Thermal oxidation of amorphous germanium thin films on SiO2 substrates

摘要

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 degrees 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 degrees 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 degrees C reducing the oxide layer to GeOx (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 0.82 +/- 0.09 eV up to 500 degrees C. Remarkably, we observed simultaneous enhancement of the oxidation and crystallization of the a-Ge in the temperature interval 450 degrees C-500 degrees C, in which the oxidation rate reaches a maximum of around 0.8 nm degrees C-1 at around 500 degrees C.