Effects of Thermal Annealing on the Density of States in Low Voltage Operating Range, High Mobility, Hf-In-ZnO/HfO2 TFTs Fabricated at Temperatures below 200 oC

Abstract

In this paper, we report the effects of thermal annealing on Poly(methyl methacrylate) (PMMA) passivated, bottom gate thin film transistors, with amorphous hafnium oxide (HfO₂) as gate dielectric and amorphous hafnium-indium-zinc oxide (a-HIZO) as semiconductor, fabricated at temperatures below 200 ^oC. It is shown that TFTs, with V_TH =0.55 V, m_FE>250 cm²/Vs, SS=200 mV/dec corresponding to D_it= 1x10¹² cm^-2eV^-1 and I_on/I_off>10⁷, can be obtained, using a thermal annealing at 200 ^oC in N₂, after the semiconductor layer is deposited. The dielectric constant of the HfO₂ layer deposited by RF sputtering was 19.5, allowing devices to work within the operating voltage range of 2 V. An important increase of the field effect mobility is obtained, combining a high-k gate dielectric and a high carrier concentration a-HIZO layer, with a lower density of localized states.