Spectro-ellipsometric modeling and optimization of two-dimensional Ge layer and three-dimensional Ge dot/island structures on SiO2 substrates

Morphological structures of two-dimensional (2D) Ge thin films and three-dimensional (3D) Ge dots/islands grown on SiO2 substrates were analyzed with UV–visible spectroscopic ellipsometry. The pseudo-dielectric functions (〈ε〉 = 〈ε 1〉 + i〈ε 2〉) were calculated under the Bruggeman effective medium approximation. The 〈ε〉 spectra of 2D films were well fitted with a simulation assuming a single-layer or two-layer model. The single-layer model has a mixture of c-Ge, a-Ge, and void components. The two-layer model has a base-layer consisting of 100% c-Ge and a surface-roughness layer consisting of a mixture of c-Ge, a-Ge, and voids. In the case of 3D films, the overall shape of the 〈ε〉 spectra could only be reproduced by using a two-layer model: a base-layer consisting of mixtures of c-Ge and a-Ge with a surface-roughness layer consisting of c-Ge, a-Ge, and voids. The degree of disagreement correlated with the root mean square surface roughness measured by atomic force microscopy.