  Surface defects created and probed with scanning tunneling microscopes are a
promising platform for atomic-scale electronics and quantum information
technology applications. Using first-principles calculations we demonstrate how
to engineer dangling bond (DB) defects on hydrogenated Si(100) surfaces, which
give rise to isolated impurity states that can be used in atomic-scale devices.
In particular we show that sample thickness and biaxial strain can serve as
control parameters to design the electronic properties of DB defects. While in
thick Si samples the neutral DB state is resonant with bulk valence bands,
ultrathin samples (1-2 nm) lead to an isolated impurity state in the gap;
similar behavior is seen for DB pairs and DB wires. Strain further isolates the
DB from the valence band, with the response to strain heavily dependent on
sample thickness. These findings suggest new methods for tuning the properties
of defects on surfaces for electronic and quantum information applications.
Finally, we present a consistent and unifying interpretation of many results
presented in the literature for DB defects on hydrogenated silicon surfaces,
rationalizing apparent discrepancies between different experiments and
simulations.
