Lattice distortion in dry-etched Si/SiGe quantum dot array studied by 2D reciprocal space mapping using synchrotron X-ray diffraction

Abstract

Using synchrotron X-ray two-dimensional reciprocal space mapping around the (004) and (224) reciprocal lattice points, we have studied the changes of the lattice strain in a 30-period 3 nm Si/3 nm Si 0.7Ge 0.3 superlattice (SL) that has been processed into a φ≈50 nm columnar dot array by e-beam patterning combined with reactive ion etching. It has been found that the superlattice within the columns was partially relaxed after nano-fabrication. The observation of high-order SL satellites from columnar dots indicates a good long-range ordering of SL layers. An analysis based on elastic theory showed that the strain relief in the superlattice occurs by a partial relaxation in SiGe layers together with a biaxial lattice expansion in the thin Si layers, i.e. a strain symmetrisation effect. A staggered band line-up between Si and SiGe is then expected. We propose that an enhanced recombination between electrons and holes confined in adjacent quantum wells at the hetero-interfaces may give rise to the observed increase in the luminescence efficiency in this type of structures.