A spectroscopic ellispometric study of the tunability of the optical constants and thickness of GeO_x films with swift heavy ions

摘要

Sub-stoichiometric GeO_x films were fabricated by electron-beam evaporation method. The films were irradiated with 100 MeV Ag~(7+) ions at fluences between 1 x 10~(12) and 1 x 10~(14) ions-cm~(-2). Spectroscopic ellipsometric measurements were performed in air at room temperature. The values of the layer thickness and refractive index were extracted from ellipsometry using a multilayer analysis and the Tauc Lorentz model. The refractive index (at 633 nm) of the as-deposited GeO_x film was estimated to be 1.860 and decreased to 1.823 for films irradiated at an ion fluence of 1 x 10~(14) ions-cm~(-2). The thickness of the films also decreased after irradiation and is due to a sputtering induced by the ion beam. The change in the refractive index with ion fluence is attributed to a stoichiometric change and structural transformation represented by GeO_x→ Ge + GeO_y (y > x) occurring due to a thermal spike induced by ion irradiation. Swift heavy ions thus provide a scope for modulating the refractive index of GeO_x films. The thickness and stoichiometric changes are supported by Rutherford backscattering measurements.