Characterization of black Ge selective absorbers

We have prepared structurally anisotropic black Ge films by rf sputtering and postdeposition etching. By careful control of various sputtering condition such as rf power, gas pressure, deposition time, and other parameters such as nature and roughness of the substrate and etching time, we can alter the morphology of these films and optimize the selective absorbing properties. Postdeposition etching in 30% H2O2 has reduced the reflectance from approximately 45% to less than 1% in the best case. This etching behavior is also observed for films immersed in water for several weeks at room temperature. This is explained on the basis of oxidation through aligned voids. The etching behavior of various films and correlation between the resulting microstructure and optical properties is discussed. Further, the various microstructures are classified and understood using the structure zone models.