Parrett, Benjamin John; (2021) Synthesis and Characterisation of Single Crystal Transparent Conducting Oxide - Gallium Doped Zinc Antimonate. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis focuses on the synthesis and characterisation of a new transparent conducting oxide (TCO) Zn1-xGaxSb2O6 (ZGSO). Polycrystalline powders of ZGSO were obtained using a novel reaction technique, designed to minimise the evaporation of antimony. Powder x-ray diffraction confirmed the powders to be phase pure and with mass increases indicating nearly complete oxidation. Out of these powders, single crystals of ZGSO were synthesised via a refi ned chemical vapour transport (CVT) technique using Cl2 as a transport agent. Investigations into the electronic properties of undoped single crystals indicated n-type semiconducting behaviour. UV - Vis spectroscopy revealed a wide optical band gap (3.38 eV) allowing transparency throughout the visible region. Substituting small amounts of Ga+3 onto the Zn+2 lattice site increased the electron carrier density by three orders of magnitude, and created a degenerate semiconductor. Low resistivities ~ 5 x 10^-4 Ω.cm were observed while retaining a wide enough band gap to prevent opacity of visible light, therefore, proving ZGSO behaves as a TCO. Energy dispersive x-ray spectroscopy (EDS) was used to quantify the cation stoichiometry in the crystals. The gallium concentration was found to vary significantly between samples and even across an individual sample indicating an erratic nature to the doping process. X-ray diffraction revealed a reduction in lattice parameters and correspondingly unit cell volume upon doping. Lab-based x-ray photoemission spectroscopy (XPS) and synchrotron-based hard x-ray photoemission spectroscopy (HAXPES) were used to measure the core levels and valence band in doped and undoped crystals. Analysis of the core peak position revealed a shift to higher binding energies under the influence of doping, a direct consequence of conduction band filling. Measurements of the valence band using HAXPES significantly enhanced the intensity of the conduction band. This is explained by the changing photoionisation cross sections as a function of photon energy. Similarly, HAXPES revealed an in-gap state that was attributed to the 5s orbital of a fraction of Sb+3 existing in the crystals. Data presented from an angle-resolved photoemission spectroscopy (ARPES) study on a (001) surface in highly doped ZGSO revealed an electron band dispersion in the valence band however displaying significantly k broadening. The absence of obvious cleavage planes in the trirutile structure prevented a quality cleaved surface for measurement. Therefore, due to the poor quality of the data, quantification of the conduction band's effective mass could not be achieved.

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