Formation mechanism of the pairs of stacking faults in pseudomorphic ZnSe epilayers on GaAs substrates

We have examined the formation mechanism of typical extended defects in semiconductor heterostructures with heterovalent interfaces, namely pairs of stacking faults in pseudomorphic ZnSe epitaxial layers grown on GaAs(001) substrate, by means of high-resolution scanning electron microscopy and high-resolution transmission electron microscopy combined with first-principles energy calculations. A stacking fault intersected the other one of the pair at the ZnSe/GaAs interface, forming an intersecting line on the interface. Around the intersecting line, Ga, As and Se atoms formed the specific reconstructed structure with relatively low formation energy, which well corresponds to the (2×4) reconstructed surface structure on pre-growth GaAs substrates with additional Se atoms. Extended defects were formed in epitaxial layers whenever their thicknesses exceeded about 30 nm. The growth surfaces of the layers were smooth. An epitaxial layer thinner than about 30 nm had an undulating surface, and no extended defect existed in the layer. These results suggest that an extended defect is introduced from an undulating surface, reducing the strain around the surface, and is stabilized on the reconstructed structure on the interface.