Gadolinium-based metal oxide for nonvolatile memory applications
Abstract In this study, the memory characteristic of a gadolinium (Gd)-based oxide charge storage layer was demonstrated. The metal/oxide/high-k/oxide/silicon (MOHOS)-type memories were fabricated by using two different charge storage layers. The Gd2O3 nanocrystal (Gd2O3-NC) was used as a charge storage layer due to the discrete nodes, while the HfGdO high-k material was used as a charge storage layer due to the existence of discrete traps. In the case of Gd2O3-NC memory, a combination of X-ray photoelectron spectroscopy (XPS) and ultraviolet (UV)–visible spectrophotometer analysis was used in this study to extract the valence band location and the band-gap of the Gd2O3-NC layer. The retention characteristic was also analyzed to extract the trapping level in Gd2O3-NC, based on the relationship between trapping energy and discharging time. A band diagram was created to characterize the memory effect of the Gd2O3-NC memory. In the case of HfGdO SONOS-type memory, the electrical and physical studies were conducted for HfGdO charge-trapping layers deposited by a dual-sputtered method for silicon–oxide–nitride–oxide–silicon (SONOS)-type nonvolatile memory. The Hf/Gd dual-sputtered power ratio and the Ar/O2 gas flow ratio were optimized. It was observed that the nonstoichiometric GdO (2 0 0) structure may be the main charge-trapping site for the memory. The memory samples with Hf/Gd = 150/150 and Ar/O2 = 20/5 exhibited better electrical performance. A physical model is proposed to further explain the retention mechanism.