Rotation anisotropy by second harmonic generation of II–VI epilayers on a GaAs {100} substrate: bare CMT and CdS on CMT

Abstract

Using second harmonic generation (SHG), the rotation anisotropy for cadmium mercury telluride, Cd_xHg_{1 –x}Te (CMT) epilayer surfaces grown on a GaAs {100} substrate has been observed. For the bare CMT {100} surface, four peaks were obtained, in agreement with similar studies on compounds with the zinc-blende crystal structure. It is shown that the overall SH response arises from an interference pattern between the bulk and the surface contributions. From the interference patterns observed in the different polarisation configurations, it is concluded that the CMT epilayer has grown following the vicinal surface of the GaAs {100} substrate. These vicinal surfaces are also responsible for the preferential symmetry exhibited by the surface. When a layer of oxide is electrochemically grown onto the CMT layer, the SH response essentially retains the underlying CMT symmetry although changes in the peak intensities were noted. These changes can arise from a modified interference pattern between the bulk and surface contributions as compared with that of the bare CMT surface. Finally, we have studied the SH response from an anodically grown CdS film on the CMT substrate. A large enhancement in the SH intensity was found, resulting from the non-linear-active CdS layer. It is shown that this layer has grown with the cubic close packing structure of the underlying CMT substrate, its anisotropic response being similar to that of the bare CMT. It is also observed that in this case, the interference pattern is much less pronounced, indicating a much weaker surface contribution. This study therefore indicates that SHG is a valuable technique for following the in situ growth of non-linear-active films used in passivating the CMT epilayers.