Formation of transparent aluminum hydroxide film with mesoscopic surface roughness by hydrothermal treatment of incompletely-nitrided sputtered aluminum film

Incompletely-nitrided Al films (Al-N film) are deposited on the glass substrate by rf sputtering with a metallic Al target and using Ar and N ₂ gas mixture. With increasing film thickness up to 300nm, the surface roughness increases. And the roughness is easily controlled. The size and the number density of surface protuberance are suitable to control diffusive optical properties in the visible and near infrared regions. The films become transparent with retained roughness by boiling in ultra pure water at 368K under atmospheric pressure. The films have been transformed from composite of Al and AlN to aluminum hydroxide (Boehmite). Total transmittance of the boiled specimens exceeded that of the glass substrate itself. These facts suggest that hydrothermally-treated Al-N films with the mesoscopic surface roughness have high potential to reduce the optical loss by reflection.