High mobility, dual layer, c-axis aligned crystalline/amorphous IGZO thin film transistor

We demonstrate a dual layer IGZO thin film transistor (TFT) consisting of a 310 degrees C deposited c-axis aligned crystal (CAAC) 20 nm thick channel layer capped by a second, 30 nm thick, 260 degrees C deposited amorphous IGZO layer. The TFT exhibits a saturation field-effect mobility of similar to 20 cm(2)/V s, exceeding the mobility of 50nm thick single layer reference TFTs fabricated with either material. The deposition temperature of the second layer influences the mobility of the underlying transport layer. When the cap layer is deposited at room temperature (RT), the mobility in the 310 degrees C deposited CAAC layer is initially low (6.7 cm(2)/V s), but rises continuously with time over 58 days to 20.5 cm(2)/V s, i.e., to the same value as when the second layer is deposited at 260 degrees C. This observation indicates that the two layers equilibrate at RT with a time constant on the order of 5 x 10(6) s. An analysis based on diffusive transport indicates that the room temperature diffusivity must be of the order of 1 x 10(-18) cm(2) s(-1) with an activation enthalpy E-A < 0.2 eV for the mobility limiting species. The findings are consistent with a hypothesis that the amorphous layer deposited on top of the CAAC has a higher solubility for impurities and/or structural defects than the underlying nanocrystalline transport layer, and that the equilibration of the mobility limiting species is rate limited by hydrogen diffusion, whose known diffusivity fits these estimates. (C) 2015 AIP Publishing LLC.