Effects of strain on the electron diffraction contrast at III-V compound heterostructure interfaces

Electron diffraction contrast effects which arise at III-V compound heterostructure boundaries with small mismatches due to small differences in the diffraction conditions on either side of the boundary are discussed in detail. These effects can be characterized by a change in the deviation parameter s and give rise to δ fringes when the boundary is tilted with respect to the electron beam and also to a displacement of equal thickness fringes. Observations of the latter phenomenon require a well defined sample geometry and layers of known composition. δ fringe profiles are evaluated by comparison with theoretical profiles computed on the basis of equations from the two-beam dynamical theory for superposed crystals. The mismatch value extracted by this method agrees with the value expected for a GaAlAs/GaAs boundary provided that a relative change in crystal orientation at the boundary is allowed for rather than a simple tetragonal distortion perpendicular to the interface. The case of a GaInAsP/InP boundary reveals complex distortions at the interface and near the specimen edge, the details of which are discussed by combining observations of δ fringes and thickness fringe displacement.