Hydrogen-terminated Silicon Surfaces Research Articles

Grafting of Organophosphonic Acid Monolayers on Hydrogen-Terminated Silicon Surface and Secondary Functionalization in Supercritical Carbon Dioxide Media.

The monolayer grafting on the oxide-free Si surface is challenging due to vulnerability of the surface against oxide formation in an ambient atmosphere. Most of the conventional studies focused on organic solvent-based chemistry and solvent and substrate interfaces, and residual solvents after the monolayer grafting play a key role in producing the highly stable monolayers. CO2 in its supercritical state (SCCO2) provides an elegant engineering solution for the problem faced as it can be used as inert processing environment and as carrier fluid for monolayer grafting taking up the role of organic solvents. In this work, monolayers of alkyl organophosphonic acids (OPAs) and functional OPAs were grafted on hydrogen-terminated oxide-free Si surfaces using the SCCO2 process. Grafted monolayers were physically and chemically characterized to verify the successful monolayer formation and determine the nature of the covalent binding configuration on the surface. To broaden the prospects of practical utility of the process and the OPA monolayer, the (3-bromopropyl)phosphonic acid (BPPA) monolayer was demonstrated to undergo secondary functionalization by terminal group substitution to convert the Br terminal group to the OH terminal group and secondary monolayer grafting to assemble 4-fluorothiophenol on top of the BPPA monolayer. The ability of monolayers to sustain secondary functionalization processing qualitatively hints toward ordered and stable monolayers of OPAs. The developed SCCO2 process in this work presents a single-step, green, and scalable method to graft the OPA monolayer on oxide-free Si which can employed in the future for monolayer doping, highly selective biochemical sensors, and targeted biological interactions.

Atomic Level Chemical and Structural Properties of Silicon Surface and Initial Stages of Oxidation

This work aims to summarize previous results reported in literature on atomic level properties of the wet chemically treated hydrogen-terminated silicon surfaces and of the Si oxidation, in comparison to a model system of ultraclean Si surfaces prepared in ultrahigh vacuum (UHV) conditions. A literature review shows that a proper wet chemical treatment of Si(111) provides an atomically smooth, high-quality surface, similar to the model template obtained in UHV conditions after high temperature heating. However, it seems that Si(111) is an exception among semiconductor surfaces concerning the effects of wet chemistry. Although the insulator films grown by the atomic layer deposition (ALD) have replaced the thermal oxide of SiO2 in many applications, still an intermediate SiO2 layer is formed and often grown intentionally beneath the ALD film to improve the device performance. However, a detailed atomic structure of the SiO2/Si interface is still debatable, which might be due to differences in atomic level smoothness of the used Si(100) starting surfaces.

Pt-Coated Silicon Nanoparticles: An Investigation into the Hydrosilylation on Hydrogen-Terminated Silicon Surfaces Using Pt(dvs).

The interaction of hydrogen-terminated silicon nanoparticles (H-SiNPs) with Karstedt's catalyst at various temperatures was investigated. The results indicate that at room temperature, the oxidative addition of Pt(0) onto H-SiNPs is irreversible, and the catalyst is not eliminated from the surface of H-SiNPs, enabling a facile synthesis of Pt-loaded SiNPs that can undergo ligand exchange. The nature of the Pt-on-Si ensemble is characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Reaction conditions that enable effective hydrosilylation are discussed. It is found that higher temperatures favor reductive elimination of the catalyst and hydrosilylation of 1-octene onto the surface of the H-SiNPs.

Resistless EUV lithography: Photon-induced oxide patterning on silicon

In this work, we show the feasibility of extreme ultraviolet (EUV) patterning on an HF-treated silicon (100) surface in the absence of a photoresist. EUV lithography is the leading lithography technique in semiconductor manufacturing due to its high resolution and throughput, but future progress in resolution can be hampered because of the inherent limitations of the resists. We show that EUV photons can induce surface reactions on a partially hydrogen-terminated silicon surface and assist the growth of an oxide layer, which serves as an etch mask. This mechanism is different from the hydrogen desorption in scanning tunneling microscopy-based lithography. We achieve silicon dioxide/silicon gratings with 75-nanometer half-pitch and 31-nanometer height, demonstrating the efficacy of the method and the feasibility of patterning with EUV lithography without the use of a photoresist. Further development of the resistless EUV lithography method can be a viable approach to nanometer-scale lithography by overcoming the inherent resolution and roughness limitations of photoresist materials.

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands.

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)5Cl to form complexes of the type Re(bpy)(CO)3Cl, which are related to known catalysts for CO2 reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO2 on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO2 reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

Stable Organic Radical for Enhancing Metal-Monolayer-Semiconductor Junction Performance.

The preparation of monolayers based on an organic radical and its diamagnetic counterpart has been pursued on hydrogen-terminated silicon surfaces. The functional monolayers have been investigated as solid-state metal/monolayer/semiconductor (MmS) junctions showing a characteristic diode behavior which is tuned by the electronic characteristics of the organic molecule. The eutectic gallium-indium liquid metal is used as a top electrode to perform the transport measurements and the results clearly indicate that the SOMO-SUMO molecular orbitals impact the device performance. The junction incorporating the radical shows an almost two orders of magnitude higher rectification ratio (R(|J1V/J-1V|) = 104.04) in comparison with the nonradical one (R(|J1V/J-1V|) = 102.30). The high stability of the fabricated MmS allows the system to be interrogated under irradiation, evidencing that at the wavelength where the photon energy is close to the band gap of the radical there is a clear enhancement of the photoresponse. This is translated into an increase of the photosensitivity (Sph) value from 68.7 to 269.0 mA/W for the nonradical and radical based systems, respectively.

Effects of dopant type and concentration on surface recombination velocity in hydrogen-terminated silicon

Isolating the effects of the type and concentration of the dopant in lightly doped regions in the reaction between hydrogen-terminated silicon surface and atmospheric impurities in air is very difficult. However, changes in the surface recombination sites can be analyzed accurately through recombination lifetime measurements performed using the microwave photoconductive decay method. Thus, we investigated variations in the effective recombination lifetime in hydrogen-terminated silicon surfaces over time in air for different dopant types and concentrations. For both p-type and n-type silicon wafers, surface recombination velocity, S, increased with decreasing resistivity, namely, increasing dopant concentration. The time-dependent variations of the S for the p-type wafers decreased, and those for the n-type wafers increased with decreasing resistivity. Thus, it was shown that the time-dependent variation of the S depends on the type and concentration of the dopant used.

Fabrication of Multiscale 1-Octadecene Monolayer Patterned Arrays Based on a Chemomechanical Method

A controlled and self-assembled micromachining system was built to fabricate a mico/nanoscale monolayer patterned array on a silicon surface using a diamond tip. The process was as follows: (1) we preprocessed a silicon wafer to obtain a hydrogen-terminated silicon surface; (2) we scratched three rectangular arrays of 10 μm × 3 μm with a spacing of 2 μm on the silicon surface with a diamond tip in 1-octadecene solution; the Si-H bonds were broken, and silicon free radicals were formed; (3) the 1-octadecene molecules were connected with silicon atoms based on Si-C covalent bonds, and the 1-octadecene nano monolayer was self-assembled on the patterned arrays of the silicon surface. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Sessile water contact angles were used to detect and characterize the self-assembled monolayers (SAMs). The XPS results showed that the Si2p peak and the O1s peak were significantly decreased after self-assembly; however, the C1s peak was successively significantly increased. Sessile water contact angles showed that the hydrophilicity was weakened after the formation of 1-octenecene SAMs on the silicon substrate. The nanofriction of the sample was measured with AFM. The change in nanofriction also demonstrated that the SAMs were formed in accordance with the patterned array. We demonstrated that, by using this method, self-assembled multiscale structures on silicon substrate can be formed quickly and conveniently.

Photoactive silicon surfaces functionalized with high-quality and redox-active platinum diimine complex monolayers

Platinum diimine complexes can covalently be grafted onto oxide-free, hydrogen-terminated silicon(111) surfaces into clean and high-quality monolayers. The so modified surfaces offer great prospects as photocathodes for solar-driven electrocatalysis.

Intertrack surface losses in miniature coplanar waveguide on silicon-on-insulator

Attenuation has been studied in different-sized coplanar waveguides (CPW) patterned onto silicon-on-insulator (SOI) for three different surfaces between the CPW tracks: a silicon dioxide/silicon interface, a hydrogen-terminated silicon surface, and a native oxide. For large-gap CPW (signal track width = 100 µm, gap width = 63.5 µm), selective removal of the silicon dioxide from between the metal tracks reduces losses from 0.79 dB mm−1 to 0.69 dB mm−1 at 50 GHz. The subsequent growth of a native oxide on the silicon surface between the tracks results in losses equal to 0.67 dB mm−1. For small-gap CPW ( = 2 µm, = 2.5 µm), losses are reduced from 5.6 dB mm−1 to 3.4 dB mm−1 at 50 GHz by selectively removing the silicon oxide between the metal tracks. However, when a native oxide is allowed to grow on the silicon surface between the tracks, the losses increase significantly to 5.8 dB mm−1. When the native oxide is removed, the losses decrease to those observed following removal of the silicon dioxide. The measurements suggest that the contribution of the intertrack surface losses is the result of free-carrier losses due to a surface inversion layer. The surface-associated losses are proportionally larger as the CPW dimensions shrink—as predicted by modelling. We suggest that technologies employing miniature CPW should take this into account. The native oxide should be routinely removed if possible—implying that appropriate chemically-resistant metallisation be chosen, or fabrication processes should incorporate stable passivation of silicon surfaces between CPW tracks to avoid native oxide growth. Conversely, the increased sensitivity of attenuation to surface effects by shrinking CPW dimensions suggests that CPW-based sensors could benefit from miniaturization.

Conformal doping strategy for fin structures: tailoring of dopant profile through multiple monolayer doping and capping layer control

Monolayer doping (MLD) has attracted attention as a next-generation doping method because of its great potential for conformal doping of non-planar nanostructures. However, fundamental limitations are identified in conformal doping of fin structures using conventional MLD process, and fin structures doped with conventional MLD presents a poor dose conformality (surface concentration ratio between the side-wall and the top of the fin) of 0.32. The poor dose conformality is mainly caused by the hydrogen-terminated silicon surface formed differently depending on the orientation of each face of the fin structure and the capping layer deposited with low step coverage on the fin structure. As a strategy to overcome the limitations without increasing process complexity, multiple-MLD for dose compensation and capping layer thickness control for adjusting the dopant diffusivity are proposed. These methods are found to be effective in tailoring doping profiles on each face of the fin structure with a fine dose controllability from about 70% to 200%. Finally, it is demonstrated that the proposed approach allows conformal doping of the fin structure with a high surface concentration of 5 × 1020 atoms cm−3 and an excellent dose conformality of 0.98.

Evaluation of alternatives to paraffin for antirelaxation coatings

Paraffin wax is currently used as an antirelaxation coating in alkali metal vapor cells since it prevents spin depolarization of alkali atoms. However, the performance of paraffin decreases rapidly above 80 °C due to degradation; thus, the inability to operate at higher temperatures motivates a search for alternatives. Here, the authors use first-principles density functional theory to study rubidium (Rb) atom interactions with paraffin, along with potential alternative coating materials such as pristine and hydrogenated graphene, molybdenum disulfide, and the hydrogen-terminated silicon (111) surface. The authors’ results are in close agreement with experiments that find an adsorption energy of 0.1 eV for Rb on paraffin. The authors also find that Rb adsorbs strongly on graphene, MoS2, and silicon surfaces, but that Rb has a low adsorption energy on hydrogenated graphene, which may be suitable as an antirelaxation coating. The authors link adsorption behavior to the charge-transfer-induced ionic bonding between Rb and the underlying material. In paraffin and hydrogenated graphene, charge transfer is avoided, leading to low Rb adsorption energies for these materials.

Deep learning-guided surface characterization for autonomous hydrogen lithography

As the development of atom scale devices transitions from novel, proof-of-concept demonstrations to state-of-the-art commercial applications, automated assembly of such devices must be implemented. Here we present an automation method for the identification of defects prior to atomic fabrication via hydrogen lithography using deep learning. We trained a convolutional neural network to locate and differentiate between surface features of the technologically relevant hydrogen-terminated silicon surface imaged using a scanning tunneling microscope. Once the positions and types of surface features are determined, the predefined atomic structures are patterned in a defect-free area. By training the network to differentiate between common defects we are able to avoid charged defects as well as edges of the patterning terraces. Augmentation with previously developed autonomous tip shaping and patterning modules allows for atomic scale lithography with minimal user intervention.

Grafting density of oligo-bottle-brushes on silicon surface: effect of mole fraction of RAFT agent-functionalized alkenes in mixed self-assembled monolayers

The purpose of the study is to prepare of novel oligo-(N-(2-hydroxypropyl) methacrylamide)-g-poly(lactic acid) bottle-brushes with different grafting density on the silicon surface. Firstly, the mixtures of 9-decen-1-ol (DO, functional interconnection molecule) and 1-undecene (UD) molecules were attached to the hydrogen-terminated silicon surface by UV irradiation under molecular self-assembly conditions. Secondary, 4-cyano-4-(propylsulfanylthiocarbonyl) sulfanylpentanoic acid RAFT agent was immobilized onto DO molecules by ester reaction, followed by the interface-mediated RAFT polymerization of pre-synthesized N-[(2-hydroxypropyl) methacrylamide]-g-poly(lactic acid) macromonomer. The systematic control of the RAFT agent-functionalized alkene concentration afforded by the mixed self-assembled monolayers on the silicon surface enabled the grafting density and the average distance between grafting points of the polymer chains to independently control. The phase image taken with atomic force microscopy has shown that the silicon surfaces were coated with the bottle-brushes.

Detecting and Directing Single Molecule Binding Events on H-Si(100) with Application to Ultradense Data Storage.

Many diverse material systems are being explored to enable smaller, more capable and energy efficient devices. These bottom up approaches for atomic and molecular electronics, quantum computation, and data storage all rely on a well-developed understanding of materials at the atomic scale. Here, we report a versatile scanning tunneling microscope (STM) charge characterization technique, which reduces the influence of the typically perturbative STM tip field, to develop this understanding even further. Using this technique, we can now observe single molecule binding events to atomically defined reactive sites (fabricated on a hydrogen-terminated silicon surface) through electronic detection. We then developed a simplified error correction tool for automated hydrogen lithography, quickly directing molecular hydrogen binding events using these sites to precisely repassivate surface dangling bonds (without the use of a scanned probe). We additionally incorporated this molecular repassivation technique as the primary rewriting mechanism in ultradense atomic data storage designs (0.88 petabits per in2).

Electrostatic Landscape of a Hydrogen-Terminated Silicon Surface Probed by a Moveable Quantum Dot.

With nanoelectronics reaching the limit of atom-sized devices, it has become critical to examine how irregularities in the local environment can affect device functionality. Here, we characterize the influence of charged atomic species on the electrostatic potential of a semiconductor surface at the subnanometer scale. Using noncontact atomic force microscopy, two-dimensional maps of the contact potential difference are used to show the spatially varying electrostatic potential on the (100) surface of hydrogen-terminated highly doped silicon. Three types of charged species, one on the surface and two within the bulk, are examined. An electric field sensitive spectroscopic signature of a single probe atom reports on nearby charged species. The identity of one of the near-surface species has been uncertain in the literature, and we suggest that its character is more consistent with either a negatively charged interstitial hydrogen or a hydrogen vacancy complex.

Binary atomic silicon logic

It has long been anticipated that the ultimate in miniature circuitry will be crafted of single atoms. Despite many advances made in scanned probe microscopy studies of molecules and atoms on surfaces, challenges with patterning and limited thermal stability have remained. Here we make progress toward those challenges and demonstrate rudimentary circuit elements through the patterning of dangling bonds on a hydrogen terminated silicon surface. Dangling bonds sequester electrons both spatially and energetically in the bulk band gap, circumventing short circuiting by the substrate. We deploy paired dangling bonds occupied by one movable electron to form a binary electronic building block. Inspired by earlier quantum dot-based approaches, binary information is encoded in the electron position allowing demonstration of a binary wire and an OR gate.

Initiating and Monitoring the Evolution of Single Electrons Within Atom-Defined Structures.

Using a noncontact atomic force microscope, we track and manipulate the position of single electrons confined to atomic structures engineered from silicon dangling bonds on the hydrogen terminated silicon surface. An attractive tip surface interaction mechanically manipulates the equilibrium position of a surface silicon atom, causing rehybridization that stabilizes a negative charge at the dangling bond. This is applied to controllably switch the charge state of individual dangling bonds. Because this mechanism is based on short range interactions and can be performed without applied bias voltage, we maintain both site-specific selectivity and single-electron control. We extract the short range forces involved with this mechanism by subtracting the long range forces acquired on a dimer vacancy site. As a result of relaxation of the silicon lattice to accommodate negatively charged dangling bonds, we observe charge configurations of dangling bond structures that remain stable for many seconds at 4.5K. Subsequently, we use charge manipulation to directly prepare the ground state and metastable charge configurations of dangling bond structures composed of up to six atoms.

Autonomous Scanning Probe Microscopy inSitu Tip Conditioning through Machine Learning.

Atomic-scale characterization and manipulation with scanning probe microscopy rely upon the use of an atomically sharp probe. Here we present automated methods based on machine learning to automatically detect and recondition the quality of the probe of a scanning tunneling microscope. As a model system, we employ these techniques on the technologically relevant hydrogen-terminated silicon surface, training the network to recognize abnormalities in the appearance of surface dangling bonds. Of the machine learning methods tested, a convolutional neural network yielded the greatest accuracy, achieving a positive identification of degraded tips in 97% of the test cases. By using multiple points of comparison and majority voting, the accuracy of the method is improved beyond 99%.

Ab initio characterization of coupling strength for all types of dangling-bond pairs on the hydrogen-terminated Si(100)-2 × 1 surface.

Recent work has suggested that coupled silicon dangling bonds sharing an excess electron may serve as building blocks for quantum-cellular-automata cells and quantum computing schemes when constructed on hydrogen-terminated silicon surfaces. In this work, we employ ab initio density-functional theory to examine the details associated with the coupling between two dangling bonds sharing one excess electron and arranged in various configurations on models of phosphorous-doped hydrogen-terminated silicon (100) surfaces. Our results show that the coupling strength depends strongly on the relative orientation of the dangling bonds on the surface and on the separation between them. The orientation of dangling bonds is determined by the anisotropy of the silicon (100) surface, so this feature of the surface is a significant contributing factor to variations in the strength of coupling between dangling bonds. The results demonstrate that simple models for approximating tunneling, such as the Wentzel-Kramer-Brillouin method, which do not incorporate the details of surface structure, are incapable of providing reasonable estimates of tunneling rates between dangling bonds. The results provide guidance to efforts related to the development of dangling-bond based computing elements.