Laser conditioning and electronic defects of HfO₂and SiO₂thin films

Abstract

Multilayer HfO2/SiO2 high reflectors (HR) and polarizers show a permanent increase in their 1064-nm damage thresholds following laser conditioning at subthreshold fluences. Threshold increases of 2-3x are typical. In an effort to better understand the conditioning effect we have made laser conditioning and electronic property measurements on single layers of these two materials. The laser damage threshold of 1-μm thick e-beam deposited SiO2 was increased by laser conditioning for wavelengths ranging from 355 to 1064 nm. The damage threshold of HfO2 single layers was not influenced by sub-threshold illumination. As-deposited thin films of a-SiO2 are known to contain paramagnetic electronic defects. We have used electron paramagnetic resonance (EPR) to study the concentrations and types of defects present in single layer and multilayer films of HfO2 and SiO2. E' and oxygen hole centers with concentrations on the order of 1017/cm3 have been measured in the SiO2 layers. A previously unreported defect has been observed for HfO2. The concentration of defects was studied both before and after laser conditioning and damage with 1064-nm photons. These electronic structure measurements are discussed in relation to an electronic defect model for laser conditioning of dielectric multilayers.