO vertically aligned nanowires, co-axial core-shell structures are grown using MOCVD and employed as a gas sensor. It is noted that the gas sensitivity of the ZnMgO/ZnO core-structures has a direct correlation with the Mg mole fraction at the core. It is also validated that the native defects, specifically, the doubly charged oxygen vacancy (Vo2+) are responsible for the trapping and de-trapping of the gas molecules.

In addition to vertically aligned nanowires, for the first time, purely ZnO horizontal nanowires in the absence of any assisting mechanism, have been grown on p-Si substrate. Typically, quasi-horizontal nanowires are grown by pre-treating the substrate using patterns, etching or catalyst. In this study it was found that a low concentration of zinc nitrate, when compare to hexamethylenetetramine needs to be maintained in the aqueous solution to preclude the growth of ZnO (002).

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