Surface doping and stabilization of Si(111) with boron

Precise determination of molecular adsorption geometries by room temperature non-contact atomic force microscopy

High resolution force measurements of molecules on surfaces, in non-contact atomic force microscopy, are often only performed at cryogenic temperatures, due to needing a highly stable system, and a passivated probe tip (typically via CO-functionalisation). Here we show a reliable protocol for acquiring three-dimensional force map data over both single organic molecules and assembled islands of molecules, at room temperature. Isolated cobalt phthalocyanine and islands of C60 are characterised with submolecular resolution, on a passivated silicon substrate (B:Si(111)-[Formula: see text]). Geometries of cobalt phthalocyanine are determined to a ~ 10 pm accuracy. For the C60, the protocol is sufficiently robust that areas spanning 10 nm × 10 nm are mapped, despite the difficulties of room temperature operation. These results provide a proof-of-concept for gathering high-resolution three-dimensional force maps of networks of complex, non-planar molecules on surfaces, in conditions more analogous to real-world application.

N-Heterocyclic Olefins on a Silicon Surface

The adsorption of N-heterocyclic olefins (NHOs) on silicon is investigated in a combined scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory study. We find that both of the studied NHOs bind covalently, with ylidic character, to the silicon adatoms of the substrate and exhibit good thermal stability. The adsorption geometry strongly depends on the N-substituents: for large N-substituents, an upright adsorption geometry is favored, while a flat-lying geometry is found for the NHO with smaller wingtips. These different geometries strongly influence the quality and properties of the obtained monolayers. The upright geometry leads to the formation of ordered monolayers, whereas the flat-lying NHOs yield a mostly disordered, but denser, monolayer. The obtained monolayers both show large work function reductions, as the higher density of the flat-lying monolayer is found to compensate for the smaller vertical dipole moments. Our findings offer new prospects in the design of tailor-made ligand structures in organic electronics and optoelectronics, catalysis, and material science.

Deciphering the Atomistic Mechanism of Si(111)-7 × 7 Surface Reconstruction Using a Machine-Learning Force Field

While the complex 7 × 7 structure that arises upon annealing the Si(111) surface is well-known, the mechanism underlying this unusual surface reconstruction has remained a mystery. Here, we report molecular dynamics simulations using a machine-learning force field for Si to investigate the Si(111)-7 × 7 surface reconstruction from the melt. We find that there are two possible pathways for the formation of the 7 × 7 structure. The first path arises from the growth of a faulted half domain from the metastable 5 × 5 phase to the final 7 × 7 structure, while the second path involves the direct formation of the 7 × 7 reconstruction. Both pathways involve the creation of dimers and bridged five-membered rings, followed by the formation of additional dimers and the stabilization of the triangular halves of the unit cell. The corner hole is formed from the joining of several five-member rings. The insertion of atoms below the adatoms to form a dumbbell configuration involves extra atom diffusion or rearrangement during the evolution of triangular halves and dimer formation along the unit cell boundary. Our findings may provide insights for manipulating the surface structure by introducing other atomic species.

Controlled growth of ordered monolayers of N-heterocyclic carbenes on silicon

N-Heterocyclic carbenes (NHCs) are promising modifiers and anchors for surface functionalization and offer some advantages over thiol-based systems. Because of their strong binding affinity and high electron donation, NHCs can dramatically change the properties of the surfaces to which they are bonded. Highly ordered NHC monolayers have so far been limited to metal surfaces. Silicon, however, remains the element of choice in semiconductor devices and its modification is therefore of utmost importance for electronic industries. Here, a comprehensive study on the adsorption of NHCs on silicon is presented. We find covalently bound NHC molecules in an upright adsorption geometry and demonstrate the formation of highly ordered monolayers exhibiting good thermal stability and strong work function reductions. The structure and ordering of the monolayers is controlled by the substrate geometry and reactivity and in particular by the NHC side groups. These findings pave the way towards a tailor-made organic functionalization of silicon surfaces and, thanks to the high modularity of NHCs, new electronic and optoelectronic applications.

Local geometry around B atoms in B/Si(1 1 1) from polarized x-ray absorption spectroscopy

The arrangement of B atoms in a doped Si(1 1 1)-[Formula: see text]:B system was studied using a near-edge x-ray absorption fine structure (NEXAFS). Boron atoms were deposited via segregation from the bulk by flashing the sample repeatedly. The positions of B atoms are determined by comparing measured polarized (angle-dependent) NEXAFS spectra with spectra calculated for various structural models based on ab initio total energy calculations. It is found that most of boron atoms are located in sub-surface L[Formula: see text] positions, beneath a Si atom. However, depending on the preparation method a significant portion of B atoms may be located elsewhere. A possible location of these non-L[Formula: see text]-atoms is at the surface, next to those Si atoms which form the [Formula: see text] reconstruction.

Hidden phase in a two-dimensional Sn layer stabilized by modulation hole doping

Semiconductor surfaces and ultrathin interfaces exhibit an interesting variety of two-dimensional quantum matter phases, such as charge density waves, spin density waves and superconducting condensates. Yet, the electronic properties of these broken symmetry phases are extremely difficult to control due to the inherent difficulty of doping a strictly two-dimensional material without introducing chemical disorder. Here we successfully exploit a modulation doping scheme to uncover, in conjunction with a scanning tunnelling microscope tip-assist, a hidden equilibrium phase in a hole-doped bilayer of Sn on Si(111). This new phase is intrinsically phase separated into insulating domains with polar and nonpolar symmetries. Its formation involves a spontaneous symmetry breaking process that appears to be electronically driven, notwithstanding the lack of metallicity in this system. This modulation doping approach allows access to novel phases of matter, promising new avenues for exploring competing quantum matter phases on a silicon platform.

Broken symmetry phases may occur in 2D materials upon doping, yet introducing doping without inducing chemical disorder remains a challenge. Here, the authors use a modulation doping approach that unveils a hidden equilibrium phase involving spontaneous symmetry breaking in a hole-doped Sn bilayer.

Switching the charge state of individual surface atoms at Si(111)-√3 × √3:B surfaces

We show that each surface atom of heavily boron-doped, (111)-oriented silicon with a √3 × √3 reconstruction has electrically switchable two charge states due to the strong electron-lattice coupling at this surface. The structural and electronic properties of the two charge states as well as their energetics are uncovered by employing scanning tunneling microscopy measurements and density functional theory calculations, which reveals that one of the two is a two-electron bound state or surface bipolaron. We also execute the single-atom bit operations on individual surface atoms by controlling their charge states while demonstrating implementation of the atomic scale memory at a silicon surface with an unprecedented recording density.

Molecular chemisorption on passivated and defective boron doped silicon surfaces: a "forced" dative bond

We investigate the adsorption mechanism of a single trans 4-pyridylazobenzene molecule (denoted by PAB) on a doped boron Si(111)√3×√3R30° surface (denoted by SiB) with or without boron-defects, by means of density functional theory calculations. The semiempirical approach proposed by Grimme allows us to take the dispersion correction into account. The role of the van der Waals correction in the adsorption geometries and energies is presented. In particular, two adsorption configurations are electronically studied. In the first one, the molecule is parallel to the surface and interacts with the SiB surface via the -N=N- bond. In the presence of a boron-defect, a Si-N chemical bond between the molecule and the surface is then formed, while electrostatic or/and van der Waals interactions are observed in the defectless surface. In the second adsorption configuration, the molecule presents different orientations with respect to the surface and interacts via the nitrogen atom of the pyridyl part of the PAB molecule. If the molecule is perpendicular to the perfect SiB surface, the lone-pair electrons associated with the heterocyclic nitrogen atom fill the empty dangling bond of a silicon adatom via a dative bond. Finally, in the presence of one boron-defect, the possibility of a "forced" dative bond, corresponding to a chemical bond formation between the PAB molecule and the silicon electron occupied dangling bond, is emphasized.

C60 molecules grown on a Si-supported nanoporous supramolecular network: a DFT study

C60 fullerene assemblies on surfaces have attracted considerable attention because of their remarkable electronic properties. Now because of the competition between the molecules-substrate and the molecule-molecule interactions, an ordered C60 array is rather difficult to obtain on silicon surfaces. Here we present density functional theory simulations on C60 molecules deposited on a TBB (1,3,5-tri(1'-bromophenyl)benzene) monolayer lying on the Si(111)-boron surface (denoted SiB). The C60 molecules are located in the nanopores formed by the TBB network. Adsorption energy calculations show that the SiB surface governs the C60 vertical position, whereas the TBB network imposes the C60 lateral position, and stabilizes the molecule as well. The low charge density between the C60 and the SiB substrate on one hand, and on the other hand between the C60 and the TBB molecules, indicates that no covalent bond is formed between the C60 and its environment. However, according to charge density differences, a drastic charge reorganisation takes place between the Si adatoms and the C60 molecule, but also between the C60 and the surrounding TBB molecules. Finally, calculations show that a C60 array sandwiched between two TBB molecular layers is stable, which opens up the way to the growth of 3D supramolecular networks.

A new assisted molecular cycloaddition on boron doped silicon surfaces: a predictive DFT-D study

In the framework of the Density Functional Theory (DFT-D), we investigate the phthalocyanine (H2Pc) molecule adsorption on SiC(0001)3 × 3 and Si(111)√3 × √3R30°-B (SiB) surfaces, and particularly compare the involved molecular adsorptions. In the H2Pc-SiC(0001)3 × 3 system, the molecular adsorption can be ascribed to a [10+2] cycloaddition. The H2Pc-SiB system is considered in three cases: defectless SiB surface (denoted SiB-0D) and SiB surfaces presenting one or two boron defects (denoted SiB-1D and SiB-2D respectively). The SiB-0D surface is passivated by a charge transfer from the Si adatoms to the boron atoms and therefore no chemical bond between the molecule and the substrate is observed. A similar molecular adsorption as already evidenced in the H2Pc-SiC(0001)3 × 3 system is involved in the SiB-2D case. In the case of the SiB-1D surface, two Si-N bonds (Si1-N1 and Si2-N2) are observed. One of them, Si1-N1, is nearly similar to that found in the H2Pc-SiB-2D system, but the Si2-N2 bond is unexpected. The Bader charge analysis suggests that, in the presence of the H2Pc molecule, the boron atoms behave like an electron reservoir whose availability varies following the involved molecular adsorption process. In the SiB-1D case, charges are transferred from the substrate to the molecule, allowing the Si2-N2 bond formation. Such a kind of molecular adsorption, not yet observed, could be designed by "assisted pseudo cycloaddition".

Full DFT-D description of a nanoporous supramolecular network on a silicon surface

We present a full density-functional-theory study taking into account the van der Waals interactions of a 2D supramolecular network adsorbed on the Si(111)√3x√3R30°-boron surface denoted SiB. We show that, contrarily to the previous calculations [B. Baris, V. Luzet, E. Duverger, Ph. Sonnet, F. Palmino, and F. Chérioux, Angew. Chem., Int. Ed. 50, 4094 (2011)] molecule-molecule interactions are attractive, thanks to van der Waals corrections which are essential to describe such systems. We confirm the importance of the substrate effect to achieve the molecular network on the boron doped silicon surface without covalent bond. Our simulated STM images, calculated in the framework of the bSKAN code, give better agreement with the experimental STM images than those obtained by the integrated LDOS calculations within the Tersoff-Hamann approximation. The tungsten tip presence is essential to retrieve three paired lobes as observed experimentally. The observed protrusions arise from the phenyl arms located above silicon adatoms.

Electronic level scheme in boron- and phosphorus-doped silicon nanowires

We report the first observation of the electronic level scheme in boron (B)- and phosphorus (P)-doped nanowires (NWs). The NWs' morphology dramatically depends on the doping impurity while a few deep electronic levels appear in both kinds of nanowires, independently of the doping type. We demonstrate that the doping impurities induce the same shallow levels as in bulk silicon. The presence of two donor levels in the lower half-bandgap is also revealed. In both kinds of NWs, B- and P-doped, the donor level (0/+) at E(v) + 0.36 eV of the gold-hydrogen complex is observed. This means that the gold diffusion from the NW tip introduces an electronically active level, which might negatively affects the electrical characteristics of the NWs. In P-doped NWs, we observed a further donor level at 0.26 eV above the valence band due to the phosphorus-vacancy pairs, the E-center, well-known in bulk silicon. These findings seriously question both diffusion modeling of impurities in NWs and the technological aspects arising from this.

Molecular doping and subsurface dopant reactivation in si nanowires

Impurity doping in semiconductor nanowires, while increasingly well understood, is not yet controllable at a satisfactory degree. The large surface-to-volume area of these systems, however, suggests that adsorption of the appropriate molecular complexes on the wire sidewalls could be a viable alternative to conventional impurity doping. We perform first-principles electronic structure calculations to assess the possibility of n- and p-type doping of Si nanowires by exposure to NH(3) and NO(2). Besides providing a full rationalization of the experimental results recently obtained in mesoporous Si, our calculations show that while NH(3) is a shallow donor, NO(2) yields p-doping only when passive surface segregated B atoms are present.

Surface segregation and backscattering in doped silicon nanowires

By means of ab initio simulations, we investigate the structural, electronic, and transport properties of boron and phosphorus doped silicon nanowires. We find that impurities always segregate at the surface of unpassivated wires, reducing dramatically the conductance of the surface states. Upon passivation, we show that for wires as large as a few nanometers in diameter, a large proportion of dopants will be trapped and electrically neutralized at surface dangling bond defects, significantly reducing the density of carriers. Important differences between p- and n-type doping are observed. Our results rationalize several experimental observations.

Contrasting growth modes of Mn on Ge(100) and Ge(111) surfaces: subsurface segregation versus intermixing

Based on first-principles total energy calculations within density functional theory, we show that a low dose of Mn on Ge(100) initiates in a novel subsurface growth mode, characterized by easy access to, and strong preference for, interstitial sites located between the two topmost Ge layers. Strikingly, such a "subsurfactant action" is preserved even during epitaxial growth of additional Ge layers, analogous to the well-known phenomenon of surfactant action. In contrast, along the [111] orientation, Mn can easily diffuse into the bulk via interstitial sites. These results are discussed within the context of dopant control in dilute magnetic semiconductors.

Understanding ultrahigh doping: the case of boron in silicon

Using first-principles calculations, we develop a theory for ultrahigh impurity doping in semiconductors. Our study of B in Si explains why boron solubility in epitaxial growth could exceed the solid solubility to reach the kinetic solubility, and, with adequate surface passivation, to reach even higher values. We further show that the partial ionization at high B concentration, C(B), observed by experiment is predominantly an electron chemical potential effect, not a boron clustering effect. Our calculated hole concentration over a wide C(B) range is in reasonable agreement with experiments.