Impact of oxide gate electrode for ferroelectric field-effect transistors with metal-ferroelectric-metal-insulator-semiconductor gate stack using undoped HfO2 thin films prepared by atomic layer deposition.

Ferroelectric field-effect transistors (FETs) with a metal-ferroelectric-metal-insulator-semiconductor (MFMIS) gate stack were fabricated and characterized to elucidate the key process parameters and to optimize the process conditions for guaranteeing nonvolatile memory operations of the device when the undoped HfO2 was employed as ferroelectric gate insulator. The impacts of top gate (TG) for the MFM part on the memory operations of the MFMIS-FETs were intensively investigated when the TG was chosen as metal Pt or oxide ITO electrode. The ferroelectric memory window of the MFMIS-FETs with ITO/HfO2/TiN/SiO2/Si gate stack increased to 3.8 V by properly modulating the areal ratio between two MFM and MIS capacitors. The memory margin as high as 104 was obtained during on- and off-program operations with a program pulse duration as short as 1 µs. There was not any marked degradation in the obtained memory margin even after a lapse of retention time of 104 s at 85 °C and repeated program cycles of 10,000. These obtained improvements in memory operations resulted from the fact that the choice of ITO TG could provide effective capping effects and passivate the interfaces.