Optical and structural characterisation of single and multilayer germanium/silicon monoxide systems

Germanium and silicon monoxide thin films prepared with different evaporation conditions are analysed. Substrate temperatures from 30 to 370 °C and deposition rates from 0.3 to 3.0 nm/s for SiO and from 0.5 to 1.5 nm/s for Ge films are considered. Optical constants in the mid-infrared range are derived from transmission spectra and their variation with deposition conditions is related to structural and morphological changes in the films. Taking into account the results of the single layers study, deposition conditions adequate for both SiO and Ge thin films in multilayer optical coatings for the mid-infrared range of the spectrum are selected. The optical characterisation of a ten-layer bandpass filter, designed considering the experimental optical constants for both materials and prepared with the selected deposition conditions, is presented. The results indicate that the departure of experimental transmittance in the mid-infrared range from the design is mainly due to thickness deviations.