Thermal oxidation of amorphous germanium thin films on SiO₂ substrates

In this work we report the thermal oxidation of amorphous germanium (a-Ge) thin films (140 nm thickness) in air. Following fabrication by conventional thermal evaporation on SiO₂ substrates, the samples were annealed in air at different temperatures ranging from 300 to 1000 °C. By means of x-ray diffraction, x-ray reflectivity, synchrotron grazing-incidence wide-angle x-ray scattering and cross-sectional transmission electron microscopy analysis it is found that the a-Ge films abruptly crystallize at 475 °C, while simultaneously increasing the thickness of the oxide (GeO₂) in a layer by layer fashion. X-ray photoemission spectroscopy reveals that the oxidation state of the Ge atoms in the GeO₂ layer is 4⁺. However, a reaction at the GeO₂/Ge interface occurs between 500 and 550 °C reducing the oxide layer to GeO_x (x<2) and containing Ge²⁺ and Ge⁺. The thickness of the oxide layer grows with the annealing temperature following an Arrhenius behavior with an activation energy of 0.820.09 eV up to 500 °C. Remarkably, we observed simultaneous enhancement of the oxidation and crystallization of the a-Ge in the temperature interval 450 °C-500 °C, in which the oxidation rate reaches a maximum of around 0.8 nm °C^-1 at around 500 °C.