Characterization and optimization of solution-derived chalcogenide glass thin films

Chalcogenide glasses have many unique properties that allow them to be used in a variety of optical applications. Infrared transparency allows them to be used in sensors for molecules that have "fingerprints" in the 2--25 &mgr;m range. By producing amorphous thin films of these materials, they can be incorporated into chemical sensors as planar waveguides and resonators. The goal of this work was to fabricate and characterize solution-derived chalcogenide thin films for use in chemical sensors with a source wavelength between 3--3.5 &mgr;m.;The structural and optical properties of the parent bulk glass were characterized and used as reference for the solution-derived thin film studies. Thermally evaporated thin films have been routinely used in chemical sensor geometries in our group and were used as a point of reference for the target roughness and surface quality of the spin-coated films. The goal was to produce films with the same structure and optical properties as the bulk glass, roughness equal to or less than TE films, and a surface free of cracks and pinholes. Glass thin films in the Ge-Sb-S family were deposited using the spin-coating method. The glass solutions were optimized by studying the effects of water content and glass loading on the post-processed films. This work has shown that the water content of the solvent influences glass dissolution and that glass loading is the key parameter for controlling film thickness. The removal of residual solvent was studied using in-situ FTIR measurements during heat treatments at various temperatures. Changes in the glass structure and solvent-glass interactions were studied through detailed analysis of the far-IR and mid-IR regions, respectively. Strong glass-solvent interactions were shown to exist and temperatures well above the boiling point of the solvent were needed to remove it. For each temperature, correlations were made between structural aspects and properties such as refractive index measured by ellipsometry, band gap by UV-vis absorption, and surface roughness by AFM. These properties were shown to be affected not only by residual solvent, but also by pores left as the solvent was removed and by consolidation of the film as the pores were eliminated.