Five-day-long ferroelectric memory effect in Pt/(Bi,La)₄Ti ₃O₁₂/HfO₂/Si structures

In order to form metal-ferroelectric-semiconductor-insulator-semiconductor (MFIS) structures, hafnium oxide (HfO₂) films were first deposited on Si(100) substrates at room temperature using an electron-beam evaporation method. Then, they were annealed in an O₂ atmosphere at 800 °C for 1 min. No hysteretic characteristics were observed in the capacitance-voltage (C-V) measurement for the metal-insulator-semiconductor capacitors. On these films, lanthanum-substituted bismuth titanate [(Bi,La) ₄ Ti₃O₁₂:BLT] films with 380 nm in thickness were deposited by a sol-gel spin-coating method and annealed in an O ₂atmosphere at 750 °C for 30 min. The MFIS sample showed the hysteretic C-V characteristics and the memory window width was about 0.9 V for the voltage sweep of ±7 V. Furthermore, the high and low capacitance values biased in the hysteresis loop were clearly distinguishable for over 5 days. The origin of the excellent data retention characteristics is discussed from a viewpoint of the transient current across the film.