Noble metal-oxide Schottky contacts on zinc oxide.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis presents a systematic investigation into the fabrication and characterisation of noble metal-oxide Schottky contacts with deliberately varied levels of oxygen incorporation to bulk single-crystal ZnO, towards the development of ZnO-based photodetectors for ultraviolet (UV) dosimetry. Noble metal and noble metal-oxide films of varied oxygen composition (as determined by Rutherford Backscattering Spectrometry) were deposited using reactive RF-sputtering (and reactive eclipse pulsed laser deposition) simultaneously with the corresponding ZnO Schottky contacts. The films were analysed for transparency, structure, surface potential, oxygen incorporation, and conductivity. The electrical characteristics of the Schottky contacts were analysed in dark conditions, under periodic exposure to visible and UV radiation, and at temperatures up to 180 °C. The electrical characteristics of the noble metal-oxide Schottky contacts to ZnO improved dramatically in ideality factor, barrier height, and rectification ratio with increasing oxygen incorporation; for example, oxidised Pt Schottky contacts exhibited current rectification of 12 orders of magnitude and effective barrier heights of up to 1.30 eV compared to the ohmic behavior of unoxidised plain-metal contacts. In addition, a AgOx Schottky contact produced the highest Schottky barrier to ZnO reported to date, with a barrier height of 1.42 eV, with several other AgOx and PtOx contacts in this work also producing very high barrier heights. A proposed model of the mechanisms responsible for the large performance gains of the oxidised noble metal-oxide Schottky contacts to ZnO in this work was developed, in which the presence of the active oxygen species during Schottky contact formation produces the following beneficial effects: (1) the removal of the hydroxyl-induced potential well and associated electron accumulation layer from the ZnO surface, (2) the passivation of interfacial oxygen vacancies, and (3) an increase in the work function and electronegativity of the oxidised Schottky contacts. The UV photoresponse of the noble metal-oxide Schottky contacts was found to improve with respect to both response time and dark-vs-photocurrent increase with increasing oxygen incorporation, and was also found to be strongly affected by device bias. Increasingly negative device biases caused increasing levels of undesirable persistent photoconductivity (PPC). In contrast, a PtOx Schottky contact showed a consistent square-wave photoresponse to 30 s intervals of UV (365 nm) radiation when biased at 0 V, with over 3 orders of magnitude of reproducible current increase on UV irradiation for over 50 exposure cycles. A proposed model of the mechanisms responsible for PPC in the metal-oxide Schottky contacts is proposed, involving the desorption of atmospheric oxygen species from the surface of ZnO and the production of metastable ionised oxygen vacancies. An IrOx Schottky contact was found to be thermally-stable to 180 °C with a barrier height of 1.07 eV and over 6 orders of magnitude rectification at 180 °C, compared to 0.91 eV and 10 orders of magnitude rectification at room temperature, which represents the highest-reported high temperature rectification performance for ZnO Schottky contacts. A PdOx Schottky contact heated to 180 °C was used to calculate an experimental Richardson constant of 36.4 Acm-2K-2, very close to the theoretical value of 32 Acm-2K-2.