Indium oxide-based thin film transistors implementing Yttria-Stabilised Zirconia gate dielectrics processed from dispersed nanoparticles by spray coating

Antoniou, Giorgos and Halcovitch, Nathan Ross and Milne, W.I. and Adamopoulos, George (2018) Indium oxide-based thin film transistors implementing Yttria-Stabilised Zirconia gate dielectrics processed from dispersed nanoparticles by spray coating. In: EMRS 2018, 2018-06-18 - 2018-06-22, Strasbourg.

Abstract

High-k dielectrics have extensively been studied as alternatives to SiO2 as gate dielectrics for the next generation of field-effect transistors employing metal-oxide semiconducting channels. Among these, ZrO2 is the most extensively studied dielectric and is widely considered to be excellent an candidate because of their relatively high dielectric constant, good thermal stability, and large band gap. The use however of substrate temperatures compatible with flexible glass i.e. up to 500 oC results in ZrO2 dielectrics of a monoclinic structure limiting its dielectric constant to 15. The desired high-temperature (>800 oC) ZrO2 with a tetragonal cubic phase can readily be stabilized, at lower temperatures by the use of different dopant elements (or compounds such as CaO, MgO, Y2O3). Because of its high dielectric constant (in excess of 24), yttria-stabilized zirconia (YSZ) has thus been suggested as a promising gate dielectric candidate. Here, we report on the deposition and characterisation of yttria-stabilised zirconia (YSZ) gate dielectrics grown by spray coating in air at moderate temperatures of about 450 oC from zirconium acetylacetonate in methanol. Y2O3 doping was achieved by blending the zirconium host precursor with Y2O3 nanoparticles (average diameter of 20 nm) dispersed in HCl solution. The YSZ films of various Y2O3 content were investigated by means of UV-Vis absorption spectroscopy, x-ray diffraction, AFM, admittance spectroscopy, spectroscopic ellipsometry, and field-effect measurements. Analyses of the YSZ films reveal smooth films (RRMS< 1 nm) of a cubic (fluorite) phase with dielectric constant in the range between 17 and 26. In2O3-based TFT transistors employing YSZ deposited at an optimal Y2O3 content of about 4 % mole, reveal excellent operation characteristics in terms of low voltage operation, low leakage currents (1 nA/cm2), high electron mobility in excess of 35 cm2/Vs and high on/off current modulation ration in the order of 10^7. These performance enhancements may be attributable to the large band offsets and small lattice mismatch between the YSZ dielectric and In2O3.