Growth of Bi ₂O ₃ ultrathin films by atomic layer deposition

Bismuth trioxide (Bi ₂O ₃) ultrathin films were successfully synthesized on silicon substrates by means of atomic layer deposition (ALD) using Bi(thd) ₃ (thd: 2,2,6,6-tetramethyl-3,5- heptanedionato) and H ₂O as precursors. The optimum ALD window was about 270-300 °C, and an ALD-type growth mechanism via surface saturation reaction was identified; the growth rate was about 0.1 Å/cycle. The X-ray diffraction and high-resolution transmission electron microscopy investigation revealed that Bi ₂O ₃ films crystallized into a predominant alpha phase above 250 °C. The resistivity at room temperature was about 1.2 × 10 ⁶ Ω·cm, which is also proof of the α-phase of as-deposited Bi ₂O ₃ films. In addition, a new method to obtain γ-Bi ₂O ₃ film was discovered. The α-Bi ₂O ₃ films (synthesized by ALD) transformed into metastable γ-Bi ₂O ₃ with preferred orientation (222) after annealing above 512 °C, and γ-phase could persist at room temperature.