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Stoichiometry control mechanisms of bias sputtered zinc oxide films Brett, Michael Julian


This thesis reports the first detailed study of the stoichiometry control mechanisms and physical properties of ZnO films deposited by dc planar magnetron sputtering of a Zn target in a reactive Ar/0₂ atmosphere. Control of film stoichiometry was achieved using a subsidiary rf discharge at the substrate and a reactive gas baffle surrounding the target. The reactive gas baffle was shown to enhance film oxidation by decreasing the metal flux to the substrate and increasing the oxygen partial pressure near the substrate. Rutherford backscattering analysis of film stoichiometry demonstrated that the effect of the rf discharge was to increase the O/Zn composition ratio. This oxidation was shown to occur through preferential resputtering and preferential evaporation of excess Zn and by activation and ion plating of oxygen species. Resputtering and evaporation rates were found to be enhanced above that expected for bulk Zn, due to the weak bonding of surface adatoms during film growth. Conducting ZnO films produced at various values of the rf-induced substrate bias voltage were characterized for electrical, optical and structural properties using Hall probe,. X-ray diffraction, electron microscope, and visible and infrared spectroscopy techniques. Films deposited at low substrate bias (0 to -50V) were found to have a large Zn excess (15%) resulting in low electron mobilities (1 cm²/Vs), high resistivities (10⁻² Ωcm) and were strongly absorbing in the visible. Films deposited at high substrate bias were nearly stoichiometric, optically transparent and had high electron mobilities (15 cm²/Vs) resulting in low resistivity (10⁻³ Ωcm). The optical properties of transparent conducting films for wavelengths 0.4 to 20 /im were modelled by the Drude theory of free electrons using measured electrical transport properties. The original goal of this work, to develop a heat mirror coating suitable for manufacture, was achieved by bias sputter deposition of ZnO onto uncooled polyester sheet at deposition rates approaching 75 nm/min. The best heat mirror films had a transmission to solar energy of 75% and an 85% reflection of 300 K blackbody radiation.

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