N⁺-ion implantation induced enhanced conductivity in polycrystalline and single crystal diamond

With the 200 keV N⁺-ion implantation technique and a systematic variation of fluence, we report on the formation of highly conducting n-type diamond where insulator-to-metal transition (IMT) is observed above a certain fluence wherein the conductivity no longer obeys the hopping mechanism of transport rather, it obeys quantum corrections to Boltzmann conductivity at concentrations of n _N ≥ 2 × 10²⁰ cm^-3. The conductivity for ultra-nanocrystalline diamond is found to be high, ∼650 Ω^-1 cm^-1 with thermal activation energy E _a ∼ 4 meV. Interestingly, with gradual increase in fluence, the conductivity in polycrystalline diamond films has been seen to progress from the hopping mechanism of transport in the case of low fluence implantation to a semiconducting nature with medium fluence and finally a semi-metallic conduction is observed where percolation occurs giving an insulator-to-metal transition. XANES confirms that the long-range order in diamond films remains intact when implanted with low and medium fluences; while implantation at sufficiently high fluences >5 × 10¹⁶ cm^-2 leads to the formation of a disordered tetrahedral amorphous carbon network leading to metallic conduction resembling a metallic glass behaviour. XPS confirms that the sp² fraction increases gradually with fluence starting from only 6% in the case of low fluence implantations and saturates at 40-50% for implantation at high fluences. A similar observation can be made for single crystal diamond when implanted at high fluence; it retains long-range order but percolative transport takes place through defects or semi-amorphized regions.