把GeC/GaP双层膜用作ZnS衬底的长波红外(8～11.5 μm波段)增透保护膜系。采用射频磁控溅射法,以高纯Ar为工作气体、单晶GaP圆片为靶制备了GaP薄膜；用射频磁控反应溅射法在高纯Ar和CH4的混合气体中,以单晶Ge圆片为靶制备了GeC薄膜。分别用柯西(Cauchy)公式和乌尔巴赫(Urbach)公式表示折射率和吸收系数,对薄膜的红外透射率曲线进行最小二乘法拟合,得到了它们的厚度及折射率、吸收系数等光学常数。GaP膜的折射率与块体材料的相近,在波长10 μm处约为2.9；GeC膜的折射率较小,在波长10 μm处约为1.78。用所得到的薄膜折射率,通过计算机膜系自动设计软件在ZnS衬底上设计并制备出了GeC/GaP双层增透保护膜系,当GaP膜厚较大时,由于吸收增大膜系增透效果较差；当GaP膜厚较小时,膜系有较好的增透效果。

GeC/GaP doublelayer thin film is used as longwave infrared (8～11.5 μm waveband) antireflective and protective thin film system for ZnS substrates. Gallium phosphide thin film has been deposited by RF magnetron sputtering with high-purity Ar as the working gas and single crystal GaP disc as the target, and germanium carbon thin film has been deposited by reactive RF magnetron sputtering in high-purity Ar and CH4 mixture with single crystal Ge disc as the target as well. The thicknesses as well as the optical parameters of both GaP film and GeC film, such as refractivity and absorption coefficient are determined by least square fit of the films' IR transmission spectra, with the refractive indexes expressed in Cauchy formula and absorption coefficients in Urbach formula. The refractive index of GaP film is about 2.9 at 10 μm wavelength and is close to that of the bulk GaP; and that of the GeC film is about 1.78 at 10 μm wavelength. With these refractive indexes, GeC/GaP double-layer antireflective and protective thin film system is designed by computer and deposited on ZnS substrate. The GeC/GaP film system shows relatively large absorption when the GaP film is relatively thick, which makes the antireflective effect poor, whereas the antireflective effect is good when the GaP film is relatively thin.