Thirty-day-long data retention in ferroelectric-gate field-effect transistors with HfO₂ buffer layers

Metal-ferroelectric-insulator-semiconductor (MFIS) diodes and p-channel MFIS field-effect transistors (FETs) were fabricated and their electrical properties were characterized. These MFIS structures were formed using HfO ₂ as an insulating buffer layer, and SrBi₂Ta ₂O₉ (SBT) and (Bi,La)₄Ti₃O ₁₂ (BLT) as ferroelectric films. HfO₂ buffer layers of about 8 nm physical thickness were deposited by ultrahigh-vacuum (UHV) electron-beam evaporation, then ferroelectric films of about 400 nm thickness were deposited by sol-gel spin coating. The fabricated p-channel MFIS-FETs with the SBT/HfO₂ gate structure exhibited a drain current on/off ratio larger than 10³ even after 30 days had elapsed. It was also found that the degradation of ferroelectricity was not pronounced even after applying 2.2 × 10¹¹ bipolar pulses.