Modeling the underlying mechanisms for organic memory devices: Tunneling, electron emission, and oxygen adsorbing

Abstract

We present a combined experimental and theoretical study to get insight into both memory and negative differential resistance (NDR) effect in organic memory devices. The theoretical model we propose is simply a one-dimensional metallic island array embedding within two electrodes. We use scattering operator method to evaluate the tunneling current among the electrode and islands to establish the basic bistable I-V curves for several devices. The theoretical results match the experiments very well, and both memory and NDR effect could be understood comprehensively. The experimental correspondence, say, the experiment of changing the pressure of oxygen, is addressed as well.PACS numbers: 73.40.Gk Memory devices based on metal-insulator-metal resistive structure with organic materials are promising in application.1 The underlying mechanism in these devices is not very clear up to now due to the complicated phenomena observed, say for example, the memory effect and the negative differential resistance (NDR) in ON state, making an obstacle for controllable improvement of the performance.2,3 Two different but correlated mechanisms have been addressed. The first one is the SimmonsVerderber (SV) model based on the space charge accumulated on some penetrated metallic islands.4 This model could be proposed to explain NDR, which is also closely related to the memory effect.2 In addition to it, electron tunneling and Coulomb blockade were also proposed, 3 indicating that the different spatial distribution of the space charge gave rise to the transition between ON and OFF state. The admittance spectroscopy provided another evidence to the SV model.5 Whereas, the basic drawback of this mechanism is that, the OFF state is always observed, in practice, during the first scanning of bias voltage, while SV model predicts the sample is initially in ON state.6 The second mechanism is the filamentary conduction, stating that there would be some penetration of metal atoms into organic layer during the deposition of top electrode, which forms some conducting filaments under relative large bias voltage, and the trap-controlled tunneling between different filaments dominates the NDR effect.7,8 The direct experimental evidence for this mechanism is from the transmission electron microscopy image, which shows the metallic filaments are formed in ON state.9 Meanwhile, the temperature insensitivity of ON state current, 10 the time dependent switch-on behavior, 11-13 and the scanning voltage and conductance dependence 14 all go together to support the filamentary mechanism. However, it is still hard to imagine the destruction and recovery of an atomic filament could respond so fast to the scanning voltage. Combining the above two mechanisms, we can get a common idea that, during the deposition of the electrode, the metallic atoms could go deeply into the organic layer and form some islands in the film, 3,15 which is crucial to the NDR effect no matter it originates from space charge or filaments.Another experiments were focusing on the ro...

Oxygen Effects on Performance of Electrically Bistable Devices Based on Hybrid Silver Sulfide Poly(N-vinylcarbazole) Nanocomposites

“…This indicates that the number of the carriers injected into the Ag 2 S/PVK layer is increased greatly when the sweeping voltage is higher than a certain voltage, and the electrons can be captured by the traps of the Ag 2 S nanocrystals with an exponential distribution in the forbidden gap, in which a large number of sulfur vacancies formed on the surface of Ag 2 S nanocrystals act as the charge traps [ 25 ]. When the devices are measured in the presence of air, the molecular oxygen may react with the sulfur vacancies to form a bridging oxide dimer to act as a potential barrier around the Ag 2 S nanocrystals [ 26 , 27 ]. Thus, the number of the injected carriers trapped by the barrier may be increased, which is responsible for the high slope of the fitting line of the devices measured in air.…”

Exciton dissociation in the presence of phonons: A reduced hierarchy equations of motion approach

“…Similarly, the NDR effect also exists in organic semiconducting devices, where it is believed to originate in the tunneling process across the energy barrier at the metal/organic interface. 39 Here, we use the WF-HEOM approach to calculate the current-voltage (I-V) curves in the NDR regime for a semiconducting material that bridges two electrodes in the presence of two Guassian-shaped interface energy barriers given by aexp [− (q − q 0 ) 2 /c 2 ] with a = 0.3 eV and c = 2 nm. The distance between these two barriers is 5 nm, and the rest of model parameters are: the electron effective mass is 0.067m 0 with m 0 the mass of free electron; the discretization number for coordinate and momentum N q and N p are 80 and 60, respectively; the mesh spacing q is 0.5 nm such that p = π /N q q .…”

“…Each data point requires more than 10 h of computation on a Linux-based computing cluster, with each CPU being a 2.67 GHz Intel processor, rendering the WF-HEOM method less efficient than the scattering-operator method. 39 However, it is easier to apply the WF-HEOM approach to our system than the scattering-operator method. In particular, the NDR effect has been reproduced for the semiconducting devices in a unified manner, while with scattering operator method, results need to be rescaled on a caseby-case basis.…”

Resistive switching based on filaments in metal/PMMA/metal thin film devices

“…Generally, filament formation is thought to be also related to the unintentional incorporation of metal particles during the thermal evaporation of the top electrode (TE). 16,17) Based on our previous work, 14,15,18) we here further elucidate the origin of unipolar resistance switching in ORS. By comparing current-voltage (I-V) characteristics of devices with conventionally evaporated top electrodes with metal transfer printed (MTP) top electrodes the hypothesis that metal implantation during the deposition of the TE is essential for switching is benchmarked.…”