Synergic strategies of composition-modified bilayer channel configuration and ozone-processed gate stacks for atomic-layer deposited In-Ga-Zn-O thin-film transistors

Composition-modified bilayer channel configuration was introduced for enhancing the device performance of In-Ga-Zn-O (IGZO) thin film transistors (TFTs), which was composed of 3-nm-thick In-rich prompt-IGZO and 12-nm-thick prime-IGZO layers for the formation of two-dimensional electron gas (2DEG) at interfaces. The cationic compositions of bilayer IGZO channels were designed by modulating the sub-cyclic ratios among the precursors during atomic-layer deposition process. To properly control the effect of modified compositions in bilayer channels, the oxidant for the formation of Al2O3 protection and gate insulator layers were chosen as ozone. The IGZO TFTs fabricated with bilayer channel configurations exhibited the carrier mobility of 45.5 cm2/Vs and the subthreshold swing of 0.24 V/dec, keeping the turn-on position at near 0 V of gate bias without performing additional heat treatments including post-annealing process at higher temperatures. From the investigations on the temperature-dependent variations in electrical conductivity and band alignments of bi-layered IGZO channels, composition controls of prime and prompt IGZO layers were found to be crucial for initiating the quantum confinement effect of 2DEG. Thus, the optimum compositions (In:Ga:Zn) of both layers were 1.4:1.0:2.0 and 5.0:1.0:3.4, respectively. The device using optimum bilayer channel composition exhibited excellent positive bias stress (PBS) stability due to the implementation of confinement barrier at interface between the prime and prompt layers, in which the threshold voltage shifts (∆VTH) were estimated to be as low as 1.34 V. Furthermore, it was found that the PBS instability was anomalously compensated even at higher temperature stresses owing to long-term hydrogen diffusion from the ozone-processed gate stacks. The ∆VTH was finally stabilized to 0.22 V at 80 °C with a lapse of stress time for 104 s.