404 research outputs found
Temperature-dependent contact resistances in high-quality polymer field-effect transistors
Contact resistances between organic semiconductors and metals can dominate
the transport properties of electronic devices incorporating such materials. We
report measurements of the parasitic contact resistance and the true channel
resistance in bottom contact poly(3-hexylthiophene) (P3HT) field-effect
transistors with channel lengths from 400 nm up to 40 m, from room
temperature down to 77 K. For fixed gate voltage, the ratio of contact to
channel resistance decreases with decreasing temperature. We compare this
result with a recent model for metal-organic semiconductor contacts. Mobilities
corrected for this contact resistance can approach 1 cm/Vs at room
temperature and high gate voltages.Comment: 10 pages, 4 figures, accepted to Appl. Phys. Let
Nonlinear charge injection in organic field-effect transistors
Transport properties of a series of poly(3-hexylthiophene) organic field
effect transistors with Cr, Cu and Au source/drain electrodes were examined
over a broad temperature range. The current-voltage characteristics of the
injecting contacts are extracted from the dependence of conductance on channel
length. With reasonable parameters, a model of hopping injection into a
disordered density of localized states, with emphasis on the primary injection
event, agrees well with the field and the temperature dependence of the data
over a broad range of temperatures and gate voltages.Comment: 7 pages, 7 figures, sub. to J. Appl. Phy
Gated nonlinear transport in organic polymer field effect transistors
We measure hole transport in poly(3-hexylthiophene) field effect transistors
with channel lengths from 3 m down to 200 nm, from room temperature down
to 10 K. Near room temperature effective mobilities inferred from linear regime
transconductance are strongly dependent on temperature, gate voltage, and
source-drain voltage. As is reduced below 200 K and at high source-drain
bias, we find transport becomes highly nonlinear and is very strongly modulated
by the gate. We consider whether this nonlinear transport is contact limited or
a bulk process by examining the length dependence of linear conduction to
extract contact and channel contributions to the source-drain resistance. The
results indicate that these devices are bulk-limited at room temperature, and
remain so as the temperature is lowered. The nonlinear conduction is consistent
with a model of Poole-Frenkel-like hopping mechanism in the space-charge
limited current regime. Further analysis within this model reveals consistency
with a strongly energy dependent density of (localized) valence band states,
and a crossover from thermally activated to nonthermal hopping below 30 K.Comment: 22 pages, 7 figures, accepted to J. Appl. Phy
Zero-bias anomalies in electrochemically fabricated nanojunctions
A streamlined technique for the electrochemical fabrication of metal
nanojunctions (MNJs) between lithographically defined electrodes is presented.
The first low-temperature transport measurements in such structures reveal
suppression of the conductance near zero-bias. The size of the zero-bias
anomaly (ZBA) depends strongly on the fabrication electrochemistry and the
dimensions of the resulting MNJ. We present evidence that the nonperturbative
ZBA in atomic-scale junctions is due to a density of states suppression in the
leads.Comment: 4 pages, 4 figure
Magnetoresistance of atomic-scale electromigrated nickel nanocontacts
We report measurements of the electron transport through atomic-scale
constrictions and tunnel junctions between ferromagnetic electrodes. Structures
are fabricated using a combination of e-beam lithography and controlled
electromigration. Sample geometries are chosen to allow independent control of
electrode bulk magnetizations. As junction size is decreased to the single
channel limit, conventional anisotropic magnetoresistance (AMR) increases in
magnitude, approaching the size expected for tunneling magnetoresistance (TMR)
upon tunnel junction formation. Significant mesoscopic variations are seen in
the magnitude and sign of the magnetoresistance, and no evidence is found of
large ballistic magnetoresistance effects.Comment: 3 pages, 3 figure
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