(INVITED) Solution Processed SiO2 and high-k Dielectrics for MO-based CMOS TFTs

Adamopoulos, George (2016) (INVITED) Solution Processed SiO2 and high-k Dielectrics for MO-based CMOS TFTs. In: 6th International Symposium on Transparent Conductive Materials (TCMs), 2016-10-09 - 2016-10-13.

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 conventional dielectrics based on thermally grown SiO2. Silicon dioxide is the most widely used dielectric material for optical and electronic applications. The SiO2 has been produced by thermal oxidation of silicon, plasma enhanced chemical vapour deposition, sputtering, electron beam evaporation, atomic layer deposition etc. The conventional production of SiO2 by thermal oxidation by necessity requires the use of Si as the substrate and the other methods either produce low quality/poor interface material and/or require high deposition temperatures (>700 oC). The first part of the present work reports on the deposition and characterisation of SiO2 gate dielectrics grown by spray coating in air at moderate temperatures i.e. 350 oC from SiCl4 solutions in pentane-2,4-dione (0.1 M). The films were investigated by means of x-ray diffraction, XPS, UFM/AFM, admittance spectroscopy, UV-Vis absorption spectroscopy, spectroscopic ellipsometry and field-effect measurements. Analyses reveal smooth films (RRMS< 1 nm) of amorphous phase with dielectric constant of 3.8, optical band gap of 8.1 eV and leakage currents of 10-7 A/cm2 at 1 MV/cm. Both XPS and FTIR measurements further confirmed SiO2 structures. Thin film transistors based on thermally grown C60 and pentacene semiconducting channels employing both spray coated as well as thermally grown SiO2 gate dielectrics exhibit identical transport characteristics in terms of hysteresis, leakage currents, carrier mobility and on/off current modulation ratio. However, TFTs employing SiO2 gate dielectrics, usually require high voltage operation and hence increased power consumption. The ever increasing demand for high performance thin film transistors based on metal oxide channels has given a boost to the development of alternatives to SiO2 gate dielectrics with desirable characteristics in terms of thermal stability, band offsets and interface quality. To that end and among the different approaches, the use of high-k dielectrics is arguably the most attractive option, since it can enable low leakage currents, through the use of physically thicker films, as well as low-voltage operation. Thus the final part of the presentation will report on the structure and properties of a wide range of solution-processed binary and ternary gate dielectrics and their implementation in TFTs employing - similarly solution processed n and p type metal oxides semiconducting channels. It will focus on the band offsets, dielectric and optical as well as the dielectric/semiconductor interface properties and their effects on device operation. References 1. M. Esro, R. Mazzocco, G. Vourlias, O. Kolosov, A. Krier, W. I. Milne and G. Adamopoulos, Solution processed lanthanum aluminate gate dielectrics for use in metal oxide-based thin film transistors, Appl. Phys. Lett. 106, 203507, 2015 2. M. Esro, S. Georgakopoulos, H. Lu, G. Vourlias, A. Krier, W. I. Milne, W. P. Gillin and G. Adamopoulos, Solution processed SnO2:Sb transparent conductive oxide as an alternative to indium tin oxide for applications in organic light emitting diodes, J. Mater. Chem. C, 4, 3563, 2016. 3. D. Afouxenidis, R. Mazzocco, G. Vourlias, P. J. Livesley, A. Krier, W. I. Milne, O. Kolosov and G. Adamopoulos, ZnO-based thin film transistors employing aluminum titanate gate dielectrics deposited by spray pyrolysis at ambient air, ACS Appl. Mater. Inter., 7, 7334, 2015. 4. M. Esro, G. Vourlias, C. Somerton, W. I. Milne and George Adamopoulos, High mobility ZnO thin film transistors based on solution-processed hafnium oxide gate dielectrics, Adv. Funct. Mater., 25, 134, 2015.