Electronic Structure of the Muonium Center as a Shallow Donor in ZnO

The electronic structure and the location of muonium centers (Mu) in single-crystalline ZnO were determined for the first time. Two species of Mu centers with extremely small hyperfine parameters have been observed below 40 K. Both Mu centers have an axial-symmetric hyperfine structure along with a $⟨0001⟩$ axis, indicating that they are located at the antibonding (${\mathrm{A}\mathrm{B}}_{\mathrm{O},\parallel }$) and bond-center (${\mathrm{B}\mathrm{C}}_{\parallel }$) sites. It is inferred from their small ionization energy ($\simeq 6$ and 50 meV) and hyperfine parameters ($\sim {10}^{-4}$ times the vacuum value) that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of $n$ type conductivity in as-grown ZnO.