Structural and electrical properties of carbon-ion-implanted ultrananocrystalline diamond films

We investigate the structural and electrical properties of carbon-ion-implanted ultrananocrystalline diamond (UNCD) films. Impedance spectroscopy measurements show that the impedance of diamond grains is relatively stable, while that of grain boundaries (GBs) (R_b) significantly increases after the C⁺ implantation, and decreases with the increase in the annealing temperature (T_a) from 650 ℃ to 1000 ℃. This implies that the C⁺ implantation has a more significant impact on the conductivity of GBs. Conductive atomic force microscopy demonstrates that the number of conductive sites increases in GB regions at T_a above 900 ℃, owing to the formation of a nanographitic phase confirmed by high-resolution transmission electronic microscopy. Visible-light Raman spectra show that resistive trans-polyacetylene oligomers desorb from GBs at T_a above 900 ℃, which leads to lower R_b of samples annealed at 900 and 1000 ℃. With the increase in T_a to 1000 ℃, diamond grains become smaller with longer GBs modified by a more ordered nanographitic phase, supplying more conductive sites and leading to a lower R_b.