Infrared transmissive and rain-erosion resistant performances of GeC/GaP double-layer thin films on ZnS substrates

Abstract GeC/GaP double layer films were deposited on ZnS substrates with radio frequency magnetron sputtering for infrared antireflective and protective purposes. Infrared transmissive and rain erosion resistant performances of the films were studied. Results show that the refractive index of the GaP films is similar to that of bulk GaP and a low refractive index of about 1.78 was obtained for GeC films. The average transmittance of GeC/GaP/ZnS over 8–11.5 μm rises by more than 4% in comparison with that of the uncoated ZnS. After rain erosion, some parts of the GeC layer were sputtered off, with discrete pits formed and the sub-layer GaP film exposed on the bottom of the pits. When the thickness of the GaP film was small, cracks were created by rain erosion on the surface of the exposed GaP films; yet the cracks disappeared with increasing the thickness of the sub-layer GaP. The damages to the uncoated ZnS surface caused by rain erosion are cracks. The lateral border of the cracks was peeled with increasing the rain erosion time. Infrared transmittance of both coated and uncoated ZnS fell after rain erosion, but the fall in infrared transmittance of uncoated ZnS was larger than that of coated ZnS, indicating the GeC/GaP films can protect the ZnS substrate from rain erosion.