Origin and removal of stacking faults in Ge islands nucleated on Si within nanoscale openings in SiO2

Abstract

We have previously reported that Ge films formed after nucleation of Ge islands within nanometer size openings in SiO2 and their subsequent coalescence over the SiO2 template exhibit threading dislocation densities below 106 cm−2. However, these films contain a density of twin/stacking fault defects on the order of 5 × 1010 cm−2 that emanate primarily from the Ge-SiO2 interface. Most of these faults self-terminate within 200 nm of the interface; however, a total of 5 × 107 cm−2 propagate to the Ge surface. These defects are found to be detrimental to the morphology and minority carrier lifetime in III-V films integrated onto the Ge-on-Si virtual substrates. We have found that annealing the Ge islands during the initial stage of coalescence eliminates stacking faults, but further Ge growth leads to a film containing a threading dislocation density of 5 × 107 cm−2. To explain the origin of the twin/stacking fault defects in the Ge films and their removal after annealing Ge islands, we have studied the Ge islands before and after annealing. Our results indicate that twin/stacking faults originate from Ge islands that nucleate within nanoscale windows in the SiO2 template, in twin relationship to the underlying Si, and their coalescence with other epitaxial Ge islands. The density of Ge islands in twin relationship is approximately 4 × 1010 cm−2. In addition to the twin-oriented Ge islands, we observe that another group of Ge islands on the order of 2 × 1010 cm−2 have a small tilt-misorientation to the underlying Si ranging from 1.8 to 5.6°. After annealing, the density of both epitaxial and twin-oriented Ge islands is significantly reduced, and only the tilt-misoriented islands remain. The reduction in epitaxial and twin-oriented Ge islands stems from the thermal desorption of SiO2 template during the annealing, which leads to the transfer of Ge by surface diffusion from these Ge islands to the freshly exposed Si. This surface diffusion, while causing dissolution of epitaxial and twin-oriented islands, creates Ge0.22Si0.78 alloy regions surrounded by the tilt-misoriented islands. The tilt-misoriented islands are stable against dissolution during annealing and grow in diameter by 30%. A Ge0.44Si0.56 alloy forms beneath the misoriented islands and relaxes by plastic deformation. The dissolution of all but the tilt-misoriented Ge islands appears to be the mechanism by which the stacking faults are removed during annealing. This finding is confirmed by experiments in which the Ge islands are first capped with spin-on-glass before annealing to prevent removal of the SiO2 template and suppress surface diffusion. After annealing, twins/stacking faults remain within the Ge islands, and the islands retain their overall density and morphology. The presence of long misfit dislocation segments, formed near the Ge-Si interface after growing thick Ge films following the annealing, bears a resemblance to graded GexSi1-x films. We attribute the long misfit dislocation segments to the growth that takes place after annealing being on a relaxed GeSi alloy layer that forms from the dissolution of the Ge islands after SiO2 desorption.