Optical Properties of Thin Germanium Films in the Wavelength Range 2000–6000 Å

Abstract

Measurements have been made of the normal incidence reflectivity and transmissivity coefficients R and T of thin germanium films. Films were deposited in vacuo on fused quartz substrates where the crystalline perfection of the film was controlled by varying the substrate temperature so that the effect of crystalline order on reflectivity could be observed. Epitaxial films were grown on cleaved CaF₂ substrates to thicknesses in the range 100–3000 Å. Structure in the reflectivity and transmission spectra showed these films to possess bulk band properties. However, the amplitudes of R and T were affected by the presence of film surface roughness believed to originate from nucleation and growth phenomena. Also, compressive strain induced by the difference in thermal explansion coefficients between Ge and CaF₂ shifted interband transition structure to slightly higher energies. Values of the optical constants were deduced from R and T. When experimental and calculational difficulties peculiar to the film method are accounted for, the results correspond closely to those of the Kramers‐Kronig analyses of bulk reflectivity data.