We report new amorphous oxide semiconductor (AOS)-based thin film phosphor, Eu-doped α-Ga2Ox (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-Ga2Ox 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.