Abstract
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 CH4 gas concentration during microwave plasma assisted chemical vapour deposition, thereby promoting the formation of sp2 carbon at the grain boundaries. The presence of sp2 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 sp2 to form. The microwave cavity perturbation results show that the measured cavity quality factor varies with CH4 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.
Highlights
Diamond films produced using microwave plasma chemical vapour deposition (MPCVD) are of great interest for a wide variety applications including but not limited to micro-electromechanical systems (MEMS) [1], electrodes [2], thermal management [3,4], terahertz applications [5] and optical windows [6]
We propose the use of microwave cavity perturbation (MCP), all within the frequency domain, to infer the electrical conductivity, minimising the complexity of the system to just a vector network analyser (VNA) and a cavity
While not nearly as sensitive as powder systems presented in previous studies, an MCP system has been demonstrated as a noncontacting probe for fast and non-invasive evaluation of the electrical conductivity of nitrogen doped NCD films
Summary
Diamond films produced using microwave plasma chemical vapour deposition (MPCVD) are of great interest for a wide variety applications including but not limited to micro-electromechanical systems (MEMS) [1], electrodes [2], thermal management [3,4], terahertz applications [5] and optical windows [6]. MCP has been a popular choice for characterising electrically conducting impurities in diamond samples due to the non-contact setup and significant dielectric contrast between sp and sp carbon in the microwave frequency range [29e31]. For NCD films synthesised using MPCVD, this is the opposite where the substrate is typically significantly larger in volume than the film as to avoid delamination caused by thermal stress and the electrical conductivity of the substrate is commonly significant, such as doped n or ptype silicon [32] In this instance, the perturbation is focused on the substrate which determines the lower conductivity detection limit. For doped diamond film applications, if the conductivity increases significantly such that the conductivity is equivalent to or much larger than that of the substrate, MCP is of great benefit through non-contact probing of electrical properties This may be achieved through both electric (E) and magnetic (H) field perturbations.
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