Application in Field-Effect Transistors
Shuhong Liu
Department of Materials Science and Engineering
Advisor: Professor Zhenan Bao
Wednesday, May 14, 2008, 1:30 pm, Packard building, Room 101
(Refershment to be served at 1:15 pm)
Abstract:
The search for low-cost, large area, flexible devices has led to a
remarkable increase in the research and development of organic
semiconductors. Single-crystal organic field-effect transistors are
ideal device structures for studying fundamental science associated
with charge transport in organic materials and have demonstrated high
mobility and outstanding electrical characteristics. For example, an
exceptionally high carrier mobility of 20 cm2/Vs has been demonstrated
for rubrene single crystal field effect transistors. However, it
remains a technical challenge to integrate single-crystal devices into
practical electronic applications. A key difficulty is that organic
single-crystal devices are usually fabricated one device at a time by
handpicking a single crystal and placing it onto the device substrate.
This makes it impossible to mass-produce at high density with
reasonable throughput. Therefore, there is a great need for a
high-throughput method for depositing large arrays of organic
semiconductor single crystals directly onto device structures.
In this talk, I?ll present several approaches towards realizing this
goal. In the first part, I?ll introduce a solution-processing
technique that relies on solvent wetting and de-wetting on substrates
with patterned wettability to selectively direct the deposition or
removal of organic crystals. The assembly of different organic
crystals over centimeter-squared areas on Au, SiO2 and flexible
plastic substrates is demonstrated. By designing line features on the
substrate, alignment of needle-like crystals is also achieved. As a
demonstration of the potential application of this approach, arrays of
organic single crystal FETs are fabricated by patterning organic
single crystals directly onto and between transistor source and drain
electrodes. Besides organic single crystals, our self-assembly
strategy is also be applicable for patterning other objects such as
metallic nanowires.
In the second part, I?ll present a vapor-processing technique that
patterns organic single crystals using carbon nanotube (CNT) bundles
as templates. Several organic semiconductor materials are successfully
patterned, including p-type pentacene, tetracene, sexiphenylene, and
sexithiophene, as well as n-type tetracyanoquinodimethane (TCNQ). This
study suggests that the selective growth of crystals onto patterned
carbon nanotubes is most likely due to the coarse topography of the
CNT bundles. Moreover, we observe that the crystals nucleate from CNT
bundles and grow onto CNT bundles in a conformal fashion. The crystal
growth can be directly applied onto transistor source-drain electrodes
and arrays of organic single-crystal field effect transistors are
demonstrated. To investigate the impact of CNTs on device performance,
CNT bundles are incorporated into thin-film FETs and a mobility
enhancement of organic semiconductors is observed.
In the third part, I?ll present a method that offers the control of
the size and shape of organic single crystals using patterned Au films
as templates. It is observed that ?-sexithiophene (?-6T) crystals
nucleate from the edge or the top surface of Au films and then grow
two dimensionally on SiO2 surface. The sizes and shapes of ?-6T
crystals are precisely determined by that of the Au patterns. After
removing Au templates, large arrays of ?-6T crystals with controlled
sizes and various shapes such as stripes, squares, hexagons, etc. are
achieved. Top-contact FETs made of ?-6T ribbons are demonstrated.
Besides organic single crystals, Au templates can also act as
templates to pattern vapor- and solution-deposited organic
semiconductor thin films. Patterned organic thin-film FETs exhibit
superior performance compared to unpatterned devices.
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