High resolution double crystal X-ray diffractometry and topography of III-V semiconductor compounds

Abstract

Double crystal diffractometry and topography are now routinely used in many laboratories for the inspection of epitaxially grown devices. However the trend towards thinner layers and more complex structures requires the continued development of novel approaches using these techniques. This thesis is concerned with the development of these approaches to study the structural uniformity of semiconductor materials. The uniformity of large single crystals of lithium niobate has been studied using synchrotron radiation and double crystal X-ray topography. This study has shown a variety of contrast features including low angle grain boundaries and non-uniform dislocation densities. The abruptness of an interface between a layer and the underlying substrate has been studied using glancing incidence asymmetric reflections. Comparisons to simulated structures revealed that a closer match was achieved by the inclusion of a highly mismatched interfacial layer. This study illustrates the need for careful comparison between experimental and simulated rocking curves as different structures may produce very similar rocking curves. A double crystal topographic study of a AlGaAs laser structure revealed X-ray interference fringes. These are shown to be produced from the interaction of two simultaneously diffracting layers separated by a thin layer. Possible formation mechanisms have been discussed showing that these fringes are capable of revealing changes in the active layer at the atomic level. A novel approach has also been developed using synchrotron radiation to study the non-stoichiometry of GaAs. This approach uses the quasi-forbidden reflections which are present in III-V semiconductors due to the differences in the atomic scattering factors. This study has also discussed the behaviour of strong and weak reflections in the region of absorption edges and modelled their behaviour using the anomalous dispersion corrections of Cromer and Liberman.