Spectroellipsometric investigation of optical, morphological, and structural properties of reactively sputtered polycrystalline AlN films

The optical and morphological properties of reactively sputtered AlN films on Si substrates have been studied in this work from a self-consistent three-layer optical model developed from spectroscopic-ellipsometry analysis and validated by observations from transmission-electron microscopy, Auger electron spectroscopy, and in situ reflectance interferometry. These properties correlate to the film microstructural properties. Accordingly, the almost thickness-independent refractive index of 2.01 of the bulk AlN layer indicates its polycrystalline microstructure. This layer also appears ungraded, homogeneous, isotropic, and free of excess Al, as if grown through a steady process. The small film absorption points to the Urbach tail states produced by the structural disorder typical of such sputtered films. The films' interface layer consists of a graded Bruggeman intermix of outdiffused Si and AlN materials spreading over 10-12 nm thickness. The surface morphology includes an Al2O3-oxidized outer rough surface gradually becoming AlN bulk with diminishing amounts of Al2O3 and inner pores. The increase in the surface-layer thickness, as the film grows, indicates further surface roughening due to enlarging crystals in a disoriented growth. This spectroscopic-ellipsometry analysis of AlN films has allowed us to study the effect of substrate biasing on the AlN microstructure and to place forward a new processing method for the surface smoothening of rough AlN and diamond films.