REVERSIBLE CHARGE TRAPPING DETRAPPING IN A PHOTOCONDUCTIVE INSULATOR OF LIQUID-CRYSTAL ZINC PORPHYRIN/

A molecular crystal of zinc octakis(beta-decoxyethyl) porphyrin (ZnODE P) is an insulator in the dark and becomes conductive under irradiatio n. An externally controllable charge trapping and detrapping within Zn ODEP thin films (similar to 1 mu m) occurs when symmetrical sandwich c ells of ITO/ZnODEP/ITO are irradiated under a proper bias voltage betw een two parallel ITO (indium-tin oxide) electrodes. The trapping and d etrapping rise time is on the nanosecond time scale. Detrapping of cha rge stored previously in the cell could be accomplished with pulse irr adiation under short-circuit conditions and gives rise to a discharge current spike. Trapped charge induced by a 10 ns laser pulse or by lon ger time irradiation with a conventional light source could be sensed by a voltage measurement at open circuit. No loss of stored charge was detectable at a 1 pA level for a period of 11 months under open circu it conditions in the dark. After charge trapping with 550 nm light irr adiation (10 mu W/cm(2)) under a bias of 0.5 V, the stored charge indu ced a voltage difference of similar to 20 mV between the two ITO elect rodes. This voltage difference was stable for at least 2000 h with no evidence of decay. These results suggest that ZnODEP as a thin film ph otoconductive insulator might serve as a memory medium for electroopti cal information storage in the form of charge. Such a data storage sys tem would be nonvolatile and rewritable. We have shown that a memory e lement could be subjected to write (trapping)/erase (detrapping) 1.5 b illion times with a readout signal that was essentially identical with the first without any evidence of deterioration. To find attainable r esolution, charge was injected with a scanning tunneling microscope ti p under different bias. For a 6 V bias, charge was trapped in an eleme nt of 40 nm diameter, equivalent to a storage density of 8 x 10(10) bi ts/cm(2).