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We studied the low frequency noise in top-contact pentacene Thin Film Transistors (TFTs). The relative spectral noise density of the drain current fluctuations SI/I2 had a form of 1/f noise in the measured frequency range 1Hz - 3.5kHz.Our studies of the noise dependencies on the gate-source VGS and drain-source VDS voltages showed that the dependencies differed from those observed for conducting polymers and resembled those reported for crystalline Si n-MOSFETs.To compare the device noise level with those of other devices and materials, we extracted the Hooge parameter α. In order to calculate the total number of carriers we used a model simulating the device DC characteristics, similar to that for amorphous Si TFTs. The extracted Hooge parameter was 0.04. For an organic material this is an extremely small value, which is three orders of magnitude smaller that the Hooge parameter values reported for conducting polymers and only several times higher than the values for amorphous Si TFTs.

We have investigated the polymeric insulators benzocyclobutene (BCB), parylene C and polyimide for use as gate dielectrics in pentacene organic thin film transistors (TFTs). Atomic force microscopy (AFM) was used to examine the surface roughness of the polymeric dielectrics and the morphology of pentacene films deposited onto them. X-ray diffraction was used to examine the molecular ordering of pentacene films deposited onto the polymeric dielectrics. We find a correlation between the surface roughness of the gate dielectric and the grain size in deposited pentacene films, with smooth surfaces yielding larger, more dendritic grains. Despite significant changes in film morphology, pentacene TFTs using BCB, parylene C, or polyimide as the gate dielectric have performance comparable to devices using SiO2 as the gate dielectric. These results suggest that there is not a strong correlation between pentacene film grain size and field-effect mobility for these devices. Pentacene TFTs using BCB as the gate dielectric had field-effect mobility as high as 0.7 cm2/V-s, on/off ratio > 107, subthreshold slope less than 2 V/decade, and negative threshold voltage, making them an attractive candidate for use in organic-based large-area electronic applications on flexible substrates.

Reducing the operating voltage and power consumption of organic-based logic circuits for portable applications is a critical step towards the commercialization of organic electronics.

We report reduced contact resistance of single-layer graphene devices by using ultraviolet ozone treatment to modify the metal/graphene contact interface. The devices were fabricated from mechanically transferred, chemical vapor deposition grown single layer graphene. Ultraviolet ozone treatment of graphene in the contact regions as defined by photolithography and prior to metal deposition was found to reduce interface contamination originating from incomplete removal of poly(methyl-methacrylate) and photoresist. Our control experiment shows that exposure times up to 10 min did not introduce significant disorder in the graphene as characterized by Raman spectroscopy. By using the described approach, contact resistance of less than 200 Ω μm was achieved for 25 min ultraviolet ozone treatment, while not significantly altering the electrical properties of the graphene channel region of devices.

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Band offsets at the interfaces of n- and p-type InP ((100) and (111)A) and atomic-layer-deposited (ALD) Al2O3 were measured with internal photoemission and spectroscopic ellipsometry. Similarly, the band offsets at the interface of semi-insulating InP (100) and ALD HfO2 were also determined. The barrier between the top of InP valence band (VB) and the bottom of Al2O3 conduction band (CB) is found to be 3.44 eV for p-type material and 3.53 eV for n-type. The photoemission thresholds are found to be sensitive to the annealing conditions, and blue shifts are observed after annealing. The offsets from InP valence band to the HfO2 conduction band for the HfO2/InP stack are found to be 3.89 eV, and we observed an increase of 60 meV if the InP surface is passivated.

We report a unique way to measure the energy band offset of a heterojunction by exploiting the light absorption profile in the heterojunction under visible-ultraviolet internal photoemission. This method was used to determine the band alignment of W/Al2O3/n+InAs/p+Al0.45Ga0.55Sb heterojunctions that are of interest for tunnel field-effect transistors. The barrier heights from the InAs and Al0.45Ga0.55Sb valence band maxima to the Al2O3 conduction band minimum are found to be 3.24 eV± 0.05 eV and 2.79 eV± 0.05 eV, respectively, yielding a 0.4 eV± 0.1 eV offset at the InAs/AlGaSb interface. This approach can readily be applied to characterize a wide range of other semiconductor heterojunctions.

We present the observation of magnetoresistance in Co/Ca/Alq3/Ca/NiFe spin-valve devices. Thin Ca layers contacting 150 nm thick Alq3 enable the injection of spin-polarized electrons into Alq3 due to the engineering of the band alignment. The devices exhibit symmetric current-voltage (I–V) characteristics indicating identical metal contacts on Alq3, and up to 4% of positive magnetoresistance was observed at 4.5 K. In contrast, simultaneously fabricated Co/Alq3/NiFe devices displayed asymmetric I–V curves due to the different metal electrodes, and spin-valve effects were not observed.

We determined the band alignment of a graphene-insulator-semiconductor structure using internal photoemission spectroscopy. From the flatband voltage and Dirac voltage, we infer a 4.6× 1011cm−2 negative extrinsic charge present on the graphene surface. Also, we extract the graphene work function to be 4.56 eV, in excellent agreement with theoretical and experimental values in literature. Electron and hole injection from heavily doped p-type silicon (Si) are both observed. The barrier height from the top of the valence band of Si to the bottom of the conduction band of silicon dioxide (SiO2) is found to be 4.3 eV. The small optical absorption in graphene makes it a good transparent contact to enable the direct observation of hole injection from Si to graphene. The barrier height for holes escaping from the bottom of Si conduction band to the top of SiO2 valence band is found to be 4.6 eV.

The fabrication of top metal contacts for organic devices represents a challenge and has important consequences for electrical properties of such systems. We report a robust, low-cost and nondestructive printing process, flip chip lamination, to fabricate top contacts on rubrene single crystals. The use of surface chemistry treatments with fluorinated self-assembled monolayers, combined with pliable substrates, and mild nanoimprint conditions, ensures conformal contact between ultrasmooth metal contacts and the organic crystal. Space-charge limited current measurements point to better interfacial electrical properties with the flip chip lamination-fabricated contacts compared to the analog architecture of e-beam evaporated top contacts.

The structural and electrical properties of a solution processable material, 2,8-difluoro-5,11-tert-butyldimethylsilylethynyl anthradithiophene (TBDMS), were measured for single crystal transistors. TBDMS is observed to readily form single crystals from physical vapor zone sublimation. A columnar packing crystal structure, with an approximate π/4 radian rotational offset between neighboring molecules, is observed. Single crystal TBDMS transistors display a maximum observed saturation mobility μS of 0.07 cm2/V s, current on-off ratio >107, and subthreshold swing S≈1 dec/V. The spectral current noises of single crystal devices display a 1/f flicker noise, while the metal-semiconductor charge injection barrier is estimated by ultraviolet photoemission spectroscopy.

We report scanning Kelvin probe microscopy (SKPM) of single crystal difluoro bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) organic transistors. SKPM provides a direct measurement of the intrinsic charge transport in the crystals independent of contact effects and reveals that degradation of device performance occurs over a time period of minutes as the diF-TESADT crystal becomes charged.

Spray-deposited poly-3-hexylthiophene (P3HT) transistors were characterized using electrical and structural methods. Thin-film transistors with octyltrichlorosilane treated gate dielectrics and spray-deposited P3HT active layers exhibited a saturation regime mobility as high as 0.1 cm2 V−1 s−1, which is comparable to the best mobilities observed in high molecular mass P3HT transistors prepared using other methods. Optical and atomic force microscopy showed the presence of individual droplets with an average diameter of 20 μm and appreciable large-scale film inhomogeneities. Despite these inhomogeneities, near-edge x-ray absorption fine structure spectroscopy of the device-relevant channel interface indicated excellent orientation of the P3HT.

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The buried interface composition of polymer-fullerene blends is found by near-edge x-ray absorption fine structure spectroscopy to depend on the surface energy of the substrate upon which they are cast. The interface composition determines the type of charge transport measured with thin film transistors. These results have implications for organic photovoltaics device design and the use of transistors to evaluate bulk mobility in blends.

Frequency dependent capacitance-voltage characteristics of organic thin-film transistors based on poly(3-hexylthiophene) as the active polymer layer are investigated. The frequency response of the channel capacitance in accumulation is examined through an analytical transmission line model, with the effect of contact resistances included in the model to account for deviations from ideal behavior. The model provides an excellent fit to the data. Furthermore, we show that the technique can be used to extract device parameters such as the mobility and the contact resistance and quantitative information on the influence of charge trapping on transport.

We report on observations of a correlation between the microstructure of organic thin films and their electronic properties when incorporated in field-effect transistors. We present a simple method to induce enhanced grain growth in solution-processed thin film transistors by chemical modification of the source-drain contacts. This leads to improved device performance and gives a unique thin film microstructure for fundamental studies concerning the effect of structural order on the charge transport. We demonstrate that the 1∕f flicker noise is sensitive to organic semiconductor thin film microstructure changes in the transistor channel.

We report on charge transport in organic field-effect transistors based on poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) as the active polymer layer with saturation field-effect mobilities as large as 1cm2V−1s−1. This is achieved by employing Pt instead of the commonly used Au as the contacting electrode and allows for a significant reduction in the metal/polymer contact resistance. The mobility increases as a function of decreasing channel length, consistent with a Poole-Frenkel model of charge transport, and reaches record mobilities of 1cm2V−1s−1 or more at channel lengths on the order of few microns in an undoped solution-processed polymer cast on an oxide gate dielectric.

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We report on a strong field-dependent mobility in organic field-effect transistors fabricated by using poly(2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT-C14) as the active polymer layer. Charge transport and mobilities in devices annealed in the mesophase show a more pronounced dependence on channel length as compared with as-cast devices. Analysis reveals that the contact effects in both sets of devices are negligible from room temperature down to ≈100K. We show that this field dependence is consistent with a Poole-Frenkel model of mobility. Finally, the nonlinear transport data for short channel devices are modeled consistently in the Poole-Frenkel framework over a broad temperature range.

We fabricated ZnO nanowire field effect transistors (FETs) and systematically characterized their low frequency (f) noise properties. The obtained noise power spectra showed a classical 1∕f dependence. A Hooge’s constant of 5×10−3 was estimated from the gate dependence of the noise amplitude. This value is within the range reported for complementary metal-oxide semiconductor (CMOS) FETs with high-k dielectrics, supporting the concept that nanowires can be utilized for future beyond-CMOS electronic applications from the point of view of device noise properties. ZnO FETs measured in a dry O2 environment displayed elevated noise levels that can be attributed to increased fluctuations associated with O2− on the nanowire surfaces.

We report on parasitic contact effects in organic thin film transistors (OTFTs) fabricated with pentacene films. The influence on the OTFT performance of the source and drain contact metal and the device design was investigated. Top contact (TC) and bottom contact (BC) gated transmission line model (gated-TLM) test structures were used to extract the combined parasitic contact resistance as a function of gate voltage swing and drain-source voltage for OTFTs with gold source and drain contacts. For comparison BC test structures with palladium contacts were studied. Differences in the bias dependence of the contact resistance for TC and BC OTFTs indicate that charge injection and device performance are strongly affected by the device design and processing. The results from this investigation show that TC and BC device performances may be contact limited for high mobility OTFTs with channel lengths less than 10μm.

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We report on the generation of a discrete trap state during negative gate bias stress in pentacene single crystal "flip-crystal" field-effect transistors with a SiO2 gate dielectric. Trap densities of up to 2*10^12 cm^-2 were created in the experiments. Trap formation and trap relaxation are distinctly different above and below ~280 K. In devices in which a self-assembled monolayer on top of the SiO2 provides a hydrophobic insulator surface we do not observe trap formation. These results indicate the microscopic cause of the trap state to be water adsorbed on the SiO2 surface., Comment: 13 pages, 4 figures, submitted to Applied Physics Letters

In order to gain further insight into the details of charge transport in organic semiconductor devices it is necessary to characterize the density of trap states at the semiconductor∕gate dielectric interface. Here we use the technique of gate bias stress to quantitatively determine the interface trap density in rubrene single-crystal field-effect transistors with two different types of interfaces. A reversible and reproducible shift of the I‐V characteristics is observed upon both negative and positive gate bias stress, whose physical origin is identified as charge trapping and detrapping at the crystal∕SiO2 insulator interface. We can thus quantify the density of interface traps that are alternately filled and emptied on a time scale of ≅1h in the energy range defined by the applied bias stress. For a typical rubrene∕SiO2 interface we extract a density of ∼2×1012cm−2 at a stress bias of ±50V, corresponding to a volume density of ≅1019∕(cm3eV). An octadecyltrichlorosilane treatment of the SiO2 dielectric...

We report on n-channel organic thin-film transistors (OTFTs) with field-effect mobility comparable to that typically reported for p-channel OTFTs fabricated from pentacene. The OTFTs were fabricated on oxidized silicon wafers using N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI–C13H27) as the semiconductor and with Au, Cr, Al, and LiF∕Al source and drain contacts. Accumulation mode n-channel transistor operation is demonstrated for all contact metals despite the large differences in their work functions. High field-effect mobility near 0.6cm2∕Vs and large Ion∕Ioff of 107 were achieved. Device performance is sufficient to demonstrate pentacene∕PTCDI–C13H27 TFT complementary inverters with record gain.

We have fabricated and demonstrated active-matrix liquid-crystal displays using organic thin-film transistors (OTFTs) on polyester substrates. This is the first reported demonstration of an OTFT active-matrix liquid-crystal display, and also the first demonstration of a TFT active-matrix liquid-crystal display of any type fabricated on a polyester substrate.