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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.

We report on the lattice location of ion-implanted Ca and Sr in thin films of single-crystalline wurtzite GaN. Using the emission channeling technique the angular distributions of $\beta\!^{-}$−particles emitted by the radioactive isotopes $^{45}$Ca(t$_{ 1/2}$=163.8 d) and $^{89}$Sr(t$_{ 1/2}$=50.53 d) were monitored with a position-sensitive detector following 60 keV room-temperature implantation. Our experiments give direct evidence that $\sim$90% of Ca and > 60% of Sr atoms were occupying substitutional Ga sites with root mean square displacements of the order of 0.15–0.30 Å, i.e., larger than the expected thermal vibration amplitude of 0.074 Å. Annealing the Ca implanted samples at 1100–1350 °C in high-pressure N$_{2}$ atmosphere resulted in a better incorporation into the substitutional Ga site. The Sr implanted sample showed a small decrease in rms displacements for vacuum annealing up to 900 °C, while the substitutional fraction remained nearly constant. The annealing behavior of the rms displacements can explain why annealing temperatures above 1100 °C are needed to achieve electrical and optical activations, despite the fact that the majority of the acceptors are already located on Ga sites immediately after ion implantation.

De groep-III-nitrides bestaan uit de halfgeleiders gallium nitride (GaN) , aluminium nitride (AlN), indium nitride (InN) en hun ternaire legering en. GaN heeft het laatste decennium heel wat aandacht gekregen omwille v an zijn grote, directe energiekloof en zijn hardheid. Deze eigenschappen , gecombineerd met het feit dat zijn energiekloof kan aangepast worden i n legeringen met InN en AlN, maken GaN een geschikt materiaal voor de fa bricage van optische componenten in het blauwe en ultraviolette deel van het elektromagnetisch spectrum, en voor microgolf en hoge-energietoepas singen. Een belangrijk aspect voor toepassingen is het introduceren van dopering selementen die de elektronische, magnetische of optische eigenschappen v an het materiaal wijzigen. In al deze gevallen zal de exacte invloed die het onzuiverheidsatoom heeft op de materiaaleigenschappen, afhangen van de roosterpositie die het inneemt in het halfgeleiderkristal. Het is du s belangrijk om te weten waar de onzuiverheden zitten en hoe hun rooster positie beïnvloed kan worden door externe invloeden om ze uiteinde lijk op actieve sites te krijgen. In dit werk hebben we een systematisch e studie ondernomen naar de roosterposities van een grote verzameling re levante onzuiverheidsatomen, met behulp van de emissiekanalisatiemethode . Deze techniek is gebaseerd op het implanteren met radioactieve isotope n, en het waarnemen van de emissiepatronen van de geladen deeltjes die d e isotopen uitzenden rondom kristalassen met een hoge-symmetrie. Deze em issiepatronen zijn immers karakteristiek voor de roosterplaats van de ra dioactieve atomen. Een probleem dat we opgemerkt hebben, is dat de metingen op AlN en in mindere mate ook GaN beïnvloed worden door de mosaïsche structuur van de nitridekristallen, wat het uitermate moeilijk maakt om fysische infor matie te achterhalen. Echter, met behulp van een theoretisch model en X- stralendiffractiedata, was het mogelijk om de metingen te corrigeren voo r dit effect, zodat we toch kwantitatieve roosterpositie-informatie kond en bepalen. Uit deze resultaten blijkt dat alle onzuiverheidsatomen met een covalent e straal die groter is dan een Ga- of Al-atoom, voornamelijk terechtkome n op substitutionele Ga- of Al-sites in, respectievelijk, GaN en AlN.&nb sp; Enkel Na een element dat een kleinere straal heeft dan Ga vertoo nt ook een meetbare interstitiële component, naast een substitutionele.& nbsp; Daarenboven werd ook de gemiddelde verplaatsingsafstand weg van de substitutionele Ga-site bepaald. Daaruit blijkt dat alle substitutionel e onzuiverheden complexen vormen met puntdefecten die gecreëerd worden t ijdens implantatie. De voornaamste kandidaten hiervoor zijn N- en Ga-vac atures in, respectievelijk, de eerste en tweede schil, en interstitiële Ga-atomen tussen de eerste en derde schil van atomen rondom de onzuiverh eden. Uitgloeistudies tonen aan dat deze complexen stabiel zijn tot 1000 -1100 °C, wat verklaart waarom het elektrisch doperen van GaN zo moeilij k is, en waarom optische doperingsatomen in verschillende luminescentiec entra worden waargenomen, alhoewel de atomen toch op substitutionele sit es zitten. Introduction Chapter 1: Group-III-nitrides 1.1 History and applications of nitrides 1.2 Nitride Growth 1.3 Properties 1.4 Doping and impurities Chapter 2: Electron emission channeling 2.1 Channeling and lattice site location 2.2 Theoretical framework 2.3 Experimental details and analysis 2.4 Scattered electron background Chapter 3: Sample characterization 3.1 RBS: composition and crystal quality 3.2 Determination of crystal mosaicity 3.3 Implications of mosaicity on channeling 3.4 Conclusion Chapter 4: Lattice location of impurities in GaN 4.1 Iso-electronic impurities 4.2 Optical doping with rare earth elements 4.3 Electrical and magnetic impurities 4.4 Light elements 4.5 Defects and impurity{defect complexes in GaN 4.6 Conclusions Chapter 5: Lattice location of impurities in AlN 5.1 Optical doping with rare earth elements 5.2 Electrical impurities 5.3 Conclusions Chapter 6: Conclusions Appendix A: Rutherford Backscattering Spectrometry (RBS) Appendix B: High Resolution X-Ray Diffraction (HRXRD) Appendix C: Wurtzite crystal axes and planes Appendix D: Probe decay schemes Appendix E: Acronyms and abbreviations Nederlandstalige samenvatting Publication List status: published

We report a detailed crystal field analysis of Pm3+ and Sm3+ as well as lattice location studies of 147Pm and 147Nd in 2H-aluminum nitride (w-AlN). The isotopes of mass 147 were produced by nuclear fission and implanted at an energy of 60 keV. The decay chain of interest in this work is 147Nd→147Pm→147Sm (stable). Lattice location studies applying the emission channeling technique were carried out using the β− particles and conversion electrons emitted in the radioactive decay of 147Nd→147Pm. The samples were investigated as implanted, and also they were investigated after annealing to temperatures of 873 K as well as 1373 K. The main fraction of about 60% of both 147Pm as well as 147Nd atoms was located on substitutional Al sites in the AlN lattice; the remainder of the ions were located randomly within the AlN lattice. Following radioactive decay of 147Nd, the cathodoluminescence spectra of Pm3+ and Sm3+ were obtained between 500 nm and 1050 nm at sample temperatures between 12 K and 300 K. High-resolution emission spectra, representing intra-4f electron transitions, were analyzed to establish the crystal-field splitting of the energy levels of Sm3+ (4f5) and Pm3+ (4f4) in cationic sites having C3v symmetry in the AlN lattice. Using crystal-field splitting models, we obtained a rms deviation of 6 cm−1 between 31 calculated-to-experimental energy (Stark) levels for Sm3+ in AlN. The results are similar to those reported for Sm3+ implanted into GaN. Using a set of crystal-field splitting parameters Bnm, for Pm3+ derived from the present Sm3+ analysis, we calculated the splitting for the 5F1, 5I4, and 5I5 multiplet manifolds in Pm3+ and obtained good agreement between the calculated and the experimental Stark levels. Temperature-dependent lifetime measurements are also reported for the emitting levels 4F5∕2 (Sm3+) and 5F1 (Pm3+).