Influence of ion bombardment on the refractive index of laser pulse deposited oxide films

Abstract

HfO 2, ZrO 2 and Y 2O 3 films for optical applications were prepared by ion assisted laser pulse deposition. The influence of oxygen ion bombardment of growing films on the refractive index was investigated in the ion energy range of 150–600 eV and in the ion current density range of 50–250 μA/cm 2. Films deposited without additional ion bombardment or using low ion energy up to 150 eV or low ion current density up to 60 μA/cm 2 possess a high bulk-like refractive index, whereas above these thresholds the refractive index steadily decreases. In the case of hafnia, for example, the highest refractive index, obtained for stoichiometrical films with low absorption, amounted to 2.15 at 600 nm wavelength. Due to relatively strong ion bombardment it decreased to 1.80. This behaviour is a result of ion induced modifications of microstructure. While films with high refractive index were of amorphous structure and had a high packing density with low porosity, increasing ion bombardment of the growing films led to increasing crystallization within the films and, finally, to polycrystalline films combined with increasing columnar film growth and rougher surfaces. Larger voids between the columns result in lower packing density and, therefore, lower refractive index. Using appropriate deposition parameters, oxide films with low absorption coefficients and high laser damage thresholds at 1.06 μm wavelength can be prepared.