We study how the (100) surface of magnetite undergoes oxidation by monitoring its morphology during exposure to oxygen at ~650 °C. Low-energy electron microscopy reveals that magnetite's surface steps advance continuously. This growth of Fe3O4 crystal occurs by the formation of bulk Fe vacancies. Using Raman spectroscopy, we identify the sinks for these vacancies, inclusions of α-Fe2O3 (hematite). Since the surface remains magnetite during oxidation, it continues to dissociate oxygen readily. At steady state, over one-quarter of impinging oxygen molecules undergo dissociative adsorption and eventual incorporation into magnetite. From the independence of growth rate on local step density, we deduce that the first step of oxidation, dissociative oxygen adsorption, occurs uniformly over magnetite's terraces, not preferentially at its surface steps. Since we directly observe new magnetite forming when it incorporates oxygen, we suggest that catalytic redox cycles on magnetite involve growing and etching crystal.
This article presents a study of the influence of the sputtering parameters on the preferred orientation of polycrystalline aluminum nitride thin films. Aluminum nitride films were grown by rf reactive sputtering of an aluminum target in an N2/Ar gas mixture for different values of the deposition parameters: total pressure, nitrogen content in the discharge gas, and substrate bias voltage. The preferred orientation of the films was analyzed by x-ray diffraction. Films with different preferred orientations were obtained, ranging from c-axis oriented films to films with the c axis tilted by up to 61.6° from the substrate normal. The different mechanisms influencing the preferred orientation of the films have been considered, especially the transfer of energy to the adatoms on the substrate by particle bombardment. An analysis of the relation between the deposition parameters and the crystal orientation has allowed us to determine the relative importance of the different particles in the supply of energy to the substrate. We have found that Ar ion bombardment of the film during growth is the most influential mechanism on the preferred orientation of the films. As bombardment becomes more energetic, microcrystals in the film tend to grow with the c axis along the surface normal. The energy of Ar bombardment can be best controlled through the substrate bias voltage, a characteristic that we have employed to obtain AlN films exhibiting pure (00.2) preferred orientation and rocking curves with a full width at half maximum as low as 4.2°.
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