(a) Schematic cross section of inverted-staggered (top contact) device geometry. Also shown are the potential probe positions at and . (b) Optical micrograph of the completed OTFT device structure showing the dielectric, patterned film, source and drain electrodes, and channel potential probes. (c) Circuit schematic for the gated four-probe TFT measurement configuration. Channel dimensions are , , , and . During sweeps, the source electrode is held at ground and is held constant as is swept. Current into the source and out of the drain electrodes is monitored. The channel potential is measured via high-impedance electrometers. Also shown is the close alignment of the film to the potential probes which ensures that the potential monitored at and is only that of the channel region.
AFM topographs of pentacene thin films deposited at a substrate temperature of on (a) and (b) . Films grown on (b) substrates (that have not seen any processing) show large grains which have a larger width-to-height aspect ratio and show sharper facets than films grown on (a) (that has been exposed to photoresist). Both show evidence of cracking due to thermal-expansion differences between the film and the substrate when cooling from to room temperature after film deposition. Maximum grain sizes observed on (a) and (b) are approximately 10 and , respectively.
Thin-film XRD (Log Counts vs ) of pentacene thin films deposited on and dielectrics. The traces are displaced vertically for clarity. Both thin-film phases are visible. Good film ordering is indicated by the four additional higher-order peaks observed.
Current vs voltage plots for pentacene thin films with Au and Ca top contacts. (a) and (b) Drain current as a function of drain voltage at different gate voltages . Note that for the Ca top contact devices, has been scaled by a factor of and for visibility. Forward and reverse sweep directions are indicated by the arrows for all plots. Each forward and reverse sweep was acquired in approximately for a total of per set (at all ). (c) and (d) Drain current as a function of gate voltage . Forward and reverse sweeps were acquired in approximately each.
Potential data and surface plots for devices with Au and Ca top contacts fabricated on . (a) and (b) Potential data acquired via the channel potential probes as a function of . Shown are the probe data, and , along with the extrapolated contact potential differences at the contacts and . The positions of and (forward sweep) are also indicated. (c) and (d) Surface plots of the channel potential as a function of gate voltage and position within the channel (d). The continuous surface shows the values of the applied source and drain potential and the channel potentials and . The solid and dashed lines, and , on the left-front and right-rear faces of each plot represent the estimated film potential just after the source and drain contacts. The differences between these values and the respective applied contact potential, and , represent the potential drops due to contact resistance (valid at only).
Film resistance and source and drain contact resistances as a function of for devices with (a) Au and (b) Ca top contacts. For Au top contacts, the contact resistance (at high ) is approximately an order of magnitude less than that for Ca top contacts.
(a) The temperature evolution of curves from for a device with Au top contacts fabricated on . (b) and as a function of temperature for devices with Au and Ca top contacts fabricated on . Both and remain fairly constant as the temperature decreases until approximately , where they begin to increase rapidly.
(a) Uncorrected and contact-corrected linear mobilities as a function of temperature for an Au top-contacted device on dielectric. (b) Activation plot showing log linear mobility as a function of . At very low temperatures , the mobility appears to be nearly temperature independent. The inset shows a close-up of the activated region from .
(a) Plot of , , and as a function of temperature for a device with Au top contacts on pentacene thin films on an dielectric. (b) Activation plot of vs at varying . Plots for all contact metal and substrate dielectric combinations yielded similar behavior. (c) for the film, source resistance, and drain resistance as a function of for Ca and Au top-contacted devices fabricated on .
Schematic of metal and pentacene energy-level alignments (a) before contact,(b)–(e) after contact. (b) and (c) Ohmic and barrier contacts formed by the Au- and Ca-pentacene contact interfaces as constructed by Mott-Schottky theory. (d) and (e) Au-pentacene and Ca-pentacene interfaces as modified by interface dipoles and interface in-gap states. If no interface states were present, holes would be injected via path 1 into pentacene. In the presence of interface states, holes may be able to travel through the barrier via path 2.
(a) Illustration of the idealized metal-organic top contact with an abrupt interface. (b) Metal-organic top contact with metal clusters intermixed in the film thickness. (c) Top contact where the electrically continuous metal contact reaches the semiconductor-dielectric interface and directly contacts the conducting channel.
(a) Film resistance and (b) total contact resistance in extrinsic and intrinsic units (in parentheses) for all contact metals and substrate dielectrics examined. Extrinsic units for all values are in ohms . Intrinsic units are given as (a) film sheet resistance and (b) contact resistivity . Resistance values were acquired at sample biases of: and for , and for .
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