Lattice site location of impurities in group III nitrides using emission channeling

The group III nitrides comprise the semiconducting materials InN, GaN, AlN and their ternary alloys. During the last decade, GaN has attracted widespread attention due to its large band gap and hardness. These properties, combined with the fact that its band gap can be adjusted by alloying it with InN and AlN, make GaN a suitable material for the fabrication of optical components that operate in the blue to ultraviolet region of the electromagnetic spectrum, and for microwave and high-power applications. Indeed, during the last couple of years, GaN-based blue and violet light-emitting devices (LEDs) and laser diodes have been realized and commercialized: the violet laser diodes will even be the keystone to the next generation of optical data storage standards, Blu-ray and HD-DVD. \\ \\ A key aspect in device production is the incorporation of dopants that can alter the electronic, magnetic or optical properties of the host material. For example, Si is often used to generate n-type GaN, while Mg is the most frequently used p-type dopant. By combining a p- and n-type doped layer, one can manufacture a p-n junction, which is one of the fundamental building blocks of semiconductor technology. On the other hand, optical doping can be achieved by introducing rare earth elements (lanthanides) into the host material, since these elements exhibit sharp emission lines in the visible and infrared region of the electromagnetic spectrum. Moreover, the incorporation of magnetic transition metals (e.g. Cr, Mn or Fe) can produce so-called dilute magnetic semiconductors, which can not only make use of the charge of electrons, but also of their spin state. Finally, impurities, like transition metals, can unintentionally be introduced during the growth process and deteriorate the electronic properties of the material. In all cases, the exact influence of the dopant or impurity on the material properties will depend on the lattice site that it occupies within the semiconductor crystal. Therefore, it is important to know where the impurities are situated and how the incorporation site can be influenced by external parameters in order to drive them into active sites. In this thesis, a systematic study of the lattice site location of impurities in GaN and AlN will be presented. \\ \\ Chapter 1 is designed as an introduction to the applications, properties and growth techniques of group III nitrides. Furthermore, an overview of the most relevant dopants and impurities will be given, focusing mainly on the influence of the lattice site on the material properties. \\ \\ A convenient way to determine the lattice site of impurities is by making use of the channeling effect. However, conventional channeling techniques based on the use of probe ion beams, like Rutherford backscattering spectrometry/channeling (RBS/C) or particle-induced X-ray emission (PIXE), require a large concentration of impurities to be introduced into the material. A large impurity concentration has the disadvantage that it might also introduce unwanted effects, such as clustering of the impurities, or crystal damage in the case implantation is used to introduce the foreign elements. One channeling based technique that can be used to investigate low concentrations of impurities is electron emission channeling. The basics of this experimental method that makes use of radioactive tracer isotopes will be explained in chapter 2. \\ \\ In contrast to other semiconductor materials, large single crystals of III-nitrides are not available. Therefore, III nitride layers are usually grown onto foreign substrates, i.e. hetero-epitaxy. As a consequence of the lattice mismatch between nitride and substrate, III nitride epitaxial layers generally have a large dislocation density, which is known to introduce effects like crystal mosaicity. Chapter 3 will focus on the characterization of the GaN and AlN layers that were used for the emission channeling experiments presented in this work. The crystal quality and the mosaicity of these layers were studied by the RBS/C method mentioned above, and by X-ray diffraction (XRD). We will also demonstrate and model the influence of mosaicity on channeling measurements. \\ \\ Chapter 4 contains the results of the electron emission channeling experiments on implanted GaN. In a first section, the lattice site of implanted Ga is discussed. Next, we have investigated how the lattice site occupation depends on the specic rare earth element that is introduced. Subsequently, the role of external parameters (like the implantation fluence, the implantation beam geometry, or co-implantation with oxygen or carbon) on the lattice site of erbium will be discussed. In a next section, we report on the lattice site location of the p-type dopants Ca and Sr, the n-type dopant Sn, and the transition metals Cu, Hg, Au and Fe. Finally, the chapter is concluded by the discussion of the lattice site of Na implanted into GaN. \\ \\ The final chapter 5 deals with the lattice site location experiments of impurities implanted into AlN. The experimental measurements were found to be distorted due to the large mosaicity in the AlN samples. Using the model that was constructed in chapter 3, we will still be able to deduce and discuss the lattice site location of Er, Nd, Pm, Ca, Fe, Cu, Sr and Ag in AlN.