Abstract
It has been a long-standing puzzle why buckled dimers of the Si(001) surface appeared symmetric below ~20 K in scanning tunneling microscopy (STM) experiments. Although such symmetric dimer images were concluded to be due to an artifact induced by STM measurements, its underlying mechanism is still veiled. Here, we demonstrate, based on a first-principles density-functional theory calculation, that the symmetric dimer images are originated from the flip-flop motion of buckled dimers, driven by quantum tunneling (QT). It is revealed that at low temperature the tunneling-induced surface charging with holes reduces the energy barrier for the flipping of buckled dimers, thereby giving rise to a sizable QT-driven frequency of the flip-flop motion. However, such a QT phenomenon becomes marginal in the tunneling-induced surface charging with electrons. Our findings provide an explanation for low-temperature STM data that exhibits apparent symmetric (buckled) dimer structure in the filled-state (empty-state) images.
Highlights
Over the last 30 years the atomic and electronic structures of the Si(001) surface have been extensively investigated because of the fundamental building block for the fabrication of electronic devices as well as for the prototypical model system of semiconductor surfaces[1,2,3,4,5,6,7,8]
To account for the symmetric dimer images observed from low-temperature scanning tunneling microscopy (STM) experiments[15,16,17,21], we investigate the flip-flop motion of buckled dimers driven by either thermal activation[32] or quantum tunneling
We present a simple picture of the quantum tunneling (QT)-driven flip-flop motion of buckled dimers with a double-well potential, we believe that it captures the microscopic mechanism underlying low-temperature symmetric-dimer STM images, as explained above
Summary
Over the last 30 years the atomic and electronic structures of the Si(001) surface have been extensively investigated because of the fundamental building block for the fabrication of electronic devices as well as for the prototypical model system of semiconductor surfaces[1,2,3,4,5,6,7,8]. At room temperature scanning tunneling microscopy (STM) experiments showed symmetric dimer images because of a thermally activated flip-flop motion of buckled dimers. Such apparent symmetric dimer images disappear below ~120 K13, forming either the c(4 × 2) structure [see Fig. 1(a)] consisting of alternatively buckled dimers along and perpendicular to the dimer rows or the p(2 × 2) one with alternatively buckled dimers along the dimer rows. Mitsui and Takayanagi[16] found that at 65 K higher tunneling currents increase the area of symmetric dimer images and the flip-flop rate of buckled dimers regardless of the polarity of the bias voltage
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