Mechanism and Properties of Nanodiamond Films Deposited by the DC-GD-CVD Process

Abstract

Nanocrystalline carbon film of a prevailing diamond character can be deposited by direct current glow discharge (DC GD) chemical vapor deposition (CVD) from a methane-hydrogen mixture. The growth of a nanodiamond film by the DC GD CVD process occurs on top of a hydrogenated sp2-coordinated carbon precursor (graphitic-like) as confirmed by near-edge x-ray adsorption fine structure (NEXAFS) and transmission electron microscopy (TEM). No surface pretreatment is necessary in order to induce film growth and the thickness of the precursor layer is ∼ 200 nm. By high resolution TEM and x-ray diffraction (XRD) it has been established the nanocrystalline nature of the films and that the average size of the diamond crystallites is about 5 nm. Preferential vertical alignment of the basal planes in the precursor was determined by cross- section HR TEM as well as by angle-resolved NEXAFS. Electron energy loss spectroscopy (EELS) demonstrated the sp2-coordinated character of the nanodiamond films surface. Secondary ion mass spectroscopy (SIMS) established a drastic increase in the adsorbed hydrogen content accompanying the nanodiamond formation. Whereas the hydrogen concentration in the precursor layer is only a few percent it increases to ∼15–20 at.% in the nanodiamond film. Density measurements X-ray reflection (XRR) of the films increases with thickness as expected from the phase composition of the films determined from the spectroscopic methods. From a microscopic perspective nanodiamond film and growth is explained as a sub-surface process in terms of a four step cyclic process: (i) Formation of a dense, hydrogenated sp2 carbon coordinated oriented layer; (ii) precipitation of sp3 clusters in this graphitic phase, (iii) growth of nanodiamond particles up to ∼5 nm in size by energetic species bombardment of the diamond/hydrogenated carbon — interface. It involves preferential displacement of sp2 carbon coordinated carbon atoms leaving sp3 coordinated carbon atoms intact, leading to expansion of the diamond phase, (iv) frustration of growth of the diamond particles and (v) growth of the film formed of an agglomerate of nanodiamond particles by a cyclic process which involves process 2–4. From a macroscopic perspective nanodiamond formation is suggested to be associated with a sub-surface nano-graphite — nanodiamond phase transition triggered by stress relaxation.