Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B22: Focus Session: Organic Electronics: FETs I |
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Sponsoring Units: DMP DPOLY Chair: Ting Xu, University of California, Berkeley Room: Morial Convention Center 214 |
Monday, March 10, 2008 11:15AM - 11:27AM |
B22.00001: In-crystal carriers in organic single crystal transistors Jun Takeya, Y. Tominari, M. Yamagishi, Y. Iwasaki The intrinsic semiconductor character of organic semiconductors, consisting of one molecular species, causes peculiar features different from those of doped inorganic semiconductors, when they are built in field-effect transistors (FETs). The effect can be most drastic with purified organic single crystal devices because of their minimized impurity (defect) concentrations of $\sim $ 10$^{14}$ cm$^{-3}$, resulting in relatively long length scale (more than 1 $\mu $m) of band bending and carrier distribution in the direction of the crystalline thickness. In this presentation, we report effects of the in-crystal carriers in two FET structures of clean rubrene single-crystals with the thickness comparable to the length scale of the distribution. With a double-gate device incorporating two transistor structures on the both sides of the crystal, we found that each transistor cross-talks with each other, inducing high-mobility (higher than 30 cm$^{2}$/Vs) carriers inside the crystal. The similar cross-talking events are observed also for a device with an acceptor layer of F$_{4}$-TCNQ on one side of the rubrene crystal. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B22.00002: Defect healing at room temperature in pentacene thin films and improved transistor performance. Wolfgang Kalb, Fabian Meier, Kurt Mattenberger, Bertram Batlogg We observed a healing of defects at room temperature in the prototypical organic semiconductor pentacene. Pentacene thin-film transistors were fabricated and characterized by in situ gated four-terminal measurements. Under high vacuum conditions (base pressure of order 10E-8 mbar), the device performance is found to improve with time. The effective field-effect mobility increases by as much as a factor of two and the contact resistance decreases by more than an order of magnitude. Oxygen/nitrogen exposure and annealing experiments show the improvement of the electronic parameters to be driven by a thermally promoted process and not by chemical doping. The spectral density of trap states was extracted from the transistor characteristics with a powerful scheme which allows for a calculation of the trap densities with high accuracy in a straightforward fashion. We show the performance improvement to be due to a reduction in the density of shallow traps $<$0.15 eV from the mobility edge, while the energetically deeper traps are essentially unaffected. This peculiar effect is a direct consequence of the weak intermolecular interaction which is characteristic of organic semiconductors. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B22.00003: Impurities and carrier trap formation in rubrene Leonidas Tsetseris, Sokrates Pantelides Rubrene has emerged recently as a very promising system for applications in organic electronics. In particular, measured carrier mobilities of rubrene films have been among the highest values reported for organic semiconductors. Here we present the results of first-principles calculations on impurities in rubrene crystals. We find that the most stable atomic H defect induces deep traps in the energy band gap of this prototype organic semiconductor, but we further show that the formation of a stable pair of vicinal H impurities eliminates these traps. We thus establish a dual role of hydrogen in carrier trap formation in rubrene devices. We also find that the incorporation of oxygen and water-related impurities is energetically favorable, and we address their possible role in the appearance of energy levels in the band gap of rubrene. Finally, we discuss the relevance of our findings for the development and operation of rubrene-based devices. This work was supported in part by DOE Grant DEFG0203ER46096. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:27PM |
B22.00004: Organic Field Effect Transistors Based on Micro and Nano-sized Single Crystalline Semiconductors. Invited Speaker: Organic single crystals are of interest for fundamental charge transport studies. Recently, we have shown that organic semiconductor single crystals can be patterned over a large area using vapor phase growth method. This opens up new possibilities of using organic single crystals for high performance devices. In this talk, I will present our recent work in solution deposition of single crystals by various methods and their device performance. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B22.00005: Field-effect modulated Seebeck coefficient in pentacene and rubrene K.P. Pernstich, B. Roessner, B. Batlogg We report on the first study of the charge carrier concentration and the temperature dependence of the Seebeck coefficient $S$ for two prototypical organic semiconductors measured in a field-effect transistor (FET) structure. As a basic transport property of solids, the Seebeck coefficient provides deep insights into the nature and dynamics of charge carriers. Using a FET structure enables the variation of the Fermi-level position in the active semiconductor region while measuring $S$, which is essential for determining individual contributions to the thermopower. The sign of the measured Seebeck coefficient is consistent with hole transport, and $S$ is in the range of 0.3-1 mV/K, it is independent of temperature between 295 K and 200 K, and interestingly it decreases for both semiconductors as $S \propto $\textit{$\vert $V}$_{g}$\textit{$\vert $}. The measured $S$ is quantitatively described by $S=(k/e)(E(V_g )/kT+A)$. The Fermi-level position $E(V_{g})$ is obtained from analyzing the transistor's characteristic which then allows to calculate the parameter $A$. For both semiconductors we find that $A$ is in the range of 1.7-3.6, just as in conventional semiconductors. The results are well described by solely considering a realistic density of in-gap trap states and band-like transport of quasiparticles that are subjected to scattering. There is no need to invoke self-trapping of massive charge carriers. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B22.00006: Contact-correlated bias stress instability in pentacene thin film transistors K. Tsukagoshi, S.D. Wang, T. Minari, T. Miyadera, Y. Aoyagi The bias stress effect and large contact resistance are the remaining problems for practical applications of organic thin film transistors (OTFTs). The bias stress effect in top-contact pentacene TFTs was observed to be correlated with the metal/organic contact. The drain current decay under the bias stress condition results from the combination of the contact resistance change and the channel resistance change arising from the threshold voltage shift in the channel. The transistors with the different contacts (gold and copper) show the very similar channel properties. On the other hand, the bias-stress-induced contact resistance change in the gold transistor is much larger than the corresponding channel resistance change, so that the bias stress effect can be even dominated by the gold contact. In contrast, the copper contact is more stable, showing the small contact resistance change. The temperature variance measurements on contact resistance indicate that, the gold contact has a wider trap distribution compared with the copper contact, that is, there are much more deep trap states at the gold contact. Therefore, the time-dependent charge trapping in the deep traps is proposed to be the mechanism of the bias stress effect in OTFTs. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B22.00007: Negative Magnetoresistnace of Organic Field Effect Transistors Masaya Nishioka, Yeonbae Lee, Allen Goldman, Yu Xia, Daniel Frisbie The magnetoresistance (MR) of organic field effect transistors has been studied. Both pentacene film, tetracene single crystal and rubrene single crystal devices exhibit negative MRs of up to 1.2 {\%} at 9 T. This has been demonstrated to not be contact related. The effect has been found to increase with decreasing temperature between 150 and 300 K. On the other hand, this effect is not strongly affected by the magnetic field direction. The phenomenon may result from the action of the magnetic field on the hopping transport of carriers. However, the possibility of a magnetocapacitance effect that would increase the number of carriers cannot be ruled out. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B22.00008: Pentacene Thin-Film Transistors With Organophosphonate Self-Assembled Monolayer Modified Gate Dielectrics Ian Hill, Matthew McDowell, Joseph McDermott, Jeffrey Schwartz, Steven Bernasek, Jaehyung Hwang, Antoine Kahn Organophosphonate-based self-assembled monolayers synthesized from aliphatic, heterocyclic aromatic, and aromatic moieties were used to modify silicon dioxide gate dielectric in pentacene organic thin-film transistors. Striking improvements in the subthreshold performance of these devices is noted, with a dependence on the molecular species used in fabricating the monolayer. All modified gate dielectrics outperform the untreated surface. As determined by parameters such as the subthreshold slope, threshold voltage magnitude and uniformity, and the on/off ratio, the aromatic species outperform the heterocyclic species, which outperform the aliphatic species. Origins of the improvement, based on energetic alignment of the molecular levels of the SAM and the pentacene, and charge trap densities will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B22.00009: Percolative Effects on Noise in Pentacene Transistors Brad Conrad, William Cullen, Winston Yan, Ellen Williams Noise in pentacene thin film transistors has been measured as a function of device thickness from well above the effective conduction channel thickness to only two conducting layers. Over the entire thickness range, the spectral noise form is 1/f, and the noise parameter varies inversely with gate voltage, confirming that the noise is due to mobility fluctuations, even in the thinnest films. Hooge's parameter varies as an inverse power-law with conductivity for all film thicknesses. The magnitude and transport characteristics of the spectral noise are well explained in terms of percolative effects arising from the grain boundary structure. http://arxiv.org/abs/0710.2700v2 [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B22.00010: Surface-Treatment Effects on the Pentacene-Based Organic Field-Effect Transistors with Anodized Gate Dielectrics Yeon Taek Jeong, Christopher Lombardo, Davianne Duarte, Ananth Dodabalapur The realization of low operating voltage organic field-effect transistors (OFETs) is technologically important with many methods having been proposed to achieve this goal. The use of anodized high-$k$ dielectrics is very promising in that the approach is applicable to obtaining gate dielectrics at low temperature. We report on the device characteristics and proper surface-treatment effects on low voltage OFETs with anodized Ta$_{2}$O$_{5}$ and SiO$_{2}$ gate dielectrics. Pentacene-based OFETs with anodized Ta$_{2}$O$_{5}$ gate dielectric obtained from an e-beam-evaporated and a sputtered Ta thin film layer exhibited the saturation mobility of 0.52 cm$^{2}$/Vs and 0.45 cm$^{2}$/Vs at $V_{ds}$ = -10V, respectively. Moreover, a hexamethyldisilazane (HMDS) and a mono-dodecyl phosphate surface treatment resulted in enhanced mobility and significantly reduced gate leakage current. In the case of anodized SiO$_{2}$ devices, an octadecyltrichlorosilane (OTS) treatment increased the saturation mobility from 0.38 cm$^{2}$/Vs to 0.88 cm$^{2}$/Vs at $V_{ds}$ = -10V. The OTS treatment also proved to reduce gate leakage current by more than 90{\%}. In related work, we will discuss the fabrication of all-organic dual-channel devices. These devices are promising because of their probable applications to organic sensing and CMOS transmission gates. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B22.00011: Drift mobility and frequency response of diode connected organic field effect transistors Brian Cobb, Ananth Dodabalapur Common methods of estimating the mobility of carriers in organic field effect transistors (OFETs) include extraction from output or transfer characteristics. Mobilities extracted from transfer curves are realistic only in certain steady-state regimes. Optical time-of-flight methods measure charge mobility in directions perpendicular to the semiconductor-dielectric interface, perpendicular to the direction of transport in a functioning OFET. We have developed a method that allows the extraction of the drift mobility of charges through an active channel in an OFET by measuring the frequency response of a diode connected device. An AC signal of varying frequency was applied to both the Gate and Drain nodes, while the rectified output voltage was measured at the Source node using an oscilloscope. This rectified voltage remains relatively steady at lower frequencies, then displays a marked fall. This pole was found to correspond to the transit time of the carriers through the channel. The fabrication of the OFETs, calculation of drift mobility from the frequency of the pole, and the effects of varying channel length will be reported. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B22.00012: Field-induced polymorphous disorder and bias-stress instability of pentacene organic thin-film transistors Masahiko Ando, Claudia Duffy, Jessica Winfield, Takashi Minakata, Henning Sirringhaus We propose a field-induced polymorphous disorder model to explain bias-stress instability in pentacene organic thin-film transistors Field-effect mobility at 0.7 cm$^2$/Vs and threshold voltage, Vth, at 0 V were obtained by using highly crystalline zone-casted pentacene semiconductor on benzocyclobutene insulator. Vth shifted up to +25V with positive gate bias-stress at +40 V for 15 hours and recovered after gate bias removal. Vth recovery was drastically accelerated by direct photo-excitation of pentacene and it indicated electrons were trapped in pentacene and not in BCB. After annealing at 130 C in N2, the initial electrical performance were recovered. Micro-Raman spectroscopy of pentacene at the channel revealed that shape of the C-H vibrational peaks at around 1160 cm$^{-2}$ changed reversibly in accordance with the positive shift and recovery of Vth. Our pentacene films with average d-spacing at 14.3 A were considered to be composed of a mixture (mosaic) of two kind of polymorphs with d-spacing at 14.1 A and 14.5 A. The polymorphous mixture should be disordered by electric field to create electron traps and induce Vth shift. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B22.00013: Prediction of the absolute charge mobility of molecular crystals Alessandro Troisi I propose a computational protocol to predict the absolute mobility of molecular semiconductors without adjustable parameters. The system dependent parameters are computed using a combination of classical molecular dynamics simulations and quantum chemical methods. The model used to connect the computable quantities with the observable temperature dependent mobility takes into account the effects of molecular reorganization energy and the fluctuation of the transfer integral due to thermal motions. The absolute value of the hole mobility, computed for the case of rubrene, is in excellent agreement with the experiments. The possibility of using computational chemistry methods to improve the theoretical models of charge transfer will be discussed in some detail. The predictive capabilities of the model presented in this work will be further validated considering the recent THz spectroscopy measurements performed by R. van Laarhoven in Eindhoven and the results in literature on temperature dependent band structure and density of state tails. [Preview Abstract] |
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