Impact of metal nano layer thickness on tunneling oxide and memory performance of core-shell iridium-oxide nanocrystals

The impact of iridium-oxide (IrO_x) nano layer thickness on the tunneling oxide and memory performance of IrO_x metal nanocrystals in an n-Si/SiO₂/Al₂O₃/IrO_x/Al ₂O₃/IrO_x structure has been investigated. A thinner (1.5 nm) IrO_x nano layer has shown better memory performance than that of a thicker one (2.5 nm). Core-shell IrO_x nanocrystals with a small average diameter of 2.4 nm and a high density of ∼2 ×10¹²/cm² have been observed by scanning transmission electron microscopy. The IrO_x nanocrystals are confirmed by x-ray photoelectron spectroscopy. A large memory window of 3.0 V at a sweeping gate voltage of ±5 V and 7.2 V at a sweeping gate voltage of ±8 V has been observed for the 1.5 nm-thick IrO_x nano layer memory capacitors with a small equivalent oxide thickness of 8 nm. The electrons and holes are trapped in the core and annular regions of the IrO_x nanocrystals, respectively, which is explained by Gibbs free energy. High electron and hole-trapping densities are found to be 1.5 ×10 ¹³/cm² and 2 ×10¹³/cm², respectively, due to the small size and high-density of IrO_x nanocrystals. Excellent program/erase endurance of >10⁶ cycles and good retention of 10⁴s with a good memory window of >1.2 V under a small operation voltage of ±5 V are obtained. A large memory size of >10 Tbit/sq. in. can be designed by using the IrO_x nanocrystals. This study is not only important for the IrO_x nanocrystal charge-trapping memory investigation but it will also help to design future metal nanocrystal flash memory.