Structural characterization and surface lattice strain determination of ZnS/GaAs heterostructures grown by metalorganic vapour phase epitaxy

We report on the structural characterization of ZnS epilayers grown on (1 0 0)GaAs by metalorganic vapour-phase epitaxy (MOVPE). The crystalline quality at the ZnS epilayer surface and the defect depth distribution was studied by Rutherford Backscattering Spectrometry (RBS)-ion channeling measurements as a function of the epilayer thickness. Transmission electron microscopy (TEM) observations were performed on selected ZnS/GaAs heterostructures. Misfit dislocations (MD) were observed at the ZnS/GaAs interface. In addition, a high density of planar defects such as stacking faults (SF) and microtwins (MT) were identified into the epilayer up to 200-300 nm. The density of these defects decreases by increasing the epilayer thickness, but a quite high and constant density of microtwins still occurs in epilayers thicker than 400 nm. However, absorption measurements point out a high optical quality for all the measured ZnS epitaxial layers. Finally, surface lattice strain was determined in the ZnS/GaAs samples by ion channeling measurements. Our data indicate that the initial lattice misfit is already fully relaxed in epilayers as thick as 400 nm and only a small residual thermal strain is measured in thicker samples.