From Atoms to Embedded Nanocrystals: Direct Observation of Defect-Mediated Cluster Nucleation

Ion implantation is widely used to introduce electrically or optically-active dopant atoms into semiconductor devices. At high concentrations, the dopants can cluster and ultimately form deactivating precipitates. On the other hand, deliberate nanocrystal formation offers an approach to self-assembled device fabrication. 

However, there is very little understanding of the early stages of how these precipitates nucleate and grow, in no small part because it requires imaging an inhomogenous distribution of defects and dopant atoms buried inside the host material. Here we demonstrate this and address the longstanding question of whether the cluster nucleation is defect-mediated or spontaneous. 

Atomicresolution illustrations are given for the chemically dissimilar cases of erbium and germanium implanted in silicon carbide. While interstitial loops act as nucleation sites in both cases, the evolution of nanocrystals is strikingly different: Er is found to gather in lines, planes and finally three-dimensional precipitates, while Ge favors compact, three-dimensional structures. 

By matching the density of the interstitial loops to the dopant atoms by co-implantation, it should be possible to control the dimensionality, and hence the electronic properties, of the resultant nanocrystals.