| A novel electrochromic thin film transistor (EC-TFT) was fabricated and characterized in this work. This concept relies on ion transport to control gating. The channel material is tungsten oxide (WOx), produced by reactive magnetron sputtering. In its oxidized state WO3 is a transparent, wide band gap insulator (> 3 eV). However upon intercalation of light ions (H+, Li+) the material becomes both electrically conducting and opaque to visible light. This allows the EC-TFT to generate a complementary optical response. We optimized the fabrication of individual layers of the EC-TFT, and found that controlling the stoichiometry of WOx is a key step. Using RF magnetron sputtering, it was found that there is a narrow window to obtain material capable of reversible switching. Fully oxidized films proved it is difficult to intercalate ions efficiently. In contrast, insufficient oxygen produced films that were always in a metallic like state. Best results were obtained with the sputter power set at 200 W using O2 fractions of 42%-46% in argon. In the preliminary studies, the device was tested in a two-step fashion. First, devices were placed in solution and cyclic voltammetry was used to set the level of ion intercalation. Samples were then removed from the electrolyte, dried, and the source/drain current was measured on a probe station. This demonstrated the concept of the EC-TFT, showing that the transistor could be turned on and off reversely with a current ratio of Ion/Ioff ~ 1000, and showed significant color change during the intercalation. However, the conductivity in off state was too high to be a promising transistor, the two step approach made the data rather noisy, it was difficult to make good contacts and this was not an in situ measurement. In order to get the in situ measurement, macrodevices with large electrodes were fabricated and characterized, which made it easier to make electrical contacts. However, after the initial cycle the macrodevices were always in on state and could not be turned off because of the dielectric. In addition, it showed no significant color change. The top source/drain structure showed similar behavior as the bottom configuration. It was found that in these configurations the supply of electrons was insufficient to allow intercalation, and permanent defects in the Al2O3 dielectric developed to compensate. To address these issues, 3-terminal devices were fabricated and characterized simultaneously. These devices showed similar behaviors with the preliminary 2-step device. The IDS increased with decreasing gate biasing with a low threshold voltage -0.8 V. In addition, we observed the color changing. However, the conduction through the electrolyte resulted very low on/off ratio (<5). In conclusion, it is the conduction through the electrolyte contributes to the low on/off ratio. |
