Inkjet-printed ZnO thin film semiconductor for additive manufacturing of electronic devices

Additive manufacturing, which is commonly known as three-dimensional (3D) printing, is a potential technology that possesses many advantages comparing with traditional fabrication technologies. The creative idea of additive manufacturing, which converts digital designs to physical objects via depositing material layer-by-layer, introduces many benefits such as capable of making complex structures, obtaining satisfactory material properties, prototyping rapidly and saving materials. Printing functional devices is an even more promising approach to utilize the benefits of additive manufacturing for micro-fabrication. Zinc oxide (ZnO) is a well-known wide bandgap semiconductor which has numerous applications in electronics, optoelectronics, piezoelectric, and piezotronics. Hence, inkjet printing of ZnO thin film is of significant interest for producing functional devices using additive manufacturing. In addition, the electrical properties of ZnO are essential to the performance of devices, such as responsivity of UV photodetector. Thus, it is important to develop an inkjet printing manufacturing process for producing ZnO thin film of desired electric properties. This thesis aims to develop and investigate the additive manufacturing process of ZnO thin film for electronic applications, and specifically to engineer the electrical properties of printed ZnO film to enhance the performance of printed UV photodetector. Owing to its fascinating features such as non-contact printing, high resolution, low cost and low-temperature process, inkjet printing is adopted as the main additive manufacturing method in this thesis. Inkjet printing process of a zinc precursor solution was successfully developed for the additive manufacturing of ZnO thin film. It was also found that inkjet-printed ZnO possesses a polycrystalline structure. To investigate the practicality of the proposed process, micro-sized flexible ZnO UV photodetectors were thoroughly prepared by a facile inkjet printing scheme. The fabricated devices can be potentially used for real-time monitoring of the UV intensity and warning users for health and safety purpose. Moreover, this additive manufacturing method can be applied to fabricate ZnO thin film on various flexible substrates, which makes the device more suitable for applications that require flexibility such as wearable devices. Electrical properties of printed UV photodetectors were characterized and studied under varied thermal annealing temperatures of up to 350 °C. It was found that both conductivity and responsivity of the printed ZnO film significantly increase along with the increase of annealing temperature. The enhancement of electrical conductivity and responsivity was attributed to band bending modification, which was reduced due to the fusing of grain boundaries under heat treatment. To explore and demonstrate the capability of integrating the inkjet printing of ZnO films with other 3D printing process, silver micro-heater was fabricated and employed to heat zinc precursor locally and form ZnO. Subsequently, electrical properties of printed ZnO films were further investigated by using localized Joule heating effect through the integrated fabrication of ZnO films and a silver microheater. The fabricated UV photodetector showed that the input power of microheater has a significant effect on the performance of the printed device. The successful inkjet printing of micro-sized ZnO thin films and the integrated photodetector has demonstrated the feasibility and great potentials of fabricating sophisticated semiconductor devices using additive manufacturing technology.Doctor of Philosoph