Solution Processed SiO
2
and high-
k
Dielectrics for Metal Oxide-based CMOS TFTs
M. Esro
1
, D. Afouxenidis
1
, W.I. Milne
2
, G. Adamopoulos
1
*
1
Engineering Department, Lancaster University, LA1 4YR Lancaster, UK
2
Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK
*E-mail:
,
Tel.:
+44 1524 594063
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 SiO
2
.
Silicon dioxide is the most widely used dielectric material for optical and electronic applications. The SiO
2
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 SiO
2
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
o
C).
The first part of the present work reports on the deposition and characterisation of SiO
2
gate dielectrics grown
by spray coating in air at moderate temperatures i.e. 350
o
C from SiCl
4
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 (R
RMS
< 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/cm
2
at 1 MV/cm. Both XPS and FTIR measurements further confirmed SiO
2
structures. Thin
film transistors based on thermally grown C
60
and pentacene semiconducting channels employing both spray
coated as well as thermally grown SiO
2
gate dielectrics exhibit identical transport characteristics in terms of
hysteresis, leakage currents, carrier mobility and on/off current modulation ratio.
However, TFTs employing SiO
2
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 SiO
2
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.
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