Transmission electron microscopy of GaN based, doped semiconductor heterostructures

Abstract

This work addresses two different topics concerning GaN based devices. The first topic discussed are pyramidal defects of p-type doped GaN:Mg, which can be formed at high doping levels. The structure of the basal plane inversion domain boundaries of these pyramidal defects is studied by quantitative comparison of simulated high resolution transmission electron microscopy images of various boundary models with images of the experimental boundary.The second topic is the measurement of the indium concentration in nanometre-sized InGaN islands grown by molecular beam epitaxy (MBE) as well as metalorganic vapour phase epitaxy (MOVPE). Transmission electron microscopy imaging conditions minimising the errors in the concentration evaluation are explained, and the errors of this method with additional regard on application to InGaN islands are discussed. For samples grown by MBE, the islands are plastically relaxed due to misfit dislocations. The In concentration varies significantly between islands and the two dimensional wetting layer in between the islands. Furthermore, the influence of GaN cap layer growth on top the islands is analysed, revealing the process of island dissolution at different stages during cap layer growth. InGaN islands grown with MOVPE relax elastically. Using strain state analysis, an indication is obtained that the In concentration within the islands increases in growth direction. This finding is confirmed by use of two different methods, which are the evaluation of the object exit wave function using focus variation image reconstruction and the 'superposition method', which was developed for this work and minimises the errors in measurement of the c and additionally the a lattice parameter. The increase of the In concentration of the elastically relaxed MOVPE grown InGaN islands as well as the plastically relaxed MBE grown islands is explained with the composition pulling effect.