Structure and properties of sputter-deposited Al-Sn-N thin films

Abstract

Coatings consisting of Al, Sn and N have been deposited using co-sputtering from Al and Sn targets in a reactive atmosphere containing N2. AlN was used as starting point, and the Sn content was gradually increased through higher cathode power on the Sn target, resulting in coatings with Sn-contents between 0 and 24 at.%. The coatings were analysed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) and also characterised using UV-vis spectroscopy and nanoindentation. All coatings show a nitrogen content of about 50 at.% and are thus fully nitrided, which is confirmed by bonding analysis with XPS. A combination of results from XRD and XPS leads to the conclusion that the coatings consist of a single phase solid solution based on wurzite (Al1-xSnx)Ny with x varying between 0 and 0.5, and y close to unity. The attained material is metastable with respect to decomposition into AlN, Sn and N2, as shown by sputter damages occurring during Ar+ ion etching. The top surface and cross sections, as observed in SEM, were found to become smoother and the columnar structure less pronounced, changing to grainy and finally glass like morphology, as the Sn content is increased. The material is hard at room temperature, with nanoindentation values of 17-24 GPa. Coatings on silica substrates are transparent and yellow to red-brown in colour. This is quantified as a shifting absorption edge, which moves from 211 to 510 nm, corresponding to an optical band gap of 5.9 and 2.4, respectively, as the Sn-content is increased. The index of refraction varies between 2.0 and 2.6. The deposited materials are thus hard, and have a tuneable absorption edge, which could be applicable in optical applications as a multifunctional optical filter with scratch resistant properties.