INFLUENCE OF ION-BOMBARDMENT ON THE REFRACTIVE-INDEX OF LASER-PULSE DEPOSITED OXIDE-FILMS

HfO2, ZrO2 and Y2O3 films for optical applications were prepared by io n assisted laser pulse deposition. The influence of oxygen ion bombard ment 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 mu A/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 mu A/cm(2) possess a high bulk-like refractive index, whereas abov e these thresholds the refractive index steadily decreases. In the cas e of hafnia, for example, the highest refractive index, obtained for s toichiometrical 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 micros tructure. While films with high refractive index were of amorphous str ucture and had a high packing density with low porosity, increasing io n bombardment of the growing films led to increasing crystallization w ithin the films and, finally, to polycrystalline films combined with i ncreasing columnar film growth and rougher surfaces. Larger voids betw een 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 mu m wavelength can be prepared.