Memory window widening of Pt/SrBi_2Ta_2O_9/HfO_2/Si ferroelectric-gate field-effect transistors by nitriding Si

摘要

The optimum temperature of rapid thermal nitridation (RTN) of Si substrates was investigated for minimizing an equivalent oxide thickness (EOT) of an interfacial layer (IL) which was grown between HfO_2 and Si of Pt/SrBi_2Ta_2O_9(SBT)/HfO_2/Si ferroelectric-gate field-effect transistors (FeFETs) during a post-annealing process. The RTN was performed in NH_3 gas at various temperatures ranging from 800 ℃ to 1190 ℃. As the RTN temperature was raised from 800 ℃ to 1080 ℃, memory windows of drain current-gate voltage curves became wider. Large memory windows were obtained at the range from 1020 ℃ to 1130 ℃. The maximum was 1.36 V obtained at 1080 ℃. It was 10% larger than the typical values of Pt/SBT/HfO_2/Si FeFETs without the RTN. At higher RTN temperatures than 1080 ℃, the memory windows tended to decrease. At 800 ℃ and 1190 ℃, all layer boundaries among SBT-HfO_2-IL-Si seemed unclear in scanning transmission electron microscopic views probably due to material diffusions. The optimum RTN temperature for minimizing the EOT of the IL and maximizing the memory window of the Pt/SBT/HfO_2/SiN_x/Si FeFET was 1080 ℃. The FeFET using the Si processed by the RTN at 1080 ℃ also showed good retentions without significant degradations over two days.