Energetically deposited ZnO-based ternary oxides: characterisation and device applications

This thesis describes the deposition, characterisation and device applications of ZnO and Zn1–xMgxO films. Energetic deposition methods have been employed since they show promise for producing high performance metal oxide films. Particular aims of this study were to establish: (1) whether energetic deposition could offer tangible benefits over other methods currently employed to prepare ZnO based films and (2) whether the control over microstructure afforded in energetic deposition could be combined with compositional control for effective bandgap tuning.<br><br>Two energetic deposition methods were used to produce ZnO and Zn1–xMgxO thin films for optoelectronic applications: filtered cathodic vacuum arc (FCVA) and high power impulse magnetron sputtering (HiPIMS). Both methods are large area and energetic. Using energetic deposition methods give gives better control of microstructures when compared to non-energetic methods. Energetic deposition causes local heating of the growing film simulating increased growth temperature without the need to hold the substrate at this temperature. This allows substrates not compatible with high temperatures to be coated.<br><br>Initially, FCVA was used to grow thin films of Zn1–xMgxO from a composite ZnMg cathode. Compositional variations were observed and attributed to cathode poisoning effects. These variations proved useful in elucidating the relationships between composition, microstructure and properties. In order to probe the microstructure at the level of atomic bonding, x-ray absorption spectroscopy was employed. These measurements revealed the presence of O-vacancies and established that the miscibility limit for single phase wurtzite Zn1–xMgxO films was x=0.37, consistent with other growth methods.<br><br>Device measurements followed in phase two. FCVA deposition was again used but this time with strategies employed to compositional variations. The as prepared Zn1–xMgxO films demonstrated excellent structural properties, moderate carrier concentrations and mobilities comparable with the best previously published. A particular aim of this phase was to demonstrate the applicability of the films to UV sensing. Visible blind UVB detectors were fabricated using graphitic contacts. These demonstrated UV to visible rejection ratios exceeding 103.<br><br>HiPIMS was used to grow ZnO films in phase three. This method has been used to produce ZnO, however, this study lacked detailed microstructural, optical and electrical characterisation. These films displayed carrier mobilities comparable or better than those achieved using conventional magnetron sputtering and moderate n–type carrier concentrations suitable for devices. Schottky diodes produced on this material, grown at the moderate temperature of 200°C, exhibited ideality factors less than 2 and rectification ratios of 103.<br><br>Finally, HiPIMS and DC magnetron sputtering were used simultaneously to co-deposit Zn1–xMgxO with a Mg fraction that varied across a sapphire wafer. This experiment enabled the determination of the bandgap, microstructure and properties as a function of Mg fraction. The as deposited film was resistive so annealing with hydrogen was performed to improve the carrier mobility. Post annealing, carrier concentrations of ~1017 cm-3 and hall mobilities of up to 4.5 cm2V-1s-1 were measured. The material was used to fabricate UVB detector which provided UV/visible rejection ratios of 103.