Table of Contents:
The successful application of several novel Transmission Electron Microscope investigation techniques (TEM) in combination with theoretical modelling is the prerequisite for the understanding of the structure and morphology of GaAs based N containing material systems. Hence the main goal of this work was the development and application of novel methods for the nanostructural analysis of these metastable, dilute N containing III/V semiconductor heterostructures.
An important step towards quantification of High Resolution TEM-images (HRTEM) was the optimisation of the TEM sample preparation for the comparatively hard, N containing material. A novel technique using Atomic Force Microscopy (AFM) allows the determination of the sample quality and beyond this the measurement of the sample relaxation of strained material systems for extremely thin sample areas (t = 20nm). Comparison to Finite Element Simulations (FE Simulations) shows excellent agreement with the expected height structure of such a sample, indicating that the observed height profile corresponds to the expected elastic relaxation of the strained thin sample.
Further TEM Dark Field imaging (DFTEM) with different reflections, which are either sensitive to the chemical composition or to the strain in the material respectively, has been used. This aims rather in gaining quantitative information on inhomogeneous distribution of the N than in the quantification of its absolute composition. To explain these results a Valence Force Field (VFF) code was developed, which allows to calculate stable configurations of the N in thermal annealed and as grown crystals under investigation.
For the absolute quantification of the N containing material systems the CELFA (Composition Evaluation of Lattice Fringe Analysis) algorithm implemented in the software package DALI (Digital Analysis of Lattice Images) has been used. This technique relies on the comparison with image intensities derived from Bloch wave calculations, which use Doyle and Turner scattering factors, derived from single atom approximation. Due to this approximation the real distribution of the electrons in the structure is neglected, which leads to erroneous results. Further more, the structure factors are influenced by the local atomic arrangement in the material. So refined structure factor calculations for N containing Ga(NAs) and (GaIn)(NAs) have been performed, which leads - as a consequence of the inclusion of the local atom arrangements in N containing material - to a more exact quantification of the N depth distribution in the introduced material systems.