Morphological and compositional variations in strain- compensated InGaAsP/InGaP superlattices

We have investigated the properties of strain-compensated InGaAsP/InGaP superlattices, grown by metalorganic vapor phase epitaxy, with and without InP interlayers inserted in the InGaP barrier. Using cross-sectional scanning tunneling microscopy and spectroscopy, we observe lateral variations in layer thickness and electronic properties. When the number of superlattice periods is increased from 8 to 16, the surface develops large undulations in the top 2–4 superlattice periods. This occurs simultaneously with reduced PL efficiency. For structures with InP layers inserted in the InGaP barrier, only slight undulations of the top superlattice periods occur, and the PL is significantly improved. We discuss the origins of the surface undulations in terms of a model which relates their formation with the elastic relaxation of strain arising from thickness and/or composition variations in the superlattice layers. Finally, we observe a 4-fold periodicity of the (001) lattice planes, presumably arising from atomic ordering in the alloys.