Selective formation of crystal polymorphs in Er3+-doped Bi2O3 thin films and their photoluminescence properties

Abstract

The process conditions for selectively forming crystal polymorphs in Er3+-doped Bi2O3 films deposited on Si, SiO2, and C-plane sapphire substrates were systematically investigated. Bi2O3:Er films were deposited at either room temperature or 300 °C and subsequently post annealed to promote crystallization. The critical factor controlling the crystal polymorphs was Er content. When the Er content was less than 1.5 at.%, only α-Bi2O3 phase nucleated upon post annealing. Deposition at 300 °C somewhat lowered the oxidization state, under which β-Bi2O3 structure appeared at lower temperatures and α-Bi2O3 structure appeared at higher temperatures. When the films were doped with over 2 at.% Er3+ ions, the excess Er2O3 stabilized the δ-Bi2O3 structure as the lowest temperature phase. The universal phase transition scheme with increasing annealing temperature was δ-Bi2O3 → β-Bi2O3 → α-Bi2O3. The δ → β transition proceeded through splitting each diffraction peak of δ-Bi2O3 into two components of β-Bi2O3, indicating a correlation between the structures of β-Bi2O3 and δ-Bi2O3. The γ-Bi2O3 phase appeared only in films on Si(100) substrates and after vacuum annealing, suggesting the formation of sillenite (Bi12SiO20). Deposition on C-plane sapphire by using H2O as the oxygen source gas produced a highly (111)-oriented δ-Bi2O3 structure, whereas deposition with O2 yielded a randomly oriented δ-Bi2O3 structure. At Er content exceeding 4 at.%, δ-Bi2O3 was the primary phase in the films on SiO2. The photoluminescence (PL) activity of dopant Er3+ under excitation at a wavelength of 532 nm strongly depended on the crystal polymorphs. α-Bi2O3:Er exhibited intense and stable PL spectra consisting of eight Stark splitting lines. PL from γ-Bi2O3:Er exhibited much weaker two emission lines. δ-Bi2O3:Er and β-Bi2O3:Er films were not emission-active at all. However, δ-Bi2O3:Er film on SiO2 with an Er content of 4 at.% exhibited an intense and broad emission at 1530 and 1560 nm.