Browsing by Author "Janssen, R. A. J."
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- Lithium fluoride injection layers can form quasi-Ohmic contacts for both holes and electronsPublication . Bory, B. F.; Rocha, Paulo R. F.; Janssen, R. A. J.; Gomes, Henrique L.; de Leeuw, D. M.; Meskers, S. C. J.Thin LiF interlayers are typically used in organic light-emitting diodes to enhance the electron injection. Here, we show that the effective work function of a contact with a LiF interlayer can be either raised or lowered depending on the history of the applied bias. Formation of quasi-Ohmic contacts for both electrons and holes is demonstrated by electroluminescence from symmetric LiF/polymer/LiF diodes in both bias polarities. The origin of the dynamic switching is charging of electrically induced Frenkel defects. The current density-electroluminescence-voltage characteristics can qualitatively be explained. The interpretation is corroborated by unipolar memristive switching and by bias dependent reflection measurements. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
- Relation between the electroforming voltage in alkali halide-polymer diodes and the bandgap of the alkali halidePublication . Bory, B. F.; Wang, J. X.; Gomes, Henrique L.; Janssen, R. A. J.; de Leeuw, D. M.; Meskers, S. C. J.Electroforming of indium-tin-oxide/alkali halide/poly(spirofluorene)/Ba/Al diodes has been investigated by bias dependent reflectivity measurements. The threshold voltages for electrocoloration and electroforming are independent of layer thickness and correlate with the bandgap of the alkali halide. We argue that the origin is voltage induced defect formation. Frenkel defect pairs are formed by electron-hole recombination in the alkali halide. This self-accelerating process mitigates injection barriers. The dynamic junction formation is compared to that of a light emitting electrochemical cell. A critical defect density for electroforming is 10(25)/m(3). The electroformed alkali halide layer can be considered as a highly doped semiconductor with metallic transport characteristics. (C) 2014 Author(s).
- Reproducible resistive switching in nonvolatile organic memoriesPublication . Verbakel, F.; Meskers, S. C. J.; Janssen, R. A. J.; Gomes, Henrique L.; Coelle, M.; Buechel, M.; de Leeuw, D. M.Resistive switching in nonvolatile, two terminal organic memories can be due to the presence of a native oxide layer at an aluminum electrode. Reproducible solid state memories can be realized by deliberately adding a thin sputtered Al2O3 layer to nominal electron-only, hole-only, and bipolar organic diodes. Before memory operation, the devices have to be formed at an electric field of 10(9) V/m, corresponding to soft breakdown of Al2O3. After forming, the structures show pronounced negative differential resistance and the local maximum in the current scales with the thickness of the oxide layer. The polymer acts as a current limiting series resistance.
- Role of hole injection in electroforming of LiF-polymer memory diodesPublication . Bory, Benjamin F.; Gomes, Henrique L.; Janssen, R. A. J.; De Leeuw, Dago M.; Meskers, S. C. J.Al/LiF/poly(spirofluorene)/Ba/Al diodes submitted to bias voltages near 15 V undergo a change to a nonvolatile memory known as electroforming. Prior to electroforming, electron trapping occurs, followed by a tunneling current due to electrons from the polymer into LiF. At the onset voltage for electroforming, a sudden electroluminescence burst originating from electronic excitations in the polymer is detected. This confirms that hole injection into the polymer through LiF occurs leading to nonvolatile resistive switching.
- Role of Hole Injection in Electroforming of LiF-Polymer Memory DiodesPublication . Bory, B. F.; Gomes, Henrique L.; Janssen, R. A. J.; de Leeuw, D. M.; Meskers, S. C. J.Al/LiF/poly(spirofluorene)/Ba/Al diodes submitted to bias voltages near 15 V undergo a change to a nonvolatile memory known as electroforming. Prior to electroforming, electron trapping occurs, followed by a tunneling current due to electrons from the polymer into LiF. At the onset voltage for electroforming, a sudden electroluminescence burst originating from electronic excitations in the polymer is detected. This confirms that hole injection into the polymer through LiF occurs leading to nonvolatile resistive switching.
- Switching dynamics in non-volatile polymer memoriesPublication . Verbakel, F.; Meskers, S. C. J.; Janssen, R. A. J.; Gomes, Henrique L.; van den Biggelaar, A. J. M.; De Leeuw, Dago M.The time dependence of resistive switching in metal-metal oxide-organic semiconductormetal diodes is investigated. The switching dynamics is controlled by two intrinsic time dependences. A single switching event occurs in a time scale of 400 nanoseconds, but the maximum repetitive switching between ON- and OFF-states is limited by a ‘‘dead time” of a few milliseconds. The dead time is the waiting time after programming in which a next switch is inhibited. Therefore, fast repetitive pulsing prevents the observation of non-volatile switching and limits the maximum clock rate at which these memories can be used. Understanding the origin of this dead time is crucial to future memory applications. Furthermore,the occurrence of a dead time is possibly the origin of the huge variation in the reported switching times.
- Trapping of electrons in metal oxide-polymer memory diodes in the initial stage of electroformingPublication . Bory, Benjamin F.; Meskers, S. C. J.; Janssen, R. A. J.; Gomes, Henrique L.; De Leeuw, Dago M.Metal oxide-polymer diodes require electroforming before they act as nonvolatile resistive switching memory diodes. Here we investigate the early stages of the electroforming process in Al/Al2O3 /polyspirofluorene /Ba/Al diodes using quasistatic capacitance-voltage measurements. In the initial stage, electrons are injected into the polymer and then deeply trapped near the polyspirofluorene-Al2O3 interface. For bias voltages below 6 V, the number of trapped electrons is found to be CoxideV/q with Coxide as the geometrical capacitance of the oxide layer. This implies a density of traps for the electrons at the polymer-metal oxide interface larger than 31017 m−2.