Heavy Hydrogen Doping into ZnO and the H/D Isotope Effect

Hydrogen (H) can drastically change the physical properties of solids by the doping of host materials with minimum perturbation to the lattice because of its small size, quantum nature, and a variety of charged states from −1 (hydride, H^–) to +1 (proton, H⁺). While the H-doping amount is limited under equilibrium conditions, H₂⁺ ion irradiation at low temperature is a promising method for introducing a large amount of hydrogen into any material. Although the application of this method offers the potential for exploring unforeseen fascinating properties, the effects of nonequilibrium H doping at very low temperature below 10 K are largely underexplored and are not well understood. In this article, we report heavy H (D) doping into ZnO films by H₂⁺ (D₂⁺) irradiation at 7 K, which resulted in metallic conductivity and an isotope effect on the conductivity at 7 K. The H/D isotope effect is attributable to metastable H (D) trapping sites generated by the effect of irradiation. The isotope effect is decreased at low acceleration voltage. Furthermore, the subsequent thermal excursion induces a large irreversible decrease in resistivity, indicating the migration of H (D) from metastable trapping sites upon heating. This work provides a new strategy to control the physical properties of materials and to investigate the H (D) migration occurring with increasing temperature after excess H doping at very low temperature.