| Space vehicles residing in the low Earth orbit (LEO) are exposed to a harsh environment that rapidly degrades their materials. The LEO ranges from 200-700km in altitude from the Earth's surface, and the temperature varies between 200 and 400K. The most hazardous species in LEO is atomic oxygen (AO) containing 5eV kinetic energy due to the high velocity of the spacecrafts (8km/s).; The goal of this research is the elucidation of the fundamental mechanisms of semiconductor degradation and passivation in LEO conditions by comparing the structural differences in the oxide films created by exposure to AO and molecular oxygen (MO). Silicon is the base material for solar cells used in LEO whereas Ge and SiOx films are common coatings to protect polymer materials that are used as structural materials in spacecrafts.; Hyperthermal AO was created by the laser detonation of MO within a high vacuum (HV) chamber, that produces a high flux of AO. A variety of nano-characterization techniques, including high resolution transmission electron microscopy (HREM), and electron energy loss spectroscopy (EELS) were used to determine the microstructure and local chemistry of the oxide and the oxide/semiconductor interface.; For Si, the amorphous silica formed by AO was nearly twice as thick, more ordered, and more homogeneous in composition, than the oxide formed by MO. The Si/SiOx interface formed by AO was atomically abrupt, with no suboxides detected near the interface or throughout the oxide. The oxide scale formed by MO on Si(100) consisted of transitional oxidation states. The oxide film formed on Ge(100) due to exposure to 5eV AO, is 2-3 times thicker and similarly to the Si/SiOx interfaces, the Ge/GeOx interface was found to be atomically abrupt.; The oxidation kinetics of Si and Ge were monitored in situ using a research quartz crystal microbalance (RQCM) that was incorporated into the AO source. The oxidation kinetics in hyperthermal AO did not follow the standard linear to parabolic Deal-Grove kinetics. A novel oxidation model, based on the oxide structure continually changing during AO exposure, is proposed to explain the unusual power law oxidation kinetics. |
