The works reported here were divided into two parts. In the first part of an
investigation, the memory effect in polymer memory diodes based on poly ( 4-
dicyanomethylene-4H-cyclopenta [1,2-b;3,4-b'] dithiophene (PCDM) is described.
The second part describes a study of a flash memory device using regioregular poly (3-hexylthiophene) (P3HT) as an organic semiconductor and polysilsesquioxane (PSQ) as an insulator. In the polymer flash memory device, a thin gold layer (20 nm) embedded in the PSQ insulator was used as a charge storage element. In both parts, the memory effect was studied by means of DC current-voltage (I-V) measurements and admittance spectroscopy. In the first part, the disappearance of the memory effect in the later PCDM films produced successively from the same monomer solution have been further studied by the FTIR and UV/Vis spectroscopy. In the second part, the precursor devices such as P3HT Schottky diode, metal-insulatormetal (MIM) and metal-insulator-semiconductor-metal (MISM) which ultimately led to the floating gate memory device were also fabricated and investigated.
The changes in the electrical characteristic of the PCDM films were consistent with the spectroscopic data. It is suggested that free BF4 anions create a space charge region at the interfaces leading to the memory effect shown in the first two PCDM films. Quaternary ammonium counter-cations become trapped in the later PCDM films as demonstrated by the FTIR spectrum and in tum bind the BF4 anions. As a result, the memory effect in the later films disappeared. In MIM device, PSQ film shows a very good insulating property in voltage and temperature range of -40 V to 40 V and 25°C to 130°C respectively. By comparing the results of the precursor devices with the flash memory device, it was shown that the embedded floating gate in the insulator created a site for charge trapping. Hysteresis was observed in both the I-V and C-V plots of the flash memory device with a threshold voltage shift of ~7.5 V in the C-V plot when the device was subjected to the voltage sweep range ±20 V. It is also suggested that the PSQ insulator only allows electron transport.