Page 32 - Organic Electronics in Sensors and Biotechnology

Scaling Effects in Organic Transistors and Transistor-Based Chemical Sensors 9

and higher substrate temperatures during sublimation tend to pro-
duce an organic layer with large grain size and planar morphology.
The substrate temperature has a stronger influence than the sublima-
tion rate. However, attempts to deposit large pentacene grains onto
nanoscale channels at elevated substrate temperatures did not yield
favorable results. This could be attributed to the repulsion of the Au
electrodes from the pentacene molecules at elevated substrate tem-
peratures. Usually 300 to 350 Å is the minimal thickness for the
organic semiconductor to uniformly cover the channel region,
whereas too thick an organic layer will not increase the gate-induced
channel conduction (the channel is only one or two monolayers deep)
but increase the non-gate-induced bulk conduction and therefore
reduce the on/off current ratio of the field-effect transistor. To form
an organic semiconductor layer with high mobility in field-effect
transistors, the requirements for the quality and the morphology of
the organic thin film are exacting. The source material has to be in
high purity, usually post-purchase purified with cycling many times
under a temperature gradient in vacuum. The grown film has to be
uniform and flat and have an ordered molecular stacking. For this
purpose, after UV ozone or gentle oxygen plasma cleaning, a surface
treatment is desired to form a well-ordered self-assembled monolayer
of alkyl groups, before depositing an organic semiconductor layer.
Two commonly used materials for a self-assembled monolayer (SAM)
on top of SiO are HMDS (hexamethyldisilazane) and OTS [trichloro
2
(octadecyl)silane], because these molecules have two functional
groups: silane group on one end to bond with SiO and alkyl group
2
on the other end to provide a hydrophobic layer for the molecules
of organic semiconductor to grow. This self-assembled monolayer
will improve the molecular ordering of the pentacene layer and
reduce the trapped charges at the dielectric-semiconductor inter-
face, which will enhance the field-effect mobility and improve the
subthreshold slope. 47, 48 Within the other type of SAM agent molecule
4-nitrobenzenethiol to treat the interface between organic semicon-
ductor and electrodes, its surface active head group (thiol) can anchor
to the electrodes through a dative bond between the sulfur atom and
the metals such as Au, Pd, Pt, and Ag. This treatment can signifi-
cantly reduce the contact resistance by a factor of 10 at room tempera-
48
ture and even more at low temperatures, which is important for the
low-temperature measurements described here. Ideally, HMDS only
forms a monolayer on SiO , without chemisorption onto electrodes,
2
and 4-nitrobenzenethiol only adsorbs onto electrodes. This ideally
gives an orthogonal self-assembly without interaction between
dielectric treatment and contact treatment. 47
For the device characterization, a Table Top Manipulated Probe
System from Desert Cryogenics was employed, at varying tempera-
4
tures and over two orders of magnitude of electric field (from 10 to
6
10 V/cm). The DC I–V characteristics for all transistors of different