(36e) Electric Field Assisted Oxidation and Oxide Growth On Metal Surfaces

Ultra-thin oxides have emerged to be of extra-ordinary interest in a wide range of problems in condensed matter physics, physical chemistry and solid-state device technologies. Examples include but are not limited to tunnel barriers in spin-based electron devices ^{1, 2}, nanoscale catalysts ³ and 2-D correlated electron systems ^{4, 5}. Given the structural and compositional complexity of oxide materials, it is not surprising that both the passive state and functional properties of these ultrathin oxides are strongly correlated to the defects which in turn is dramatically influenced by the synthesis conditions ⁶. For example, the formation of point defects such as oxygen vacancies depends both on thermodynamics as well as growth kinetics, particularly in non-equilibrium processing ⁷. Oxygen point defects play a fundamental role in determining the physical and chemical properties of complex oxides and their interfaces. One of the approaches to tune oxide non-stoichiometry at room temperature involves the use of electric field to stimulate oxide growth beyond that possible through thermal diffusion ^{8, 9}. Atomistic simulations employing dynamic charge transfer between atoms are used to investigate ultra-thin oxide growth on Al(100) metal substrates in the presence of an ac electric field. In the range of 1-10 GHz frequencies, the enhancement in oxidation kinetics by ~12% over natural oxidation can be explained by the Cabrera-Mott mechanism. At field frequencies approaching 0.1-1 THz, however, we observe a dramatic lowering of the kinetics of oxygen incorporation by ~ 35% compared to the maximum oxidation achieved, resulting in oxygen non-stoichiometry near the oxide-gas interface (O/Al ~ 1.0), which is attributed to oxygen desorption from the oxide surface. These results suggest a general strategy to tune oxygen concentration at oxide surfaces using ac electric fields that could be of great interest in diverse topics such as complex oxide hetero-interfaces, tunnel barriers and gate dielectrics.

 

 

References

 

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