Structural characterization of bismuth zinc oxide thin films grown by plasma-assisted molecular beam epitaxy

We report the structural characterization of Bi _xZn _1-xO thin films grown on c-plane sapphire substrates by plasmaassisted molecular beam epitaxy. By increasing the Bi flux during the growth process,Bi _xZn _1-xO thin films with various Bi contents (x = 0~13.17 atomic %) were prepared. X-ray diffraction (XRD) measurements revealed the formation of Bi-oxide phase in (Bi)ZnO after increasing the Bi content. However, it was impossible to determine whether the formed Bi-oxide phase was the monoclinic structure α-Bi ₂O ₃ or the tetragonal structure β-Bi ₂O ₃ by means of XRD Θ-2Θ measurements, as the observed diffraction peaks of the 2Θ value at ~28 were very close to reflection of the (012) plane for the monoclinic structure α-Bi2O3 at 28.064 and the reflection of the (201) plane for the tetragonal structure β-Bi ₂O ₃ at 27.946. By means of transmission electron microscopy (TEM) using a diffraction pattern analysis and a high-resolution lattice image, it was finally determined as the monoclinic structure α-Bi ₂O ₃ phase. To investigate the distribution of the Bi and Bi-oxide phases in BiZnO films, elemental mapping using energy dispersive spectroscopy equipped with TEM was performed. Considering both the XRD and the elemental mapping results, it was concluded that hexagonal-structure wurtzite Bi _xZn _1-xO thin films were grown at a low Bi content (x = ~2.37 atomic %) without the formation of α-Bi2O3. However, the increased Bi content (x = 4.63~13.17 atomic %) resulted in the formation of the α-Bi ₂O ₃ phase in the wurtzite (Bi)ZnO matrix.