Elastic and plastic strain relaxation in ultrathin CdS/ZnS quantum-well structures grown by molecular-beam epitaxy

We investigate the growth of highly strained ultrathin CdS/ZnS quantum well structures by molecular-beam epitaxy with an emphasis on structural aspects such as surface morphology of the growing layer and strain relaxation. As shown by quantitative reflection high-energy electron diffraction (RHEED) measurements, no true three-dimensional nucleation of CdS takes place despite the high mismatch relative to the ZnS buffer. Nevertheless, the CdS surface reveals a high density of very small typically monolayer islands, leading to a strong elastic relaxation at the surface. A new RHEED technique is applied to distinguish this elastic deformation from plastic relaxation through misfit dislocations. The critical thickness for the onset of the latter is found to be three monolayers. This result is confirmed by transmission electron microscopy and photoluminescence measurements carried out for further characterization, proving the presented RHEED technique to be reliable.