The Effect of Interface Modification on the Properties of Organic Thin-Film Transistors

In organic thin-film transistors (OTFTs), the conducting channel is located near the interface between the organic semiconductor and the dielectric; this interface is crucial for transistor performance. The goal of this thesis is to study the effect of this interface, especially when it is modified either by surface treatments or self-assembled monolayers. For this purpose, OTFTs based on pentacene and doped silicon wafer gate electrode and gold top contacts are fabricated. The dielectric is either silicon oxide (SiO2) or polyimide (PI). The dielectric surface affects both the growth of the pentacene film on the top of it and the electric performance of the transistor. The pentacene film morphology is studied on treated and untreated SiO2 and PI surfaces by AFM and water contact angle measurements. Depending on the nature of the dielectric surface, traps or charge transfer centers are introduced which then affect the performance of the transistor. The transistor performance is determined by measuring the drain current as it is dependent upon the drain and gate voltage. The contributions of the channel and the contacts are separated by 4-probe measurements (at floating and non-floating gate). The transistors with untreated dielectrics are first characterized. The dependence on the channel length, the thickness and the morphology of the pentacene film is studied. The short channels (2 – 20 µm) are patterned by stencil masks; the long channels (100 – 600 µm) by steel masks. The results show that the transistors with a channel length of 20 µm and smaller are limited by the contacts, while the transistors with a channel length between 100 and 600 µm are dominated by the channel, and thus the traps and residual carriers in the channel. The pentacene film morphology has more influence on the contact resistance than on the channel behavior. To study the effect of the interface modifications, the transistor behavior should be dominated by the channel and the contact effects minimized; hence, long-channel transistors are used. The dielectric surfaces are treated by plasma or pH solutions prior to the pentacene deposition resulting in a variety of defects. The defects are distinguished by those which lead to a change in the film growth and those which affect the electric performance by introducing traps or charge transfer centers. To overcome the influence of the dielectric surface, the oxide dielectric is passivated by a self-assembled monolayer (SAM). The neutral SAM acts as a spacer between the dielectric and the pentacene film and thus enhances the transistor performance. The polar SAMs also introduce a dipole moment. A series of molecules with a similar length, but different end groups, are used to investigate the effect of the dipole moment. The main result of this study is that the nature and the quality of the gate and the contact interfaces are often more important for the performance of the transistors than the thin film morphology.

Organic thin-film transistors: the passivation of the dielectric-pentacene interface by dipolar self-assembled monolayers

Franziska Dora Fleischli, Stéphane Suarez, Libero Zuppiroli

In organic thin-film transistors (OTFTs), the conducting channel is located near the interface between the organic semiconductor and the oxide dielectric; this interface is crucial for transistor performance. Self-assembled monolayers (SAMs) on the interface reduce the negative influences of the oxide dielectric surface by decreasing the coupling of the carriers at the gate and the role of the active surface defects on transfer. In this paper, we show that SAMs carrying a dipole moment determine the OTFT performance by controlling the charge transfer between the oxide dielectric and the semiconductor. The charges introduced into the semiconductor by this transfer (i.e., residual carriers) lead to a threshold shift to positive values, as well as a decrease in the contact resistance and an increase in the apparent mobility. In this study, other effects of the SAMs, such as the gate potential shift in the channel or a direct reaction between semiconductor and SAM molecules, can be excluded as dominant processes.

American Chemical Society2010

Transistors organiques ultraminces à base de pentacène

Adrian von Mühlenen

The goal of this thesis is to contribute to the understanding of charge transport in organic field-effect transistors (OFETs) made of pentacene. Organic thin-film transistors (OTFTs) with active layers thicknesses of 5, 10, 20, and 100 nm were fabricated in order to examine their structure and electrical characteristics and identify the key parameters that affect the charge transport in these devices. The conductivity of the pentacene films determined via four probe measurements suggests the presence of extrinsic charge carriers, or residual carriers, which have a large influence on device performance and electrical characteristics. The origin of these carriers is discussed in terms of a charge-transfer process occurring between the organic semiconductor and the electron acceptor states of the gate oxide surface. The gate interface is studied by varying the density of residual carriers via modification of the oxide surface by different plasma treatments with or without using subsequent deposition of various molecular monolayers. The OFETs yielded residual carrier densities ranging from 5 × 109 et 1 × 1013 cm-2 depending on the gate interface modification. The electrical characteristics such as the film conductivity and field-effect mobility are shown to be dependent on the density of residual carriers. Based on the measured density of residual carriers the OFETs are classified into two groups : devices based on doped and on un-doped pentacene. Temperature-dependent measurements of the field-effect mobility and the film conductivity performed on these devices reveal a thermally-activated character. This suggests that charge transport in the examined devices occurs via hopping between localized states. Based on the activation energies of the film conductivity and the field-effect mobility, the difference in energy between the Fermi and the transport level Δε is estimated to be between 10 and 160 meV depending on the density of residual carriers. This range of values is in agreement with thermoelectric power measurements performed on the same devices. The thermoelectric power is discussed in terms of two contributions : one that is dependent on Δε and another constant term that is independent of temperature and gate interface modification. The latter contribution was measured to be 265 ± 40 μV/K and originates from the creation of the phonon cloud associated with local changes in intermolecular interaction. This suggests that the charge carrier in the channel is dressed with a cloud of polarization, an electronic polaron.

EPFL2007

Electrical properties of functionalized nanowire field effect transistors

Ralf Thomas Weitz

The utilization of functional organic materials holds great promise for applications in electronic devices. Semiconducting organic molecules are frequently used as channel material in field effect transistors, due to the ease by which they can be assembled as such components, and the ease with which their properties can be specifically tailored. An extension of the use of organic materials in field effect transistors with the potential to substantially improve the performance of such devices is investigated in this thesis. It is the miniaturization of the functional parts of these devices, including their conductive channel or the gate dielectric, down to nanoscopic dimensions, which is important for their integration into complex device architectures. The materials under consideration offer advantages over conventional field effect transistor channels, including improved processibility, high mobility and electrical stability. The first part of the thesis is dedicated to the fabrication and electrochemical functionalization of nanoscopic field effect transistors comprising individual semiconducting single-walled carbon nanotubes. The combination of an ultra-thin gate dielectric based on an organic self-assembled monolayer is used for the fabrication of high-performance field effect transistors that show large on-state transconductance, as well as a large on/off ratio of 107, negligible hysteresis in the drain current, and a subthreshold swing that approaches the room temperature limit of 60 mV/decade. Two different types of self-assembled monolayers are used. The first, a silane-based self-assembled monolayer, is utilized to build field effect transistors in a global back gate geometry. This layout is useful for demonstrating the benefits of the use of a self-assembled monolayer for nanotube field effect transistors. Secondly, a gate dielectric incorporating a self-assembled monolayer composed of an alkane phosphonic-acid is exploited for the fabrication of high-performance field effect transistors with patterned aluminum gate electrodes. This geometry allows for the fabrication of more than one gate electrode per substrate and thus for the realization of logic circuits with all field effect transistors located on the same substrate. The technological relevance of nanotube field-effect transistors is proven by showing that nanotube field effect transistors can be cycled for more than 104 times between their on and off state and stored in ambient air for more than 200 days without degradation of their electrical performance. The electrochemical functionalization of individual single-walled carbon nanotubes with the organic coordination magnet Prussian Blue is investigated in the second part of this thesis. While this modification leaves metallic tubes unaffected, semiconducting SWCNTs become strongly p-doped during the electrochemical process. The temperature-dependent electrical behavior of Prussian Blue-functionalized semiconducting tubes can be understood by the freeze-out of the transfer of holes from the Prussian Blue below 150 K. In the third part of this thesis, novel core-cyanated perylene carboxylic diimides end-functionalized with substituents of varying flourine content and geometry are utilized as active material in air-stable n-channel thin film field- effect transistors. The relation between the thin-film structure and the air stability of the charge carrier mobility is investigated. Contrary to previous reports, it is found that the air stability is not related to the redox potential of the molecules. In addition, one of the compounds is used for the fabrication of field-effect transistors with a nanoribbon channel. These nanoribbons self-assemble from the solution phase, and, owing to their high crystalline order, field-effect transistors with mobilities of up to 0.2 cm2/Vs in air could be realized. The air stability of their mobility is found to be superior to that of thin-film transistors based on the same compound. Since the investigated perylene derivatives offer the possibility of fabricating thin-film as well as nanobelt field-effect transistors, they represent a valuable model system for the investigation of the relation between the crystalline order, mobility, stability and the performance of organic n-channel field-effect transistors. The last part is dedicated to a novel method for the fabrication of nanoscale polymer wires of diameters as small as 25 nm that offer potential as channel material for nanofiber field-effect transistors. These fibers emerge from a polymer solution during a standard spin-coating process. The fiber formation relies upon the Raleigh-Taylor instability of the spin-coated liquid film that arises due to a competition of the centrifugal force and the Laplace force induced by the surface curvature.