Tsuneyuki Research Group

Research

"Charge states of impurity hydrogen in rutile oxide semiconductors"

Impurity hydrogen (H) can take three charge states (H⁰, H⁺, H^-) and affects the electronic properties of materials. It has been known for passivation of doped semiconductors and semiconductor surfaces, while it is also gathering attention as an electron dopant for oxides where an O^2- ion is substituted by a H^- ion [1]. Controlling the electrical conductivity is important for the electronic device applications.

We study stability of H in oxide semiconductors with rutile structure MO₂ (M=Si, Ge, Sn, Pb, Ti, Zr, Hf) by using first-principles calculation based on density functional theory (DFT). The charge states are determined by Bader analysis [2] after the optimization of SiO₂ with a H atom. Figure shows the behavior of H in SiO₂. A neutral H atom is localized in interstice as shown in Fig. (a). When H is in H⁺ state, the H atom is located near the O atom as shown in Fig. (b). The distance between H and O atoms is about 1.03Å, which is almost the same as the O-H bond length in H₂O of 0.96Å. Similarly, when H is in H^- state in Fig. (c), the H atom forms a Si-H bond of 1.33Å. SiO₂ and HfO₂ take three charge states (H⁰, H⁺, H^-), ZrO₂ takes H⁺ and H^-, and the others take H⁺ only. Thus, H exhibits different behavior in these semiconductors.

[1] S. Iimura, S. Matuishi, H. Sato, T. Hanna, Y. Muraba, S.W. Kim, J.E. Kim, M. Takata, and H. Hosono, Nat. Commun. 3, 943 (2012).

[2] R. F. W. Bader, Atoms in Molecules - A Quantum Theory (Oxford University Press, Oxford, 1990).