Anisotropic diffusion between the step-up and the step-down directions on a Si(001) surface

When a Si(001) substrate is heated by passing a direct current through it, the electromigration of Si atoms occurs. The activation energies of Si migration on the surface were investigated by a reflection electron microscope. In the growth process of the 2 x 1 terrace, Si atoms are released from the S-b substrate steps in the step-up direction, and the activation energy is determined to be about 1.60 eV. In the growth process of the 1 x 2 terrace, Si atoms are released from the S-a substrate steps in the step-down direction and the activation energy is about 1.52 eV. Here, the front edge of the 2 x 1 terrace is the S-a step parallel to the dimer and the front edge of the 1 x 2 terrace is the S-b step perpendicular to it. In electromigration the difference in the activation energies is caused by the difference in the release energies from steps. The release energy of Si atoms from the S-b step in the step-up direction is approximately 0.08 eV larger than the release energy of them from the S-a step in the step-down direction. Si atoms are easily released from steps in the step-down direction on the surface. However, there is the difference in the frequency factors; the frequency factor from S-b steps in the step-up direction is about twice as large as the frequency factor from S-a steps in the step-down direction.