Effect of the Thickness of Insulator Polymeric Films on the Memory Behavior: The Case of the Polymethylmethacrylate and the Polystyrene

The effect of thickness variation on the memory behavior of the polymethylmethacrylate-(PMMA)-based devices has been investigated. The PMMA film thicknesses have been varied between 5 to 300？nm, and we have found that the film thickness determines the type of behavior: ohmic, write-once-read-many-times (WORM) memory with two ON states, WORM memory with a negative differential resistance (NDR) region, and WORM memory without NDR region. The fact that similar results were obtained using different solvents to dilute PMMA (chlorobenzene, chloroform, and dimethyl sulfoxide), as well as using an other insulating polymer such as polystyrene (PS), leads to the conclusion that the phenomenon of memory depends on the aluminum electrodes, organic film thickness, and the compliance current used during the electroformation whereas the type of organic layer (PMMA or PS) has minor influence. From here, we conclude that the conductivity switching of the insulator organic film is due to the injection of aluminum particles into the film during the first voltage cycle. 1. Introduction Since the last decade, there has been an intensive research in the field of organic electronics, the organic memory devices being one of the emerging research areas in this field. A considerable number of architectures as well as materials for the development of devices with memory effect has been proposed [1–3]. Among the principal thin-film structures that have been reported are metal-organic insulator-metal (MIM) [4–14], metal–organic–metal–organic–metal [15–17], metal nanoparticles embedded in the organic layer in MIM architecture [18], devices where the embedded metal nanoparticles are replaced with organic particles such as fullerenes [19–21], and carbon nanoshells [22]. From all, the simplest architecture and one of the first to show memory behavior was the MIM architecture. One of the pioneer works was realized by Simmons and Verderber [23], who used a thin film of SiO2 inorganic insulator with a thickness in the order of nanometers between two metallic electrodes. The authors explained this phenomenon as the creation of charge traps in the inorganic film due to an electric field. However, different theories have been proposed in order to explain such phenomena, one of them by Thurstans and Oxley [24] who proposed the effect of electroformation. They mentioned that once a voltage was applied on the device, an electric field was induced and this field moves the metal particles coming from the electrodes into the organic film, where they form metallic islands. These islands are