Rare earth-doped aluminum nitride thin films for optical applications

Abstract

This project is dedicated to study the optical properties of rare earth-doped aluminum nitride thin films. In particular, to investigate the luminescence mechanisms of selected RE elements incorporated in AlN thin films. Reactive magnetron sputtering (RMS) technique is used to synthesize the undoped and doped AlN thin films. The effect of sputtering conditions on the structure and optical properties of the prepared samples are investigated. In addition, the optimum experimental conditions that will be used during this work are determined. The structure and composition analyses have been investigated by several means, such as transmission electron microscopy (TEM), Energy-dispersive X-ray spectroscopy (EDS), and Rutherford backscattering spectrometry (RBS). The optical properties of the films are characterized by UV-Visible transmission, Ellipsometry spectroscopy, and Photoluminescence spectroscopy. For undoped AlN, well crystallized AlN thin films with high degree of c-axis orientation were prepared. Controlling the preferred orientation by only tuning the N 2 % in the gas phase has been achieved. It was found that, the synthesis of highly c-axis oriented crystalline AlN is favored by depositing the coatings in nitrogen-rich reactive ambiance. The results have been interpreted on the basis of an improved mobility of adatoms assisted by the bombardment of the films by fast particles. The optical constants (n, k) and bandgap of the prepared films have been modeled from spectroscopic ellipsometry measurements in transmission and reflection modes. It is found that the refractive index can be tuned with the crystal orientation while keeping constant the bandgap. Our findings suggest that, the optical properties of the AlN films can be tuned via their crystallographic orientation which, in turn, varied by the amount of nitrogen in the gas phase. For doped AlN, crystalline Ce-doped AlN were prepared. The crystal structure of the prepared samples and the compositions have been examined by TEM, RBS, EDS and EELS analyses. It was found that, presence of oxygen in this material is essential for sensitizing the photoluminescence. Oxygen has been found playing major role note only in converting Ce ions from the optically inactive state +4 to the optically active one +3, but also leaded to the formation of defect complexes with Al vacancies. These defect complexes were found contributed in the excitation mechanism of Ce ions. Therefore, an optical excitation and emission mechanisms have been proposed based on the role of oxygen. Based on that, manipulation of the PL was achieved and different colors (blue, and green) were clearly observed by the naked eye. In addition, white light emission approach has been presented and strong eye observed white light was obtained. Yb-doped and (Ce, Yb) co-doped Al(O)N systems are prepared. The crystal structures and compositions of the prepared samples have been investigated. Indirect optical excitation of the NIR emission of Yb ions is achieved. The excitation mechanisms of the NIR and visible emission of single and co-doped samples are discussed. It is found that, the similarity between the PLE spectra of Ce and Yb results in the possibility of achieving energy transfer between the two ions in the co-doping system. The type of energy transfer mechanism is found consistence with the one-to-one down conversion 176 via charge transfer state mechanism. The PL thermal quenching has been studied by following the PL evolution with low temperatures. Keywords: AlN thin films, rare earth doped AlN, optical properties, magnetron sputtering

Dieses Projekt widmet sich der Untersuchung der Lumineszenzmechanismen von Cerium- (Ce) und Ytterbium-Ionen (Yb), die in Aluminiumnitrid-Dünnschichten (AlN)einzeln und co- dotiert sind. Die reaktive Magnetronsputter-Technik (RMS) wird verwendet, um die undotierten und dotierten AlN-Dünnschichten zu synthetisieren. Die Auswirkung der Prozessbedingungen und der Konzentrationen von Ce und Yb auf die Struktur und die optischen Eigenschaften der hergestellten Proben werden untersucht. Die Struktur- und Zusammensetzungsanalysen wurden mit verschiedenen Methoden wie Transmissionselektronenmikroskopie (TEM), energiedispersive Röntgenspektroskopie (EDS) und Rutherford-Rückstreu-Spektrometrie (RBS) durchgeführt. Die optischen Eigenschaften der Filme wurden durch Spektralphotometrie, Ellipsometrie und Photolumineszenz- Spektroskopie (PL) charakterisiert. Ein vorgeschlagener Mechanismus zur Verbesserung und Löschung der PL wird diskutiert. Zusätzlich wurde ein Energieübertragungsprozess zwischen Ce- und Yb-Ionen erreicht. Basierend auf dem vorgeschlagenen Mechanismus konnten wir die Emission dieses Materials manipulieren, um blaue, grüne und weiße Farben zu erhalten. Unser Befund kann als Richtlinie zum besseren Verständnis des optischen Verhaltens von Seltenerdionen in AlN verwendet werden.