Strain relaxation by coherent three-dimensional islanding in molecular-beam epitaxy of EuTe on PbTe(111)

By use of in situ reflection high-energy electron diffraction (RHEED), the heteroepitaxial growth of strained-layer EuTe on PbTe(111) was investigated. From the determination of the in-plane EuTe surface lattice constant as a function of the layer thickness, the onset of EuTe strain relaxation (at critical layer thickness ${\mathit{h}}_{\mathit{c}}$) was obtained. Time-dependent intensity measurements of different features in the RHEED patterns indicate that the critical layer thickness essentially coincides with an abrupt roughening of the EuTe surface. In spite of the observation that for certain growth conditions a change in the damping of the RHEED intensity oscillations occurs at ${\mathit{h}}_{\mathit{c}}$, generally, the damping behavior does not lead to the correct critical layer thickness. With increasing substrate temperatures, we not only observe a strong increase in the adatom surface diffusion lengths indicated by the damping of the RHEED intensity oscillations, but also a drastic decrease in the EuTe critical layer thickness. These facts indicate that the mechanism of initial strain relaxation is due to formation of coherent three-dimensional (3D) islands on the surface and their relaxation by elastic lateral deformation. Based on these results, we propose a new interpretation for the limits of 2D layer-by-layer heteroepitaxial growth of EuTe on PbTe(111), the limits being mainly a result of the strain-induced tendency toward 3D islanding on the surface.