Effect of Annealing Temperature on Optical Properties and Surface Structure of Germanium Thin Films

The influence of annealing temperature on optical properties and surface structure of Ge films prepared by electron beam evaporation was investigated. Ge films with a thickness of about 850 nm were prepared on silicon substrate and annealed at 350, 400, 450, and 500 degrees C. The transmittance of the film was measured by infrared spectrometer. The variation of refractive index and extinction coefficient of thin films was obtained by spectral inversion method. The crystal properties and surface morphology of the specimens were analyzed by X-ray diffraction and atomic force microscope. Results show that compared with the properties of films before annealing, the transmittance of films after annealing is increased, while the refractive index and extinction coefficient become decreased. When the annealing temperature increases from 350 degrees C to 500 degrees C, the transmittance and refractive index gradually decrease, while the extinction coefficient gradually increases. Crystallization occurs in the films after annealing above 400 degrees C and the Ge(111) crystal plane is the preferred growth orientation. With increasing the annealing temperature, the grain size becomes larger, the granular particles appear on the film surface, and the surface roughness is increased.