Graduate Thesis Or Dissertation

 

The Local Bonding Environment of Amorphous In-Zn-O Films Studied by X-ray Absorption Fine Structure and Total X-ray Scattering Using Synchrotron Radiation Public Deposited

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https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/d504rk72v
Abstract
  • Amorphous transparent conducting oxides (a-TCOs) are a class of materials becoming increasingly important for their use in transparent electronic devices. Depending on the application, these materials can be tuned to behave as conductors, semiconductors, or even insulators. These materials have been gaining interest because they exhibit several desirable properties that are limiting factors in more common crystalline TCOs. The amorphous nature of these films means that they have no grain boundaries. This is beneficial because grain boundaries act as defects, decreasing the mobility of charge carriers in the material. Grain boundaries also provide a path for contaminants from the surrounding environment, such as water vapor, to transport through the films. This can be particularly detrimental to the performance of photovoltaics. Typically, these materials are very smooth and are mechanically robust, making them well suited for use in the emerging field of flexible electronics.

    An archetypical a-TCO is amorphous indium zinc oxide (a-IZO). This material can be produced with a wide range of compositions and conductivities while not displaying any evidence of crystallization. The electrical and optical properties of a-IZO have been well characterized under a variety of different growth methods and conditions. Despite being well characterized from the standpoint of device performance, the structure of a-IZO has not, until recently, been thoroughly investigated. The native oxide of indium (In) is bixbyite, a cubic structure, while zinc (Zn) naturally crystallizes as wurtzite, a hexagonal structure. These two crystal structures are incompatible. It is believed that the inclusion of Zn atoms in IZO “frustrates” the crystal structure, hindering the creation of any long-range periodicity. This is because, when deposited at low temperatures, both Zn and In try to form their respective native oxides. The structural description of sputtered a-IZO thin films is of interest for this work.

    The amorphous nature of these materials means that any structural description will be statistical in nature, and traditional methods to measure structure, such as X-ray diffraction, will not be particularly illuminating. Despite the lack of long-range periodicity, a-TCOs still exhibit short-range ordering that closely resembles what would be expected in a crystalline material. Two methods well suited for structural determinations of amorphous materials are the Pair-Distribution Function (PDF), which is derived from the total scattering spectrum, and X-ray Absorption Fine Structure spectroscopy (XAFS). XAFS is capable of probing the structure of materials on very short (≲ 5 Å) length scales surrounding a specific element of interest. The PDF method is capable of probing the structure over longer distances than XAFS. Both techniques have strengths and weaknesses that will be discussed.

    Both XAFS and PDF methods require highly monochromatic X-rays with widely tunable energy ranges and high fluxes. These requirements necessitate the use of synchrotron-based X-ray sources. Although synchrotron based measurements are becoming more routine in the scientific community, these methods still present significant difficulties in the determination and interpretation of results. The Stanford Synchrotron Radiation Lightsource and the Advanced Photon Source were both used in the collection of experimental data for this work.

    Previous XAFS results from similar systems has shown that the oxygen coordination around each metal site is close to that of the native oxide, however current literature for XAFS analysis of aIZO is lacking. The PDF results confirm that the arrangement of nearest and next-nearest neighbors in a-IZO is similar to what would be expected of crystalline In2O3. From the XAFS results, it has been concluded that InO6 octahedra and ZnO4 tetrahedra form edge-sharing linkages in a-IZO. Supporting this conclusion, EXAFS analysis provided a measure of the local cation (Zn/In) ratio which was consistent with the measurement of the bulk cation ratio.

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  • 2011
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  • 2020-02-11
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