The semiconductor of an organic field‐effect transistor is stripped with adhesive tape, yielding an exposed gate dielectric, accessible for various characterization techniques. By using scanning Kelvin probe microscopy we reveal that trapped charges after gate bias stress are located at the gate dielectric and not in the semiconductor. Charging of the gate dielectric is confirmed by the fact that the threshold voltage shift remains, when a pristine organic semiconductor is deposited on the exposed gate dielectric of a stressed and delaminated field‐effect transistor.

The threshold voltage is an important property of organic field-effect transistors. By applying a self-assembled monolayer (SAM) on the gate dielectric, the value can be tuned. After electrical characterization, the semiconductor is delaminated. The surface potentials of the revealed SAM perfectly agree with the threshold voltages, which demonstrate that the shift is not due to the dipolar contribution, but due to charge trapping by the SAM.

Encapsulation of organic (opto-)electronic devices, such as organic light-emitting diodes (OLEDs), photovoltaic cells, and field-effect transistors, is required to minimize device degradation induced by moisture and oxygen ingress. SiNx moisture permeation barriers have been fabricated using a very recently developed low-temperature plasma-assisted atomic layer deposition (ALD) approach, consisting of half-reactions of the substrate with the precursor SiH2(NH(t)Bu)2 and with N2-fed plasma. The deposited films have been characterized in terms of their refractive index and chemical composition by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). The SiNx thin-film refractive index ranges from 1.80 to 1.90 for films deposited at 80 °C up to 200 °C, respectively, and the C, O, and H impurity levels decrease when the deposition temperature increases. The relative open porosity content of the layers has been studied by means of multisolvent ellipsometric porosimetry (EP), adopting three solvents with different kinetic diameters: water (∼0.3 nm), ethanol (∼0.4 nm), and toluene (∼0.6 nm). Irrespective of the deposition temperature, and hence the impurity content in the SiNx films, no uptake of any adsorptive has been observed, pointing to the absence of open pores larger than 0.3 nm in diameter. Instead, multilayer development has been observed, leading to type II isotherms that, according to the IUPAC classification, are characteristic of nonporous layers. The calcium test has been performed in a climate chamber at 20 °C and 50% relative humidity to determine the intrinsic water vapor transmission rate (WVTR) of SiNx barriers deposited at 120 °C. Intrinsic WVTR values in the range of 10(-6) g/m2/day indicate excellent barrier properties for ALD SiNx layers as thin as 10 nm, competing with that of state-of-the-art plasma-enhanced chemical vapor-deposited SiNx layers of a few hundred nanometers in thickness.

The huge impact of nitrogen dioxide (NO 2 ) emission on public health and the environment is motivating extensive scientific and technological research in the field of NO 2 sensing. Field-effect transistors have emerged as a sensitive and reliable technology for monitoring air quality, due to their amplified sensor response. In this review article, the NO 2 detection mechanism with field-effect transistors is discussed. The origin is charge trapping at the gate dielectric, yielding a threshold voltage shift. The dynamic response can be described by an analytical model. Implementation in a sensor protocol allows for the fabrication of a functional demonstrator. The sensor, based on a ZnO field-effect transistor, is capable of detecting concentrations as low as 40 ppb of NO 2 in real-time. The sensor operates in ambient air, and apart from drying, no further precautions are taken, showing that the fabricated sensor is selective for NO 2 . The results reviewed in this paper set a precedent for sensing with field-effect transistors.

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Detection of nitrogen dioxide, NO2, is required to monitor the air‐quality for human health and safety. Commercial sensors are typically chemiresistors, however field‐effect transistors are being investigated. Although numerous investigations have been reported, the NO2 sensing mechanism is not clear. Here, the detection mechanism using ZnO field‐effect transistors is investigated. The current gradually decreases upon NO2 exposure and application of a positive gate bias. The current decrease originates from the trapping of electrons, yielding a shift of the threshold voltage towards the applied gate bias. The shift is observed for extremely low NO2 concentrations down to 10 ppb and can phenomenologically be described by a stretched‐exponential time relaxation. NO2 detection has been demonstrated with n‐type, p‐type, and ambipolar semiconductors. In all cases, the threshold voltage shifts due to gate bias induced electron trapping. The description of the NO2 detection with field‐effect transistors is generic for all semiconductors and can be used to improve future NO2 sensors.

Water permeation in inorganic moisture permeation barriers occurs through macroscale defects/pinholes and nanopores, the latter with size approaching the water kinetic diameter (0.27 nm). Both permeation paths can be identified by the calcium test, i.e., a time-consuming and expensive optical method for determining the water vapor transmission rate (WVTR) through barrier layers. Recently, we have shown that ellipsometric porosimetry (i.e., a combination of spectroscopic ellipsometry and isothermal adsorption studies) is a valid method to classify and quantify the nanoporosity and correlate it with the WVTR values. Nevertheless, no information is obtained about the macroscale defects or the kinetics of water permeation through the barrier, both essential in assessing the quality of the barrier layer. In this study, electrochemical impedance spectroscopy (EIS) is shown as a sensitive and versatile method to obtain information on nanoporosity and macroscale defects, water permeation, and diffusivity of moisture barrier layers, complementing the barrier property characterization obtained by means of EP and calcium test. EIS is performed on thin SiO2 barrier layers deposited by plasma enhanced-CVD. It allows the determination of the relative water uptake in the SiO2 layers, found to be in agreement with the nanoporosity content inferred by EP. Furthermore, the kinetics of water permeation is followed by EIS, and the diffusivity (D) is determined and found to be in accordance with literature values. Moreover, differently from EP, EIS data are shown to be sensitive to the presence of local macrodefects, correlated with the barrier failure during the calcium test.

Dynamics of charge carrier trapping in NO2 sensors based on ZnO field-effect transistors Andringa, Anne-Marije; Vlietstra, Nynke; Smits, Edsger C. P.; Spijkman, Mark-Jan; Gomes, Henrique L.; Klootwijk, Johan H.; Blom, Paul W. M.; de Leeuw, Dago M. Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Andringa, A-M., Vlietstra, N., Smits, E. C. P., Spijkman, M-J., Gomes, H. L., Klootwijk, J. H., ... de Leeuw, D. M. (2012). Dynamics of charge carrier trapping in NO2 sensors based on ZnO field-effect transistors. Sensors and actuators b-Chemical, 171(27), 1172-1179. DOI: 10.1016/j.snb.2012 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Here we investigate the dynamics of charge trapping and recovery as a function of temperature by monitoring the threshold voltage shift. The threshold voltage shifts follow a stretched-exponential time dependence with thermally activated relaxation times. We find an activation energy of 0.1 eV for trapping and 1.2 eV for detrapping. The attempt-to-escape frequency and characteristic temperature have been determined as 1 Hz and 960 K for charge trapping and 10 11 Hz and 750 K for recovery, respectively. Thermally stimulated current measurements confirm the presence of trapped charge carriers with a trap depth of around 1 eV. The obtained functional dependence is used as input for an analytical model that predicts the sensor's temporal behavior. The model is experimentally verified and a real-time sensor has been developed. The perfect agreement between predicted and measured sensor response validates the methodology developed. The analytical description can be used to optimize the driving protocol. By adjusting the operating temperature and the duration of charging and resetting, the response time can be optimized and the sensitivity can be maximized for the desired partial NO 2 pressure window.