Quantum dots of Si(1-x-y)Ge(x)C(y )alloys with high Ge contents were grown on Si(311) and Si(001) substrates by solid source molecular beam epitaxy and were measured by atomic force microscopy. The quantum dot layers had a nominal thickness (equivalent two-dimensional) of 4 nm. The smallest quantum dots occurred for the composition Si0.09Ge0.9C0.01 on Si (311), and had a 40 nm mean diameter, an 8 nm mean height, and a density of 3.3 x 10(10) cm(-2). Quantum dots on Si(001) were larger and had less regular spacing than quantum dots on Si(311) with the same composition. Carbon decreased both the mean size and spacing of SiGe quantum dots and the ratio of size deviation to mean diameter. The presence of small uniform quantum dots for particular compositions is attributed to a reduction in the surface migration of adatoms due to decreased atomic surface diffusivity. These results suggest that quantum dot organization is controlled by composition, substrate orientation, strain, and surface diffusion.