Inhomogeneous strain relaxation and defect distribution of ZnTe layers deposited on (100)GaAs by metalorganic vapor phase epitaxy

The structural characterization of ZnTe epilayers grown on (100)GaAs by metalorganic vapor-phase epitaxy is reported. A detailed study of the ZnTe/GaAs heterostructure based on both high-resolution and conventional electron microscopy and ion channeling Rutherford backscattering spectrometry allows correlation of the type and spatial distribution of the extended defects occurring at or close to the ZnTe/GaAs interface with the amount of residual lattice strain into the ZnTe epilayers. Both pure edge Lomer and 60”-mixed misfit dislocations were identified at the interface along with partial dislocations bounding stacking faults, their overall density and distance distribution indicating the occurrence of a residual compressive strain at the heterostructure interface. By comparing this interface strain to the corresponding surface value of the same samples the occurrence of an inhomogeneous strain relaxation along the growth direction is clearly demonstrated. It is shown that such a strain gradient should be entirely ascribed to threading dislocations occurring into the ZnTe epilayers, their distribution being strictly correlated to the amount of residual strain along the epilayer growth direction. The conclusions are further supported by the analysis of the ZnTe surface strain, whose dependence on the epilayer thickness is consistent with that expected on the basis of a phenomenological model for the epilayer dislocations.