Optical Absorption Edge Broadening of Evaporated Germanium Films

The absorption spectra of evaporated films of germanium, about 5 microns thick, are examined at room temperature and at 90 °K in the spectral range 0.5 to 1.0 eV. An absorption edge, considerably broader than the single-crystal edge and with no line structure, is seen in all cases. The broadening is studied as a function of substrate temperature during the film deposition, evaporation rate, and annealing time, and is interpreted in terms of the internal strains in the evaporated films.It is found that information on the quality of an evaporated film may be obtained from the study of the absorption spectrum.