Investigation of etch pits on KDP crystals with porous sol-gel coatings

Abstract

We have investigated etch-pit formation on potassium dihydrogen phosphate (KDP) crystals with porous anti-reflective coatings. Etch pits develop beneath the sol–gel coatings after exposure to ambient humidity. The etch pits are homogeneously distributed with a density and an average size governed by the relative humidity and the coating thickness. Furthermore, the etch pits are self-similar in shape and possess facets corresponding to low-energy planes of KDP. Results from optical microscopy, light scattering, and atomic force microscopy support the following model for etch-pit formation in this system. Water adsorbed from the environment into the porous sol–gel coating contacts the crystal surface, causing etch-pit nucleation at high undersaturation. The plume of KDP rising from an etch pit slowly diffuses laterally, reducing the undersaturation and shutting off nucleation in the surrounding area. Because surface kinetics are rapid compared to mass transport through the sol–gel, etch pits continually undergo equilibration to maintain a low-energy geometry and generate an average spacing. Growth continues until the reservoir of water in the sol–gel is saturated with KDP. Coarsening is only observed in high relative humidity environments.