Session B17: Focus Session: Pentacene and Field Effect Transistors

Organic Field Effect Transistors

This talk will present an introduction to organic field effect transistors (OFETs) including results for pentacene thin film transistors, which have become a benchmark for organic electronics. We will also discuss the use of high capacitance gate dielectrics in OFETs to achieve large two dimensional carrier densities and metallic conductivity in OFETs.   

Growth of pentacene on inorganic and organic dielectrics and sub-micron channel oTFT fabrication

We have fabricated pentacene thin films on different organic and inorganic dielectrics at four substrate temperatures with different film thicknesses ranging from the submonolayer over the multilayer to the ``thick'' film regime. These films were characterized by AFM and analyzed by means of scaling and rate equation theory in order to deduce the molecular growth dynamics. We found that on all substrates and in a certain substrate temperature range the growth can be well described as diffusion-limited aggregation. A critical island size was deduced from the scaling of the distribution density of the grain areas and the power-law dependence of the saturated nucleation density on the deposition rate. OTFTs with a channel length down to 300 nanometers have been fabricated by nanoimprint-lithography, using stamps made by e-beam-lithography and reactive ion etching. Due to a combination of different effects these transistors show high quality electrical characteristics. In conclusion, we observed no principal limitation for the downscaling of pentacene-based oTFTs due to short channel effects concerning all relevant parameters such as threshhold voltage, mobility, on-current and on-off ratio as long as the morphology is characterized by large and well-ordered crystallites.   

The improvement of out of plane crystalline size of pentacene thin films on plastic substrates by transfer printing

Pentacene thin films on plastic substrates were fabricated using the transfer printing method. [1] The out of plane structural order, structure disorder factor and crystalline size were studied by X-ray diffraction.[2] The effects of transfer printing control parameters, such as temperature and pressure, on the crystalline size and structural disorder were quantified using paracrystal theory. The calculated average number of reflecting net planes in the crystalline domains N and the disorder factor g$_{II}$ agree with the $\alpha^{\star}$ law. The 12$\sim$16\% improvement of out of plane crystalline size was observed for pentacene printed at 100 -120 $ ^{o}C$. Higher printing pressure 600 PSI improved the crystallinity above that obtained with low pressure 100 PSI. Pentacene printed at 120 $ ^{o}$C and 600 PSI showed both optimal growth axis crystalline size of 283 $\AA$ and mobility 0.237 cm$^{2}$/Vs, respectively. The optimized crystalline size shows a direct correlation with the improvement of the carrier mobility of pentacene thin film transistors. [1] D. R. Hines $\textit {et al}.,$ Appl. Phys. Lett. 86, 163101 (2005). [2] Y. Shao $\textit {et al}.,$ J. Appl. Phys. 100, 44512 (2006).   

Enantiotropic Polymorphs in Pentacene

The high temperature structural phase transformation in bulk pentacene has been characterized by X-ray single crystal and powder diffraction. A well-defined transition temperature of 463K was observed in single crystals. In contrast, pentacene powders heated above the phase transformation temperature do not always fully convert, and upon cooling, coexistence of the two polymorphs in varying concentrations is observed down to room temperature. The 1$^{st}$ order phase transformation is isostructural, whereby the close packed herringbone-type layers shift against each other, keeping the same symmetry. The present results demonstrate that the structure of pentacene first reported in 1961 is actually stable only at high temperatures, and thus metastable at room temperature.   

Effect of impurities on pentacene thin film growth for field-effect transistors

The presence of impurities in organic semiconductors is one of the factors that limit device performance. Among all organic semiconductors, pentacene has shown the highest mobility reported to date. The effect of a controlled introduction in pentacene thin films of a well characterized impurity, 6,13-pentacenequinone (PnQ), was investigated. Since the majority of charge carriers in organic field effect transistors (OTFT) are located at the semiconductor-dielectric interface, this work focuses on the correlation between pentacene ultrathin film morphology and the overall OTFT device performance. The introduction of large amounts of PnQ revealed the presence of crystalline domains characteristic of pure PnQ submonolayer growth. The change of crystalline structure of the initial submonolayer for smaller amounts of PnQ is being investigated. The transistor mobility is dramatically reduced by increasing the degree of PnQ in the source material.   

Aggregation of pentacene molecules on SiO2 substrates and its influence on the FET characteristics.

Pentacene is one of the most promising materials for organic field effect transistors (OFETs). In order to improve the FET performance, dielectric layers, such as SiO2, are commonly modified by the self-assembled monolayers (SAMs), such as hexamethyldisilazane (HMDS). Owing to utilization of these SAMs, the performance of the pentacene FET has exceeded that of amorphous Si FET. However, we have found that pentacene molecules deposited on HMDS-treated SiO2 substrates aggregate with time even under ultra-high-vacuum (UHV) and ambient temperature conditions. We constructed an in situ atomic force microscopy (AFM)-FET measurement system and found that the FET mobility significantly decreased with the aggregation. Thus, this aggregation should be one of the major origins of the instability and irreproducibility of pentacene-based devices. In order to reveal the mechanism of the aggregation, we carried out an in situ and real time observation of growth and the aggregation of pentacene molecules on the several substrates, such as clean SiO2 and HMDS, under UHV conditions with low energy electron microscope (LEEM). We have found that pentacene tends to aggregate on the substrate with lower surface energy.   

Orientation of Pentacene Molecules on SiO$_{2}$: From a Monolayer to the Bulk

The orientation of pentacene films on SiO$_{2}$ is studied for the thickness range from a monolayer to 150 nm by polarization-dependent NEXAFS spectroscopy (Near Edge X-ray Absorption Fine Structure). All films exhibit a strong polarization dependence of the $\pi ^{\ast }$ orbitals, which indicates that the pentacene molecules are highly oriented. However, the degree of orientation varies with the rate at which pentacene molecules are deposited. This difference can be explained by a previously-proposed mixture of the bulk phase and a metastable thin film phase. Faster rates favor the thin film phase and slower rates the bulk phase. Our NEXAFS results extend previous structural observations down to the first layer at the oxide interface, which is critical for the performance of devices. Including a finite distribution of the molecular tilt angles in the data analysis accounts for residual disorder. Damage to the molecules by hot electrons from soft x-ray irradiation eliminates the splitting between nonequivalent $\pi ^{\ast }$ orbitals, indicating a breakup of the pentacene molecule.   

Structure of a pentacene monolayer deposited on SiO$_{2}$: Role of trapped interfacial water

\textit{In situ }synchrotron x-ray reflectivity is used to probe the early stages of pentacene growth in real time, under conditions relevant to the fabrication of organic thin film transistors. The results reveal that there is an interfacial water layer initially present on the SiO2 substrate and that this water layer is still present at the interface after the deposition of a pentacene thin film. The thickness of the trapped interfacial water layer does not significantly change subsequent to film deposition, even after exposure to atmospheric pressure or during vacuum annealing at 70 \r{ }C. However, a water layer is observed to form on the free surface of pentacene after sufficient exposure to water vapor, and the thickness of this layer can be reduced by subsequent vacuum annealing. These observations are correlated with organic thin film transistor mobilities measured at atmospheric pressure versus under vacuum.   

Molecular Scale Structure of Pentacene Interfaces

The adsorption of organic molecules on metals, insulators, and semiconductors has been an important issue in the organic semiconductor research. The morphology and crystal structure of the first few molecular layers at organic-inorganic interfaces, in particular, affects the electrical properties of organic thin films. The first upright layer of pentacene on Si (111) forms on top of a disordered layer on which strongly bonded pentacene molecules are formed. The microstructures of interfaces between organic molecules and insulators lack understanding in molecular orientation, packing and degree of disorder. Low temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements were performed to probe molecular-scale structures of an upright layer of pentacene molecules. Our approach uses the disordered layer as a conducting analog of an insulating surface to enable a high-resolution structural study of the relevant crystalline phase of pentacene. STM and STS can be used to gain further understanding of structural defects such as vacancies, dislocations and grain boundaries within and between islands.   

Pentacene Molecules on Inert Surfaces

We study the energetics and dynamics of pentacene molecules in vacuum and saturated diamond (111) surface and silica surfaces. Force field molecular dynamics simulations are applied to capture the van de Waals type interactions among the pentacene molecules and the substrates. The herringbone arrangement of the molecules is found to be optimal both in vacuum and on various inert surfaces. A 90 degree rotation of the entire structure relative to that experimentally reported is identified on the silica surfaces.   

Charge-Transport Parameters in Molecular Organic Semiconductors.

In this contribution we will discuss the present state-of-the-art in the derivation of electronic and electron-phonon coupling constants in organic semiconductors from quantum-chemical calculations. We will reveal some of the shortcomings of the current models used to depict organic semiconductors and also the paths to be followed to achieve significant improvements. The contributions of both intra-molecular and inter-molecular vibrations to the electron-phonon interaction will be discussed in detail. Our results show that for an adequate description of the charge transport in organic semiconductors both local and non-local electron-phonon mechanisms should be taken into account. In the case of oligoacene crystals several phonon modes that contribute most strongly to the modulation of the transfer integrals were found to display large nonlinear electron-phonon couplings.   

Scanning Tunneling Microscopy and Spectroscopy of Pentacene films Deposited on SiC

Among various organic semiconductors, pentacene (Pn) has attracted much attention because of its ability of form ordered structures and its relatively high electron and hole mobilities. We have used SiC surfaces etched at 1600\r{ }C in 1 atm of hydrogen to form atomically flat substrates for Pn deposition. Oxidizing these substrates prior to Pn deposition electronically decouples the molecular films from the substrate. Scanning tunneling microscopy (STM) and spectroscopy (STS) was performed at room temperature on in-situ deposited Pn films. STM reveals a dendritic morphology of the films, consistent with prior reports [1]. We find a step height of 1.43$\pm $0.10 nm indicating that the Pn molecules are standing up, confirming the relatively weak interaction between the substrate and the film. STS reveals a band gap of about 2.0 eV, which is attributed to the edges of HOMO and LUMO bands of the molecules. Measurements over a wide range of tunnel currents are in progress, in an effort to deduce any transport limitations in the films. Supported by NSF. [1] F.-J. Meyer zu Heringdorf et al., Nature \textbf{412}, 517 (2001)   

Time Resolved Microscopy of Charge Trapping in Polycrystalline Pentacene

The microscopic mechanisms by which charges trap in organic electronic materials are poorly understood. Muller and Marohn recently showed that electric force microscopy (EFM) can be used to image trapped charge in working pentacene thin-film transistors [E. M. Muller \textit{et al.}, \textit{Adv. Mater.} \textbf{17} 1410 (2005)]. We have made a new discovery by imaging trapped charge in pentacene films with much larger grains. In contrast to the previous study in which charge was found to trap inhomogeneously throughout the transistor gap, we find microscopic evidence for a new trapping mechanism in which charges trap predominantly at the pentacene/metal interface in large-grained devices. We conclude that at least two charge trapping mechanisms are at play in polycrystalline pentacene. We have made localized measurements of the trap growth over time by performing pulsed-gate EFM experiments. Trap formation is not instantaneous, taking up to a second to complete. Furthermore, the charge-trapping rate depends strongly on gate voltage (or hole concentration). This kinetics data is consistent with the hypothesis that traps form by chemical reaction.