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/n(PVD) transparent conductive oxide (TCO) materials (e.g. ZnO, In2O3 and SnO2)/nalso serve as the active layer in thin-film transistor (TFT) technology for modern liquidcrystal/ndisplays. Relative to a-Si:H and organic TFTs, commercial TCO TFTs have reduced/noff-state leakage and higher on-state currents. Additionally, since they are transparent, they/nhave the added potential to enable fully transparent TFTs which can potentially improve/nthe power efficiency of existing displays./nIn addition to PVD, solution-processing is an alternative route to the production of displays/nand other large-area electronics. The primary advantage of solution-processing is in/nthe ability to deposit materials at reduced-temperatures on lower-cost substrates (e.g. glass,/nplastics, paper, metal foils) at high speeds and over large areas. The versatility offered by/nsolution-processing is unlike any conventional deposition process making it a highly attractive/nemergent technology./nUnfortunately, the benefits of solution-processing are often overshadowed by a dramatic reduction/nin material quality relative to films produced by conventional PVD methods. Consequently,/nthere is a need to develop methods that improve the electronic performance of/nsolution-processed materials. Ideally, this goal can be met while maintaining relatively low/nprocessing temperatures so as to ensure compatibility with low-cost roll-compatible substrates./nMobility is a commonly used metric for assessing the electronic performance of semiconductors/nin terms of charge transport. It is commonly observed that TCO materials exhibit significantly/nhigher field-effect mobility when used in conjunction with high-k gate dielectrics (10/nto 100 cm2 V−1/ns/n−1/n) as opposed to conventional thermally-grown SiO2 (0.1 to 20 cm2 V−1/ns/n−1/n)./nDespite the large amount of empirical data documenting this bizarre effect, its physical ori-/n2/ngin is poorly understood./nIn this work, the interaction between semiconductor TCO films and high-k dielectrics is/nstudied with the goal of developing a theory explaining the observed mobility enhancement./nElectrical investigation suggests that the mobility enhancement is due to an effective doping/nof the TCO by the high-k dielectric, facilitated by donor-like defect states inadvertently/nintroduced into the dielectric during processing. The effect these states have on electron/ntransport in the TCO is assessed based on experimental data and electrostatic simulations/nand is found to correlate with negative aspects of TFT behavior (e.g. frequency dispersion,/ngate leakage, hysteresis, and poor bias stability)./nBased on these findings, we demonstrate the use of an improved device structure, analogous/nto the concept of modulation doping, which uses the high-k dielectric film as an encapsulate,/nrather than a gate-dielectric, to achieve a similar doping effect. In doing so, the enhanced/nmobility of the TCO/high-k interface is retained while simultaneously eliminating the negative/ndrawbacks associated with the presence of charged defects in the gate dielectrics (e.g./nfrequency dispersion, gate leakage, hysteresis, and poor bias stability). This demonstrates/nimproved understanding of the role of solution-processed high-k dielectrics in field-effect/ndevices as well as provides a practical method to overcome the performance degradation/nincurred through the use of low-temperature solution-processed TCOs.

Advisor: Subramanian, Vivek