In order to investigate the relationship between atomic diffusion and electronic structure in the epitaxial AlInP/GaAs system, the authors have performed cross-sectional cathodoluminescence spectroscopy and secondary ion mass spectrometry measurements of a SiOx-capped, lattice-matched Al0.52In0.48P/GaAs double heterostructure. The authors measure atomic diffusion of over 100 nm resulting from annealings ranging from 650 to 850 degrees C. An similar to 40 meV increase in the emission energy of AlInP is observed after the highest temperature annealings. This increase is consistent with an increase in the Ga concentration of the ternary layer at the expense of In, and as a result of diffusion from neighboring GaAs layers. Additionally, the authors observe a broad, AlInP-localized feature at similar to 1.98 eV. The intensity of this emission relative to the AlInP band-edge emission depends sensitively on the annealing temperature and, therefore, the amount of cross diffusion, and corresponds well energetically to literature reports of P vacancies. These results clarify cross diffusion and defect emission in AlInP/GaAs, and demonstrate that these effects can have a significant impact on the electronic structure of lattice-matched III-V heterostructures. (c) 2007 American Vacuum Society.