Understanding the Enhanced Mobility of Solution-Processed Metal-Oxide Thin-Film Transistors Having High-k Gate Dielectrics

Primarily used as transparent electrodes in solar-cells, more recently, physical vapor deposited(PVD) transparent conductive oxide (TCO) materials (e.g. ZnO, In2O3 and SnO2)also serve as the active layer in thin-film transistor (TFT) technology for modern liquidcrystaldisplays. Relative to a-Si:H and organic TFTs, commercial TCO TFTs have reducedoff-state leakage and higher on-state currents. Additionally, since they are transparent, theyhave the added potential to enable fully transparent TFTs which can potentially improvethe power efficiency of existing displays.In addition to PVD, solution-processing is an alternative route to the production of displaysand other large-area electronics. The primary advantage of solution-processing is inthe ability to deposit materials at reduced-temperatures on lower-cost substrates (e.g. glass,plastics, paper, metal foils) at high speeds and over large areas. The versatility offered bysolution-processing is unlike any conventional deposition process making it a highly attractiveemergent technology.Unfortunately, the benefits of solution-processing are often overshadowed by a dramatic reductionin material quality relative to films produced by conventional PVD methods. Consequently,there is a need to develop methods that improve the electronic performance ofsolution-processed materials. Ideally, this goal can be met while maintaining relatively lowprocessing temperatures so as to ensure compatibility with low-cost roll-compatible substrates.Mobility is a commonly used metric for assessing the electronic performance of semiconductorsin terms of charge transport. It is commonly observed that TCO materials exhibit significantlyhigher field-effect mobility when used in conjunction with high-k gate dielectrics (10to 100 cm2 V−1s−1) as opposed to conventional thermally-grown SiO2 (0.1 to 20 cm2 V−1s−1).Despite the large amount of empirical data documenting this bizarre effect, its physical ori-2gin is poorly understood.In this work, the interaction between semiconductor TCO films and high-k dielectrics isstudied with the goal of developing a theory explaining the observed mobility enhancement.Electrical investigation suggests that the mobility enhancement is due to an effective dopingof the TCO by the high-k dielectric, facilitated by donor-like defect states inadvertentlyintroduced into the dielectric during processing. The effect these states have on electrontransport in the TCO is assessed based on experimental data and electrostatic simulationsand is found to correlate with negative aspects of TFT behavior (e.g. frequency dispersion,gate leakage, hysteresis, and poor bias stability).Based on these findings, we demonstrate the use of an improved device structure, analogousto the concept of modulation doping, which uses the high-k dielectric film as an encapsulate,rather than a gate-dielectric, to achieve a similar doping effect. In doing so, the enhancedmobility of the TCO/high-k interface is retained while simultaneously eliminating the negativedrawbacks associated with the presence of charged defects in the gate dielectrics (e.g.frequency dispersion, gate leakage, hysteresis, and poor bias stability). This demonstratesimproved understanding of the role of solution-processed high-k dielectrics in field-effectdevices as well as provides a practical method to overcome the performance degradationincurred through the use of low-temperature solution-processed TCOs