Search

Control of dopants in silicon remains the most important approach to tailoring the properties of electronic materials for integrated circuits, with Group V impurities the most important n-type dopants. At the same time, silicon is finding new applications in coherent quantum devices, thanks to the magnetically quiet environment it provides for the impurity orbitals. The ionization energies and the shape of the dopant orbitals depend on the surfaces and interfaces with which they interact. The location of the dopant and local environment effects will therefore determine the functionality of both future quantum information processors and next-generation semiconductor devices. Here we match observed dopant wavefunctions from low-temperature scanning tunnelling microscopy (STM) to images simulated from first-principles density functional theory (DFT) calculations. By this combination of experiment and theory we precisely determine the substitutional sites of neutral As dopants between 5 and 15A below the Si(001):H surface. In the process we gain a full understanding of the interaction of the donor-electron state with the surface, and hence of the transition between the bulk dopant (with its delocalised hydrogenic orbital) and the previously studied dopants in the surface layer., 12 pages; accepted for publication in Phys. Rev. B

We characterize the incorporation of Ba adatoms into the Ge(001) surface, resulting in the formation of one-dimensional structures with an internal 2×3 periodicity, after the deposition of Ba atoms at 970 K or at room temperature followed by a 770 K anneal. Scanning tunneling microscopy (STM) data were compared with theoretically simulated STM images generated by density functional theory electronic structure calculations. Excellent agreement between experiment and simulation was found when using an adopted structural model that assumes partial removal of the surface Ge dimers in the [1–10] surface direction and subsequent addition of a single Ba atom to the substrate second layer. Structural assignments for a number of defects observed within regions of the 2×3 reconstruction were also obtained.

An ordered alkaline-earth submonolayer on a clean $\mathrm{Si}(001)$ surface provides a template for growth of the atomically sharp, crystalline Si-oxide interface that is ubiquitous in the semiconductor device industry. It has been suggested that submonolayers of Sr or Ba on $\mathrm{Ge}(001)$ could play a similar role as on structurally identical $\mathrm{Si}(001)$, overcoming known limitations of the $\mathrm{Ge}(001)$ substrate such as amorphization of its oxidation layers. In this paper the initial stage of the Ba oxidation process, i.e., adsorption and organization of Ba atoms on the $\mathrm{Ge}(001)$ surface as a function of temperature $(270\ensuremath{-}770\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ for coverage 1.0 monolayer (ML) and $0.15\ensuremath{-}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$, is studied using scanning tunneling microscopy (STM) and density functional theory (DFT). Three types of features have been identified on the Ba-covered $\mathrm{Ge}(001)$ surface. They originate from isolated Ba adatoms, isolated Ba ad-dimers, and the Ba ad-dimers assembled into short-range, randomly distributed chains that run across the Ge dimer rows. We find from both STM measurements and DFT calculations that the latter is the dominant structure on $\mathrm{Ge}(001)$ with increasing coverage.

The extended Brenner potential has been shown to provide a good description of the (001) and (111) surfaces of silicon. In this paper, this empirical potential is employed to study the energetics of steps on the Si(001)2x1 surface. Particular attention has been paid to the dependence of the step energies on the width of the terraces and the number of substrate layers. Formation energies for both the single-layer S-A and S-B steps, and the double layer D-A and D-B steps, have been determined. All of the formation energies are found to be negative. Values of the step-step and surface stress interaction energy coefficients are also determined. The correlation of these results with experiment and previous theoretical calculations is discussed.

Using the acetic acid/Si(001) system as an illustrative example, we discuss the limits and opportunities of "chemical analogy" as a paradigm to rationalize chemisorption processes on surfaces. Recent proposals that acetic acid chemisorption results in a bidentate, single-dehydrogenated product are based on earlier findings for the acetic acid/Ge(001) system. In contrast, the well-characterized reaction of acetone with Si( 001) suggests that acetic acid chemisorption leads to the loss of two hydrogen atoms from the molecule. Density-functional calculations resolve this ambiguity, finding the latter structure model to be thermodynamically preferred and kinetically viable.

Strain induced by antiphase boundaries (APBs) in the Si(111) 2 x 1 surface is investigated using scanning tunneling microscopy (STM), laterally resolved scanning tunneling spectroscopy (STS), and density functional theory (DFT). We determine the structure of all identified APB reconstructions and show that a band shift of states close to the Fermi energy leads to the previously observed electronic contrast. The orientation of the band shift and the observed movement of APBs within the surface are explained by surface strain resulting from the excess free energy of the boundary. We demonstrate that the location of APBs and their associated strain can be precisely manipulated, making them an ideal model system to study and control strain at the atomic scale.

The recent literature has now firmly attributed the common C defect on the Si(001) surface to the dissociative adsorption of water. This work, by examining the dynamical properties of the C defect at elevated temperatures (450 K), establishes the missing mechanistic link between. dissociative water adsorption and wet surface oxidation. Scanning tunneling microscopy and density functional theory in combination reveal in detail the various paths by which a water molecule breaks apart on the surface and inserts oxygen atoms into the surface.

Chemisorption of a single hydrogen atom on the $n$-type Si(001) surface is investigated by scanning tunneling microscopy (STM) and first-principles density functional theory (DFT) calculations. The STM experiments show that the formation of a hemihydride induces static buckling of the neighboring Si-Si dimers and suggest that different buckling configurations of these dimers are observed at negative and positive biases. They also show that the appearance of an isolated Si-Si-H hemihydride on Si(001) exhibits a complex voltage dependence with the brightness of the dangling bond of the hemihydride changing significantly at negative sample bias. DFT calculations predict two stable, ground state atomic configurations for the hemihydride on Si(001). These correspond to parallel and antiparallel configurations of the Si-Si-H hemihydride with respect to the neighboring bare Si-Si dimers. In order to understand the bias-dependent appearance in the STM images of the $n$-type Si(001) surface, we include the effect of hemihydride charging due to tip-induced band bending. In filled state, the STM images are shown to result from the electronic and structural features that originate from the charge-dependent parallel configuration. In empty state, the energetics and STM measurements support the charge independent antiparallel configuration, while either structure can produce simulated images consistent with experiment.

It was recently shown that low-coverage $\mathrm{P}{\mathrm{H}}_{3}$ dosing of the Si(001) surface is fully dissociative at room temperature with $\mathrm{P}{\mathrm{H}}_{2}+\mathrm{H}$, $\mathrm{P}\mathrm{H}+2\mathrm{H}$, and $\mathrm{P}+3\mathrm{H}$ as intermediate species. Here, we consider high-coverage $\mathrm{P}{\mathrm{H}}_{3}$ dosing and show that the increased density of adsorbates leads to qualitatively different behavior due to competition between thermal dissociation and desorption. Using a combination of existing temperature-programmed desorption data and density functional theory simulations, we present a detailed mechanistic understanding of phosphine adsorption, dissociation, and desorption on the surface. This understanding provides a consistent interpretation of existing infrared and x-ray spectroscopic data, as well as an explanation of the dependence of the phosphorus saturation coverage on dosing conditions.

We present a detailed voltage-dependent scanning tunneling microscopy study of a single phosphorus atom in the Si(001) surface. Using density functional theory calculations we show that tip-induced charging results in reversible structural and electronic changes. These changes are caused by charge transfer from delocalized surface states to localized states associated with the presence of the phosphorus atom. While two stable geometric configurations are predicted, only the higher energy configuration is consistent with experiment.

We have used scanning tunneling microscopy to identify phosphorus that is present at the clean silicon $(100)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ surface as a result of the thermal cycling necessary for preparation of samples cut from heavily doped wafers. Substitutional phosphorus is observed in top layer sites as buckled $\mathrm{Si}\text{\ensuremath{-}}\mathrm{P}$ heterodimers. We also observe a second type of feature that appears as a single depressed dimer site. Within this site, the atoms appear as a pair of protrusions in the empty states and a single protrusion in the filled states. These properties are not consistent with known adsorbate signatures or previously reported observations of $\mathrm{P}\text{\ensuremath{-}}\mathrm{P}$ dimers on the $(100)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ surface. The lack of other impurity sources suggests that they are due to either phosphorus or silicon. The symmetry of the features and their magnitude are consistent with one of those elements residing in an interstitial site just below the top layer of atoms. To identify the type of interstitial, we performed density functional theory calculations for both phosphorus and silicon located below a surface dimer. The resulting charge density plots and simulated STM images are consistent with interstitial phosphorus and not interstitial silicon.

We report a comprehensive ab initio survey of possible dissociation intermediates of phosphine $({\mathrm{PH}}_{3})$ on the Si(001) surface. We assign three scanning tunneling microscopy (STM) features, commonly observed in room-temperature dosing experiments, to ${\mathrm{PH}}_{2}+\mathrm{H}$, $\mathrm{PH}+2\mathrm{H}$, and $\mathrm{P}+3\mathrm{H}$ species, respectively, on the basis of calculated energetics and STM simulation. These assignments and a time series of STM images which shows these three STM features converting into another, allow us to outline a mechanism for the complete dissociation of phosphine on the Si(001) surface. This mechanism closes an important gap in the understanding of the doping process of semiconductor devices.

Dimer vacancy (DV) defect complexes in the Si(001)2×1 surface are investigated using high-resolution scanning-tunneling microscopy and first-principles calculations. We find that under low-bias filled-state tunneling conditions, isolated “split-off” dimers in these defect complexes are imaged as pairs of protrusions, while the surrounding Si surface dimers appear as the usual “bean-shaped” protrusions. We attribute this to the formation of π-bonds between the two atoms of the split-off dimer and second-layer atoms, and present charge density plots to support this assignment. We observe a local brightness enhancement due to strain for different DV complexes and provide the first experimental confirmation of an earlier prediction that the 1+2-DV induces less surface strain than other DV complexes. Finally, we present a previously unreported triangular shaped split-off dimer defect complex that exists at S-type step edges, and propose a structure for this defect involving a bound Si monomer.

The quest to build a quantum computer has been inspired by the recognition of the formidable computational power such a device could offer. In particular silicon-based proposals, using the nuclear or electron spin of dopants as qubits, are attractive due to the long spin relaxation times involved, their scalability, and the ease of integration with existing silicon technology. Fabrication of such devices however requires atomic scale manipulation - an immense technological challenge. We demonstrate that it is possible to fabricate an atomically-precise linear array of single phosphorus bearing molecules on a silicon surface with the required dimensions for the fabrication of a silicon-based quantum computer. We also discuss strategies for the encapsulation of these phosphorus atoms by subsequent silicon crystal growth., Comment: To Appear in Phys. Rev. B Rapid Comm. 5 pages, 5 color figures