Formation of inorganic electride thin films via site-selective extrusion by energetic inert gas ions

Inert gas ion implantation (acceleration voltage 300 kV) into polycrystalline 12CaO·7 Al2 O3 (C12A7) films was investigated with fluences from 1× 1016 to 1× 1017 cm-2 at elevated temperatures. Upon hot implantation at 600 °C with fluences greater than 1× 1017 cm-2, the obtained films were colored and exhibited high electrical conductivity in the as-implanted state. The extrusion of O2- ions encaged in the crystallographic cages of C12A7 crystal, which leaves electrons in the cages at concentrations up to ~1.4× 1021 cm-3, may cause the high electrical conductivity. On the other hand, when the fluence is less than 1× 1017 cm-2, the as-implanted films are optically transparent and electrically insulating. The conductivity is enhanced and the films become colored by irradiating with ultraviolet light due to the formation of F+ -like centers. The electrons forming the F+ -like centers are photo released from the encaged H- ions, which are presumably derived from the preexisting OH- groups. The induced electron concentration is proportional to the calculated displacements per atom, which suggests that nuclear collision effects of the implanted ions play a dominant role in forming the electron and H- ion in the films. The hot ion implantation technique provides a nonchemical process for preparing electronic conductive C12A7 films.