Ion Implantation in Ge: Structural and electrical investigation of the induced lattice damage & Study of the lattice location of implanted impurities

The past two decades, germanium has drawn international attention as one of the most promising materials to replace silicon in semiconductor applications. Due to important advantages with respect to Si, such as the increased electron and hole mobility, Ge is well on its way to become an important material in future high-speed integrated circuits. Although the interest in this elemental group IV semiconductor is increasing rapidly nowadays, the number of publications about this material is still relatively scarce, especially when compared to Si. The most widely used technique to dope semiconductors is ion implantation, due to its good control of the dopant concentration and profile, and the isotopic purity of the implanted species. However, there is a major lack of knowledge of the fundamental properties of ion implantation in Ge, which has triggered the research presented in this thesis. One of the most important and generally unwanted properties of ion implantation is the creation of damage to the crystal lattice, ranging from simple point defects such as vacancies and self-interstitials, over small and large defect clusters to even fully amorphous layers of material. These structural defects give rise to electronic (deep) levels in the semiconductor band gap, altering the electrical properties of the material, eventually resulting in the degradation of the semiconductor device. During implantation, the energetic ions travel through the host material while losing energy and creating defects, until they come to rest and occupy a certain lattice site within the crystal structure. The exact location of the implanted ions is known to influence the (electrical, optical and magnetic) properties of the doped material to a large extent, which makes it a very important, but barely-studied topic in Ge. It is generally assumed that most impurities simply replace the Ge host atoms - i.e. they are located on a substitutional Ge site - but the lack of accurate results on this issue puts this assumption at least into question. In this work, we present an investigation of these two important ion implantation-related issues in germanium. First of all, we will study the accumulation of, and the recovery from implantation-induced defects in Ge, both from structural and from electrical point of view. Secondly, we present a detailed lattice location study of a number of relevant and interesting impurities in germanium. In chapter 1, the research presented in this thesis is situated in its scientific and technological context. First of all, the status of germanium throughout the semiconductor history is highlighted, starting at the late 40s, followed by its revival during the past two decades and concluding with the current issues and problems to integrate Ge in the semiconductor technology. Secondly, the most widely used technique to dope semiconductors, i.e. ion implantation, is put forward, emphasizing the different types of implantation-induced defects. We also present an extended overview of the current knowledge of implantation-related damage in Ge. The third section of this chapter contains an overview of the most relevant impurities in Ge, i.e. electrical dopants, isovalent elements, metal impurities and optical dopants. The overall objectives of the thesis are explained in chapter 2. The sample preparation, which includes the ion implantation and the subsequent annealing process, is presented in chapter 3, while chapter 4 contains relevant information about the experimental methods that have been used in this research. First, the techniques to structurally and electrically characterize the implantation-induced damage are introduced - i.e. Rutherford backscattering and channeling spectrometry (RBS/C), X-ray diffraction (XRD) and deep level transient spectroscopy (DLTS) - followed by emission channeling (EC) and ab initio calculations, which aim at studying the lattice location of implanted impurities. Finally, chapter 5 contains a short summary and a discussion of the results that have been obtained within the framework of this research. A more elaborate explanation and discussion of the results can be found in the articles that have been published and in the manuscripts that have been prepared for publication. These articles and manuscripts are added at the end of this thesis.