Ultrathin oriented BiFeO ₃ films from deposition of atomic layers with greatly improved leakage and ferroelectric properties

Highly (001)-oriented BiFeO₃ ultrathin films of total thickness of less than 10 nm were deposited on Si(001) substrates via deposition of atomic layers (ALD) with a LaNiO₃ buffer. A radio-frequency (RF)-sputtered sample of the same thickness was prepared for comparison. The ALD combined with interrupted flow and an exchange reaction between Bi and Fe precursors provides a superior method to grow ternary compounds. According to X-ray diffraction, upon deposition at a temperature of less than 550 C, the only phase in the film was BiFeO₃. Anomalous fine structure from synchrotron X-ray diffraction certified the valence bonding through the BiFeO₃ (001) diffraction signal. The stoichiometric ratio of BiFeO₃ obtained from X-ray photoelectron spectroscopy indicated that ALD has a proportion much improved over the RF preparation, and this is also in agreement with the results for diffraction anomalous fine structure. The use of high-resolution transmission electron and atomic force microscopes showed that the layer structure and morphology from ALD presented a satisfactory coverage, more conformal than that with the RF method. The BiFeO₃ thin film deposited with ALD shows excellent leakage, improved at least 1000 times with respect to the RF preparation, making this method suitable for the fabrication of ferroelectric random-access memory devices. From the hysteresis loop, the largest remanent polarization was observed as 2P_r = 2.0 μC cm ^-2.