Cedric Rolin

On the Extraction of Charge Carrier Mobility in High‐Mobility Organic Transistors

Transistor parameter extraction by the conventional transconductance method can lead to a mobility overestimate. Organic transistors undergoing major contact resistance experience a significant drop in mobility upon mild annealing. Before annealing, strong field-dependent contact resistance yields nonlinear transfer curves with locally high transconductances, resulting in a mobility overestimate. After annealing, a contact resistance below 200 Ω cm is achieved, which is stable over a wide V(G) range.

Unraveling the Mechanism of Molecular Doping in Organic Semiconductors

The mechanism by which molecular dopants donate free charge carriers to the host organic semiconductor is investigated and is found to be quite different from the one in inorganic semiconductors. In organics, a strong correlation between the doping concentration and its charge donation efficiency is demonstrated. Moreover, there is a threshold doping level below which doping simply has no electrical effect.

Noise-Margin Analysis for Organic Thin-Film Complementary Technology

Parameter variation in organic thin-film transistor (OTFT) technology is known to limit the yield of digital circuits. It is expected that complementary OTFT technology (C-TFT) will reduce the sensitivity to parameter variations. In this paper, we quantify the dependence of yield on transistor parameter variations for C-TFT and compare it to unipolar logic. First, a basic inverter model is developed and fitted to measured transfer characteristics of organic complementary inverters. Next, the inverter model is used in numerical simulations to determine how the noise margin of the inverter, a measure for its reliable operation, changes as a function of transistor parameter variations. The noise margin is significantly improved with respect to p-type-only inverters with similar parameters. Finally, we perform circuit-level yield predictions as a function of parameter spread using the noise-margin simulations performed earlier.

Pentacene devices and logic gates fabricated by organic vapor phase deposition

An organic vapor phase deposition (OVPD) tool has been developed and optimized for the deposition of pentacene thin films. Pentacene is grown with a good thickness uniformity, a good material consumption efficiency, and deposition rates up to 9.5 A/s. Top-contact transistors based on OVPD-grown pentacene show high mobilities (up to 1.35 cm(2)/V s) and excellent characteristics, even at high deposition rates. Elementary circuit blocks have also been produced using an OVPD-deposited pentacene film. A five-stage ring oscillator features a stage delay of 2.7 mu s at a supply voltage of 22 V. (c) 2006 American Institute of Physics.

Predictive Model for the Meniscus-Guided Coating of High-Quality Organic Single-Crystalline Thin Films

A model that describes solvent evaporation dynamics in meniscus-guided coating techniques is developed. In combination with a single fitting parameter, it is shown that this formula can accurately predict a processing window for various coating conditions. Organic thin-film transistors (OTFTs), fabricated by a zone-casting setup, indeed show the best performance at the predicted coating speeds with mobilities reaching 7 cm2 V-1 s-1 .

Charge carrier mobility in thin films of organic semiconductors by the gated van der Pauw method

Thin film transistors based on high-mobility organic semiconductors are prone to contact problems that complicate the interpretation of their electrical characteristics and the extraction of important material parameters such as the charge carrier mobility. Here we report on the gated van der Pauw method for the simple and accurate determination of the electrical characteristics of thin semiconducting films, independently from contact effects. We test our method on thin films of seven high-mobility organic semiconductors of both polarities: device fabrication is fully compatible with common transistor process flows and device measurements deliver consistent and precise values for the charge carrier mobility and threshold voltage in the high-charge carrier density regime that is representative of transistor operation. The gated van der Pauw method is broadly applicable to thin films of semiconductors and enables a simple and clean parameter extraction independent from contact effects.

Highly Crystalline C8-BTBT Thin-Film Transistors by Lateral Homo-Epitaxial Growth on Printed Templates

Highly crystalline thin films of organic semiconductors offer great potential for fundamental material studies as well as for realizing high-performance, low-cost flexible electronics. The fabrication of these films directly on inert substrates is typically done by meniscus-guided coating techniques. The resulting layers show morphological defects that hinder charge transport and induce large device-to-device variability. Here, a double-step method for organic semiconductor layers combining a solution-processed templating layer and a lateral homo-epitaxial growth by a thermal evaporation step is reported. The epitaxial regrowth repairs most of the morphological defects inherent to meniscus-guided coatings. The resulting film is highly crystalline and features a mobility increased by a factor of three and a relative spread in device characteristics improved by almost half an order of magnitude. This method is easily adaptable to other coating techniques and offers a route toward the fabrication of high-performance, large-area electronics based on highly crystalline thin films of organic semiconductors.

High-speed growth of pentacene thin films by in-line organic vapor phase deposition

Taking another step towards industrial production of devices based on organic semi-conductors, this work presents an extension of the organic vapor phase deposition technique to in-line geometry. A study of the in-line tool operation is carried out. It leads to the definition of a specific in-line deposition rate that qualifies the coating speed. It also allows for an understanding of processing parameter variations that lead to high deposition speeds. As a consequence, pentacene films are grown at in-line deposition rates of up to 1055μm2/s. This corresponds to web speeds of 2.1 m/min, equivalent to an average deposition rate of 105 Å/s in a static system. These films present a high uniformity, with a thickness standard deviation below 1.2% over 4 inch diameter substrates. Moreover, with transistor mobilities of up to 1.5 cm2/Vs, these pentacene films are of excellent electrical quality. This quality is conserved up to the highest deposition rates. Finally, 5-stage ring oscillators on foil based on a pentacene thin film deposited by in-line OVPD achieve a frequency of 24 kHz at a supply voltage of 20 V.

Functional Pentacene Thin Films Grown by In-Line Organic Vapor Phase Deposition at Web Speeds above 2 m/min

We show in this paper that the organic vapor phase deposition technique can advantageously be extended to an in-line system, where a susceptor moves at a constant speed underneath an elongated showerhead. Highly uniform pentacene films are grown at web speeds of up to 2.1 m/min, equivalent to an average deposition rate of 105 angstrom/s in a static system. These pentacene films are of high electrical quality as proven by transistor mobilities of up to 1.5 cm(2) V-1 s(-1) and five-stage ring oscillators on foil that achieve a frequency of 24 kHz at a supply voltage of 20 V

Contact resistance characterization in organic thin film transistors (Conference Presentation)

Proper thin film transistor (TFT) operation requires that its contact resistance Rc remains only a fraction of its channel resistance Rch. The integration of thin films based on latest generation organic semiconductors into downscaled TFTs with short channel length and high capacitance dielectric results in devices with very low Rch. Matching this with a low enough Rc is very challenging, due to the notoriously poor charge injection into organic semiconductors. The viability of integrated circuit technologies based on organic TFTs hinges on solving this critical contact resistance issue. To properly address this, it is important to use a common metric based on simple, comparable contact resistance measurements. Rc is commonly measured using the Transfer Length Method (TLM) that involves the characterization of TFTs of different channel lengths in the linear regime. We find, however, that the precision and the absolute value of the extracted Rc is greatly influenced by the conditions used to characterize each TFT. This seriously complicates the comparison to other literature values. In this talk, we present an in-depth study of the TLM technique aimed at solving these particular problems. Our TLM structures are based on high mobility organic TFTs, fabricated with different technologies and topologies. We conduct a systematic comparison of voltage- and current-controlled measurements with constant lateral electric field and charge density. As a result, we delineate the conditions to conduct TLM characterization and data treatment for clean Rc extraction. We also identify the measurement parameters that count in establishing a good Rc benchmark.