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Title: A study into the growth kinetics and properties of thin film zinc oxide deposited by MOCVD
Author: Lamb, D. A.
ISNI:       0000 0001 3604 2358
Awarding Body: University of Wales, Bangor
Current Institution: Bangor University
Date of Award: 2004
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A study of the deposition of thin film ZnO from diethylzinc (DEZn) and a novel oxygen precursor n-butanol (n-BuOH) has been carried out, using atmospheric metal organic chemical vapour deposition (MOCVD). ZnO thin films have been identified as candidates for transparent conducting oxides (TCOs) for solar cells. The n-BuOH precursor is suggested to be a viable alternative to the more commonly used alcohol t-BuOH having the process-simplifying advantage of being liquid at room temperature. No prereaction was observed using these precursors. The purpose-designed and built MOCVD reactor incorporated in situ monitoring of the thin film growth via laser interferometry. Growth rates as high as 0.6 nm/s were achieved at temperatures below 250°C and precursor ratio II/VI = 4. Polycrystalline zinc oxide thin films were deposited at temperatures as low as 67°C. The IINI precursor ratio was found to strongly influence the growth rate of the zinc oxide films. A rate equation has been proposed which identifies two rate constants; the first, kA a slow step that relates to the decomposition of the alkylzinc alkoxide intermediate via a radical mechanism, and the second faster step ks that occurs under higher than equimolar II/VI ratios. The mechanism proposed in the second step identifies mono-ethylzinc radicals, physisorbed on the surface, as initiators for the decomposition of the alkylzinc alkoxide. It is proposed that when deposition takes place at above equimolar VI/II ratios excess n-BuOH acts as a site blocking molecule lowering the thin film growth rate. A nucleation delay in fi lm growth was measured via in situ interferometry. This delay was found to be independent of temperature but decreased two fold when moving from equimolar partial pressures to zinc rich conditions. A UV Nis lamp, filtered to exclude gas phase photolysis, was also employed to study the photoassisted deposition of zinc oxide. The photo-energy, supplied to the growing film via a flexible liquid light guide, was selected to promote a surface photo-catalytic reaction. An increase in the bandgap energy, compared to non-photoassisted films, was observed when films were deposited at 308°C and 36.8 mW/cm2 irradiation. A rapid change in surface morphology was noted between macro-smooth, below 260°C, to a rough textured film above 260°C. The average transmission in the visible wavelengths for the macro-smooth films was shown to be above 90%. A shift to lower bandgap energies was observed as deposition temperature increased indicating the possibility of a change in stoichiometry within the thin film. From the transmission data, a well-defined and sharp absorption edge was shown to begin at around 360 nm. The films generally displayed high transmission of wavelengths from 360 nm and through the visible spectrum to 900 nm (the limit of the spectrometer). For use in thin film photovoltaics the ZnO films would allow a large range of energy (< 3.3 eV) through to the p-n junction.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available