ZnO-based thin film transistors employing Nb1-xAlxOy high-k dielectrics processed from solutions in ambient air

Afouxenidis, Dimitrios and Vourlias, G. and Milne, W.I. and Adamopoulos, George (2016) ZnO-based thin film transistors employing Nb1-xAlxOy high-k dielectrics processed from solutions in ambient air. In: EMRS 2016 Spring Meeting, 2016-05-02 - 2016-05-06.

Abstract

In thin film transistors (TFTs) a wide range of binary metal oxides, such as Al2O3, Y2O3, ZrO2, HfO2, TiO2, Nb2O5, have already been demonstrated as promising gate dielectrics with superior properties compared with SiO2. 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 point constitutes a significant drawback when wide band gap semiconducting channels need to be employed as such dielectrics cannot possess at the same time wide band gaps and high permittivity. The obvious solution to both the low dielectric constant and narrow band gap issues could be the use of a composite dielectric material, i.e. a ternary oxide that combines wide band gap, high permittivity and low leakage current. To that end, the use of ternary oxides, such as La2O3.Al2O3[1] and Al2O3.TiO2[2]has already been reported. Here we report on the deposition and characterisation of Nb1-xAlxOy gate dielectrics as a function of the Nb to Al atomic ratio and their implementation in TFTs employing ZnO semiconducting channels. The films were deposited by spray coating at moderate substrate temperatures (< 400 oC) in air and characterised by UV-Vis, FT-IR, impedance spectroscopy, AFM, XRD and field-effect measurements. Analyses showed Nb1-xAlxOy smooth films with dielectric constant in the range between 9 and 40 and optical band gap between 6.1 eV and 4.2 eV. TFTs employing Nb1-xAlxOy dielectrics and ZnO semiconducting channels show low leakage currents (<6 nA/cm2), low subthreshold swing, high on/off current modulation ratio (>10^6) and electron mobilities in excess of 10 cm2 V−1 s−1. [1] D. Afouxenidis, R. Mazzocco, G. Vourlias, A. Krier, W. I. Milne, O. Kolosov and G. Adamopoulos, ACS Applied Materials and Interfaces, 7, 7334, 2015. [2] M. Esro, R. Mazzocco, G. Vourlias, O. Kolosov, A. Krier, W. I. Milne and G. Adamopoulos, Appl. Phys. Lett. 106, 203507, 2015.s