Black a-Si solar selective absorber surfaces

Amorphous Si films display a unique anisotropic etching phenomenon in which a columnar microstructure results. The alignment of the anisotropic microstructure is related to the average angle of the depositing material. Since cross-sectional spacing of the columns is less than the wavelength of light, a continuous grading of the refractive index occurs and results in a drastic reduction in the total reflectance from 40 to 50% in the as-deposited film to <2% in the etched film. The etchant, HF/HNO3, apparently oxidizes rapidly down aligned void networks followed by an equally rapid dissolution and removal step. Such a semiconductor film on a low ir emmisivity metallic substrate is a potentially efficient photothermal selective absorber surface.