Monolayer superlattices and heterostructures studied by surface-sensitive XAFS

Abstract

The local structures in semiconductor monolayer superlattices (MSL's) and monolayer heterostructures (MHS's) have been investigated by surface-sensitive XAFS (X-ray absorption fine structure). In this paper, EXAFS (extended X-ray absorption fine structure) studies of GeSi MSL's on Si(100), (GaAs) 1(InAs) 1 MSL's on InP(100) and GaAs/Si(100) MHS are reviewed and their structural features are discussed in relation with the tetragonal strain and the interface mixing. The EXAFS results demonstrate that the tetragonal strain in GeSi MSL is accommodated by the ordered rearrangement of Ge atoms parallel with the superlattice direction. We find that the interface mixing is strongly dependendent on the growth condition. A mechanism of interface mixing through a site exchange between the nearest-neighbor atoms is proposed. The interface mixing at high temperatures modifies the nature of the tetragonal strain on the Ge atoms which deeply intervene into the Si lattice as evidenced by the near-edge structure. The observed displacement of atoms in strained MSL's such as GeSi and (GaAs) 1(InAs) 1 are generally in agreement with the valence force field calculation. In case of GaAs/Si(100) MHL however, the interface between the first heteropolar layer and the homopolar substrate is strongly modified by a repulsive Coulomb interaction at the heteropolar interface.