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We investigate the adsorption and thermal decomposition of triphenyl bismuth (TPB) on the silicon (001) surface using atomic-resolution scanning tunneling microscopy, synchrotron-based X-ray photoelectron spectroscopy, and density functional theory calculations. Our results show that the adsorption of TPB at room temperature creates both bismuth–silicon and phenyl–silicon bonds. Annealing above room temperature leads to increased chemical interactions between the phenyl groups and the silicon surface, followed by phenyl detachment and bismuth subsurface migration. The thermal decomposition of the carbon fragments leads to the formation of silicon carbide at the surface. This chemical understanding of the process allows for controlled bismuth introduction into the near surface of silicon and opens pathways for ultra-shallow doping approaches.

Over the past 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 the 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 ± 0.04 monolayers. 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 confinement as good as similarly prepared phosphorus layers, while still retaining >80% carrier activation and sheet resistances of <2 kΩ/square. These excellent characteristics of arsenic represent opportunities to enhance existing capabilities of atomic-scale fabrication of dopant structures in silicon, and may be important for three-dimensional devices, where vertical control of the position of device components is critical.

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.

This study investigates the adsorption and thermal transformations of a bicyclic tertiary amine, triethylenediamine, on the clean Si(100)-2 × 1 surface. Below room temperature, triethylenediamine adsorption leads to the formation of a strong dative bond between one of the nitrogen atoms of this compound and the silicon surface. In contrast to previously studied amines, the datively adsorbed triethylenediamine features a second tertiary amine entity that is not bonded to the surface, with a lone pair orbital that is directed away from the surface and is available for further reactions. The thermal chemistry and electronic properties of triethylenediamine on silicon are studied using thermal desorption spectroscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy. Near-edge X-ray absorption fine structure measurements are utilized to clarify the geometry of the adsorbates at room temperature. Density functional theory calculations are used to describe the binding geometry and electronic properties of the resulting surface species and the likely reaction paths at elevated temperatures.

It is now well established that an isolated C-defect on the Si(001) surface corresponds to a dissociated water molecule with the H and OH species bonded to silicon atoms on the same side of two adjacent surface dimers. In this paper we show that there are two distinct types of such C-defects at both low temperature (∼100 K) and room temperature. These defects differ in the buckling orientation of the Si–Si dimers adjacent to the defect and/or the distribution of their localized electron spins. Changes in the buckling configuration of the bare Si–Si dimers neighboring the defect modifies the interaction between its dangling bonds and the surface and leads to significant variations in the defect charge density distributions.

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 the molecular states with the substrate. This is supported by the absence of any significant features in STS curves recorded over the adsorbate. The structure of this adsorbate and its robust attachment to silicon makes it attractive for future STM/semiconductor-based molecular conductance measurements.

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.

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 slabs. Recent work has shown that while the geometry of the clean Ge(001) surface is reliably reproduced by relatively thin asymmetric slabs, the associated electronic structure can be quite sensitive to the number of layers in the slab. Here we show that the changes in the character and energy of the states near the Fermi energy of such slabs are related to the description of the bulk states and the width of the valence band, both of which are sensitive to the thickness of the slabs. Calculations performed for an isolated H or Cl atom adsorbed on the surface of a Ge(001) slab show that the effect of bulk electron confinement within thin slabs significantly influences the geometries, electronic structures, and relative thermodynamic stability of the single-atom adsorbates and the associated reconstructed surface. By contrast, the corresponding Si(001) surface does not exhibit any significant slab thickness dependence. We believe that these differences are a direct consequence of the nature of the states in the vicinity of the Fermi energy. Our results highlight the importance of employing an appropriate number of layers in slab calculations of the Ge(001) surface and imply that the surface electronic properties of Ge(001) slabs may be manipulated by employing a thin film technology.

We present the discovery of a charge density wave (CDW) ground state in heavily electron-doped molybdenum disulfide (MoS2). This is the first observation of a CDW in any d2(column 6) transition metal dichalcogenide (TMD). The band structure of MoS2is distinct from the d0and d1TMDs in which CDWs have been previously observed, facilitating new insight into CDW formation. We demonstrate a metal–insulator transition at 85 K, a 25 meV gap at the Fermi level, and two distinct CDW modulations, (2√3 × 2√3) R30° and 2 × 2, attributable to Fermi surface nesting (FSN) and electron–phonon coupling (EPC), respectively. This simultaneous exhibition of FSN and EPC CDW modulations is unique among observations of CDW ground states, and we discuss this in the context of band folding. Our observations provide a route toward the resolution of controversies surrounding the origin of CDW modulations in TMDs.

Nominally pure black phosphorus (BP) is commonly found to be a p-type semiconductor, suggesting the ubiquitious presence of impurity species or intrinsic, charged defects. Moreover, scanning tunneling microscopy (STM) images of black phosphorus reveal the presence of long-range double-lobed defect features superimposed onto the surface atomic lattice. We show that both the p-type doping of BP and the defect features observed in STM images can be attributed to substitutional tin impurities. We show that black phosphorus samples produced through two common synthesis pathways contain tin impurities, and we demonstrate that the ground state of substitutional tin impurities is negatively charged for a wide range of Fermi level positions within the BP band gap. The localized negative charge of the tin impurities induces hydrogenic states in the band gap, and it is the 2p level that sits at the valence band edge that gives rise to the double-lobed features observed in STM images.