Microwave cavity perturbation of nitrogen doped nano-crystalline diamond films

Non-contact and non-destructive electrical conductivity measurements of nitrogen doped nano-crystalline diamond films have been demonstrated using a microwave cavity perturbation system. The conductivity of the films was controlled by simply varying the CH ₄ gas concentration during microwave plasma assisted chemical vapour deposition, thereby promoting the formation of sp ² carbon at the grain boundaries. The presence of sp ² carbon is verified through Raman spectroscopy, x-ray photoelectron spectroscopy and electron energy loss spectroscopy, while scanning electron microscopy confirms an increasing surface area for sp ² to form. The microwave cavity perturbation results show that the measured cavity quality factor varies with CH ₄ concentration. The extraction of conductivity is achieved through a depolarisation model, which must be considered when the sample is smaller than the cavity and through both electric and magnetic field perturbations. The microwave measurements are comparable to contacting and damaging measurements when the film conductivity is greater than the substrate, thus demonstrating an invaluable method for determining conductivity without the need for depositing any electrodes on the film.