Stefan Knobelspies

Micromachined Hotplate Platform for the Investigation of Ink-Jet Printed, Functionalized Metal Oxide Nanoparticles

This paper describes a novel micromachined platform serving as an interface between nanosized, gas sensitive metal oxide particles, and the macroscopic world. Through a combination of ink-jet printing and microelectromechanical systems technologies, it thus becomes possible to quickly test and characterize new nanosized metal oxide particles with respect to their gas sensitivity. Within the framework of this report, we describe the design considerations, thermal finite-element method simulations, processing, characterization, and utilization of the platform. Due to the low-power consumption, the hotplate provides an experimental platform to test nanoparticle-based metal oxide gas sensors for mobile systems.

Metal‐Halide Perovskites for Gate Dielectrics in Field‐Effect Transistors and Photodetectors Enabled by PMMA Lift‐Off Process

Metal-halide perovskites have emerged as promising materials for optoelectronics applications, such as photovoltaics, light-emitting diodes, and photodetectors due to their excellent photoconversion efficiencies. However, their instability in aqueous solutions and most organic solvents has complicated their micropatterning procedures, which are needed for dense device integration, for example, in displays or cameras. In this work, a lift-off process based on poly(methyl methacrylate) and deep ultraviolet lithography on flexible plastic foils is presented. This technique comprises simultaneous patterning of the metal-halide perovskite with a top electrode, which results in microscale vertical device architectures with high spatial resolution and alignment properties. Hence, thin-film transistors (TFTs) with methyl-ammonium lead iodide (MAPbI3 ) gate dielectrics are demonstrated for the first time. The giant dielectric constant of MAPbI3 (>1000) leads to excellent low-voltage TFT switching capabilities with subthreshold swings ≈80 mV decade-1 over ≈5 orders of drain current magnitude. Furthermore, vertically stacked low-power Au-MAPbI3 -Au photodetectors with close-to-ideal linear response (R2 = 0.9997) are created. The mechanical stability down to a tensile radius of 6 mm is demonstrated for the TFTs and photodetectors, simultaneously realized on the same flexible plastic substrate. These results open the way for flexible low-power integrated (opto-)electronic systems based on metal-halide perovskites.

Charge Trapping Mechanism Leading to Sub-60-mV/decade-Swing FETs

In this paper, we present a novel method to reduce the subthreshold swing (SS) of FETs below 60 mV/decade. Through modeling, we directly relate trap charge movement between the gate electrode and the gate dielectric to SS reduction. We experimentally investigate the impact of charge exchange between a Cu gate electrode and a 5-nm-thick amorphous Al2O3 gate dielectric in an InGaZnO4 thin-film transistor. Positive trap charges are generated inside the gate dielectric while the semiconductor is in accumulation. During the subsequent detrapping, the SS diminishes to a minimum value of 46 mV/decade at room temperature. Furthermore, we relate the charge trapping/detrapping effects to a negative capacitance behavior of the Cu/Al2O3 metal–insulator structure.

Buckled Thin-Film Transistors and Circuits on Soft Elastomers for Stretchable Electronics

Although recent progress in the field of flexible electronics has allowed the realization of biocompatible and conformable electronics, systematic approaches which combine high bendability (<3 mm bending radius), high stretchability (>3-4%), and low complexity in the fabrication process are still missing. Here, we show a technique to induce randomly oriented and customized wrinkles on the surface of a biocompatible elastomeric substrate, where Thin-Film Transistors (TFTs) and circuits (inverter and logic NAND gates) based on amorphous-IGZO are fabricated. By tuning the wavelength and the amplitude of the wrinkles, the devices are fully operational while bent to 13 μm bending radii as well as while stretched up to 5%, keeping unchanged electrical properties. Moreover, a flexible rectifier is also realized, showing no degradation in the performances while flat or wrapped on an artificial human wrist. As proof of concept, transparent TFTs are also fabricated, presenting comparable electrical performances to the nontransparent ones. The extension of the buckling approach from our TFTs to circuits demonstrates the scalability of the process, prospecting applications in wireless stretchable electronics to be worn or implanted.

Positive charge trapping phenomenon in n-channel thin-film transistors with amorphous alumina gate insulators

In this work, we investigate the charge trapping behavior in InGaZnO4 (IGZO) thin-film transistors with amorphous Al2O3 (alumina) gate insulators. For thicknesses ≤10 nm, we observe a positive charge generation at intrinsic defects inside the Al2O3, which is initiated by quantum-mechanical tunneling of electrons from the semiconductor through the Al2O3 layer. Consequently, the drain current shows a counter-clockwise hysteresis. Furthermore, the de-trapping through resonant tunneling causes a drastic subthreshold swing reduction. We report a minimum value of 19 mV/dec at room temperature, which is far below the fundamental limit of standard field-effect transistors. Additionally, we study the thickness dependence for Al2O3 layers with thicknesses of 5, 10, and 20 nm. The comparison of two different gate metals shows an enhanced tunneling current and an enhanced positive charge generation for Cu compared to Cr.

Photo-Induced Room-Temperature Gas Sensing with a-IGZO Based Thin-Film Transistors Fabricated on Flexible Plastic Foil

We present a gas sensitive thin-film transistor (TFT) based on an amorphous Indium–Gallium–Zinc–Oxide (a-IGZO) semiconductor as the sensing layer, which is fabricated on a free-standing flexible polyimide foil. The photo-induced sensor response to NO2 gas at room temperature and the cross-sensitivity to humidity are investigated. We combine the advantages of a transistor based sensor with flexible electronics technology to demonstrate the first flexible a-IGZO based gas sensitive TFT. Since flexible plastic substrates prohibit the use of high operating temperatures, the charge generation is promoted with the help of UV-light absorption, which ultimately triggers the reversible chemical reaction with the trace gas. Furthermore, the device fabrication process flow can be directly implemented in standard TFT technology, allowing for the parallel integration of the sensor and analog or logical circuits.

Ge2Sb2Te5 p-Type Thin-Film Transistors on Flexible Plastic Foil

In this work, we show the performance improvement of p-type thin-film transistors (TFTs) with Ge2Sb2Te5 (GST) semiconductor layers on flexible polyimide substrates, achieved by downscaling of the GST thickness. Prior works on GST TFTs have typically shown poor current modulation capabilities with ON/OFF ratios ≤20 and non-saturating output characteristics. By reducing the GST thickness to 5 nm, we achieve ON/OFF ratios up to ≈300 and a channel pinch-off leading to drain current saturation. We compare the GST TFTs in their amorphous (as deposited) state and in their crystalline (annealed at 200 °C) state. The highest effective field-effect mobility of 6.7 cm2/Vs is achieved for 10-nm-thick crystalline GST TFTs, which have an ON/OFF ratio of ≈16. The highest effective field-effect mobility in amorphous GST TFTs is 0.04 cm2/Vs, which is obtained in devices with a GST thickness of 5 nm. The devices remain fully operational upon bending to a radius of 6 mm. Furthermore, we find that the TFTs with amorphous channels are more sensitive to bias stress than the ones with crystallized channels. These results show that GST semiconductors are compatible with flexible electronics technology, where high-performance p-type TFTs are strongly needed for the realization of hybrid complementary metal-oxide-semiconductor (CMOS) technology in conjunction with popular n-type oxide semiconductor materials.

N-type to p-type transition upon phase change in Ge6Sb1Te2 compounds

In this work, we study the electronic properties of Ge6Sb1Te2 compounds in thin-film transistor architectures on plastic substrates, which enable the extraction of field-effect mobility μFE, carrier density, and polarity in highly resistive thin-films. We find that the Ge-rich compound exhibits n-type conductivity in the amorphous phase with a gradual transition to p-type behavior upon thermal annealing. At a temperature of 350 °C, the material undergoes a phase change, which is confirmed by x-ray diffraction measurements. After the phase change, μFE and the conductivity increase and the polarity becomes p-type, while the carrier density does not change significantly. Furthermore, we compare the properties of Ge6Sb1Te2 to the commonly studied material composition of Ge2Sb2Te5 in the Hall measurement and find that the carrier density of the Ge-rich compound is reduced by 2 orders of magnitude, which indicates that the significantly lower concentration of Ge vacancies leads to a reduction of p-type doping.In this work, we study the electronic properties of Ge6Sb1Te2 compounds in thin-film transistor architectures on plastic substrates, which enable the extraction of field-effect mobility μFE, carrier density, and polarity in highly resistive thin-films. We find that the Ge-rich compound exhibits n-type conductivity in the amorphous phase with a gradual transition to p-type behavior upon thermal annealing. At a temperature of 350 °C, the material undergoes a phase change, which is confirmed by x-ray diffraction measurements. After the phase change, μFE and the conductivity increase and the polarity becomes p-type, while the carrier density does not change significantly. Furthermore, we compare the properties of Ge6Sb1Te2 to the commonly studied material composition of Ge2Sb2Te5 in the Hall measurement and find that the carrier density of the Ge-rich compound is reduced by 2 orders of magnitude, which indicates that the significantly lower concentration of Ge vacancies leads to a reduction of p-type doping.

Geometry-Based Tunability Enhancement of Flexible Thin-Film Varactors

In this letter, flexible voltage-controlled capacitors (varactors) based on an amorphous Indium–Gallium–Zinc–Oxide (a-IGZO) semiconductor are presented. Two different varactor designs and their influence on the capacitance tuning characteristics are investigated. The first design consists of a top electrode finger structure which yields a maximum capacitance tunability of 6.9 at 10 kHz. Second, a novel interdigitated varactor structure results in a maximum tunability of 93.7 at 100 kHz. The design- and frequency-dependencies of the devices are evaluated through C–V measurements. Furthermore, we show bending stability of the devices down to a tensile radius of 6 mm without altering the performance. Finally, a varactor is combined with a thin-film resistor to demonstrate a tunable RC-circuit for impedance matching and low-pass filtering applications. The device fabrication flow and material stack are compatible with standard flexible thin-film transistor fabrication which enables parallel implementation of analog or logic circuitry and varactor devices.