Strain and defects depth distributions in undoped and boron-doped Si1−xGex layers grown by solid phase epitaxy

Abstract

A detailed characterization of undoped and heavily boron-doped Si1−xGex layers with x=0.21, 0.26, and 0.34 grown on (001) Si wafers by solid phase epitaxy is presented. The starting material for solid phase epitaxial growth was prepared by amorphization of epitaxial SiGe-Si heterostructures by ion implantation. In order to obtain doped layers, boron was also implanted into some of the amorphous samples. After regrowth, the strain depth distributions of the SiGe layers were measured using axial channeling angular scans and the defect distributions were observed by high-resolution electron microscopy. A defect-free region ranging from 0 nm (undoped layer of x=0.34) to 30 nm (doped layer of x=0.21) in thickness was observed next to the layer-substrate interface. In the upper region of the layers, strain-relieving defects, identified as planar faults, were observed. Some isolated defects were also present at the layer-substrate interfaces of most of the samples. The measured strain depth profiles show that (i) the defect-free regions are not always fully strained; the defects located at the interfaces being responsible for this partial relaxation; (ii) the strain is almost constant throughout the defect-free layers because it cannot be relieved due to the absence of defects; and (iii) the strain progressively decreases towards the sample surfaces in the region of the layer where the strain-relieving defects are located. Comparison between the undoped and boron-doped layers show the consequences of the strain compensation effect due to the incorporation of boron atoms into the lattice. The defect-free regions of the doped layers are thicker and closer to coherency than those in the undoped layers of the same composition and the defect density in the upper region of the layers is significantly reduced. As a result of the strain compensation effect, a 30-nm-thick heavily doped layer with x=0.21 is found to be defect free and fully strained throughout its whole thickness although the corresponding undoped layer was partially relaxed and showed strain-relieving defects.