Electrical and Optical Properties of Ga-Doped Mg1-xZnxO System

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Title: Electrical and Optical Properties of Ga-Doped Mg1-xZnxO System
Author: Wei, Wei
Advisors: Jagdish Narayan, Committee Chair
Roger Narayan, Committee Co-Chair
Abstract: The primary aim in this thesis is to investigate Ga-doped Mg1-xZnxO, as well as undoped Mg1-xZnxO for the application of transparent conducting oxide. For this thesis work, the films have been grown on sapphire using pulsed laser deposition technique. The films were grown under various deposition conditions in order to understand the effect of processing on the film properties. The grown films have been characterized using various techniques, including XRD, TEM, XPS, 4-probe resistivity measurements, Hall measurements and absorption/transmission spectroscopy. Undoped Mg1-xZnxO films have been grown at several temperatures between room temperature and 750 oC. Photoluminescence was correlated with Urbach energy values which were determined from absorption spectrum. The film grown at 350 oC exhibited lowest band-tail parameter values and highest photoluminescence values than the other films. The optical and electrical properties of heavily Ga-doped MgxZn1-xO thin films were investigated. The film transparency is greater than 90% in the visible spectrum range. The absorption can be extended to lower wavelength range with higher magnesium concentration, which can improve the transparency in the ultraviolet wavelength range; however, conductivity is decreased. The optimum Ga concentration was found to be 0.5 at. %. At this Ga concentration, the film resistivity increased from 1.9×10-3 to 3.62×10-2 Ω.cm, as the magnesium concentration increased from 5 at. % to 15 at. %. The optical and electrical properties of Ga-doped MgxZn1-xO thin films were investigated systematically. In these films, the Ga content was varied from 0.05 at.% to 7 at.% and the Mg content was varied from 5 at.% to 15 at.%. X-ray diffraction showed that the solid solubility limit of Ga in MgxZn1-xO is less than 3 at.%. The absorption spectra were fitted to examine Ga doping effects on bandgap and band tail characteristics. Distinctive trends in fitted bandgap and band tail characteristics were determined in films with Ga content below 3 at.% and Ga content above 3 at.%. The effects of bandgap engineering on optical transparency were evaluated using transmission spectra. Carrier concentration and Hall mobility data were obtained as functions of Ga and Mg content. The electrical properties were significantly degraded when the Ga content exceeded 3 at.%. Correlations between conduction mechanisms and Ga doping of MgxZn1-xO thin films were described. In addition, the effect of bandgap engineering on the electrical properties of epitaxial single crystal Ga-doped MgxZn1-xO thin films was discussed. Mott transition in Ga-doped MgxZn1-xO thin films was investigated. 0.1 at.%, 0.5 at.% and 1 at.% Ga-doped Mg0.1Zn0.9O films were selected for resistivity measurements in the temperature range from 250 K to 40 mK. The 0.1 at.% Ga-doped Mg0.1Zn0.9O thin film showed typical insulator-like behavior and the 1 at.% Ga-doped Mg0.1Zn0.9O thin film showed typical metal-like behavior. The 0.5 at% Ga-doped Mg0.1Zn0.9O film showed increasing resistivity with decreasing temperature; resistivity was saturated with a value of 1.15 × 10-2 Ω•cm at 40 mK, which is characteristic of the metal-insulator transition region. Temperature dependent conductivity σ(T) in the low temperature range revealed that the electron-electron scattering is the dominant dephasing mechanism. MgxZn1-xO/TiN/Si(111) heterostructures were fabricated using pulsed laser deposition. X-ray diffraction and transmission electron microscopy studies showed that both TiN and MgxZn1-xO were grown epitaxially on Si(111). A thin spinel layer (~5 nm) was formed after deposition at the MgxZn1-xO and TiN interface. Current-voltage measurements showed that the electrical contact between MgxZn1-xO and TiN is ohmic contact. These results suggest that the TiN provides a buffer layer to integrate MgxZn1-xO thin films with silicon substrate.
Date: 2010-04-05
Degree: PhD
Discipline: Materials Science and Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/6252

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