A Structural Comparison of Si(100) Oxidized by Atomic and Molecular Oxygen
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A Structural Comparison of Si(100) Oxidized by Atomic and Molecular Oxygen Maja Randjelovic and Judith C. Yang Materials Science and Engineering Department, 848 Benedum Hall, University of Pittsburgh, Pittsburgh PA 15213 USA ABSTRACT We compared the structural characteristics of a silica layer formed on Si(100) by oxidation in hyperthermal atomic oxygen and molecular oxygen at 493K. The laser detonation method was used to create primarily neutral atomic oxygen with kinetic energy of 5.1eV. The silicon oxides were characterized by High Resolution Transmission Electron Microscopy (HRTEM), Atomic Force Microscopy (AFM), Rutherford Backscattering Spectrometry (RBS), Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS). We determined that atomic oxygen forms amorphous silica that is almost twice as thick and nearly double the surface roughness as compared to molecular oxygen - formed silica at the same temperature and time conditions. INTRODUCTION Space vehicles in the low earth orbit are exposed to a harsh environment that can significantly enhance their failure. The low earth orbit ranges from 200 to 700km in altitude, with average surface temperatures that vary between 200 and 400K [1]. Atomic oxygen is considered to be the most hazardous factor causing corrosion and degradation of spacecraft materials, acting either alone or in synergy with other components, such as debris impacts, ultraviolet radiation, solar irradiation, electron and proton exposures and fluctuations in temperature. As a direct consequence of high velocity of orbiting spacecrafts (8km/sec at 250km altitude), the atomic oxygen flux is very high (1014 atoms cm-2sec-1) and has kinetic energy of about 5eV. We used a laser detonation source to generate 5.1 eV neutral atomic oxygen with a reasonable flux. The pulsed detonation source creates hyperthermal atomic oxygen by using a pulsed CO2 laser to detonate the oxygen gas to create an oxygen plasma, and this wave of atomic oxygen is neutralized and strikes the sample before re-combining into oxygen gas. We studied the passivation of silicon, as a model semiconductor material, by hyperthermal atomic oxygen and compared with the oxidation in molecular oxygen. Furthermore, silicon forms a uniform passive and protective oxide that could be used as a coating material for polymer materials such as Kapton or FEP Teflon that degrade rapidly in low earth orbit. Their protection is essential since these materials are used as thermal blankets and structural components. Thin oxide protective coatings are required to be atomic oxygen resistant, sufficiently thick to be an effective diffusion barrier, but also thin enough in order to not affect the required functional properties of the underlying material such as solar absorptance and thermal emittance. Uniformity of the oxide coating is needed since small pinholes in the coating can lead to the erosion of the underlying material due to the direct contact with atomic oxygen [2]. Quantitative determination of the thickness of the coati
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