Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A25: Focus Session: Organic Field Effect Transistors |
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Sponsoring Units: DPOLY DMP Chair: David Martin, University of Michigan Room: Baltimore Convention Center 322 |
Monday, March 13, 2006 8:00AM - 8:36AM |
A25.00001: Organic Semiconductors and Nanodielectrics for Flexible, Low Voltage Thin-Film Transistors Invited Speaker: Molecular materials scientists are skilled at designing and constructing individual molecules with the goal of imbuing them with predetermined chemical and physical properties. However, the subsequent task of rationally assembling them into organized, functional supramolecular architectures with precise, nanometer-level control of bulk opt-electronic properties presents another level of challenge. In this lecture, synthetic and computational approaches to addressing such problems are described in which the ultimate goal is the fabrication of flexible electronic circuits employing unconventional materials classes and unconventional fabrication techniques. The issues here concern not only the rational design, realization, and understanding of high-mobility p- and n-type organic semiconductors, but also robust enabling nanoscopic gate dielectrics having ultra-high capacitance, low leakage, and high breakdown fields. In the former area, routes to and properties of, new high-mobility heterocyclic materials are described. These materials are then used to fabricate high-performance organic thin film transistors and CMOS circuits. In the latter topic, the design, synthesis, and characterization of new high-k nanoscopic gate dielectrics are described. It is then shown how these dielectrics can be employed to significantly enhance the performance of thin-film transistors and other devices fabricated from a wide variety of both organic as well as inorganic semiconductors. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 8:48AM |
A25.00002: Complex Organic Semiconductor Devices Utilizing Threshold Voltage Shifting and Carrier Sign Reversal Howard Katz, Cheng Huang, James West We consider a range of materials and mechanisms that lead to nonvolatile switching in field-effect transistors made from organic semiconductors (OFETs). Charging of gate dielectrics, secondary dielectrics, and/or the semiconductor/dielectric interface can greatly shift the threshold voltage in these OFETs. The dielectrics can consist of nonpolar electrets or ferroelectric polymers. In the particular case of SiO$_{2}$ electret coated with a hydrophobic surface layer, the shift is permanent on the timescale of years and is in quantitative agreement with measured surface charge. The semiconductor perfluoro copper phthalocyanine is shifted from electron- to hole-carrying by application of sufficient field from a corona apparatus. Inverter circuits were built from pairs of OFETs based on a single semiconductor, differing only in the quantity of stored gate charge. [Preview Abstract] |
Monday, March 13, 2006 8:48AM - 9:00AM |
A25.00003: Improving the electrical characteristics of a solution-processable, anthradithiophene organic semiconductor by solvent vapor annealing Kimberly Dickey, John Anthony, Yueh Lin Loo The development of organic semiconductors is driven by the promise of low-cost device applications. To fully realize cost-effective organic electronics, solution-processable materials need to be developed. While several solution-processable materials have been demonstrated, these materials often suffer from significantly reduced carrier mobilities due to defects and grain boundaries introduced during the deposition process. We have been studying triethylsilyl anthradithiophene (TES ADT), a solution-processable, p-type organic semiconductor. Transistors fabricated with spun-cast TES ADT exhibit low carrier mobilities (0.002cm$^{2}$/V-s). Subjecting the fabricated transistors to dichloroethane solvent vapor annealing, however, yields average carrier mobilities of 0.2cm$^{2}$/V-s. Additionally, the current hysteresis observed during device operation is eliminated with this annealing. This dramatic improvement in transistor performance is solvent choice dependent, and can be directly correlated with morphological transformations in the thin film. [Preview Abstract] |
Monday, March 13, 2006 9:00AM - 9:12AM |
A25.00004: Dielectric relaxation in semiconducting regioregular poly(3-hexylthiophene) Tatiana Psurek, Jan Obrzut Regioregular effect transistor applications. While, the electrical mobility properties of P3HT have been examined$^{ }$the dielectric poly(3-hexylthiophene) (P3HT) is a material typically used as an organic semiconductor in thin film field- relaxation processes have not been systematically studied. We investigated the dielectric response of P3HT films in the temperature range of 240~K to 315~K and at frequencies up to 12 GHz. In the low frequency range P3HT shows a semi-conducting character and the overall dielectric response is dominated by dc-conductivity with $E_{a}$~=~17~kJ/mol. The apparent semi-conducting character cease at certain frequency above which, the material becomes a dielectric. At room temperature the semi-conducting to dielectric transition takes place at about 5 kHz. In the range of 10 kHz to 12 GHz the dielectric dispersion is insignificant and the dielectric constant is about 4. The dielectric relaxation process, with \textit{$\tau $} = 3*10$^{-5}$s and $E_{a}$~=~24~kJ/mol, can be attributed to local molecular motion of thiophene rings. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:24AM |
A25.00005: All Electronic Time of Flight Mobility Measurements in Pentacene Organic Field Effect Transistors Lawrence Dunn, Debarshi Basu, Liang Wang, Ananth Dodabalapur One of the most important figures of merit for an organic semiconductor is its mobility, which is the proportionality constant relating the velocity of charge carriers to the electric field in the material. We present an all-electronic time of flight method for measuring the in-plane mobility of charge carriers using a voltage pulse (rise time $<$60 ns) applied to the source of an OFET at temperatures between 300 K and 77 K. The transit time mobilities extracted at all temperatures are compared with the DC mobilities extracted from the OFET transfer characteristics. We observe significantly higher transit time mobilities at higher temperatures that merge with the DC mobilities at lower temperatures. This is noteworthy because the transit time technique measures the mobility of the fastest carriers, which at higher temperatures seems to be uninfluenced by the interaction of charge carriers with deep traps, and also before the bias stress effect influences transport. By comparing the DC and transit time mobilities at varying temperatures we may gain an understanding of the time scale of the charge carriers' interaction with traps in the semiconducting material. [Preview Abstract] |
Monday, March 13, 2006 9:24AM - 9:36AM |
A25.00006: Infrared Imaging of Charge Injection Landscape in Organic Field-Effect Transistors Zhiqiang Li, Guangming Wang, Na Sai, Daniel Moses, Michael Martin, Massimiliano Di Ventra, Alan Heeger, Dimitri Basov We report on infrared (IR) spectro-microscopy study of charge injection phenomena in organic field-effect transistors (OFET) based on regioregular poly(3-hexylthiophene) (P3HT). An analysis of the oscillator strength of the spectroscopic signatures of charge injection allowed us to quantify the density of the injected carriers and examine its evolution with applied voltages. Using IR microscopy, the uniformity of the field- induced charge layer in OFETs with high dielectric constant insulator TiO$_{2} $ as well as SiO$_{2}$ as gate dielectrics was explored. These data uncover the critical role of the gate insulator in defining the relevant charge injection length scales. This work demonstrates the unique potential of IR spectroscopy for the investigation of the electronic excitations in nanometer-thick accumulation layers in OFET devices. [Preview Abstract] |
Monday, March 13, 2006 9:36AM - 9:48AM |
A25.00007: Oligothiphene nanostructure evolution in transition from monolayer to multilayers. Geetha Dholakia, M. Meyyappan, Antonio Facchetti, Tobin Marks Organic electronics offers tremendous potential in applications requiring structural flexibility and large area coverage, with the added advantage of low fabrication cost. While the mobilities of organic field effect transistors (OFETs) have recently increased to the level needed for practical applications, differences in the orientation of interchain stacking, as well as defects in the film interfaces and the presence of grain boundaries act as scattering centers and charge traps, hence degrade their performance. Here we present tunneling spectroscopic results and STM studies at the nanoscale analyzing the evolution in morphology and grain connectivity of fluoroalkyl-functionalized DFH-4T (a, w-diperfluorohexyl-4T) oligothiophene films on Au(111) substrates, as the thickness is varied from one monolayer to many multilayers. Monolayer thick DFH-4T films exhibit a featureless morphology except for a number of pits similar to those in self-assembled thiol monolayers, while multilayer films have a drastically different terraced morphology consisting of overlapping platelets. This study provides important information on the nature of the interface between organic semiconductors and the source$\backslash$drain electrodes in OFET devices. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:00AM |
A25.00008: Charge injection and band alignment in organic field effect transistors. Behrang Hamadani, Huanjun Ding, Jacob Ciszek, Yongli Gao, James Tour, Douglas Natelson We have studied metal/organic semiconductor charge injection in poly(3-hexylthiophene) (P3HT) field effect transistors with Pt and Au electrodes as well as Au electrodes modified by self-assembled monolayers (SAMs) as a function of annealing in vacuum. At low impurity dopant densities, Au/P3HT contact resistances show nonohmic behavior, whereas Pt/P3HT contacts remain ohmic. Ultraviolet photoemission spectroscopy (UPS) reveals that metal/P3HT band alignment shifts dramatically as samples are dedoped, leading to an increased injection barrier for holes. We also present the results of a preliminary study in which we modified the Au surface with electron poor (rich) SAMs, resulting in an effective increase (reduction) of the workfunction of Au and drastically changing the nature of charge injection at the contacts. Our results indicate that understanding the band alignment at the metal/polymer interface is an important step towards understanding the nature of charge injection in these devices. [Preview Abstract] |
Monday, March 13, 2006 10:00AM - 10:12AM |
A25.00009: Structural Determination of Interfaces in Organic Semiconductors using Coherent Bragg Rod Analysis Brandon Chapman, Ronald Pindak, Yizhak Yacoby, Julie Cross, Edward Stern, Christian Kloc The low charge carrier mobilities long associated with organic semiconductors have increased by several orders of magnitude in recent years. Charge mobilities in field-effect transistors (FETs) using single-crystals of rubrene (5,6,11,12-tetra-phenyl-tetracene) have been reported with values in the range from 5 to 30 cm2/Vs. However, little information is known about the integrity of interfaces formed by electrical contacts in organic FETs, which can play a significant role in determining device performance. We are using a novel approach called Coherent Bragg Rod Analysis (COBRA) to determine the atomic structure of interfaces in organic FETs. COBRA uses the measured x-ray diffraction intensities along the substrate defined Bragg (crystal truncation) rods to investigate the electron density near the surface, interface and throughout the thin-film. The COBRA method is uniquely suited for probing buried interfaces formed by electrical contact materials on molecular crystals. Here, we present preliminary measurements of Bragg rods from the free surface of rubrene single-crystals and we discuss progress toward measuring a complete set of inequivalent Bragg rods from rubrene that will provide a model-independent determination of the surface structure. [Preview Abstract] |
Monday, March 13, 2006 10:12AM - 10:24AM |
A25.00010: Transfer Printing of Pentacene Thin-Film Transistors onto Flexible Substrates Daniel Hines, Vince Ballarotto, Ellen Williams Transfer printing methods were optimized to fabricate high quality organic thin-film transistors (TFT) on flexible substrates. Flexible TFT devices fabricated from a 50 nm thick pentacene (Pn) semiconductor layer, a 600 nm thick polymer dielectric layer and Au electrodes on a polyethylene terephthalate (PET) substrate constitute a model system. A set of TFT devices with length L = 1 $\mu $m to 45 $\mu $m and width W = 100 $\mu $m were fabricated with both poly(methyl methacrylate) (PMMA) and poly(4-vinylphenol) (PVP) as the dielectric material. Output and transfer IV characteristics of these devices were measured as a function of L. The data sets were analyzed to determine both contact resistance and mobility. The Pn/PMMA/PET device set exhibited a contact resistance of 2 M$\Omega $ and a mobility of 0.237 cm$^{2}$/Vs while the Pn/PVP/PET device set exhibited a contact resistance of 0.5 M$\Omega $ and a mobility of 0.2 cm$^{2}$/Vs. A detailed comparison of these transfer printed device sets will be presented. *Work supported by the Laboratory for Physical Sciences, College Park, MD and ARDA. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 10:36AM |
A25.00011: Optical Probe of the Density of Defect States in Organic Thin-Film Transistors Mihaela Breban, Danilo Romero, Vincent Ballarotto, Ellen Williams We investigate the role of defect states associated with different gate dielectric materials on charge transport in organic thin film transistors. Using a modulation technique we measure the magnitude and the phase of the photocurrent$^{1}$ in pentacene thin film transistors as a function of the modulation frequency. The photocurrent generation process is modeled as exciton dissociation due to interaction with localized traps. A time domain analyses of this multi-step process allows us to extract the density of defect states. We use this technique to compare the physical mechanism underlying performances of pentacene devices fabricated with different dielectric materials. *Supported by the Laboratory for Physical Science $^{1}$ M. Breban, et al. ``Photocurrent probe of field-dependent mobility in organic thin-film transistors'' Appl. Phys. Letts. 87, 203503 (2005) [Preview Abstract] |
Monday, March 13, 2006 10:36AM - 10:48AM |
A25.00012: Architecture Effects for Transfer-Printed Carbon Nanotube Mat Transistors Vinod Sangwan, Dan Hines, Vincent Ballarotto, Gokhan Esen, Michael Fuhrer, Ellen Williams Transfer printing methods have been developed to assemble both top gate -- top source/drain and bottom gate -- bottom source/drain carbon nanotube (CNT) thin-film transistors (TFT) onto flexible substrates. The devices consist of CNT mats as the semiconductor layer, a poly(methyl methacrylate) dielectric layer and gold electrodes on a polyethylene terephthalate substrate. As a function of gate voltage (V$_{G})$, the top gate -- top source/drain devices exhibit ambipolar behavior with no hysteresis$^{\ast }$. This is in contrast to bottom gate -- top source/drain CNT mat TFTs with a SiO$_{2}$ dielectric layer which show only p-type characteristics and a large hysteresis. Fabrication and characterization of both CNT TFT architectures fabricated on flexible substrates via transfer printing will be presented and discussed. $^{\ast }$ D. R. Hines et al., Appl. Phys. Lett. 86, 163101 (2005). [Preview Abstract] |
Monday, March 13, 2006 10:48AM - 11:00AM |
A25.00013: ESR Study of Electric-Field Controlled Conductance of Fully-doped Polymers in a Transistor Structure Fang-Chi Hsu, Arthur J. Epstein It was recently reported that use of doped ``metallic'' polymer as the active channel in a field effect transistor structure results in unexpected ``normally on`` transistor-like behavior. Epstein \textit{et al}. proposed [1] that ion motion is involved in the conductance modulation of the ``metallic'' polymer. We study here the role of ion migration in the ``metallic'' polymer (poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid) (PEDOT:PSS)) based transistors by employing ESR spectroscopy. We found that approximately 18{\%} reduction of localized polaron concentration in PEDOT:PSS caused by ions insertion results in 10$^{3}$ times decrease in the polymer conductance. The $N(E_{F})$ determined from the Pauli susceptibility remains essentially unchanged after ions are inserted into the active channel. This indicates that the major conductance suppression in PEDOT:PSS occurs in the disordered regions. We proposed that the inserted ions modulate the charge carrier hopping distance in the disordered regions of the PEDOT:PSS resulting in a conductor-nonconductor transition. [1]A. J. Epstein \textit{et al.}, Curr. Appl. Phys. \textbf{2}, 339 (2002). [Preview Abstract] |
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