Electrical bistable properties of nonvolatile memory device based on hybrid ZCIS NCs:PMMA film

A write-once-read-many-times (WORM) electrical bistable device was fabricated utilizing hybrid quaternary Zn-Cu-In-S nanocrystals (ZCIS NCs) and poly(methyl methacrylate) (PMMA) as active layer. A low turn-on voltage of 1.0 V and the maximum ON/OFF current ratio of 4×103 were found from current-voltage (I-V) characteristics. The current stability in the ON and OFF state has a fluctuation loss less than 5% in the measurement duration of 104 s. The electron transport and memory mechanism are described based on the fitted I-V curves and the energy band diagrams. The conduction models in the OFF state from 0 V to 1.1 V are described as thermionic emission (TE), space-charge-limited current (SCLC) and Fowler-Nordheim (FN) tunneling, respectively. And the Ohmic conduction dominates when the device is transited to the ON state at the voltage over 1.1 V. The high-density defects in quaternary ZCIS NCs and the high barrier between ZCIS NCs and the PMMA are considered to contribute to the electron retention behavior. For device performance optimization, an additional PMMA layer is pre-deposited on the ITO substrate surface. The 50% reduction at turn-on voltage and over 50 times enhancement in ON/OFF ratio are reasonably attributed to the better film quality and decreased series resistance from the decrease of root-mean-square average surface roughness (RMS) of the ITO surface from 0.877 to 0.216 nm. The better retention capacity is considered to relate to thickened tunnel barrier by PMMA inserted layer.