Thickness dependence of Al₂O₃/HfO₂/Al₂O₃ stacked tunneling layers on gadolinium oxide nanocrystal nonvolatile memory

Characteristics of Gd₂O₃-NC memories with multiple tunneling layers of Al₂O₃/HfO₂/Al₂O₃ (AHA) have been investigated. It can be found that the Gd₂O₃-NC memory with thin bottom and thin top Al₂O₃ film of AHA stacked tunneling layers exhibits superior programming and erasing (P/E) properties, respectively. Compared with the memory with SiO₂ tunneling layer, the retention characteristics of Gd₂O₃-NC memories with AHA stacked tunneling layers are significantly improved. In addition, for the memories with AHA stacked tunneling layers, the trapping energy level (E_t) of shallow-trap (ST) electron loss is decreased but that of deep-trap (DT) electron loss is increased due to some shallow traps within HfO₂ film and the midgap interface states at HfO₂/Si interface respectively. Further, the dependence of AHA stacked layer thickness on memory characteristics can be explained by the band engineering of tunneling layer. The Gd₂O₃-NC memories with AHA stacked tunneling layers can sustain a stable memory window of more than 1.6 V after a P/E cycling test of 10⁴ times.