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Title: Atomic layer deposition of alumina and zinc oxide for optoelectronic devices
Author: Burgess, Claire Hannah
ISNI:       0000 0004 7228 8592
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Atomic layer deposition (ALD) uses surface reactions of gaseous precursors to grow thin films of materials. Exceptional uniformity and thickness control is possible due to the separation of the different precursors, allowing each to undergo reactions with substrate surface groups until, on chemisorption of a complete monolayer, self-termination occurs. Purging after the precursor release removes by-products and remaining precursor, ensuring that the material growth proceeds layer-by-layer. These and other benefits such as relatively low deposition temperatures, and the ability to coat large areas and complex 3D surfaces, have led to ALD being a key and increasingly popular technique in research and industry. One area of application of ALD is in optoelectronics; this thesis describes the design and construction of an ALD system for research into optoelectronics, including thin film transistors, solar cells and photoelectrodes. A conventional, thermal ALD system was custom-built, and the deposition of Al2O3, ZnO and Al doped ZnO (AZO) using trimethyl aluminium, diethyl zinc and H2O was characterised through techniques including X-ray diffraction, UV-visible spectroscopy, cross-sectional TEM, resistivity and Hall measurements. The growth per cycle and the properties of the materials were consistent with literature values, confirming self-saturating ALD was achieved. The conductivity of ZnO was seen to increase with temperature and doping, and the ZnO (wurtzite crystal structure) out-of-plane preferential orientation became increasingly [100] at lower temperature and with higher Al content. Reducing purge times below 10 s resulted in a slight increase in ZnO film thickness due to a chemical vapour deposition contribution to growth, but the more significant effect was a reduction in mobility and carrier concentration, and a higher hysteresis when the ZnO was employed as a thin film transistor channel layer. The crystal orientation of ALD ZnO deposited at 100 °C was investigated on different substrates such as amorphous quartz, polycrystalline ITO and single crystal sapphire. A method of using multilayers of [100] oriented AZO, ultrathin Al2O3 (< 20 ALD cycles) and ZnO was developed as an alternative way to control the ZnO orientation whilst ensuring compatibility with devices. A dependency on ZnO crystal orientation was seen for CH3NH3PbI3 perovskite degradation rate when CH3NH3PbI3 was deposited on top of an ALD ZnO layer. The stability of CH3NH3PbI3 was studied by temperature controlled XRD and film colour change; Al doping of ZnO reduced degradation rates. High aspect ratio track-etched membranes were coated by ALD to produce nanotubes, and Cu2ZnSnS4 nanoparticle films were coated with Al2O3 layers which increased stability as a photocathode. The ALD system built and the principles explored for tuning material properties presented in this thesis can be used to further optimise optoelectronic devices in the future.
Supervisor: McLachlan, Martyn ; Alford, Neil Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral