Soeren Steudel

Influence of the dielectric roughness on the performance of pentacene transistors

The properties of the dielectric strongly influence the performance of organic thin-film transistors. In this letter, we show experimental results that quantify the influence of the roughness of the dielectric on the mobility of pentacene transistors and discuss the cause of it. We consider the movement of charge carriers out of the “roughness valleys” or across those valleys at the dielectric–semiconductor interface as the limiting step for the roughness-dependent mobility in the transistor channel.

Correlation between bias stress instability and phototransistor operation of pentacene thin-film transistors

The authors study the use of pentacene thin-film transistors as phototransistors. The shift in turn-on voltage (Von), responsible for the high photosensitivity of these devices, is shown to be strongly dependent on illumination time and applied gate voltage. The time dependence of this process is similar to the shift in Von during bias stress experiments in the dark, and illumination can simply be accounted for as an acceleration factor for bias stress instability. By comparing the characteristics of devices with different gate dielectrics, trapping of electrons by OH groups at the gate dielectric interface is indicated as a main origin for these shifts.

Comparison of organic diode structures regarding high-frequency rectification behavior in radio-frequency identification tags

In this article, we compare the direct current (dc) and high-frequency performance of two different organic diode structures, a vertical diode and an organic field effect transistor (OTFT) with shorted drain-gate contact, regarding their application in a rectifying circuit. For this purpose, we fabricated both diode structures using the organic semiconductor pentacene. dc measurements were performed showing a space-charge-limited current mobility of more than 0.1cm2∕Vs for the vertical diode and a field effect mobility of 0.8cm2∕Vs for the OTFT with shorted source-drain. High-frequency measurements of those diode structures in a rectifier configuration show that both types of diodes are able to follow the base-carrier frequency of 13.56MHz which is essential for viable radio-frequency-identification (rf-ID) tags. Based on those results we evaluate the performance limits and advantages of each diode configuration regarding their application in an organic rf-ID tag.

Low voltage complementary organic inverters

We have developed a method for integrating n- and p-type organic thin-film transistors (OTFTs) on the same substrate. An integrated shadow mask was used for the n- and p-type semiconductor patterning. The integrated shadow mask can be aligned with submicron accuracy relative to the OTFT substrate. This allows for the integration at transistor level of n- and p-type OTFTs on the same substrate. A complementary inverter was fabricated, showing excellent performance while operating at a supply voltage of 2V.

An integrated double half-wave organic Schottky diode rectifier on foil operating at 13.56 MHz

We demonstrate an integrated organic double half-wave rectifier for use in organic radio frequency identification (RFID) tags. This rectifier comprises two organic Schottky diodes, each followed by a capacitor, integrated on the same foil. This rectifier delivers approximately twice the dc voltage of single half-wave rectifiers. Its offset voltage is merely 2 V. It is able to generate voltages of 10–14 V, which are necessary for driving current organic RFID multibit code generators, from an ac-input voltage of only 8–10 V amplitude, which are generated at rf magnetic fields of 0.9–1.3 A/m. Such fields are below the minimum required rf magnetic field strength set by standards.

Integrated shadow mask method for patterning small molecule organic semiconductors

We have developed a simple and efficient method for patterning small molecule semiconductors for applications in the field of organic electronics. In our approach, a profile is created using a single layer of photoresist, defining the regions where the organic semiconductor is to be deposited. Subsequent deposition of a small molecule semiconductor results in a discontinuity of the semiconductor film at the photoresist edge. The resulting transistor characteristics have an off current that is systematically below 1pA. We demonstrate both p-type and n-type organic thin-film transistors using this method, using pentacene and copper hexadecafluorophthalocyanine (F16CuPc), respectively.

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.

Patterned growth of pentacene

We propose a way of patterning organic small molecule thin films without requiring a hardmask and therefore more compatible with printing technologies. Active and passive areas for transistors are predefined by different surface chemistries. The subsequent growth takes place under conditions that cause the formation of a high mobility two-dimensional film in the active area and a disconnected three-dimensional film or no film in the passive area. This concept is founded on the basic theory of nucleation of organic small molecules on inert substrates and applied to the growth of patterned pentacene layers.

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.

High-performance a-In-Ga-Zn-O Schottky diode with oxygen-treated metal contacts

High-performance Schottky diodes based on palladium blocking contacts were fabricated upon depositing indium-gallium-zinc oxide (IGZO) with high oxygen content. We find that an oxygen treatment of the palladium contact is needed to achieve low off currents in the Schottky diode, and rationalize this by relating an increased oxygen content at the Pd/IGZO interface to a lower interfacial trap density. Optimized IGZO films were obtained with a record high ratio of free charge carrier density to subgap traps. The rectification ratios of diodes with such films are higher than 107 with current densities exceeding 103 A/cm2 at low forward bias of 2 V.