Tracking defect type and strain relaxation in patterned Ge/Si(001) islands by x-ray forbidden reflection analysis

Abstract

Plastic relaxation and formation of defects are crucial issues in the epitaxial growth of nanoparticles and thin films. Indeed, defects generate local stress in the crystalline lattice, which affects their surroundings and may lead to undesired effects such as reduced charge-carrier lifetime or nonradiative recombinations. Here, we use a nondestructive method based on x-ray diffuse scattering close to forbidden reflections to identify the defect types with a high sensitivity and quantify their average size and strain field. Combined with transmission electron microscopy, it offers opportunities to track both ensemble average and single defects inside three-dimensional structures. These techniques have been applied to partially embedded and high-Ge-content (x(Ge) = 0.87 +/- 0.06) dots selectively grown in 20-nm-sized pits on Si(001) surfaces through openings in a SiO(2) template. The stress in the 20-nm-wide Ge islands is relaxed not only by interfacial dislocations but also by microtwins and/or stacking faults located at the interface, proving the importance of {111} planes and twinning in the relaxation process of nanometer-size Ge dots.