Contact Resistance and Stability Analysis of Oxide-Based Thin Film Transistors

摘要

This thesis focuses on 2 aspects of oxide-based thin-film transistors (TFTs), contact resistance and instability assessment. Determination of the contact resistance of indium tin oxide (ITO) on two wide- band gap semiconductors, zinc oxide (ZnO) and indium gallium oxide (IGO), is attempted and the effects of contact resistance on device performance is investigated. Both transistor and transfer length method structures are used and 3 material systems are employed: ZnO on SiO2, ZnO on aluminum titanium oxide (ATO), and IGO on SiO2. It is found the measured resistance is not dominated by contact resistance effects. It is concluded that the device dimensions used in this study are too large to yield an accurate estimate of the contact resistance. Second, a methodology for assessing the stability of oxide-based TFTs is developed and implemented. This stability assessment strategy is first applied to 3 semiconducting materials: ZnO, zinc indium oxide (ZIO), and IGO, using thermal silicon oxide as the gate dielectric. Relatively stable devices are obtained after post-deposition annealing at a temperature of approximately 600 deg. C for ZnO and IGO TFTs, and approximately 400 deg. C for ZIO TFTs. The presence of instability in these devices, which is more pronounced at a lower annealing temperature, results in a positive shift in the turn-on voltage and clockwise hysteresis in the drain current-gate voltage transfer curve. Such an instability is attributed to electron trapping near the channel/insulator interface. The stability a ZnO TFT fabricated using a spin-coat synthesized aluminum phosphate (AlPO) as the gate dielectric is also investigated. It is shown that stable TFTs can be fabricated with oxide-based channel layers if a high quality insulator is available and if a post-deposition anneal at an elevated temperature is employed.