Ge concentration profiles and defect characterization in high Ge content Si/SiGe heterostructure

Defect reduction in high Ge content Si/SiGe heterostructures is a critically important issue for the successful application of strained Si ULSI technology, considering that a high Ge content is needed for larger and more symmetric enhancements of electron and hole mobilities, and the fact that the defect density usually increases with increasing Ge content. In this paper, two wafers with 30 and 41% target Ge contents, respectively, were characterized by Raman microspectroscopy performed on beveled surfaces, cross-sectional TEM, and preferential etching/Nomarski optical microscopy. It was found that the 41% wafer is continuously graded and the 30% wafer is stepwise graded. This was further confirmed by TEM. Preferential etching shows that the density of threading dislocations in the 30% Ge wafer is approximately 1 to 2×10⁵ cm^-2, which is about one order lower than that in the 41% Ge sample. Thus, the stepwise grading structure appears to have a positive impact on threading dislocation reduction. Finally, in addition to a cross-grid misfit array of dislocations, truncated line structures were observed in the 30% Ge sample.