Abstract
We present a combined experimental and theoretical study of the electronic properties of close-spaced dangling-bond (DB) pairs in a hydrogen-passivated Si(001):H p-doped surface. Two types of DB pairs are considered, called “cross” and “line” structures. Our scanning tunneling spectroscopy (STS) data show that, although the spectra taken over different DBs in each pair exhibit a remarkable resemblance, they appear shifted by a constant energy that depends on the DB-pair type. This spontaneous asymmetry persists after repeated STS measurements. By comparison with density functional theory (DFT) calculations, we demonstrate that the magnitude of this shift and the relative position of the STS peaks can be explained by distinct charge states for each DB in the pair. We also explain how the charge state is modified by the presence of the scanning tunneling microscopy (STM) tip and the applied bias. Our results indicate that, using the STM tip, it is possible to control the charge state of individual DBs in complex structures, even if they are in close proximity. This observation might have important consequences for the design of electronic circuits and logic gates based on DBs in passivated silicon surfaces.
Highlights
In this paper we study close-spaced DB pairs (i.e., DBs located in nearest-neighbor Si dimers) created with atomic precision on a moderately p-doped silicon substrate
We have demonstrated that STS spectra of close-spaced DB pairs at our p-doped Si(001):H surface exhibit a remarkable resemblance but appear shifted relative to each other by a constant energy that depends on the relative positions of those DBs
This spontaneous and robust asymmetry persists over extended periods of time—it can be repeatedly probed by STS measurement without being changed
Summary
In this paper we study close-spaced DB pairs (i.e., DBs located in nearest-neighbor Si dimers) created with atomic precision on a moderately p-doped silicon substrate. The STS spectra taken on different DBs of the pair are almost identical, except for a constant energy shift whose magnitude depends on the pair type This is to the best of our knowledge the first focused study of pairs of nearest-neighbor DBs. Schofield et al.[20] reported STS measurements for different DB structures in n-doped Si(001):H. These DB-dimers behave quite distinctly from the pairs of the more separate DBs considered here They exhibit a very different electronic structure, with DB-derived states much closer to the edges of valence and conduction bands. They undergo high frequency fluctuations during STM imaging, a feature which is not observed for the systems studied here. In order to understand the persistent, but moderate asymmetry of the experimental dI/dV curves, it is necessary to invoke two ingredients: (i) the ground state of the formed DB pairs in our p-doped substrate being inherently asymmetric, with one of the DB always positively charged; (ii) the easy transition of the charge state of those positive DBs to a neutral charge state under the influence of the proximity of the STM tip
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