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The coverage-dependent behavior of p-methoxyacetophenone on the clean Si(001) surface was followed using X-ray photoelectron spectroscopy and supporting density functional theory calculations. Unlike other multifunctional organic molecules, this compound exhibits a high selectivity of adsorbate species formation by forming only two distinct adsorbate structures at low coverage, with a third configuration forming at high coverages. At low coverage, surface chemisorption is driven by methoxy group dissociation. However, at high coverage, the surface footprint required for this process is no longer available, leading to the formation of less thermodynamically stable adsorbates that are datively bonded to the surface with a smaller footprint. This coverage-dependent but well-defined behavior is promising in designing functional organic–inorganic interfaces on silicon.

The adsorption configurations of a technologically relevant model organic adsorbate on the silicon (001) surface were studied using energy scanned x-ray photoelectron diffraction (PhD). Previous work has established the existence of an interesting vertically-aligned (‘flagpole’) configuration, where the acetophenone attaches to Si(001) via the acetyl group carbon and oxygen atoms. Density functional theory calculations have predicted two energetically similar variants of this structure, where the phenyl ring is orientated parallel or perpendicular to the rows of silicon dimers on this reconstructed surface. However, previously published experimental measurements, including scanning tunnelling microscopy, x-ray photoelectron spectroscopy, and near-edge x-ray absorption fine structure investigations were unable to distinguish between these two configurations. Here, we apply the unique experimental capabilities of the PhD technique to this system and demonstrate that the dominant adsorption configuration has the phenyl ring parallel to the dimer rows (the end-bridge structure). This information in turn facilitates the determination of the dominant reaction pathway for acetophenone on Si(001), which has remained elusive until now. Information about subtle preferences in reaction pathways that affect the alignment and orientation of organic adsorbates such as acetophenone on technologically-relevant semiconductor surfaces such as Si(001) is critical for the fabrication of future atomically-precise atomic and molecular-scale electronic devices utilising the organic-silicon interface, and this work demonstrates the unique and complementary capabilities of PhD for providing this information.

The growth of multi-layer germanium-tin (GeSn) quantum wells offers an intriguing pathway towards the integration of lasers in a CMOS platform. An important step in growing high quality quantum well interfaces is the formation of an initial wetting layer. However, key atomic-scale details of this process have not previously been discussed. We use scanning tunneling microscopy combined with density functional theory to study the deposition of Sn on Ge(1 0 0) at room temperature over a coverage range of 0.01 to 1.24 monolayers. We demonstrate the formation of a sub-2% Ge content GeSn wetting layer from three atomic-scale characteristic ad-dimer structural components, and show that small quantities of Sn incorporate into the Ge surface forming two atomic configurations. The ratio of the ad-dimer structures changes with increasing Sn coverage, indicating a change in growth kinetics. At sub-monolayer coverage, the least densely packing ad-dimer structure is most abundant. As the layer closes, forming a two-dimensional wetting layer, the more densely packing ad-dimer structure become dominant. These results demonstrate the capability to form an atomically smooth wetting layer at room temperature, and provide critical atomic-scale insights for the optimization of growth processes of GeSn multi-quantum-wells to meet the quality requirements of optical GeSn-based devices.

Over the last two decades, prototype devices for future classical and quantum computing technologies have been fabricated, by using scanning tunneling microscopy and hydrogen resist lithography to position phosphorus atoms in silicon with atomic-scale precision. Despite these successes, phosphine remains the only donor precursor molecule to have been demonstrated as compatible with the hydrogen resist lithography technique. The potential benefits of atomic-scale placement of alternative dopant species have, until now, remained unexplored. In this work, we demonstrate successful fabrication of atomic-scale structures of arsenic-in-silicon. Using a scanning tunneling microscope tip, we pattern a monolayer hydrogen mask to selectively place arsenic atoms on the Si(001) surface using arsine as the precursor molecule. We fully elucidate the surface chemistry and reaction pathways of arsine on Si(001), revealing significant differences to phosphine. We explain how these differences result in enhanced surface immobilization and in-plane confinement of arsenic compared to phosphorus, and a dose-rate independent arsenic saturation density of $0.24{\pm}0.04$ ML. We demonstrate the successful encapsulation of arsenic delta-layers using silicon molecular beam epitaxy, and find electrical characteristics that are competitive with equivalent structures fabricated with phosphorus. Arsenic delta-layers are also found to offer improvement in out-of-plane confinement compared to similarly prepared phosphorus layers, while still retaining >80% carrier activation and sheet resistances of $

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

The attachment of organic molecules to semiconductor surfaces and their measurement using scanning tunneling microscopy and spectroscopy (STM/STS) is considered to be a potential route toward conductance measurements of single molecules with known structural and electronic configuration. Here, we investigate a model system—acetophenone on Si(001)—and demonstrate that this adsorbate can be manipulated using the STM such that it adopts a configuration where it stands upright with a free-standing phenyl ring and strong Si–O linkage to the substrate. For the structural identification we combine STM imaging with density functional theory (DFT) calculations to describe the adsorbate structures that were observed in our experiments and the reaction pathways that link them; of these the upright configuration is the most thermodynamically stable. The chemical structure of the upright configuration suggests that π-conjugation within the adsorbate extends to the silicon surface resulting in strong hybridization of t...

In this work we combine scanning tunneling microscopy, near-edge X-ray absorption fine structure spectroscopy, X-ray photoemission spectroscopy and density functional theory to resolve a long-standing confusion regarding the adsorption behaviour of benzonitrile on Si(001) at room temperature. We find that a trough-bridging structure is sufficient to explain adsorption at low coverages. At higher coverages when steric hindrance prevents the phenyl ring lying flat on the surface, the 2+2 cycloaddition structure dominates.

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.

Quantum confinement effects on the electronic structure of thin Ge(001) slabs with one clean and one H-terminated surface are discussed based on density functional theory calculations for periodic ...

The adsorption of acetone [(CH 3 ) 2 CO] on the Si(0 0 1) surface has been investigated using density functional theory (DFT) and scanning tunneling microscopy (STM). Three distinct features have been observed in the experimental STM images – one single-dimer and two two-dimer wide features. We have considered 32 different possible adsorbate configurations corresponding to the [2 + 2] cycloaddition, dative bonded, α-Hydrogen cleavage, C H bond cleavage, methyl cleavage, cross-dimer bridging, O-dimer insertion, OH cleavage, alcohol-like, end-bridging and dimer-bridging structures. The optimised geometry, binding energy, and simulated filled- and empty-state STM images for each of these possible configurations are presented. These results have facilitated the identification of the three experimentally-observed acetone adsorbate structures.

We present the results of a combined study using scanning tunneling microscopy (STM) and density functional theory (DFT) of the interaction of acetone [(CH3)2CO] with the Si(0 0 1) surface. Three distinct adsorbate features were observed using atomic-resolution STM. One of the features appears as a bright protrusion located above a Si–Si dimer, while the other two are asymmetric about the dimer row and involve a second neighboring Si–Si dimer. One of the two asymmetric features has a protrusion located between the two dimers, while the other has a protrusion which is located at the site of a single dimer and exhibits a dimer sized depression on the adjacent dimer. DFT calculations have been performed for two structures; the four-membered ring structure and dissociation structure. Our calculations show that the bright single-dimer sized feature observed in the STM images could be attributed to either of these two calculated structures. However, neither of the two calculated structures can explain the appearance of the two-dimer wide asymmetric features observed in the experiment.

The ability to covalently attach organic molecules to semiconductor surfaces in a controllable and selective manner is currently receiving much attention due to the potential for creating hybrid silicon-organic molecular-electronic devices. Here we use scanning tunneling microscopy (STM) and density functional theory calculations to study the adsorption of a simple ketone [acetone; (CH(3))(2)CO] to the silicon (001) surface. We show both bias and time-dependent STM images and their agreement with total energy DFT calculations, simulated STM images, and published spectroscopic data. We investigate the stability of the resulting adsorbate structures with respect to temperature and applied STM tip bias and current. We demonstrate the ability to convert from the kinetically favored single-dimer alpha-H cleavage adsorbate structure to thermodynamically favored bridge-bonded adsorbate structures. This can be performed for the entire surface using a thermal anneal or, for individual molecules, using the highly confined electron beam of the STM tip. We propose the use of the carbonyl functional group to tether organic molecules to silicon may lead to increased stability of the adsorbates with respect to current-voltage characterization. This has important implications for the creation of robust single-molecule devices.

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.

We report that solitary bismuth and antimony atoms, incorporated at Si(111) surfaces, induce either positive or negative charge states depending on the site of the surface reconstruction in which they are located. This is in stark contrast to the hydrogenic donors formed by group V atoms in silicon bulk crystal and therefore has strong implications for the design and fabrication of future highly scaled electronic devices. Using scanning tunnelling microscopy (STM) and density functional theory (DFT) we determine the reconstructions formed by different group V atoms in the Si(111)2 × 1 surface. Based on these reconstructions a model is presented that explains the polarity as well as the location of the observed charges in the surface. Using locally resolved scanning tunnelling spectroscopy we are furthermore able to map out the spatial extent over which a donor atom influences the unoccupied surface and bulk electronic states near the Fermi-level. The results presented here therefore not only show that a dopant atom can induce both positive and negative charges but also reveal the nature of the local electronic structure in the region of the silicon surface where an individual donor atom is present.

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 adsorption of isolated H atoms on the Ge(001) surface is studied using density functional theory (DFT) and scanning tunneling microscopy (STM). Two stable adsorption positions that are found in DFT correspond to H atom attachment to an up-or down-buckled Ge dimer atom, respectively. Surprisingly, in the case where H bonds to the down-buckled Ge atom, we find that there is a redistribution of a unit of charge which leaves the net charge of the doubly occupied dangling bond of the unreacted Ge atom intact. This configuration is found to be the more stable of the two structures. Comparison to filled- and empty-state STM images confirms that this lowest energy structure is observed at room temperature. These results represent a fundamentally different picture of the physics and chemistry of H adsorption to Ge(001) compared with previous work.

Using density functional theory, we report detailed reaction path calculations for the reaction of acetone with the silicon (001) surface. We identify the key reaction intermediates of dissociative adsorption and the transition states between them. This resolves the identity of the one-dimer intermediate observed in STM experiments and its role in the formation of several two-dimer-wide end products of dissociation. Key to the understanding of the dissociation mechanism is the ambiphilic character of the two reactants, that is the simultaneous expression of electrophilic and nucleophilic reactivities in both the surface and the acetone molecule.

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.

Using density functional theory and guided by extensive scanning tunneling microscopy (STM) image data, we formulate a detailed mechanism for the dissociation of phosphine (PH3) molecules on the Si(001) surface at room temperature. We distinguish between a main sequence of dissociation that involves PH2+H, PH+2H, and P+3H as observable intermediates, and a secondary sequence that gives rise to PH+H, P+2H, and isolated phosphorus adatoms. The latter sequence arises because PH2 fragments are surprisingly mobile on Si(001) and can diffuse away from the third hydrogen atom that makes up the PH3 stoichiometry. Our calculated activation energies describe the competition between diffusion and dissociation pathways and hence provide a comprehensive model for the numerous adsorbate species observed in STM experiments.

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.

A detailed understanding of the chemical reactions of organic molecules with semiconductor surfaces will greatly aid schemes for the incorporation of organic functionality into existing technologies. In this paper we report on the reaction of acetaldehyde ( CH 3 CHO ) with silicon (001) as revealed by a combination of temperature-dependent scanning tunneling microscopy(STM) experiments and density functional theory(DFT). We observe that low-coverage exposures at room temperature result almost exclusively in the formation of a single adsorbate species. Conversion of this structure into thermodynamically favored bridge-bonded structures is achieved through temperature anneals between 150 – 250 ° C . We determine the chemical identity of each of the experimentally observed species by comparison with DFT total energy calculations and simulated STM images. Calculations of transition states are used to formulate a full reaction pathway explaining the formation of the observed species. Excellent agreement is found between our experimental measurements and theoretical calculations. The results also present a picture consistent with our previous work on acetone and reveal a general reaction pattern for molecules containing the acetyl COCH 3 functional group, where the initial attachment to the surface is mediated by a carbonyl C = O group. This suggests that modification of the residue R will facilitate in binding other electronically active molecules to the surface in a controlled fashion.

The properties of an isolated dangling bond formed by the chemisorption of a single hydrogen atom on a dimer of the Ge(001) surface are investigated by first-principles density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements. Two stable atomic configurations of the Ge-Ge-H hemihydride with respect to the neighboring bare Ge-Ge dimers are predicted by DFT. For both configurations, the unpaired electron of the HGe(001) system is found to be delocalized over the surface, rendering the isolated dangling bond of the hemihydride unoccupied. However, local surface charge accumulation, such as may occur during STM imaging, leads to the localization of two electrons onto the hemihydride dangling bond. The calculated surface densities of states for one of the charged Ge-Ge-H hemihydride configurations are found to be in good agreement with atomic-resolution STM measurements on n-type Ge(001). Comparison with a Si-Si-H hemihydride of the Si(001) surface shows similarities in structural properties, but substantial differences in electronic properties.

Using first-principles density functional theory, we discuss doping of the Si(001) surface by a single substitutional phosphorus or arsenic atom. We show that there are two competing atomic structures for isolated Si-P and Si-As heterodimers, and that the donor electron is delocalized over the surface. We also show that the Si atom dangling bond of one of these heterodimer structures can be progressively charged by additional electrons. It is predicted that surface charge accumulation as a result of tip-induced band bending leads to structural and electronic changes of the Si-P and Si-As heterodimers which could be observed experimentally. Scanning tunneling microscopy (STM) measurements of the Si-P heterodimer on a n-type Si(001) surface reveal structural characteristics and a bias-voltage dependent appearance, consistent with these predictions. STM measurements for the As:Si(001) system are predicted to exhibit similar behavior to P:Si(001).