Semiconductor Device Group

 

Ge has a 4.2% larger lattice constant than Si and therefore only a certain number of heterolayers can be grown coherently on top of Si before the energy is so large that defects and misfit dislocations form to relieve the strain. On bulk Si one can grow above the equilibrium critical thickness forming a metastable layer but this will over time or under high stress relax. Therefore for all electronic or optical devices which must complete lifetime trials before products can be marketed require to be below the Matthews and Blakeslee equilibrium critical thickness (see right).

 

 

 

 

 

 

If heterolayers are grown on a relaxed virtual substrate then there is no metastable critical thickness as there are many nucleation sites from the threading dislocations. The equilibrium thickness will be different from the bulk silicon substrate values (above) due to the difference in bulk moduli values as shown on the left.