Mechanism of nanodiamond film formation by stress relaxation on a preferentially oriented vertical basal plane graphitic precursor

Abstract

Nanocrystalline carbon films possessing a prevailing diamond or a graphite character, depending solely on the substrate temperature, can be deposited from a methane–hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. While in a narrow temperature window around 880 °C a nanodiamond film composed of an agglomerate of diamond particles 3–5 nm in size embedded in an amorphous matrix is obtained, at higher and lower deposition temperatures the films maintain their graphitic character throughout. The nanodiamond film forms on top of a thin graphitic precursor layer of 150–200 nm thickness (critical thickness of the precursor). It was also found that the formation of the nanodiamond phase is initially accompanied by an increase in surface roughness which decreases with film growth. The graphitic precursor film displays a preferred spatial alignment of its basal planes perpendicular to the silicon substrate surface. The reason for this alignment is suggested to be associated to a stress relaxation mechanism in the graphitic films during growth. Beyond a “critical thickness” where compressive stress has built up in the layer to an extent that it must be relaxed, stress relaxation is governed by the formation of a nanodiamond film. By cross sectional and high resolution transmission electron microscopy analysis the microstructure of the films as a function of distance from the silicon substrate interface was investigated. The alignment of the graphitic precursor within the surface near region of the films as a function of deposition time was investigated by angle-resolved near edge x-ray adsorption fine structure. Atomic force microscopy was applied to study the morphological evolution of the films.