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Wafer bonding is increasingly used to fabricate heterostructures which cannot be obtained by conventional epitaxial growth because they involve highly mismatched materials. We have optimised the GaAs/InP bonding process in order to realise long wavelength microcavity devices combining advantageously high reflectivity AlGaAs Bragg reflectors with an InP-based active material. The bonded interface has been studied by transmission electron microscopy. Three dislocation networks are found at the interface. The first one accommodates both the lattice mismatch and the inevitable twist between the two crystals whereas the second one accommodates the tilt resulting from the slight vicinality of the initial surfaces. Our experiments strongly suggest that the third network forms upon cooling the structure. Between the dislocation lines, the crystal structure is perfectly reconstructed across the interface, with no amorphous materials present, except for cavities which occupy a few percent of the bonded area. The geometry and distribution of these cavities can be studied by imaging the first dislocation network. In light of these results, we discuss the mechanisms operating during the formation of the interface. © 1997 American Institute of Physics.