Memory window widening of Pt/SrBi2Ta2O9/HfO2/Si ferroelectric-gate field-effect transistors by nitriding Si

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 HfO2 and Si of Pt/SrBi2Ta2O9(SBT)/HfO2/Si ferroelectric-gate field-effect transistors (FeFETs) during a post-annealing process. The RTN was performed in NH3 gas at various temperatures ranging from 800 degrees C to 1190 degrees C. As the RTN temperature was raised from 800 degrees C to 1080 degrees C, memory windows of drain current-gate voltage curves became wider. Large memory windows were obtained at the range from 1020 degrees C to 1130 degrees C. The maximum was 1.36 V obtained at 1080 degrees C. It was 10% larger than the typical values of Pt/SBT/HfO2/Si FeFETs without the RTN. At higher RTN temperatures than 1080 degrees C, the memory windows tended to decrease. At 800 degrees C and 1190 degrees C, all layer boundaries among SBT-HfO2-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/HfO2/SiNx/Si FeFET was 1080 degrees C. The FeFET using the Si processed by the RTN at 1080 degrees C also showed good retentions without significant degradations over two days.