TEM-investigation on the critical thickness anisotropy of MBE-grown ZnSe/GaAs and Zn 1− xMg xSe/GaAs

Abstract

The critical thickness anisotropy of the Zn 1− x ,Mg xSe/GaAs(0 0 1) and ZnSe/GaAs(0 0 1) systems grown by molecular beam epitaxy (MBE) is investigated by in situ reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM). The increasing FWHM of the specular spot in the RHEED-pattern during growth indicates the initial relaxation stages in strained epitaxial layers as described in an earlier publication [Reisinger et al., Mater. Sci. Forum 182–184 (1995) 147]. Plan-view TEM-micrographs of ZnSe/GaAs samples show that the perfect 60°-misfit dislocations in the [1 – 10]-direction nucleate prior to the [1 1 0]-direction due to their higher mobility. According to the TEM investigation the FWHM increase of the specular spot is connected with the formation of 60°-dislocations perpendicular to the electron beam, whereas dislocations aligned parallel to the beam leave the FWHM unaffected. Finally, a thickness range of the ZnSe-layer from about 225–337 nm is found with misfit dislocations aligned only in the [1 – 10]-direction. Furthermore, we present a new in situ RHEED-method to determine the critical thickness of the Zn 1− x Mg x Se/GaAs system separately in both 〈1 1 0〉-directions without changing the experimental set-up. The application of this method to various Zn 1− x Mg x Se-samples with different Mg-content x confirms the observation that different critical thicknesses in [1 1 0] and [1 - 1 0] are measured related to two different glide systems as observed in ZnSe. Plan-view TEM-micrographs reveal the dislocation structure during the different states of relaxation.