Y2O3 nanoparticles-induced stabilisation of solution-processed ZrO2 high-k gate dielectrics for metal oxide thin film transistors

Antoniou, Giorgos and Halcovitch, Nathan Ross and Milne, W.I. and Adamopoulos, George (2018) Y2O3 nanoparticles-induced stabilisation of solution-processed ZrO2 high-k gate dielectrics for metal oxide thin film transistors. In: 7th International Symposium on Transparent Conductive Materials, 2018-10-14 - 2018-10-19, Chania, Crete, Greece.

Abstract

Oxide-based TFTs implementing high -k dielectrics have received much attention and a number of high mobility, low-voltage devices have been demonstrated. Among these high-k dielectrics, ZrO2 is the most extensively studied dielectric and is widely considered to be an excellent candidate due to its relatively high dielectric constant, good thermal stability, and wide band gap. The deposition ZrO2 at substrate temperatures compatible with flexible glass (i.e. up to 500 oC), yield a monoclinic ZrO2 phase, which imposes a limit of 15 for the dielectric constant. Doping Zirconia with CaO, MgO, CeO2 or Y2O3, the high temperature (>800 oC) tetragonal phase can be stabilised at considerably lower temperatures. In this study, the fabrication of YSZ-based TFTs by spray coating in ambient air at moderate temperatures (of about 450 oC) is being reported. ZrO2 has been deposited from solutions of zirconium acetylacetonate in methanol whereas Y2O3-assisted stabilisation was achieved by simple chemical blending of the aforementioned solution with Y2O3 nanoparticles (nanoparticle size < 20 nm) dispersion in low concentration HCl solutions. YSZ films’ physical properties were investigated by UV-Vis absorption spectroscopy, AFM, X-Ray diffraction, spectroscopic ellipsometry, admittance spectroscopy and field-effect measurements. The results reveal a phase transition from a monoclinic undoped ZrO2 to cubic Y2O3-stabilised ZrO2 at a Y2O3 content of about 4%. The YSZ films were characterised by a very smooth surface (RRMS < 1 nm), dielectric constant in the range between 17 and 26, low leakage currents (< 1 nA/cm2), wide band gaps and considerably improved long-range order. Thin film transistors employing YSZ and solution-processed In2O3 semiconducting channels showed excellent operation characteristics such as high on/off current modulation ratio (>107), negligible hysteresis, electron mobility in excess of 40 cm2 /Vs and incredible stability under constant bias stress.