Structural and dielectric properties of yttria-stabilised zirconia deposited from solutions at moderate temperatures in air

Antoniou, Giorgos and Vourlias, G. and Milne, W.I. and Adamopoulos, George (2017) Structural and dielectric properties of yttria-stabilised zirconia deposited from solutions at moderate temperatures in air. In: EMRS 2017, 2017-05-22 - 2017-05-26, FRANCE.

Abstract

Solution processed ZrO2 and its implementation as the active channel material in TFTs has already been demonstrated. The use however of substrate temperatures compatible with flexible glass substrates i.e. up to 500 oC resulted in ZrO2 dielectrics of a monoclinic structure (the “distorted” phase of a cubic structure) limiting its dielectric constant to 15. The desired “high-temperature” 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). This can also be achieved by reducing the crystallite size. Because of its high dielectric constant, in theory 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 yttrium and zirconium b-diketones in methanol. The YSZ films of various yttrium 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<2 nm) of a tetragonal (fluorite) phase with dielectric constant in the range between 16 and 21. TFT transistors employing YSZ deposited with an optimal Y content of about 4 % mole, reveal excellent operation characteristics in terms of low voltage operation, low leakage currents, high electron mobility in excess of 35 cm2/Vs and high on/off current modulation ration in the order of 107. These performance enhancements may be attributable to the large band offsets and small lattice mismatch between the YSZ dielectric and ZnO.