HARD ANTIREFLECTING PECVD SILICON-NITRIDE COATINGS ON POLYCRYSTALLINEGERMANIUM

In thermal imaging, germanium has found its important application in t he form of windows, lenses and detectors. Roughening on the external s urface of the exterior optical elements made from germanium is the mos t common form of damage. The surface roughening causes reduced transmi ssion through the external optical components. In the present work, we have developed a thin film coating that protects the surface of germa nium and also increases its transmission in the lower wavelength range , i.e. 2.5 to 7 mu m. The polycrystalline germanium ingots grown by a directional solidification technique and the silicon nitride films use d in the present studies have been grown using the plasma enhanced che mical vapor deposition (PECVD) system. Infrared transmission measureme nts were carried out in the 2.5-25 mu m wavelength range. It has been found that this coating enhances the transmission in the 2.5-7 mu m wa velength range and also provides a good hard protective coating on the germanium surface.