XPS Characterization of Materials for Photonic Applications

X-rays can be utilized to induce electron emission from the material surface. The material properties are probed measuring the energy of photoelectrons produced. The photoelectron energy, in fact, mirrors the electronic structure of the atoms thus making the X-ray photoelectron spectroscopy (XPS) a powerful, technique to investigate the physical properties of the material. Referring to photonics, the emission properties depend on a list of elements such as the nature of the luminescent material, on its chemical state, on the host chemical composition and structure, on the presence of defects etc. Synthesis conditions, heat treatments and any kind of processing affecting the dopant and host energy levels strongly affect the material photoluminescence. Hence, XPS probing the electronic structure of the elements and their oxidation state is one of the primary techniques to assess their optical properties. XPS is widely utilized to determine the material composition which corresponds to the identification of all the chemical elements constituting the material together with their atomic concentrations. In the following we will present some example showing how XPS is crucial to explain the optical properties of materials. In the first example we will show the application of XPS to x HfO
₂
-(100 - x) SiO
₂
(x = 10,20,30 mol%) glass-ceramic planar waveguides. We will focus on the high resolution spectra of Si, O and Hf elements and how they change as a function of the Hf concentration. The analysis of these spectra allowed the identification of the chemical bonds formed among these elements. These changes were correlated to the presence of a spinodale decomposition occurring at the higher Hf concentrations and formation of a glass-ceramic structure. The structural changes occurring in the waveguides were utilized to explain the improvements in the Er
³⁺
emission. In the second example XPS and Auger spectroscopy were fundamental to study the behavior of Sn in SiO
₂
-SnO
₂
: Er3+ waveguides produced by sol gel technique. Waveguides were fabricated with a sequence of 25 depositions of thin SiO
₂
-SnO
₂
layers followed by annealing. Unbalances in the waveguide surface composition led to hypothesize the presence of a segregation of Sn below the surface of each of the layers. This hypothesis was then confirmed by depth profiling analyses which clearly show an oscillating layer by layer trend of the Si and Sn abundances. These results allowed explaining the optical properties of the waveguides. Finally, XPS was utilized to analyze sodalime glasses and the fate of silver atoms which were introduced in this glassy matrix via an ion-exchange process. Ag was introduced in the glass to enhance the emission from Er
³⁺
dopants. However, the plasmonic properties strongly depend on the chemical state of the Ag atoms introduced, if they coalesce in nanostructures and on their size. XPS was crucial to determine these parameters as a function of the temperature. A detailed analysis of the Ag and O spectra and of the valence bands enabled the identification of the silver chemical state and the transition from oxidized to metallic character by increasing the temperature. In their turn, these results were correlated with the material plasmonic properties.