Electronic And Optical Properties Of Oxide Thin Films

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Off-axis sputtering has been used to study two oxide thin film systems. The first study involved developing an epitaxially grown NiO/SrTiO3 heterostructure to be used for a negative index of refraction. The second used off-axis sputtering as a combinatorial approach to study the effects of doping and codoping of transparent conducting ZnO. A negative index of refraction should be possible by combining only intrinsic material resonances. We proposed that combining NiO, which has an antiferromagnetic resonance in the far infrared, and SrTiO3, which has a dielectric resonance in the far-infrared, can be used to achieve a negative refractive index of refraction in the far-infrared. These resonances can be shifted through changing temperature, doping, or through applying a magnetic field. A high quality epitaxially grown composite heterostructure and bulk ceramics have been fabricated. Preliminary measurements of the permeability and permittivity been taken for a bulk composite structure. The SrTiO3 ionic resonance can be seen in a reflection technique while the NiO antiferromagnetic resonance is easily seen in a transmission technique ZnO is a potential inexpensive replacement for indium tin oxide (ITO) as a high-end transparent conductor. However, ZnO has not replaced ITO in many applications such as flat-panel displays due to high resistivity and poor thermal stability. ZnO has been doped with many elements including Al, In and Ga. We proposed that codoping with both Al and In might result in size compensation which would lead to improved dopant solubility and therefore better electrical conductivity and stability. A high-throughput combinatorial approach was used to study codoping of ZnO with both Al and In. Measurement of the deposited composition spread suggests that codoping results in much improved conductivity. By adding 1% In to highly Al-doped ZnO (3-6%) the conductivity increases by an order of magnitude. Thermal annealing in a variety of atmospheres including air, vacuum and hydrogen results in some understanding of the difference in the two dopants. It was found that Al doping results in a higher carrier concentration than In doping, but any excess Al greatly decreases the mobility. In doping improves the mobility but with lower carrier concentrations. Calculations based on density functional theory support the experimental data, showing that Al is a shallower dopant than In. Unit cell volume comparisons between theory and experiment correspond well. The interplay between substitutional doping, oxygen vacancies, and possible adventitious H doping makes the doped ZnO system difficult to analyze definitively.

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