Low-energy charge-transfer state and optical properties of Eu ³⁺-doped GaN

We argue that the charge-transfer state of Eu³⁺ due to an electron transfer from N to Eu plays important roles in the luminescence properties of GaN:Eu³⁺ grown by gas-source molecular beam epitaxy. On the basis of the excitation spectrum of the Eu³⁺ luminescence, the excitation mechanism of Eu³⁺ under the GaN interband excitation is interpreted in terms of the energy transfer from the above charge-transfer state. The intra-4f shell ⁵D₀-⁷F₂ transition strength of Eu³⁺ in GaN, which is responsible for red luminescence, is large compared with those of most Eu³⁺-doped oxide materials. This fact can be attributed to the low-energy position of the charge-transfer state of Eu³⁺ in GaN.