Transmission electron microscopic observation of grain boundaries in CVD diamond thin films

Polycrystalline diamond thin films were grown by electrocyclotron resonance chemical vapor deposition (ECR-CVD). Electron diffraction, conventional transmission electron microscopy and high-resolution electron microscopy were used to characterize the grain boundary structure in the films. The films showed microstructures consisting of micrometer scale grains and twins inside these grains. Most of the boundaries were [110] tilt boundaries even though twist boundaries were also observed. The low-angle tilt boundaries were usually curved and sometimes deviated from the [110] axis. The high-angle boundaries were mainly {111} Sigma 3 coincidence-site-lattice (CSL) boundaries. Other Sigma 3, Sigma 9 and Sigma 27 CSL boundaries were usually observed to be connected with {111} Sigma 3 CSL boundaries. The Sigma 9 CSL boundaries often appeared parallel to {114} planes instead of {221} planes. The relation between the grain boundary structure and film growth procedure is discussed.