N-type conductivity and phase transition in ultrananocrystalline diamond films by oxygen ion implantation and annealing

Ultrananocrystalline diamond (UNCD) films were implanted by oxygen ion and annealed at different temperatures. The electrical and structrual properties of O⁺-implanted UNCD films were investigated by Hall effects, high-resolution transmission electron microscopy (HRTEM) and uv Raman spectroscopy measurements. The results show that O⁺-implanted nano-sized diamond grains annealed at 800 °C and above give n-type conductivity to the sample and the UNCD film exhibits n-type resistivity with the carrier mobility of 1∼11 cm² V^-1s^-1. With O⁺ dose increasing from 10¹⁵ to 10¹⁶ cm^-2, diamond phase transits to the amorphous carbon phase accompanied by n-type semiconduction transforming to metallic conduction. In the 10¹⁴ cm^-2 O⁺-implanted UNCD film, some amorphous carbon at grain boundaries transits to diamond phase with annealing temperature (T_a) increasing from 500 °C to 800-900 °C, and some of diamond grains are found to be converted to amorphous carbon phase again after 1000 °C annealing. This phase transition is closely relative to the n-type conductivity of the UNCD films, in which n-type conductivity increases with the amorphous carbon phase transiting to diamond phase in the T_a range of 500-900 °C, and it decreases with diamond phase transiting to amorphous carbon phase in the case of 1000 °C annealing. It is indicated that the O-implanted nano-sized diamond grains dominantly control the n-type conductivity of UNCD film in the T_a range of 800-900 °C, while the grain-boundary-conduction controls the n-type conductivty in UNCD film annealed at 1000 °C. In this case, a novel conduction mechanism that O ⁺-implanted nano-sized diamond grains supply n-type conductivity and the amorphous carbon grain boundaries give a current path to the UNCD films is proposed. © 2011 American Institute of Physics.