Abstract
Metal Oxide for Solid State Electronics: Transparent Electrodes, Photoanodes and Transistors
by
Hyun Sung Park
Doctor of Philosophy in Engineering - Mechanical Engineering University of California, Berkeley
Professor Liwei Lin, Chair
Metal oxides have unique characteristics for a variety of modern applications such as thin films and nano-structured materials in optoelectronic devices and renewable energy productions. In this dissertation, three different metal oxides are developed: solution processed antimony doped tin oxide (ATO) thin films for transparent electrodes; high aspect-ratio ZnO nanowires with ALD (Atomic Layer Deposition) TiO2 for stabilizations as photo-anodes; and spin-coated SnO2 films for TFTs (Thin Film Transistors) in the applications of display devices.
In the first part of this dissertation, solution processed ATO electrodes show high conductivity (~21000 S/m) and optical transparency (~95%) suitable for the state-of-art optoelectronic devices such as displays, solar cells, and smart windows. The high quality thin films are made by the low-cost solution-based process with multiple spin-coating steps using sol-gel precursors. In order to obtain better electrical and optical performances, a parametric study has been performed on four key processing parameters: doping concentration, film thickness, ambient gases and temperatures. Studies on the film surface morphology using scanning electron microscopy together with electrical, and optical characterizations elucidate the dominant factors for the conductivity and transmittance of the films. Further examinations under X-ray photoelectron spectroscopy reveal the required annealing temperature to form Sb+5 species in the Sn+4 lattices. It is found that improved mechanical flexibility is achieved and the elastic modulus of 35GPa is obtained by using the nano-indentation test. A 3D finite element analysis shows ATO films can resist 4x more deformation than those of commercial vacuum-processed indium tin oxide (ITO) films under the four-point bending simulations.
The high aspect-ratio and vertically ordered ZnO nanowires as long as 10μm in length and 50nm in the cross-sectional width are constructed by using a hydrothermal process. A conformal TiO2 film of 40nm in thickness is deposited afterwards by ALD to stabilize the ZnO nanowires in a solar-powered hydrogen gas harvester. Two key innovations have been achieved using the favorable geometry and high quality nanowires in this work: (1) improved stability over bare ZnO nanowires during the photocatalytic reactions, and (2) excellent low bias voltages.
In the work of using SnO2 films to make TFTs for display devices, a multiple spin coating is developed in order to obtain better gate modulation properties. The effects of annealing condition on the electrical performances and the surface morphologies of TFTs have been investigated. Experimentally, a value of 6.3cm2/Vs of the extracted field effect mobility for solution processed SnO2 TFTs has been achieved and this value is 6x higher than that of a-Si:H TFTs.
