Highly conductive ultrananocrystalline diamond thin films via nitrogen incorporation

Diamond has many superlative properties that make it an ideal candidate for a number of different mechanical and electronic applications. One of the biggest limiting factors has been the lack of n-type diamond films with suitable conductivity. This work demonstrates a method to fabricate a highly conductive, n-type diamond film via nitrogen doping. Ultrananocrystalline diamond (UNCD) films with up to 0.2% total nitrogen content were synthesized by a microwave plasma-enhanced chemical-vapordeposition method using a CH₄/Ar gas mixture with added nitrogen gas. The electrical conductivity of these nitrogen-doped UNCD films increases by five orders of magnitude, up to 143 Ω −1·cm−1 when nitrogen is introduced. Conductivity and Hall measurements indicate that these films have the highest n-type conductivity and carrier concentration demonstrated for diamond films to date. UNCD thin films typically consist of 2–5 nm grains of pure sp3-bonded carbon and atomically abrupt grain boundaries with a disordered mixture of sp2 and sp3-bonded carbon. The grain size and grain boundary widths increase with the addition of N ₂, as studied using high-resolution transmission electron microscopy. In order to clarify the roles these factors play in the conduction mechanisms of nitrogen-doped UNCD, near edge x-ray absorption fine structure, electron energy loss spectroscopy, and soft x-ray fluorescence were carried out to ascertain the bonding structure of the films. These measurements indicate that the overall grain boundary volume of nitrogen-doped UNCD is increasing, while the diamond grains themselves remain pure diamond. Additionally, the visible Raman spectra of UNCD thin films were studied in this work. In order to correctly interpret the Raman spectral features of UNCD thin films, a series of films spanning the range of structures from microcrystalline to UNCD was studied using visible and UV Raman spectroscopy. For UNCD, we find that although the sample has been verified to be composed of ∼95% sp3-bonded carbon by other techniques, none of the spectral features observed using visible Raman spectroscopy can be attributed to sp3-bonded carbon.