tesi R. Miscioscia.pdf - EleA@UniSA

86 Chapter 3
to reduce gate currents by de-doping the insulating films from
undesired contaminants such as oxygen and moisture contents
[3][5][6]. The aim of this study is to obtain an electrical model
describing the effects of the analyzed non-idealities.
3.2 Samples preparation
For the gate contact, we have used commercial glass substrates
coated with Indium Tin Oxide (ITO) film (thickness 130 nm)
purchased from Delta Technologies, Ltd and processed after standard
cleaning procedures. Three kinds of polymeric materials have been
used to obtain the gate dielectric layers: Polyimide PI2556 from HD
Microsystems (PI), Polystyrene (PS) Poly(1-phenylethane-1,2-diyl))
and AZ5214E photoresist (PR) from Clariant. Gate dielectrics were
solution-processed by spin-coating technique, to obtain 1 µm-thick
films for each material. PI was cured as specified by the Manufacturer
[4] and PR was processed and hard-baked for 30’ at 115°C without
UV-light exposure to obtain a stable film. Pentacene was purchased
by Sigma-Aldrich and processed under class-100 clean room
environment and deposited without further purification.
Pentacene thermal evaporation was carried out at a base pressure
of 2·10 -7 mbar from an alumina-coated crucible. Pentacene layers
were deposited on the three samples in the same evaporation run,
obtaining 65 nm-thick films at the average growth rate of 0.7 Å/s. The
deposition process was performed at room temperature, to obtain
polycrystalline thin film phase according to [9]. Source and drain gold
contacts were deposited, after vacuum breaking, by thermal
evaporation through a shadow mask, at a base pressure of 10 -6 mbar.
The 50 nm-thick electrodes, on the top of pentacene film, were
deposited at the rate of 0.5 Å/s to fabricate a bottom-gate top-contacts
OTFT structure (figure 1). Devices geometries were: channel length L
= 500 µm, channel width W = 1200 µm, Drain (and Source) length LD
= LS = 1000 µm.
The Pentacene films were characterized by means of UV-vis and
near-infrared (NIR) optical absorbance, using films deposited on
quartz with a Perkin-Elmer Lambda 900 spectro-photometer in the
wavelength range of 200 ÷ 800 nm. The crystalline film structure was