The surface of a polycrystalline germanium film, vacuum-deposited onto a heated substrate in a narrow range of temperatures, takes on a regular structure exhibiting optical interference effects whose nature is quite sensitive to the angle at which the film is deposited. For all films deposited between 550° and 620°C, Fraunhofer diffraction effects are observed which are independent of crystalline texture. In the case of perpendicular deposition, the diffraction is nearly isotropic about the normal to the film, while in the case of deposition at an acute angle, the diffraction effects are nonisotropic and the observed optical behavior is analogous to that observed with blazed gratings; electron-microscope observations confirm these particular surfaces to be “blazed” in character. The “blue-grey” character of germanium films and the “blue haze” observed on other surfaces is explained on the basis of these results.
JOHN E. DAVEY and TITUS PANKEY, "Anomalous Optical Effects in Germanium Films," J. Opt. Soc. Am. 56, 1331-1336 (1966)