High-k dielectrics based on solution processed composite oxides for metal oxide thin film transistors

Afouxenidis, Dimitrios and Milne, W.I. and Bin Esro, Mazran and Adamopoulos, George (2018) High-k dielectrics based on solution processed composite oxides for metal oxide thin film transistors. In: EMRS 2018, 2018-06-18 - 2018-06-22, Strasbourg.

Abstract

Whilst progress on solution-processed oxide semiconductors has been rapidly advancing, research efforts towards the development of dielectric materials has been relatively slow, with most of the reported work performed using dielectrics based on SiO2 that results in high voltage transistor operation and hence increased power consumption. Gate dielectric materials with high dielectric constants are desirable, but the band offset condition that requires a reasonably large band gap should also be satisfied. The latter is of particular concern when wide band gap (>3 eV) metal oxide semiconducting channels are being employed as the number of suitable candidate gate dielectric materials to be used will be significantly reduced. The obvious solution to both the low dielectric constant and narrow band gap issues could be the use of a composite dielectric material with combined high permittivity, wide band gap and low leakage current. Additionally, despite their extraordinary performance, vacuum-based deposition techniques still suffer from high manufacturing costs and limited large-area deposition capabilities. To overcome this issue, significant research has been focused on the development of alternative deposition processes based on solutions. Based on the above, here we report on the structural, optical, electronic and dielectric properties of a number of dielectric composites based on the combination of a high-k/low band gap dielectric such as Nb2O5, TiO2 and La2O3 with a low-k/wide band gap such as Al2O3. The Al2O3.TiO2, Al2O3.Nb2O5 and LaAlO3 composite gate dielectrics were grown from soluble precursors by spray coating in air at moderate temperatures (400 oC) varying the Al/Ti, Al/Nb and Al/La atomic ratios. Each compound was investigated by a wide range of characterisation techniques including UV−vis absorption spectroscopy, spectroscopic ellipsometry, atomic force microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy and impedance spectroscopy. The above-mentioned gate dielectrics were further implemented in thin film transistors employing spray coated ZnO semiconducting channels. Analyses reveal dielectric compounds that for optimised stoichiometries show very smooth (RRMS< 2nm) amorphous films with excellent dielectric properties i.e. high dielectric constants of about 15 combined with wide band gaps of 6 eV, low leakage currents in the order of 5 nA/cm2 and excellent environmental stability. The related thin film transistors employing these gate dielectrics and ZnO semiconducting channels exhibit excellent electron transport characteristics which are hysteresis-free, have tunable operation voltage (5-10 V), high on/off current modulation ratios of >10^6, and electron mobility of about 12 cm2 V-1 s-1.