Amorphous gallium oxide as an improved host for inorganic light-emitting thin film semiconductor fabricated at room temperature on glass

We report new amorphous oxide semiconductor (AOS)-based thin film phosphor, Eu-doped α-Ga₂O_x (α-GO:Eu), to solve the issues of previously reported α-In-Ga-Zn-O:Eu (α-IGZO:Eu). The internal quantum efficiencies (IQE) of α-GO:Eu (2.3% for unannealed, 8.3% for annealed films) are improved from those of α-IGZO:Eu (0.9% and 1.6%, respectively) because of the much wider bandgap (4.26 eV), subsequent low residual electron density, and higher available annealing temperature. We found that the annealing temperature to improve IQE is limited by crystallization temperature. Another issue of α-IGZO:Eu is that the initial state of Eu³⁺ 4f is deeper than the valence band maximum (VBM), which is not suitable for light-emitting diode. We expected that Eu³⁺ 4f would locate above the VBM in α-GO:Eu because the VBM of α-Ga₂O_x is ∼0.8 eV deeper than that of α-IGZO. However, resonant photoemission spectroscopy revealed that the Eu 4f states are bound more to the O 2p valence band than to the vacuum level, and the Eu³⁺ 4f states in α-GO:Eu are still buried in the valence band.