Sarah Schols

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.

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.

The characterization of chloroboron (III) subnaphthalocyanine thin films and their application as a donor material for organic solar cells

Chloroboron (III) subnaphthalocyanine (SubNc) films have been characterized by ellipsometry, absorption, photoluminescence measurements, and atomic force microscopy. The films strongly absorb red light, as the extinction coefficient k peaks at 1.4 at a wavelength of 686 nm. Planar bilayer heterojunctions with fullerene (C60) on top of SubNc are measured under AM 1.5 simulated illumination at various light intensities, leading to an open-circuit voltage (Voc) of 790 mV and a power conversion efficiency of 2.5%. The external and internal quantum efficiencies peaked at 36% and 70%, respectively. The combination of a strong red absorption and high Voc make SubNc an interesting material for organic solar cells, in particular for tandem cells.

Light-emitting organic field-effect transistor using an organic heterostructure within the transistor channel

The authors have realized a light-emitting organic field-effect transistor. Excitons are generated at the interface between a n-type and a p-type organic semiconductor heterostructure inside the transistor channel. The dimensions and the position of the p-n heterostructure are defined by photolithography. The p-n heterostructure is at a distance of several microns from the metal electrodes. Therefore, the exciton and photon quenching in this device is reduced. Numerical simulations fit well with the experimental data and show that the light-emitting zone can move within the transistor channel.

Origin of multiple memory states in organic ferroelectric field-effect transistors

In this work, we investigate the ferroelectric polarization state in metal-ferroelectric-semiconductor-metal structures and in ferroelectric field-effect transistors (FeFET). Poly(vinylidene fluoride-trifluoroethylene) and pentacene was used as the ferroelectric and semiconductor, respectively. This material combination in a bottom gate—top contact transistor architecture exhibits three reprogrammable memory states by applying appropriate gate voltages. Scanning Kelvin probe microscopy in conjunction with standard electrical characterization techniques reveals the state of the ferroelectric polarization in the three memory states as well as the device operation of the FeFET.

Influence of the contact metal on the performance of n-type carbonyl-functionalized quaterthiophene organic thin-film transistors

Organic thin-film transistors using 5, 5‴-diperfluorohexylcarbonyl-2,2′:5′,2″:5″,2‴-quaterthiophene (DFHCO-4T) as the electron conducting organic semiconductor are fabricated and the performance of these transistors with different top-contact metals is investigated. Transistors with Au source-drain top contacts attain an apparent saturation mobility of 4.6 cm2/V s, whereas this parameter is 100 times lower for similar transistors with Al/LiF top contacts. We explain this lower performance by the formation of a thin interfacial layer with poor charge injection properties resulting from a redox reaction between Al and DFHCO-4T.

Low-temperature formation of source–drain contacts in self-aligned amorphous oxide thin-film transistors

We demonstrated self-aligned amorphous-Indium-Gallium-Zinc-Oxide (a-IGZO) thin-film transistors (TFTs) where the source–drain (S/D) regions were made conductive via chemical reduction of the a-IGZO via metallic calcium (Ca). Due to the higher chemical reactivity of Ca, the process can be operated at lower temperatures. The Ca process has the additional benefit of the reaction byproduct calcium oxide being removable through a water rinse step, thus simplifying the device integration. The Ca-reduced a-IGZO showed a sheet resistance (RSHEET) value of 0.7 kΩ/sq., with molybdenum as the S/D metal. The corresponding a-IGZO TFTs exhibited good electrical properties, such as a field-effect mobility (μFE) of 12.0 cm2/(V s), a subthreshold slope (SS−1) of 0.4 V/decade, and an on/off current ratio (ION/OFF) above 108.

Flexible NAND-Like Organic Ferroelectric Memory Array

We present a memory array of organic ferroelectric field-effect transistors (OFeFETs) on flexible substrates. The OFeFETs are connected serially, similar to the NAND architecture of flash memory, which offers the highest memory density of transistor memories. We demonstrate a reliable addressing scheme in this architecture, without the need for select or access transistors. As proof of principle, a 1 × 4 NAND-like string is fabricated and characterized. Retention up to one month and endurance up to 2500 cycles are shown. Read and write disturb measurements show that the memory array can potentially be scaled up to 8 kbits.

Integrated Line Driver for Digital Pulse-Width Modulation Driven AMOLED Displays on Flex

An integrated scan-line driver, driving half a QQVGA flexible AMOLED display using amorphous-IGZO backplane technology on foil, has been designed and measured. A pulse-width modulation technique has been implemented, enabling to drive the OLEDs with a duty cycle up to almost 100%. The digital driving method also results in a 40% static power reduction of the display. Dynamic logic and bootstrapping techniques enabled the use of clock frequencies up to 300 kHz in unipolar amorphous-IGZO technologies on foil.

Integrated UHF a-IGZO energy harvester for passive RFID tags

We present an energy harvester composed of a dipole antenna, matching network and a rectifier based on thin-film metal-oxide (amorphous Indium-Gallium-Zinc Oxide, IGZO) semiconductor Schottky diodes, operating at 868MHz. S-parameters measurements show that the integrated single IGZO Schottky diodes have a cutoff frequency over 1.8GHz at 0V bias. Large signal analysis of the integrated double-half wave rectifier shows a cut-off frequency of 1.1GHz. Our harvester generates 1.3V at 15 cm from an emitter emitting 15dBm at 868MHz.