Hard antireflecting PECVD silicon nitride coatings on polycrystalline germanium

In thermal imaging, germanium has found its important application in the form of windows, lenses and detectors. Roughening on the external surface of the exterior optical elements made from germanium is the most common form of damage. The surface roughening causes reduced transmission through the external optical components. In the present work, we have developed a thin film coating that protects the surface of germanium 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 used in the present studies have been grown using the plasma enhanced chemical vapor deposition (PECVD) system. Infrared transmission measurements 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 wavelength range and also provides a good hard protective coating on the germanium surface.