Very hard and durable single layer antireflection coatings for Ge and Si have been made which result in a peak transmittance of better than 98% at any desired wavelength in the infrared up to about 15 µ, Silicon monoxide makes an excellent antireflection coating for silicon and germanium in the near infrared, but cannot be used in the 10-µ region where it becomes strongly absorbing. Zinc sulfide has been found to be the most suitable antirefiecting material at the longer wavelengths. Under proper conditions zinc sulfide films, which in the past have frequently exhibited unsatisfactory durability, can be prepared as very hard and adherent antireflection coatings for germanium and silicon. The best zinc sulfide films are obtained when the substrate is heated to about 150°C and cleaned with a dc glow discharge just before the evaporation is made. The coatings can stand several hours boiling and several days immersion in water without damage and have been used for periods up to one year without showing any deterioration.
J. T. Cox and G. HASS, "Antireflection Coatings for Germanium and Silicon in the Infrared," J. Opt. Soc. Am. 48, 677-678 (1958)