Wilson, Rachel Lyndsey; (2017) Deposition of ultra-thin metal oxide films for gas sensing applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The aim of this research project was to investigate the use of Atomic Layer Deposition (ALD) and Chemical Vapour Deposition (CVD) to deposit n- and p-type metal oxide thin films for use in gas sensing applications, with the long term goal to identify the materials which provide maximum sensitivity and selectivity. Two ALD reactors have been designed and constructed specifically for this project. N-type TiO₂ thin films have been deposited by ALD of titanium(IV) isopropoxide and water, where film growth was shown to proceed via a self-limiting mechanism. Films were characterised using AFM, XRD, XPS and Raman, which confirmed anatase phase on the film surfaces. TiO₂ films of various thickness were deposited onto gas sensor substrates and exposed to a range of test gases in order to evaluate their gas sensitivity at operating temperatures of 350 °C and 480 °C at several different relative humidity’s. Electrical resistance changes were observed for a 50 nm TiO₂-coated sensor in response to NH₃, where the sensor response was found to decrease with increasing relative humidity. However for a 10 nm film, whose thickness was most consistent with reported literature values of the Debye length for TiO₂ was the not the most sensitive. Attempts to deposit p-type NiO films via ALD were less successful. However two novel nickel complexes were synthesised: [Ni(dmamp)₂] and [Ni{(NiPr₂)₂CNEt₂}₂], whose volatility was greater than some of the other commonly used nickel precursors for ALD and CVD applications. These precursors, along with [Ni(thd)₂] and [Ni(Cp)₂], have been screened for their use in the deposition of NiO thin films via ALD with water. However, XPS analysis confirmed nickel metal and/or Ni(OH)₂ on the film surfaces, which has been attributed to both a lack of reactivity between the nickel precursors and water and issues with the reactor design. Separate CVD experiments performed with [Ni(dmamp)₂] and [Ni{(NiPr₂)₂CNEt₂}₂] resulted in the deposition of NiO films, as confirmed by XRD and XPS. Under the CVD conditions used, film growth could be controlled relatively easily, as compared to other conventional CVD methods.

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