Dislocations, twins, and grain boundaries in CVD diamond thin films: Atomic structure and properties

We have used transmission electron microscopy techniques to study the nature of dislocations, stacking faults, twins, and grain boundaries in CVD (chemical-vapor-deposition) diamond thin films. Perfect a/2〈110〉 and partial a/6〈112〉 and a/3〈111〉 type dislocations are observed; the partial dislocations are also associated with twins and stacking faults. The most common defect in diamond thin films, particularly in 〈110〉 textured films, is Σ = 3 grain boundary or the primary twin. These twins in 〈110〉 textured films can lead to formation of fivefold microcrystallites. We have also investigated the splitting of Σ = 9 grain boundary (second order twin) into two Σ = 3 boundaries or primary twins via reaction Σ9 = 2Σ3. A rapid thermal annealing treatment has been shown to result in annealing of Σ = 3 boundaries and produce “defect-free” regions in thin films. A mechanism of annealing (removal) of Σ = 3 boundaries is discussed. Atomic structure and energetics of dislocations, twins, and grain boundaries are calculated using Tersoff potentials. The calculated atomic structure for Σ = 3 boundary is compared with high-resolution TEM images and a good agreement is obtained. These boundaries consist of periodic units of 5–7 rings which are similar to the core structure of 90° a/2〈110〉{001} dislocations. The energy of the 5–7 rings in the grain boundaries is considerably lower, due to overlapping and strain cancellation effects, than that associated with single dislocations. The 5–7 ring energy and consequently the boundary energy increases as the overlapping effects decrease. An interesting analogy between the diamond and silicon results is drawn.