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 degrees C and above give n-type conductivity to the sample and the UNCD film exhibits n-type resistivity with the carrier mobility of 1 similar to 11 cm(2) V-1 s(-1). With O+ dose increasing from 10(15) to 10(16) cm(-2), diamond phase transits to the amorphous carbon phase accompanied by n-type semiconduction transforming to metallic conduction. In the 10(14) cm(-2) O+-implanted UNCD film, some amorphous carbon at grain boundaries transits to diamond phase with annealing temperature (T-a) increasing from 500 degrees C to 800-900 degrees C, and some of diamond grains are found to be converted to amorphous carbon phase again after 1000 degrees 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 degrees C, and it decreases with diamond phase transiting to amorphous carbon phase in the case of 1000 degrees C annealing. It is indicated that the O+-implanted nano-sized diamond grains dominantly control the n-type conductivity of UNCD film in the Ta range of 800-900 degrees C, while the grain-boundary-conduction controls the n-type conductivty in UNCD film annealed at 1000 degrees 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. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3556741]