ELASTIC STRAIN RELAXATION IN SI1-XGEX LAYERS EPITAXIALLY GROWN ON SI SUBSTRATES

We have investigated the elastic strain relaxation in Si1-xGex layers grown by the molecular beam epitaxy (MBE) technique and in situ contro lled with RHEED. Up to approximate to 0.8% critical lattice mismatch ( about 20% Ge) uniform strained and flat layers were grown both on (111 ) and on (001) Si substrates. Calculations of the elastic constants ev idenced a tetragonal distortion about 50% higher on (001) than on (111 ) in the same experimental conditions. At higher misfits (and/or thick nesses) a growth instability was evidenced only on (001) Si substrates . Si1-xGex layers there displayed a surface layer undulation. On the c ontrary, Si1-xGex layers grown on (111) Si substrates remained smooth throughout the growth up to the plastic relaxation of the layers. To d etermine stress fields in the Si1-xGex layers, a high spatial resoluti on convergent beam electron diffraction (CBED) experiment was performe d with a field effect analytical microscope. The CBED technique was ap plied to two typical cases: totally strained layer and undulated dislo cation-free layer. In the latter case, CBED patterns recorded on nanom eter scale areas of an undulation crest (cross-section sample) showed a gradual elastic relaxation mainly directed along the growth axis (z) . Moreover a triclinic distortion of the unit cell was pointed out. Th ese results were confirmed on a plane view sample. In conclusion, our results show that the driving force for the undulation is not the in-p lane elastic relaxation since CBED experiments proved an important ela stic relaxation of the (001) Si1-xGex layers along the z axis. This wa s in agreement with the calculations of the elastic constants. We thin k that this could be at the origin of the undulation.