Microstructure and phase composition evolution of nano-crystalline carbon films: Dependence on deposition temperature

Abstract

Nano-crystalline carbon films possessing a prevailing diamond or a graphite character, depending solely on substrate temperature and deposition time, can be deposited from a methane–hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. In this study we investigate the evolution of nano-crystalline carbon films deposited in the 800–950 °C temperature range onto silicon substrates aiming to enlight the physicochemical processes leading to the formation of nano-diamond films. While at a deposition temperature of ∼880 °C the formation of a thin precursor graphitic film is followed by deposition of a film of diamond character, at higher and lower temperatures the films maintain their graphitic character. The morphology of the films and their growth rate vary with deposition temperature: slower growth rates and higher film roughness are obtained at lower temperatures suggesting the importance of kinetic effects during the growth process. For deposition times longer than ∼60 min, similar morphologies are obtained irrespectively of the deposition temperature. A preferred spatial alignment of the basal planes of the graphitic film at the interface with the silicon substrate was determined. The alignment was found to differ with deposition temperature: at 800 and 880 °C the alignment occurs along the graphitic â axis perpendicular to the silicon substrate, while at 950 °C the ĉ axis is aligned perpendicular to the silicon substrate. However, it was determined that for films a few hundred nm thick close to the evolving surface the films display a preferred alignment of the basal planes vertical to the surface, irrespectively of their orientation at the interface. The reason for this alignment is suggested to be associated with a stress relaxation mechanism in the graphitic films. It was determined that film growth is accompanied by the evolution of large local stresses which obtain a maximum value for the films deposited at 880 °C. The relaxation of these stresses is suggested to lead to the transformation of the graphitic material into the diamond phase. The narrow range of temperatures (880+/−10 °C) which enables the formation of the diamond phase indicates the importance of hydrogen adsorption/desorption processes in the nucleation and growth of the nano-crystalline diamond films. The morphological evolution of the films was analyzed by atomic force microscope. By electron diffraction and high-resolution transmission electron microscopy the phase composition of the films and their microstructure were examined. The alignment of the graphitic films within the near-surface region of the evolving films as a function of the deposition time and temperature was investigated by angle-resolved near edge x-ray absorption fine structure measurements. Raman spectroscopy was applied to determine the presence of stresses within the films and their phase composition.