Abstract Gems & Gemology, Summer 2013, Vol. 49, No. 2

Growth Strategy for Controlling Dislocation Densities and Crystal Morphologies of Single Crystal Diamond by Using Pyramidal-Shape Substrates

A single synthetic diamond crystal can be used as an electronic device because of its properties, such as a high breakdown field and thermal conductivity. However, defects, such as dislocation bundles, which were formed during growth process, can cause current leakage, higher bandgap, and birefringence. Defect-free single diamond crystal that is large enough to be used as an electronic device is difficult to grow using current methods. For example, the “threading dislocations” can be formed in a CVD diamond using (001)-oriented substrate. The authors demonstrated that these defects could be reduced by modifying substrate’s shape and orientations.
Two type Ib HPHT-grown, pyramidal substrates, type A and type B, were used to grow single CVD crystals in this research. The {100} directions in type A substrate and {110} directions in type B were polished into inclined faces with 20°, 30°, and 40° angles. Both possessed a square top measuring 200 x 200 µm2. Crystals were grown on these substrates using plasma-assisted CVD technique with power density of 100 W/cm3 and methane concentration of 5% at 850°C, achieving a growth rate of 13 microns per hour. 
After removal of the substrate, the fully grown CVD crystals showed high transparency. Nitrogen-related defects were not detected. The inclined faces in type A substrate helped prevent twin sectors from reaching the top face. The growth rates of the lateral and top faces were 32.5 and 13.5 µm per hour, respectively. When thickness reached about 490 microns, the inclined faces started to disappear. The dislocations, originating from the square top of a pyramidal substrate, were observed in a crystal grown on type A substrate with 20° dislocations. However, these dislocations were diverted toward the edges rather than the top. Photoluminescence image showed that the inclined faces effectively diverted the direction of threading dislocations from {100} to {110} direction. Absence of sharp edges in these pyramidal substrates also helped in reducing dislocations. The lateral surfaces had the highest growth rate and possessed “fish scale” morphology in type A grown crystals, a morphology not observed in type B crystals. A maximum thickness of 2.2 mm could be achieved with a dislocation of 40° on type B substrate. A crystal 1.70 mm thick containing very low defects was achieved using type B substrate with 20° or 30° dislocations.
Abstracted by Kyaw Soe Moe