Properties and Characterisation of Sputtered ZnO
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The aim of this work was the study of sputtered zinc oxide (ZnO) film deposition, the optimisation and characterisation of film properties and applications as a sensing material. In recent years there has been increased interest in ZnO in terms of its potential applications as piezoelectric films (or coatings) for surface acoustic wave devices (SAW), for IR and visible light emitting devices and UV sensing. The electrical, optoelectronic and photochemical properties of undoped ZnO have resulted in its use for solar cells, transparent electrodes and blue/UV light emitting devices. ZnO is a unique material that exhibits both semiconducting and piezoelectric properties. In the past decade, numerous studies have been made on both production and application of one-dimensional ZnO. Compared with other semiconductor materials, ZnO has a higher exciton binding energy of 60 meV, which gives it a high potential for room temperature light emission, is more resistant to radiation, and is multifunctional as it has piezoelectric, ferroelectric, and ferromagnetic properties. ZnO-based semiconductor and nanowire devices are also promising for the integration on a single chip. So far, the various applications of ZnO nanomaterials such as biosensors, UV detectors and field emission displays are being developed. In this work, ZnO was sputtered using both DC and RF magnetron sputtering. Reactive DC sputtering was performed with a Zn target and oxygen plasma, while RF sputtering was performed with a ZnO target. Comparisons between films deposited under different conditions on different substrates were employed to assess film properties. Several experiments were performed on as-grown films as a control for subsequence treatments, other samples were post-annealed in N2 at temperatures up to 1200 ºC, the highest reported annealing temperature and the quality of the deposited films was determined using PL, RBS, XRD, SEM and AFM. The piezoelectric properties (d33) of selected films were determined using single beam interferometry, double beam interferometry, and for the first time, using piezoelectric force microscopy (PFM). It was found that DC sputtered films yielded better quality films as evident by PL and XRD analysis and higher piezoelectric response than RF sputtered films. Films deposited using DC sputtering on Si substrates and followed by post-annealing in N2 atmosphere at 1100 ºC showed the highest recorded PL response, while films deposited on sapphire showed good PL response without any need for post-annealing. The d33 of selected films were determined first using single beam interferometry and inflated results were reported, caused by sample bending/buckling. Double beam interferometry results confirmed d33 values in the range of 3.3 to 4.3 pm/V. Piezoelectric force microscopy (PFM) which is based on AFM, was employed to investigate the local electromechanical (piezoelectric) properties of the ZnO films. UV sensing was demonstrated using Schottky contacts and SAW devices on ZnO deposited on Si and post-annealed. In the first instance, Schottky contacts were fabricated on the films and the I V characteristics determined under exposure of various light sources. The current increased up to one order of magnitude during exposure with a halogen light bulb, which is known to emit energy in the UV band. Another experiment was performed using surface acoustic wave (SAW) devices which were fabricated on the films and interrogated using a network analyser. These SAW devices contain an interdigitated transducer and two reflectors each. The signals sent back from the two reflectors were analysed under various light conditions and gave lower readings during exposure to UV light. In order to enable device fabrication of UV sensors a novel “super coating”, achieving both optimised PL and d33 properties, was designed, fabricated and tested. The structure is based on optically transparent Quartz substrate. During this experiment the first DC sputtered coat was optimised to have high PL response by post-annealing at 900 ºC. Afterwards, the second coat was left as-sputtered in order to have highly piezoelectric properties. Preliminary analysis using XRD showed two peaks corresponding to the annealed and not annealed coat, which suggest the super coating combines the properties of the two individual films. This configuration has the potential to be used as UV sensing material and as piezoelectric substrate for SAW devices.