Defect characterization of low temperature solution grown ZnO
Date of Issue2016-02-29
School of Materials Science and Engineering
Zinc oxide (ZnO) is a wide bandgap semiconducting oxide with many potential applications in optoelectronic devices such as light emitting diodes (LEDs) and field effect transistors (FETs). Controlling the conductivity of ZnO is a foremost concern in the optoelectronic industry. Understanding the nature of ZnO native defects is vital for the application of ZnO in the semiconductor industry. Many electronic properties depend significantly on the intrinsic defect concentration as it affects the doping efficiency and the growth of the material. These properties also depend on the type and amount of impurities and degree of crystallinity. There is a need to understand the function of native point defects and the incorporation of impurities, with the intention for a better control of the conductivity of ZnO. However, one major issue in determining whether it is zinc or oxygen deficiency that provides ZnO its unique properties remains. In this thesis, ZnO films and powders were synthesized via a low temperature aqueous solution chemical bath deposition route and its short to medium range structure order characterized. In addition, Ga-doped ZnO films and powders were also synthesized as Ga has been found to be an excellent donor, increasing the conductivity of ZnO films. All samples were post-growth thermally annealed in air in order to improve the electrical and optical properties, and correlate the annealing temperature with the defect structure. The epitaxial nature of the ZnO films (undoped and Ga-doped) were determined via high resolution X-ray diffraction (HRXRD). The morphology of the films was studied via field emission gun scanning electron microscopy (FEGSEM) showed that the films were smooth and continuous on the plane-view. Cross-sectional images showed that the films were about 2 µm thick after a growth time of 4 hours at 90°C. Visible pores were also observed in the films after post-growth thermal annealing at temperatures of above 200°C. Electrical properties were obtained via Hall measurements showed characteristic n-type conductivity and a carrier concentration of the order of 1020 cm-3 after post-growth thermal annealing at 400°C in air for 1 h for the Ga-doped ZnO films. For the ZnO powders, phase identification was determined by conventional powder XRD methods. The morphologies were obtained via FEGSEM and showed different nanorods morphologies for the different precursors used for the syntheses. Internal pores were observed in the ZnO nanorods upon thermal annealing at >300 °C and were examined using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Three-dimensional electron tomography reconstruction shows that pores exist within the nanorods. These pores coalesce and also increase in size as the ZnO nanorods were annealed at higher temperatures or for extended periods of time, as observed by an in situ TEM study. As such, this pore formation phenomenon will significantly affect the utilisation of solution-synthesized ZnO for optoelectronic devices, but new application fronts such as gas sensors and photo/electro catalysis will benefit due to its high light absorption properties and high surface areas. Further defect characterization was performed for the all the ZnO samples, largely using synchrotron-based characterization techniques, namely X-ray absorption spectroscopy (XAS) and X-ray pair distribution function (PDF) techniques. XAS is an ideal, atom specific characterization technique that is able to probe defect structure in many materials, including ZnO. Comparative studies of commercially available ZnO and aqueous solution grown (≤ 90°C) ZnO powders using XAS and PDF has allowed for the study of short to medium range structure order. The ZnO films were studied via ex situ XAS and the ZnO powder samples were studied via ex situ and in situ XAS and in situ PDF experiments. The local structure order of the ZnO films and powders were determined. From XAS of the Ga-doped ZnO samples, it was evidently shown that Ga3+ resides tetrahedrally in the Zn2+ site within the ZnO lattice. The understanding of the role of defects and the short to medium range order and structure within the ZnO system has provided useful insight for further exploitation of the material and is invaluable for the exploitation of ZnO for further development of optoelectronic device applications.