Semiconductor Device Group

Misfit dislocations are basically where there is a missing or dangling bond in the lattice between two layers with different lattice constant (see left). In Si/SiGe heterostructures, when SiGe is grown on Si above the critical thickness 60 degree misfit dislocations result due to the silicon lattice.

 

For every misfit dislocation there will always be two threading dislocations at the ends of the misfit which must thread to a surface (see right) or form a loop so that the two ends of the dislocation can join. These threads are at 60 degree since they glide on the Si (111) lattice plane.

 

 

 

 

 

 

 

The TEM image of a virtual substrate on the left clearly shows the 60 degree threading dislocations and a large number of misfit dislocations especially in the graded SiGe region of the buffer. The trick to growing high quality virtual substrates is to spread the misfits out so that when the threads glide, they can easily thread to the edge of the wafer and not interact with any other dislocations. This reduces the threading dislocation density at the surface of the wafer.