Radiation Hard Thin Film Transistors and Logic Circuits Based on ZnO and Related Materials

Abstract

Thin film and thin film transistors (TFTs) and logic circuits based on zinc oxide (ZnO) and related materials, such as zinc tin oxide (ZTO), are designed, fabricated and characterized in this Ph.D. dissertation. ZnO as a metal oxide semiconductor provides many advantages such as non-toxicity, environmental stability, and transparency. Its wide band gap, high displacement threshold energy and large free-exciton binding energy make it a viable candidate for space-based radiation-hard electronics and optoelectronics. Therefore, it is meaningful to study and optimize the process of ZnO TFTs fabrication. Focused on this work, most ZnO TFTs have been produced by low cost, simple set-up and high throughput sol-gel spin-coating. Temperature dependent IV measurement as a convenient and precise method for localized subgap density of states (DOSs) extraction has been applied to ZnO (especially sol-gel derived ZnO) transistors for the first time. According to the DOSs distribution calculation, the influence of annealing temperature on ZnO sol-gel derived transistors has been determined and gave a glean of optimizing ZnO TFTs fabrication process. Sol-gel derived ZnO TFTs always have a negative threshold voltage, while RF magnetron sputtered ZnO TFTs are usually normally off. Besides that, the field effect mobility of RF magnetron sputtered ZnO is also larger than that in sol-gel derived ZnO TFTs. With the limitation of p-type ZnO and ZnO TFTs stably produce, sol-gel derived and RF magnetron sputtered n-type ZnO are combined to fulfill the basic function of logic circuits. Based on ZnO radiation-hard characteristics, gamma-ray irradiation effect has been investigated. After the different doses of radiation, ZnO TFTs survive but show various degradations. ZTO, which is a ZnO related material, was first carried out to overcome the low field-effect mobility of sol-gel derived ZnO TFTs. Compared to ZnO TFTs, ZTO TFTs with proper fabrication process demonstrate extremely high stability in gamma-ray irradiation.