Atmosphere-Induced Preferentially Oriented Growth: Crystalline Evolution and a Tunable Optical Band Gap in Bismuth-Derived β-Bi2O3 Films

The controllable synthesis of β-Bi2O3 thin films with specific crystallographic orientations is essential for optoelectronic device applications. However, achieving precise orientation control by modulating the growth conditions remains a significant challenge. In this work, we demonstrate that the oxidizing atmosphere is a critical determinant in the oxidation of bismuth (Bi) metal, dictating a transition from thermodynamics-driven epitaxial growth to kinetics-dominated non-epitaxial growth of β-Bi2O3 films. In an oxygen-deficient atmosphere, the reaction is thermodynamically controlled. In this regime, β-Bi2O3 grows via a mechanism on the Bi(003) surface, guided by lattice matching, to form a dense film with a dominant (201) orientation. This process follows a specific crystallographic relationship: Bi(003)//β-Bi2O3(002). In contrast, under oxygen-rich and air atmospheres, the reaction becomes kinetically controlled, where the competition among growth rates of different crystal facets prevails. This leads to a shift in the preferred orientation toward a mixed feature of the (002) and (220) planes. Such atmosphere-induced microstructural evolution directly modulates the optical properties of the films. The preferred oriented-growth (201) film exhibits an optical band gap of approximately 2.64 eV, which is slightly wider than the intrinsic value. Conversely, films grown under oxygen-rich and air atmospheres show a broader band gap of approximately 2.83 eV. This work elucidates the physical origin of orientation competition during Bi oxidation and establishes a universal strategy for tailoring the crystal orientation, microstructure, and optical band gap of metal-oxide films simply by engineering the oxidizing atmosphere.