Titanium dioxide: functional electronic materials for IoT devices

Abstract

The era of the Internet of Things (IoT) calls for advancements in device level to fulfill the diversified applications. Titanium dioxide (TiO2), belongs to the family of metal oxides, has attracted extensive research interests in the fields of IoT devices such as optical sensor, solar cell, thin film transistor (TFT), and memristive device due to its high transparency to visible light, large refractive index, low-cost growth method, great chemical, and mechanical stability. ☐ In this dissertation, the possibility of TiO2 as the functional electronic material was explored rigorously. First, TiO2-based TFTs with unprecedented electrical performance were achieved by the high-temperature oxygen annealing of TiO2 channel and the utilization of ZrO2 as the high-k gate dielectric, revitalizing the traditional image of TiO2 as the channel material in TFT applications. ☐ Next, the effects of annealing ambient on TiO2 TFT performance were studied. The O2-annealed TiO2 TFTs exhibit enhanced performances compared to N2-annealed TiO2 TFTs, suggesting the importance of passivation effects of oxygen gas on TFT performance. Then, the effects of annealing temperature of TiO2 TFT performance were examined. The conductivity of the TiO2 channels were found to transition from insulating to semiconducting at 300 ºC, highlighting the important role of crystallinity in the electrical properties of TiO2. After that, the impact of ZrO2 dielectrics thickness was also systematically investigated. It is found that the ZrO2 possesses an outstanding thickness scalability and TiO2 TFTs exhibit enhanced performance with the reduced ZrO2 dielectric thickness. ☐ Through these studies, an optimized fabrication process was developed based on a 300 ºC O2-annealed TiO2 channel and a 10 nm ZrO2 gate dielectric. The resulting TFTs exhibited a high device performance under an ultra-low voltage of 1 V. These one-volt TiO2 TFTs with much-reduced thermal budget show a great potential in emerging IoT applications, such as foldable displays and wearable sensors. ☐ Apart from the fabrication process optimization, a post-fabrication superacid treatment was also tentatively applied in TiO2 TFTs. The current drivability was found to enhance by nearly two folds for TiO2 TFTs with offset regions after the treatment, bringing out new possibility to future IoT device applications. ☐ Finally, the possibilities of TiO2 in other device applications, such as buffer layers in solar cells and gate dielectrics in transistors, were also explored. The successful implementation of TiO2 in other device structure suggests the versatility of TiO2 films as functional electronic materials in IoT devices.