Current Conduction Mechanism for Low-molecular Organic Nonvolatile Memory

摘要

Organic devices fabricated with a top metal layer/conductive organic layer/middle metal layer/conductive organic layer/bottom metal layer structure have been reported to demonstrate nonvolatile memory behavior such as an after writing (I_(on))/after erasing (I_(off)) performance of > 1 × 10~1 and a response time of ～10 ns, when the organic conductive layers were AIDCN (2-amino-4, 5-imidazoledicarbonitrile), Alq_3 (Aluminum tris(8-hydroxyquinoline)), or α-NPD. We fabricated an organic nonvolatile memory device with a structure of α-NPD/Al nanocrystals surrounded by Al_2O_3/α-NPD/Al, where α-NPD was N,N'-bis(1-naphthyl)-1,1'biphenyl4-4' diamine. A layer of Al nanocrystals, confirmed by a 1.25-MV high voltage transmission-electron-microscope, was uniformly produced between the a-NPD layers by Al layer evaporation at 1.0 ?/sec on the α-NPD followed by O_2 plasma oxidation. We confirmed a conduction bistability of ～10~2 and a threshold voltage for a set state of 3 V. Al nanocrystals surrounded by amorphous Al_2O_3 were formed in the a-NPD. They presented seven different reversible current paths for an electron charge or discharge on the nanocrystals. The current slightly increased with an applied bias from 0 V to V_(th), (a high resistance state (I_(off))), abruptly increased with an applied bias from V_(th), to V_p, decreased with an increasing applied bias from V_p to V_e (a negative differential resistance (NDR) region), and slightly increased with an applied bias above V_e. After sweeping the first applied voltage from 0 to 10 V (erase), a second applied bias was swept from 0 to V_p (program), where the current followed a high resistance state (I_(off)). Next, a third applied bias was swept from 0 to V_p again, where the current followed a low resistance state (I_(on)). Surprisingly, the ratio of I_(on) to I_(off) was ～1×10~2, which is enough current difference to be nonvolatile memory behavior. These I-V characteristics under a positive applied bias were symmetrically repeated under a negative applied bias. All the current sweeping paths were reproducible and symmetrical for an applied bias polarity. In particular, our device demonstrated multi-level nonvolatile memory behavior. It also revealed the current conduction mechanism for each of its operation regions. We observed that the high resistance and low resistance regions followed space-charge-limited current conduction, the V_(th) to V_p and V_(NDR) to V_e regions followed precisely thermionic-field-emission current conduction, and the above V_e regions followed space-charge-limited current conduction.