A study on the resistive switching characteristics in Non-volatile Organic (PEDOT:PSS) memory devices of various structure

DoctorThe paper investigated resistive switching characteristics of the various polymer memory structures using poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS).Cross-point array memory device didn’t show the memory switching because there was leakage current due to the thinly coated polymer at the edge of bottom electrode. Therefore, planarization structure using CMP (chemical mechanical polishing) process was fabricated, where unipolar switching characteristic has been observed although there was very little chance. It means that uniform polymer film helps to improve the poor switching properties. We fabricated other structures to examine the memory characteristics of micrometer-sized device. The active areas of the device were defined as the size of TEOS hole etched by photolithography process. Polymer was filled in the TEOS hole by spin-coating. In this structure, a 45㎛ cell shows clear unipolar switching characteristics in spite of shadowmask mismatch. Especially, the ON voltage (3.5V) and OFF voltage (0.8V) are similar to those of 1㎜-sized cell.Next structure was improved to resolve the mismatching problem of the small electrode by inverting top and bottom electrodes. We found that when the top probe was located above (top electrode/polymer/bottom electrode) layer, the device represented certain unipolar switching curve. At that location, huge amount of current paths are intensively created. Also, transparent device using both top and bottom indium tin oxide (ITO) electrodes was fabricated on the glass. The devices show bipolar memory switching. The data retention test and write-read-erase-read cycle test were performed under ambient air condition successfully. The ON and OFF were occurred by reduction and oxidation mechanisms of the PEDOT chain and each interface structure had an influence on the ON-OFF voltage Subsequently, A flexible and transparent polyethylene naphthalate (PEN) film was applied instead of a glass substrate. The PEN film has a better thermostability than other plastic films. An ITO/PEDOT:PSS/ITO/PEN film device was fabricated using the same process as the glass substrate device, with the exception of the bottom ITO, which was sputtered on the PEN film at room temperatureFinally, unipolar switching characteristics were observed only when Al was used as bottom electrode. By experiments, Al2O3 layer plays an important role in continuing formation and destruction of the filamen