Fig:(top) Control of the alignment of the O2 molecular axis by the magnetic field direction and (bottom) time evolution of the O2 adsorption probability on the Si(100) surface, showing that the reaction probability changes greatly when the O2 alignment is changed in accordance with a control signal.
research group consisting of Dr. Mitsunori Kurahashi, Principal
Researcher, and Dr. Yasushi Yamauchi, Group Leader of the Spin
Characterization Group, both of the Nano Characterization Unit of the
National Institute for Materials Science, developed the world’s first O2
molecular beam which enables us to designate the alignment of the
molecular axis and spin direction. The NIMS researchers applied this
beam to the surface oxidation of silicon, and discovered that only
oxygen molecules with the molecular axis nearly parallel to the surface
contribute to the silicon oxidation.
oxygen (O2) is one of the most important chemical species in virtually
all fields of fundamental science and materials development. O2 has an
anisotropic shape, i.e., is a linear molecule, and possesses spin
originating from two unpaired electrons. However, until now, there have
been no experimental methods which enable us to investigate how the
shape and spin of an O2 molecule influence oxidation reactions. Even
though the initial oxidation of silicon has been studied in detail to
understand the thermal oxidation of silicon used for fabricating gate
insulator films, the origin of the particularly low initial reaction
probability has not been understood well.
the magnetic hexapolar field technique, the team headed by Dr.
Kurahashi developed the world’s first single quantum state-selected O2
beam which enables us to designate both the alignment of the molecular
axis and the spin direction. By applying this beam to the surface
oxidation of silicon, Dr. Kurahashi’s team discovered that only O2
molecules with the molecular axis nearly parallel to the surface
contribute to the silicon oxidation. This research clarified that the
silicon oxidation is inefficient due to the stringent geometrical
requirement for the O2 axis direction, and only those molecules that
satisfy a certain angular condition can participate in the reaction.
research established a new experimental technique for analyzing the
effects of the O2 alignment and the spin direction on oxidation
reactions, and elucidated the origin of the inefficiency in the silicon
oxidation. This technique not only enables us to scrutinize the
mechanism of oxidation, but also opens up a possibility to control
reactions or create new high quality materials by controlling the O2
alignment and/or spin direction.
research results were achieved as part of the NIMS 3rd Mid-Term Program
Project “Development and Application of Advanced Material
Characterization Technologies” (Leader: Daisuke Fujita) and the Japan
Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific
Research/Basic Research (B) “Elucidation of Spin/Steric Effect in
Surface Oxidation using Single Quantum State-Selected Triplet Oxygen
Molecular Beam” (Research Representative: Mitsunori Kurahashi). These
results were published on April 19 in the online edition of Physical Review B (Rapid Communication) of the American Physical Society.