Hydrogen-passivated Surfaces Research Articles

Evaluation of critical supersaturation ratios during epitaxial growth of hexagonal SiC {0001} under carbon- and hydrogen-rich conditions

Critical supersaturation ratios α crit during epitaxial growth on 6H-SiC (0001) and (000) under carbon- and hydrogen-rich conditions were formerly estimated by assuming disc-shaped critical nuclei whose side surfaces were terminated with silicon and carbon atoms, respectively. Since this assumption contradicts the atomic configurations of possible hexagonal-prism-shaped critical nuclei, we evaluated α crit by using the reported surface free energies for hydrogen-passivated 3C-SiC surfaces. To account for the observed α crit being much larger on 6H-SiC (0001) than on 6H-SiC (000), we considered that silicon atoms on the side surfaces of hexagonal-prism-shaped critical nuclei be terminated not by carbon but by hydrogen atoms.

Explainable prediction of N-V-related defects in nanodiamond using neural networks and Shapley values

Although the negatively charged nitrogen-vacancy (N-V − ) defect in nanodiamonds is desirable for a variety of biomedical applications, a range of other defect complexes involving nitrogen and/or vacancies can also exist, depending on their relative stability. Using machine learning, a re-usable model is developed to predict the likelihood of a particular defect complex being stable at a given depth below reconstructed or hydrogen-passivated surfaces. A neural network is used to generate a system of equations that can be easily implemented in any workflow, and explainable artificial intelligence (XAI) methods are used to provide insights into which structural features and defect configurations are most responsible for the model prediction. It is found that, although the number of nitrogen atoms present in the defect is the most important feature determining the defect likelihood, the most influential data instances are the unlikely defects, providing a type of baseline for comparison. • An accurate neural network predicts the likelihood of N-V defects in nanodiamond • Explainable AI identifies important features and influential defect configurations Although the negatively charged nitrogen-vacancy defect in nanodiamonds is useful, a range of other defect complexes involving nitrogen and/or vacancies can also exist. Here, Barnard uses machine learning to develop a re-usable model to predict the likelihood of a particular defect complex being stable at a given depth below a given surface.

Superlow Friction of a-C:H Coatings in Vacuum: Passivation Regimes and Structural Characterization of the Sliding Interfaces

A combination of atomistic simulations and vacuum tribometry allows atomic-scale insights into the chemical structure of superlubricious hydrogenated diamond-like carbon (a-C:H) interfaces in vacuum. Quantum molecular dynamics shearing simulations provide a structure-property map of the friction regimes that characterize the dry sliding of a-C:H. Shear stresses and structural properties at the sliding interfaces are crucially determined by the hydrogen content CH in the shear zone of the a-C:H coating. Extremely small CH (below 3 at.%) cause cold welding, mechanical mixing and high friction. At intermediate CH (ranging approximately from 3 to 20 at.%), cold welding in combination with mechanical mixing remains the dominant sliding mode, but some a-C:H samples undergo aromatization, resulting in a superlubricious sliding interface. A further increase in CH (above 20 at.%) prevents cold welding completely and changes the superlubricity mechanism from aromatic to hydrogen passivation. The hydrogen-passivated surfaces are composed of short hydrocarbon chains hinting at a tribo-induced oligomerization reaction. In the absence of cold welding, friction strongly correlates with nanoscale roughness, measured by the overlap of colliding protrusions at the sliding interface. Finally, the atomistic friction map is related to reciprocating friction experiments in ultrahigh vacuum. Accompanying X-ray photoelectron and Auger electron spectroscopy (XPS, XAES) analyses elucidate structural changes during vacuum sliding of a hydrogen-rich a-C:H with 36 at.% hydrogen. Initially, the a-C:H is covered by a nanometer-thick hydrogen-depleted surface layer. After a short running-in phase that results in hydrogen accumulation, superlubricity is established. XPS and XAES indicate a non-aromatic 1–2-nm-thick surface layer with polyethylene-like composition in agreement with our simulations.

A molecular dynamics analysis of the effect of surface passivation on the adhesion, deformation behavior and structure stability of amorphous carbon ultrathin films

Basic knowledge of the interfacial interactions, contact deformation, and structure changes in amorphous carbon (a-C) ultrathin films due to irreversible deformation is of critical importance to the protective effectiveness of these films. This paper presents a molecular dynamics (MD) analysis that reveals the role of surface passivation on the contact deformation and adhesion characteristics of ultrathin (<30 Å) a-C films possessing a layered structure consisting of intermixing, bulk, and surface layers. MD simulations reveal much higher interfacial adhesion, destabilization of the film structure resulting in partial sp3-to-sp2 rehybridization, and film delamination in the intermixing layer for unpassivated compared to hydrogen-passivated diamond surfaces. The results of this study illuminate the adhesion and contact deformation behaviors of ultrathin a-C films with layered structures, which are impossible to track experimentally due to the extremely small spatiotemporal scales.

Gas sensing properties of silicon nanowires with different cross-sectional shapes toward ammonia: a first-principles study

Nanostructure-based sensors working based on the adsorption-desorption mechanism received much attention in a wide variety of applications as a way to achieve both ultrasensitive sensing and small dimensions. In this paper, the adsorption of ammonia molecule on silicon nanowire surface with square, circular, and triangular cross-sectional shapes, which has almost similar cross-sectional area (1.2 nm2), is studied. The effects of various adsorption configurations are considered on the sensing properties, utilizing first-principles ab initio density functional theory calculations. Structural properties such as bond lengths and bond angles are obtained along with electronic properties such as band structure, charge transfer, and projected density of states for several adsorption configurations. The most stable and most sensitive structure is calculated to be the triangular nanowire considering formation enthalpy as a stability factor. For this nanowire, adsorption energy, covalent bond length, and charge transfer between ammonia and nanowire are calculated to be 1.296 eV, 3.124 A, and 0.227 |e|, respectively. We also consider nanowires with fully hydrogen-passivated surface and nanowires with a dangling bond. It has been shown that nanowires with a dangling bond are more sensitive toward ammonia than fully passivated nanowires.

Stability, electronic and vibrational properties of GaAlN wurtzoid molecules and nanocrystals: A DFT study

Electronic structure and vibrational characteristics of GaAlN wurtzoids at the molecular-nanoscale limit are investigated. The investigated properties include energy gap, vibrational properties, and phase stability against transition to GaAlN diamondoids. At the molecular-nanoscale limit, wurtzite GaAlN nanocrystals are represented by wurtzoids. The properties of GaAlN molecules and nanocrystals as a function of Ga and Al contents are shown. The results show that the energy gap depends on size, shape and surface conditions. Molecules with hydrogen passivated surface have wide energy gap while bare molecules have smaller energy gaps. On the other hand, vibrational longitudinal optical mode of bare molecules experiences a redshift after surface hydrogen passivation with smaller vibrational reduced masses and force constant at a given frequency. Differences between GaAlN two limits (GaN and AlN) are explained by the existence of d orbitals in Ga atoms. Calculated Gibbs free energy of atomization show the stability of wurtzoids against transition to the diamondoids at the molecular-nanoscale limit for both bare and hydrogen passivated cases.

Surface recombination velocity imaging of wet-cleaned silicon wafers using quantitative heterodyne lock-in carrierography

InGaAs-camera based heterodyne lock-in carrierography (HeLIC) is developed for surface recombination velocity (SRV) imaging characterization of bare (oxide-free) hydrogen passivated Si wafer surfaces. Samples prepared using four different hydrofluoric special-solution etching conditions were tested, and a quantitative assessment of their surface quality vs. queue-time after the hydrogen passivation process was made. The data acquisition time for an SRV image was about 3 min. A “round-trip” frequency-scan mode was introduced to minimize the effects of signal transients on data self-consistency. Simultaneous best fitting of HeLIC amplitude-frequency dependencies at various queue-times was used to guarantee the reliability of resolving surface and bulk carrier recombination/transport properties. The dynamic range of the measured SRV values was established from 0.1 to 100 m/s.

Shallow V-Shape Nanostructured Pit Arrays in Germanium Using Aqua Regia Electroless Chemical Etching.

Due to its high refractive index, reflectance is often a problem when using Germanium for optoelectronic devices integration. In this work, we propose an effective and low-cost nano-texturing method for considerably reducing the reflectance of bulk Germanium. To do so, uniform V-shape pit arrays are produced by wet electroless chemical etching in a 3:1 volume ratio of highly-concentrated hydrochloridric and nitric acids or so-called aqua regia bath using immersion times ranging from 5 to 60 min. The resulting pit morphology, the crystalline structure of the surface and the changes in surface chemistry after nano-patterning are all investigated. Finally, broadband near-infrared reflectance measurements confirm a significant reduction using this simple wet etching protocol, while maintaining a crystalline, dioxide-free, and hydrogen-passivated surface. It is important to mention that reflectance could be further reduced using deeper pits. However, most optoelectronic applications such as photodetectors and solar cells require relatively shallow patterning of the Germanium to allow formation of a pn-junction close to the surface.

Atomic White-Out: Enabling Atomic Circuitry through Mechanically Induced Bonding of Single Hydrogen Atoms to a Silicon Surface

We report the mechanically induced formation of a silicon-hydrogen covalent bond and its application in engineering nanoelectronic devices. We show that using the tip of a noncontact atomic force microscope (NC-AFM), a single hydrogen atom could be vertically manipulated. When applying a localized electronic excitation, a single hydrogen atom is desorbed from the hydrogen-passivated surface and can be transferred to the tip apex, as evidenced from a unique signature in frequency shift curves. In the absence of tunnel electrons and electric field in the scanning probe microscope junction at 0 V, the hydrogen atom at the tip apex is brought very close to a silicon dangling bond, inducing the mechanical formation of a silicon-hydrogen covalent bond and the passivation of the dangling bond. The functionalized tip was used to characterize silicon dangling bonds on the hydrogen-silicon surface, which was shown to enhance the scanning tunneling microscope contrast, and allowed NC-AFM imaging with atomic and chemical bond contrasts. Through examples, we show the importance of this atomic-scale mechanical manipulation technique in the engineering of the emerging technology of on-surface dangling bond based nanoelectronic devices.

Automated extraction of single H atoms with STM: tip state dependency

The atomistic structure of the tip apex plays a crucial role in performing reliable atomic-scale surface and adsorbate manipulation using scanning probe techniques. We have developed an automated extraction routine for controlled removal of single hydrogen atoms from the H:Si(100) surface. The set of atomic extraction protocols detect a variety of desorption events during scanning tunneling microscope (STM)-induced modification of the hydrogen-passivated surface. The influence of the tip state on the probability for hydrogen removal was examined by comparing the desorption efficiency for various classifications of STM topographs (rows, dimers, atoms, etc). We find that dimer-row-resolving tip apices extract hydrogen atoms most readily and reliably (and with least spurious desorption), while tip states which provide atomic resolution counter-intuitively have a lower probability for single H atom removal.

Functionalization of silicon nanowires by conductive and non-conductive polymers

The work reports on the development of hybrid devices based on silicon nanowires (SiNW) with polymers and the difference obtained when using conductive and non-conductive polymers. SiNW have attracted much attention due to their importance in understanding the fundamental properties at low dimensionality as well as their potential application in nanoscale devices as in field effect transistors, chemical or biological sensors, battery electrodes and photovoltaics. SiNW arrays were formed using metal assisted chemical etching method. This process is simple, fast and allows obtaining a wide range of silicon nanostructures. Hydrogen-passivated SiNW surfaces show relatively poor stability. Surface modification with organic species confers the desired stability and enhances the surface properties. For this reason, this work proposes a covalent grafting of organic material onto SiNW surface. We have chosen a non-conductive polymer polyvinylpyrrolidone (PVP) and conductive polymers polythiophene (PTh) and polypyrrole (PPy), in order to evaluate the electric effect of the polymers on the obtained materials. The hybrid structures were elaborated by the polymerization of the corresponding conjugated monomers by electrochemical route; this electropolymerization offers several advantages such as simplicity and rapidity. SiNW functionalization by conductive polymers has shown to have a huge effect on the electrical mobility. Hybrid surface morphologies were characterized by scanning electron microscopy (SEM), infrared spectroscopy (FTIR-ATR) and contact angle measurements.

Influence of hydrogen surface passivation on Sn segregation, aggregation, and distribution in GeSn/Ge(001) materials

Plane-wave density functional theory is used to investigate the impact of hydrogen passivation of the p(2×2) reconstructed Ge1−xSnx surface on Sn segregation, aggregation, and distribution. On a clean surface, Sn preferentially segregates to the surface layer, with surface coverages of 25%, 50%, and 100% for total Sn concentrations of 2.5%, 5.0%, and 10.0%, respectively. In contrast, a hydrogen passivated surface increases interlayer migration of Sn to subsurface layers, in particular, to the third layer from the surface, and results in surface coverages of 0%, 0%, and 50% corresponding to Sn concentrations of 2.5%, 5.0%, and 10.0%, respectively. Hydrogen transfer from a Ge-capped surface to the one enriched with increasing Sn surface coverage is also an unfavorable process. The presence of hydrogen therefore reduces the surface energy by passivating the reactive dangling bonds and enhancing Sn interlayer migration to the subsurface layers. For both clean and hydrogenated surfaces, aggregation of Sn at the surface layer is also not favored. We explain these results by considering bond enthalpies and the enthalpies of hydrogenation for various surface reactions. Our results thus point to reduced Sn segregation to the surface in a Ge1−xSnx epitaxial thin film if CVD growth, using hydride precursors in the hydrogen limited growth regime, is used. This would lead to a more abrupt interface and is consistent with recent experimental observation. Hydrogenation is therefore a promising method for controlling and manipulating elemental population of Sn in a Ge1−xSnx epitaxial thin film.

Topologicalα-Sn surface states versus film thickness and strain

The theoretical prediction that gray tin represents a strong topological insulator under strain [L. Fu and C.L. Kane, Phys. Rev. B 76, 045302 (2007)] is proven for biaxially strained $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ layers with varying thickness by means of a generalized density functional theory with a nonlocal exchange-correlation potential that widely simulates quasiparticle bands and a tight-binding method including intra- and interatomic spin-orbit interaction. Hydrogen-passivated surfaces are modeled by symmetric slabs. In contrast to the conventional picture of a topological insulator, we find topological gapless surface states below the strain-induced bulk gap, in agreement with photoemission studies. The topological surfaces states do not emerge out of the lower ${\ensuremath{\Gamma}}_{8}^{+}$ states but are strongly influenced by the inverted bulk ${\ensuremath{\Gamma}}_{7}^{\ensuremath{-}}$ band, i.e., lie within the negative bulk $sp$ gap. We show that the position of the Dirac point and the dispersion of the surrounding bands depend on the thickness of the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ layer but are less influenced by strain. The resulting Fermi velocities agree well with available photoemission data.

Tuning the transport properties of graphene films grown by CVD on SiC(0001): Effect ofin situhydrogenation and annealing

The structural, optical, and transport properties of graphene grown by chemical vapor deposition (CVD) of propane under hydrogen on the Si face of SiC substrates have been investigated. We show that little changes in temperature during the growth can trigger the passivation of the SiC surface by hydrogen. Depending on the growth condition, hole or electron doping can be achieved, down to a few ${10}^{11}$ cm${}^{\ensuremath{-}2}$. When the growth temperature is high ($T\ensuremath{\approx}1500--1550{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C), we obtain electron-doped graphene monolayers lying on a buffer layer. When the growth temperature is slightly lowered ($T\ensuremath{\approx}1450--1500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C), hole-doped graphene layers are obtained, lying on a hydrogen-passivated SiC surface, as confirmed by the enhancement of the mobility (of the order of 4500 cm${}^{2}/$Vs) and the persistence of weak localization almost up to room temperature (250 K). The high homogeneity of this graphene allows the observation of the half-integer quantum Hall effect, typical of graphene, at the centimeter scale in the best cases. The influence of the SiC steps on the transport properties is discussed.

Contacting a Conjugated Molecule with a Surface Dangling Bond Dimer on a Hydrogenated Ge(001) Surface Allows Imaging of the Hidden Ground Electronic State

Fabrication of single-molecule logic devices requires controlled manipulation of molecular states with atomic-scale precision. Tuning molecule-substrate coupling is achieved here by the reversible attachment of a prototypical planar conjugated organic molecule to dangling bonds on the surface of a hydrogenated semiconductor. We show that the ground electronic state resonance of a Y-shaped polyaromatic molecule physisorbed on a defect-free area of a fully hydrogenated surface cannot be observed by scanning tunneling microscopy (STM) measurements because it is decoupled from the Ge bulk states by the hydrogen-passivated surface. The state can be accessed by STM only if the molecule is contacted with the substrate by a dangling bond dimer. The reversibility of the attachment processes will be advantageous in the construction of surface atomic-scale circuits composed of single-molecule devices interconnected by the surface dangling bond wires.

Environmental photostability of SF6-etched silicon nanocrystals

We report on the long-term environmental stability of the photoluminescent (PL) properties of silicon nanocrystals (SiNCs). We prepared sulfur hexafluoride (SF6) etched SiNCs in a two-stage plasma reactor and investigated their PL stability against UV irradiation in air. Unlike SiNCs with hydrogen-passivated surfaces, the SF6-etched SiNCs exhibit no photobleaching upon extended UV irradiation despite surface oxidation. Furthermore, the PL quantum yield also remains stable upon heating the SF6-etched SiNCs up to 160 °C. The observed thermal and UV stability of SF6-etched SiNCs combined with their PL quantum yields of up to ∼50% make them attractive candidates for UV downshifting to enhance the efficiency of solar cells. Electron paramagnetic spin resonance indicates that the SF6-etched SiNCs have a lowered density of defect states, both as-formed and after room temperature oxidation in air.

Theoretical analysis of fluorine-passivated germanium surface for high- k/Ge gate stack by molecular orbital method

Energy state and coordination of fluorine (F)-passivated Ge surface have been theoretically analyzed by semi-empirical molecular orbital method in comparison with hydrogen-passivated Ge surface to predict usefulness of F for passivation element and surface stabilization. Heat of formation for the reaction of F atoms and Ge layer system decreased simultaneously without energy barrier. Resultantly, F–Ge bonds were formed on Ge layer system and Ge surface dangling bonds were passivated by F dissimilar to the reaction of H atoms and Ge layer system. Furthermore, it was confirmed experimentally that the electrical properties of HfO 2/Ge gate stack were improved by F 2-ambient treatment of Ge substrate prior to HfO 2 deposition. It is concluded that F-passivation of Ge surface is useful in making stable and low-defective Ge substrate for high- k dielectric layer deposition.

Origin of the isotope effect on solid friction

Friction of hydrogen-passivated surfaces can be reduced by replacing terminating hydrogen atoms with heavier deuteriums. Using molecular-dynamics simulations, we show that this experimentally observed isotope effect can be explained quantitatively by small differences in surface coverage, which are due to isotope-dependent bond stabilities. We also demonstrate that a change in vibrational frequencies alone does not account for an isotope-dependent solid friction in our model system.

Atomic-Scale Templates Patterned by Ultrahigh Vacuum Scanning Tunneling Microscopy on Silicon

The ultrahigh vacuum (UHV) scanning tunneling microscope (STM) enables patterning and characterization of the physical, chemical, and electronic properties of nanostructures on surfaces with atomic precision. On hydrogen-passivated Si(100) surfaces, selective nanopatterning with the STM probe allows the creation of atomic-scale templates of dangling bonds surrounded by a robust hydrogen resist. Feedback-controlled lithography, which can remove a single hydrogen atom from the Si(100):H surface, demonstrates high-resolution nanopatterning. The resulting patterns can be used as templates for a variety of materials to form hybrid silicon nanostructures while maintaining a pristine background resist. The versatility of this UHV-STM nanolithography approach has led to its use on a variety of other substrates, including alternative hydrogen-passivated semiconductor surfaces, molecular resists, and native oxide resists. This review discusses the mechanisms of STM-induced hydrogen desorption, the postpatterning deposition of molecules and materials, and the implications for nanoscale device fabrication.

Photoexcited silicon nanocrystals: mediators for the excitation of organic molecules

We report on spin-flip activation of organic molecules from the ground state to one of the excited states having different electronic spin configurations. Based on photoexcitations in silicon (Si) nanocrystals, stored electronic energy can be transferred to other substances. The huge hydrogen passivated surface being accessible for acceptor molecules, the tunable confinement energy of excitons and their very long radiative lifetime allow Si nanocrystals to act efficiently as photosensitizers. We demonstrate that the energy transfer is governed by electron exchange mechanism. Therefore a good overlap of the electron wave functions, of donor and acceptor, is crucial for the efficiency of this type of interaction. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)