Heinz von Seggern

Photostimulable x-ray storage phosphors: a review of present understanding

Photostimulable x-ray storage phosphors in form of image plates are a promising alternative to conventional two-dimensional x-ray detectors. By absorption of ionizing radiation, electrons and holes are generated and captured locally to form a dose proportional latent image. The stored information is read out by scanning with a focussed HeNe laser. This leads to a local excitation of the trapped electrons which subsequently recombine with the trapped holes causing the emission of light. The information is then recorded by means of a photomultiplier, digitized by an A/D converter and displayed with the aid of a computer. The present state of the field of storage phosphors is reviewed in the following areas: the image principle, todays understanding of the nature of the storage centers, their physical generation, and the optically stimulated charge transfer paths. In addition, improvements and applications of these phosphors will be discussed for existing and future commercial devices.

A pentacene ambipolar transistor: Experiment and theory

An ambipolar pentacene transistor with top-gold and top-calcium contacts has been realized by utilizing a parallactic shadow mask effect during vapor deposition. The pentacene deposited on top of a silicon dioxide gate insulator is doped by Ca at the pentacene/SiO2 interface in order to compensate electron traps. An equivalent circuit model based on a resistor-capacitor network has been developed to describe the basic electrical properties of the transistor. Shockley-like analytical expressions for the output and transfer characteristic, as well as an analytical expression for the potential and charge-carrier distribution in the channel, are derived under the assumption of a high electron-hole recombination probability. The model has been fitted to our experimental results and yields comparable mobilities for both holes and electrons in the order of 0.1cm2∕Vs. The increasing threshold voltages, with an increase in gate voltage, are discussed as an indication for trapped charge carriers within the insulato...

Light emission from a polymer transistor

We report on light emission from a polymeric transistor that utilizes interdigitated source and drain electrodes with channel length of 5 μm in a bottom gate configuration based on a Si/SiO2 substrate. The polymer investigated is poly[9,9-di(ethylhexyl)fluorene] deposited by spin coating from chloroform solution to achieve an active layer thickness of 40 nm. Light emission occurs above drain source voltages of −60 V and the light intensity can be controlled by the gate voltage. Emission occurs close to the drain electrode as determined by optical microscopy. The transistor operates in hole accumulation mode without saturation of the output characteristics.

Nondestructive Laser Method for Measuring Charge Profiles in Irradiated Polymer Films

Relative charge-density distributions in the thickness direction of thin (>10¿m) polymer films are determined directly and accurately with a new laser-induced pressure-pulse (LIPP) method: A pressure pulse is generated in the sample by applying a short (70ps) and energetic (1-11mJ) light pulse from a Nd:YAG laser to a specially coated surface of the sample. Stress effects within this surface layer and possibly the recoil caused by ablation of some target material generate the desired pressure pulse of less than 1ns duration which propagates through the sample. From the electrode currents, the charge distribution can be evaluated. The new method is described in detail and some results for 20 to 50¿m thick films of Teflon FEP, Mylar PETP, and Kynar PVDF are given.

n-type organic field-effect transistor based on interface-doped pentacene

The realization of an n-type pentacene field-effect transistor based on interface-doped pentacene is demonstrated, laying a headstone for an organic complementary-metal–oxide–semiconductor technology. The doping is performed by depositing traces of calcium onto the gate insulator before applying the organic semiconductor. Electron field-effect mobilities of 0.19cm2V−1s−1 are achieved. The field effect, i.e., the electron accumulation behavior, is studied by impedance spectroscopy and charge measurements on a metal–insulator–semiconductor (MIS) diode. A good correlation between the physical properties of the transistor and the MIS diode can be reported. A temporal dynamics and a hysteresislike accumulation behavior are observed, both explainable by the influence of deep electron traps.

Identification of TSC peaks and surface‐voltage stability in Teflon FEP

In this paper we examine the stability of corona‐ and electron‐beam‐charged 25‐μm FEP‐A in open circuit by using thermally stimulated (TSC) measurements. We show that electrets charged with electron beams of 10 keV or higher are more stable than corona‐charged foils. The inferior stability of corona‐charged samples is due to the filling of surface traps as opposed to volume traps in electron‐beam‐charged samples. A peak at 155 °C was attributed to surface traps located at a depth of 0–0.5 μm, while a peak at 170 °C is due to charge stored 0.5–1.8 μm beneath the nonmetallized surface. The volume traps yield a peak at 200 °C. The total volume trap density was estimated to 1.4×1014 traps/cm3.

Pentacene field-effect transistors with sub-10-nm channel lengths

The field effect in pentacene thin-film transistors was studied using bottom-contact devices with channel lengths below 10nm. To suppress spreading current in these devices, which have a small channel width-to-length (W-L) ratio, we employed a pair of guarding electrodes as close as 20nm to the two sides of the channel. The responses of these nanometer scale transistors exhibit good gate modulation. Mobilities of 0.046cm2∕Vs and on/off ratios of 97 were achieved in sub-10-nm transistors. We find that the device response is strongly influenced by the nature of the metal-semiconductor contact.

A new model of isothermal charge transport for negatively corona‐charged Teflon

In this paper we study theoretically and experimentally the transport of electrons in Teflon FEP type A under isothermal conditions (145 °C) in open circuit. We suggest a theoretical model including surface, shallow, and deep traps. The model leads to a system of partial differential equations which was solved numerically using a finite‐step algorithm. By fitting the theoretical to the experimental surface‐voltage decay curves we found a shallow‐trap‐modulated mobility μQ=1.6×10−12 cm2/V sec, a deep‐trap‐modulated transit time tT=1.25×105 sec in the voltage range of −200 to −400 V for electrons in 25‐μm‐thick material. We also determined the time constants for surface injection, trapping, and detrapping. The deep‐trap‐modulated transit time is in good agreement with measured values. Further we found a voltage‐independent mean free path for the shallow‐trapped carriers λ=6.5 μm. These traps were assumed to be in thermal equilibrium with the free carriers in the conduction band.

Order induced charge carrier mobility enhancement in columnar liquid crystal diodes.

Discotic molecules comprising a rigid aromatic core and flexible side chains have been promisingly applied in OLEDs as self-organizing organic semiconductors. Due to their potentially high charge carrier mobility along the columns, device performance can be readily improved by proper alignment of columns throughout the bulk. In the present work, the charge mobility was increased by 5 orders of magnitude due to homeotropic columnar ordering induced by the boundary interfaces during thermal annealing in the mesophase. State-of-the-art diodes were fabricated using spin-coated films whose homeotropic alignment with formation of hexagonal germs was observed by polarizing optical microscopy. The photophysical properties showed drastic changes at the mesophase-isotropic transition, which is supported by the gain of order observed by X-ray diffraction. The electrical properties were investigated by modeling the current-voltage characteristics by a space-charge-limited current transport with a field dependent mobility.

Complementary inverter based on interface doped pentacene

An organic complementary metal–oxide–semiconductor (O-CMOS) inverter is presented, which is based on a single pentacene layer acting both as n- and p-type organic semiconductors. The circuit consists of two spatially separated transistors realized by pairs of calcium and gold source and drain electrodes, respectively. The p transistor is obtained by utilizing the conventional pentacene∕SiO2 channel interface whereas the n transistor is realized by doping the SiO2 interface with traces of calcium prior to pentacene deposition. Both, n and p transistors work exclusively in unipolar mode within the range of the supply voltage of 60 V. The O-CMOS inverter works reliably with a gain in between 17 and 24, and the respective electron and hole mobilities were found around 0.1cm2V−1s−1. The circuit shows hysteresis, which can be explained by a gate voltage-dependent electron trapping in the n channel. Electron accumulation can also be realized by the application of a polymethylmethacrylate interlayer between SiO2 ...