H-terminated Si Research Articles

Metallic Nanostructure Formation Limited by the Surface Hydrogen on Silicon

Constant miniaturization of electronic devices and interfaces needed to make them functional requires an understanding of the initial stages of metal growth at the molecular level. The use of metal-organic precursors for metal deposition allows for some control of the deposition process, but the ligands of these precursor molecules often pose substantial contamination problems. One of the ways to alleviate the contamination problem with common copper deposition precursors, such as copper(I) (hexafluoroacetylacetonato) vinyltrimethylsilane, Cu(hfac)VTMS, is a gas-phase reduction with molecular hydrogen. Here we present an alternative method to copper film and nanostructure growth using the well-defined silicon surface. Nearly ideal hydrogen termination of silicon single-crystalline substrates achievable by modern surface modification methods provides a limited supply of a reducing agent at the surface during the initial stages of metal deposition. Spectroscopic evidence shows that the Cu(hfac) fragment is present upon room-temperature adsorption and reacts with H-terminated Si(100) and Si(111) surfaces to deposit metallic copper. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) are used to follow the initial stages of copper nucleation and the formation of copper nanoparticles, and X-ray energy dispersive spectroscopy (XEDS) confirms the presence of hfac fragments on the surfaces of nanoparticles. As the surface hydrogen is consumed, copper nanoparticles are formed; however, this growth stops as the accessible hydrogen is reacted away at room temperature. This reaction sets a reference for using other solid substrates that can act as reducing agents in nanoparticle growth and metal deposition.

Negative differential resistance in electrografted layer of N-(2-(4-diazoniophenyl)ethyl)- N′-hexylnaphthalene-1,8:4,5-tetracarboxydiimide tetrafluoroborate on Si

We demonstrate electrochemical deposition of a specially designed and synthesized σ–π–σ molecular architecture, that is, N-(2-(4-diazoniophenyl)ethyl)- N′-hexylnaphthalene-1,8:4,5-tetracarboxydiimide tetrafluoroborate (DHTT), on H-terminated Si substrates. Electrografting of DHTT on Si at 5 and 27 °C leads to the formation of monolayer and multilayers, respectively. DHTT monolayer exhibits a pronounced negative differential resistance (NDR) in the current–voltage characteristics with peak-to-valley current ratio of ∼10. Theoretical simulations studies show that the observed NDR effect is intrinsic to the DHTT molecules. NDR effect has been explained using the ab initio molecular-orbital theoretical calculations.

AFM measurement of atomic-scale Si surface etching by active oxidation

Atomic-scale etching of a clean Si surface by active oxidation with oxygen molecules was examined using ex-situ atomic force microscopy (AFM). The etch rate was directly determined by measuring the etch depth with AFM. A SiO 2 anti-etching mask was used on a H-terminated Si(001)-2 × 1:H surface prepared by low pH HF treatment followed by annealing in H 2. The etch rate under active oxidation conditions was almost proportional to the O 2 pressure, which was consistent with previous reports. The etch rate exhibited a weak temperature dependence with an apparent activation energy of 0.2 eV. A distinct transition of the reaction mode from etching to oxide formation was observed in detail as an abrupt decrease of the etch rate by lowering the temperature near the boundary condition between the etching and oxide formation conditions.

Chemical routes to fine tuning the redox potential of monolayers covalently attached on H–Si(1 0 0)

A closely related series of three ferrocene derivatives, bearing a short carbon–carbon tethering arm for covalent attachment to H-terminated Si(1 0 0), has been used to produce electroactive hybrid materials. Three different grafting procedures have been applied for the first time on this type of molecules in order to obtain the full series of possible unsaturations in the C–C lateral chain bound to Si: ethyl, vinyl and ethynyl group, respectively, associated with the surface species (η 5-C 5H 5)Fe 2+(η 5-C 5H 4)–CH 2–CH 2–Si (EtFC/Si), (η 5-C 5H 5)Fe 2+( η 5-C 5H 4)–CH CH–Si (VFC/Si), and (η 5-C 5H 5)Fe 2+(η 5-C 5H 4)–C C–Si (EFC/Si). The surface reactions here applied involve Grignard derivatization, Lewis acid catalysis and nucleophilic substitution by an acetylide anion. The redox potentials of the three molecule/surface hybrids, measured by cyclic voltammetry, are 0.059, 0.136 and 0.251 V vs. Ag/Ag +, respectively, from EtFC/Si to EFC/Si and are supposedly following the increasing trend of the extent of unsaturation in the tethering arm. The above trend in redox potentials parallels the one experimentally found and theoretically reproduced for the corresponding isolated molecules, which shows that the adopted functionalization procedures may be used to control the number of residual unsaturations in the C–C arm. The presence and chemical nature of the redox species covalently attached to Si are strongly supported after evaluation of XPS spectra and electrochemical data. The formation of electroactive hybrids on Si with tuneable well-defined switching potentials can be highly beneficial to the development of Si-based memory devices.

Binding of Ag Adatoms to Dangling Bond Defects on a H-terminated Si(001) Surface

Using a first-principles calculation method, we investigate the binding of Ag adatoms with dangling bond (DB) defects on a H-terminated Si(001) surface and the diffusion of a single Ag adatom near the DB defects. We find that the Ag adatoms saturate DBs with binding energies ∼2.0 eV/Ag, much larger that on a perfect H/Si(001) surface. This means that Ag atoms preferentially bind to the DB sites on the surface. The addition energy, which reflects the energy gain when a Ag atom is added from the perfect region (0.35 ∼ 1.72 eV), decreases very much when the number of Ag atoms exceeds the number of DBs. This suggests that the Ag atoms cover the DB areas to form Ag nanostructures. The simulated scanning-tunneling microscopy (STM) images of the Ag/H/Si(001) surfaces with the DB defects reproduce the experimental ones well. It is also found that a Ag adatom diffuses anisotropically (along the dimer rows) and that the Schwoebel barriers are small near the DB defects.

Silicon surface transformations: From initial phases of pore formation to nanoscopic metal–insulator–semiconductor junctions

The functioning and fabrication of nanoemitter solar energy converting structures is described. Metal nanoisland formation on a nanoporous anodic oxide template is investigated in model systems where Pt is deposited onto H-terminated single crystal Si electrodes. The formation of an oxide layer, concurrent with the noble metal electrodeposition (Pt, Ir) is revealed by synchrotron radiation photoelectron spectroscopy (SRPES) and TEM. Scanning tunnelling spectroscopy (STS) on the metal islands and at the adjacent oxide covered surface shows rectifying behaviour at the nanoscopic silicon-oxide-metal contact. In the divalent dissolution regime in fluoride containing solution, initial pores form at re-entrant sites on the surface. Interestingly, the adsorption of enzymes such as the reverse transcriptase (RT) of the avian myeloblastosis virus (AMV) also occurs preferably at similar sites on a step-bunched Si surface. The results are discussed based on highly local nanoscopic electrical fields in the semiconductor and in the adjacent electrolytic double layer.

Nanoscale Shape and Size Control of Cubic, Cuboctahedral, and Octahedral Cu−Cu2O Core−Shell Nanoparticles on Si(100) by One-Step, Templateless, Capping-Agent-Free Electrodeposition

Cu-Cu2O core-shell nanoparticles (NPs) of different shapes over an extended nanosize regime of 5-400 nm have been deposited on a H-terminated Si(100) substrate by using a simple, one-step, templateless, and capping-agent-free electrochemical method. By precisely controlling the electrolyte concentration [CuSO4 x 5H2O] below their respective critical values, we can obtain cubic, cuboctahedral, and octahedral NPs of different average size and number density by varying the deposition time under a few seconds (<6 s). Combined glancing-incidence X-ray diffraction and depth-profiling X-ray photoelectron spectroscopy studies show that these NPs have a crystalline core-shell structure, with a face-centered cubic metallic Cu core and a simple cubic Cu2O shell with a CuO outerlayer. The shape control of Cu-Cu2O core-shell NPs can be understood in terms of a diffusion-limited progressive growth model under different kinetic conditions as dictated by different [CuSO4 x 5H2O] concentration regimes.

Surface Radical Chain Reaction Revisited: Comparative Investigation of Styrene and 2,4-Dimethyl-Styrene on Hydrogenated Si(001) Surface from Density Functional Theory Calculations

We report on periodic Density Functional Theory calculations of the adsorption and desorption of styrene and dimethyl-styrene on the H-terminated Si(001)-2 × 1 surface. Experimentally, the adsorption of these molecules on H−Si(001)-2 × 1 results in the formation of one-dimensional lines parallel to the Si dimer rows. At higher temperature, desorption of the molecules at the end of the lines is observed, leading to a shortening of the lines. The desorption temperature is ∼300 K for dimethyl-styrene and ∼400 K for styrene. To obtain insights into the atomic scale mechanisms of these processes, we have studied the initial steps of the radical chain and inverse chain reactions for the two molecules. Due to the existence of several metastable states, different competing reaction paths exist in both cases. We have found differences between styrene and dimethyl-styrene desorption barriers that are in agreement with experiments.

Amorphization of Si(100) by Ar+-ion bombardment studied with spectroscopic and time-resolved second-harmonic generation

The surface and interface sensitive technique of optical second-harmonic generation (SHG) has been applied spectroscopically and time-resolved before, during, and after low energy (70–1000 eV) Ar+-ion bombardment of H-terminated Si(100). The photon energy range of the fundamental radiation was ℏω=0.76–1.14 eV. Besides physical sputtering of the silicon, ion bombardment of crystalline silicon damages and amorphizes the top layer of the sample and thereby creates a layered structure of amorphous silicon (a-Si) on crystalline silicon. The SHG radiation, which is sensitive to the Ar+-ion flux, ion energy, and the presence of reactive gas species, originates from the top surface of the sample and from the interface between a-Si and c-Si. From a comparison with the SHG results obtained at a fundamental radiation of ℏω=1.3–1.7 eV, it is concluded that the SHG radiation during and after creation of this structure dominantly originates from the tails of electronic transitions in the E0′/E1 energy region rather than from silicon dangling bonds.

Growth and stability of Langmuir-Blodgett films on OH-, H-, or Br-terminated Si(001)

Growth of Langmuir-Blodgett (LB) films of nickel arachidate (NiA) on differently terminated (OH-, H-, or Br-terminated) Si(001) substrates and their structural evolution with time have been investigated by x-ray reflectivity technique and complemented by atomic force microscopy. Stable and strongly attached asymmetric monolayer (AML) of NiA is found to grow on freshly prepared oxide-covered Si substrate while unstable and weakly attached symmetric monolayer (SML) of NiA grows on H-terminated Si substrate, corresponding to stable hydrophilic and unstable hydrophobic natures of the substrates, respectively. The structure of LB film on Br-terminated Si substrate, however, shows intermediate behavior, namely, both AML and SML are present on the substrate, indicative of coexisting (hydrophilic and hydrophobic) nature of this terminated surface. Such coexisting nature of the substrate shows unusual growth behavior of LB films: (i) hydrophilic and hydrophobic attachments of NiA molecules in single up stroke of deposition and (ii) growth of few ring-shaped large-heights islands in subsequent deposition. These probably occur due to the presence of substrate-induced perturbation in the Langmuir monolayer and release of initially accumulated strain in the film structures near hydrophilic/hydrophobic interface, respectively, and provide the possibility to grow desired structures (AML or SML) of LB films by passivation-selective surface engineering.

Neutron Reflectometry of Buried Monatomic Hydrogen Layer at the Hetero-Interface in a Highly Mismatched Sr and H-terminated Si(111)

Strontium (Sr) is a well-known template on Si for a highly-desirable transistor gate material SrTiO3. Sr layers are grown epitaxially on hydrogen-terminated Si(111) surface despite the large lattice mismatch of 12%. However, there are still many unclear points concerning the specific interface structure. We need to study how the buried monatomic hydrogen layer behaves to manage the large mismatch. In order to clarify its buried hetero-interface structure related to monatomic hydrogen layer, Si-H bonding states are in situ monitored during Sr growth by Multiple Internal Reflection Fourier Transform Infrared (MIR-FTIR) spectroscopy, and the buried hetero-interface between Sr layer and Si(111) surface is investigated by ex situ neutron reflectometry (NR). FTIR has shown change of Si-H bonding states caused by the Sr growth at the initial monolayer growth stage. Furthermore, we have found the difference in neutron reflectivity profiles between the Sr layers grown on H- and D-terminated Si substrates. These results suggest the existence of buried monatomic hydrogen layer at the hetero-interface acting as an effective component of interface structure to manage the high lattice mismatch.

Photoetching of Silicon by N-Fluoropyridinium Salt

A photoetching method with -fluoropyridinium salts is proposed in this study. Si is etched by applying -fluoropyridinium salts to its surface and exposing the surface to light. The etched surface that uses liquid -fluoropyridinium salts is smoother than when solid -fluoropyridinium salts are used. The etching depth increases with exposure time. H-terminated hydrophobic Si is easier to etch than OH-terminated hydrophilic Si. is produced by photoetching. This suggests that -fluoropiridinium salts receive excited electrons from Si and supply Si with active F species.

Diodes based on bilayers comprising of tetraphenyl porphyrin derivative and fullerene for hybrid nanoelectronics

Abstract We demonstrate synthesis of diodes based on donor–acceptor bilayers consisting of fullerene (C 60 ) and tetraphenyl porphyrin derivative, that is, 5,10,15,20 tera(3-fluorophenyl)-porphyrin (TFPP), grafted on Si. These bilayer diodes were synthesized in two steps: (i) electrografting of a ∼3.5 nm C 60 layer on H-terminated Si and (ii) deposition of TFPP layers on C 60 layer by self-assembly process. Fourier-transform infrared spectroscopy data show that C 60 layer forms a supramolecular complex with TFPP at the interface. The current rectification ratio (defined as: RR = | I −1.8V /| I +1.8V ) for TFPP/C 60 molecular diodes was ∼1500. The rectification behavior has been understood using the ab initio molecular-orbital theoretical calculations.

비휘발성 메모리 응용을 위한 ALD법을 이용한 Al₂O₃ 절연막의 특성

We have successfully demonstrated of metal-insulator-semiconductor (MIS) capacitors with Al₂O₃/p-Si structures. The Al₂O₃ film was grown at 200 ℃ on H-terminated Si wafer by atomic layer deposition (ALD) system. Trimethylaluminum [Al(CH₃)₃, TMA] and H₂O were used as the aluminum and oxygen sources. A cycle of the deposition process consisted of 0.1 s of TMA pulse, 10 s of N₂ purge, 0.1 s of H₂O pulse, and 60 s of N₂ purge. The 5 nm thick Al₂O₃ layer prepared on Si substrate by ALD exhibited excellent electrical properties, including low leakage currents, no mobile charges, and a good interface with Si.

Au 8 cluster adsorption on Si(100):2 × 1 asymmetric surface studied by a first principles calculation

In this work, we investigate the adsorption of Au 8 cluster onto H-terminated Si(100):2 × 1 asymmetric surface by density functional theory within local density and generalized gradient approximations. We study the site-dependent shape of Au 8 cluster, adsorption energies, band structures and the corresponding charge distribution. We show that the electronic properties of the cluster and substrate complex change with the location of the cluster on the surface.

Quantum size effects in competing charge and spin orderings of dangling bond wires on Si(001)

Using spin-polarized density-functional theory calculations, we investigate the competition between charge and spin orderings in dangling-bond (DB) wires of increasing lengths fabricated on an H-terminated Si(001) surface. For wires containing less than ten DBs as studied in recent experiments, we find antiferromagnetic (AF) ordering to be energetically much more favorable than charge ordering. The energy preference of AF ordering shrinks in an oscillatory way as the wire length increases and preserves its sign even for DB wires of infinite length. The oscillatory behavior can be attributed to quantum size effects as the DB electrons fill discrete quantum levels. The predicted AF ordering is in startling contrast with the prevailing picture of charge ordering due to Jahn-Teller distortion or Peierls instability for wires of finite or infinite lengths, respectively.

Temperature-dependent transport in a sixfold degenerate two-dimensional electron system on a H-Si(111) surface

Low-field magnetotransport measurements on a high-mobility $(\ensuremath{\mu}=110,000\text{ }{\text{cm}}^{2}/\text{Vs})$ two-dimensional electron system on a H-terminated Si(111) surface reveal a sixfold valley degeneracy with a valley splitting $\ensuremath{\le}0.1\text{ }\text{K}$. The zero-field resistivity ${\ensuremath{\rho}}_{xx}$ displays strong temperature dependence for $0.07\ensuremath{\le}T\ensuremath{\le}25\text{ }\text{K}$ as predicted for a system with high degeneracy and large mass. We present a method for using the low-field Hall coefficient to probe intervalley momentum transfer (valley drag). The relaxation rate is consistent with Fermi-liquid theory but a small residual drag as $T\ensuremath{\rightarrow}0$ remains unexplained.

Diffusion and clustering of Au adatoms on H-terminated Si(111)-(1×1): A first principles study

We have examined the diffusion and agglomeration of Au adatoms on the H-terminated Si(111)-(1x1) surface using periodic slab density functional theory calculations. We find that a single Au atom favorably resides atop a surface Si atom by breaking an original identical withSi-H bond while the H atom is bonded to the Au atom in the vertical direction, leading to the identical withSi-Au-H state. Starting from the most favorable on-top (T) site, a Au adatom is predicted to undergo diffusion by moving in and out of the T site without disrupting surface Si-H bonds. The predicted overall activation energy for the Au diffusion is 0.5 eV. Our calculations show that Au agglomeration leads to libration of H atoms from the Au/Si interface, while the H atoms are weakly bound to Au clusters and subsequently undergo associative H(2) desorption with no significant barrier. Based on charge density analysis we also discuss bonding mechanisms for Au on H-terminated Si(111)-(1x1). Our findings are as a whole consistent with experimental results available in literature.

Surface Potential Changes Induced by Physisorption of Si-tagged Protein A on HF-last Si(100) and Thermally Grown SiO2 surfaces

We successfully detected Si-tagged protein A, attached to OH-terminated SiO2 and H-terminated Si surfaces by surface potential measurements using an AFM/Kelvin probe technique. Significant change in topographic images were observed after scratching the surface by contact mode AFM measurements indicating formation of multilayer film for Si-tagged protein A solution with concentration 100mg/l. The surface potential increased by a factor of 3 and 5 for OH-terminated SiO2 and H-terminated Si surfaces after the attachment of the Si-tagged protein A. We also observed significant change in surface potential after attachment of Si-tagged protein A with different pH. The surface potential increased with the increase of the pH of the Si-tagged protein A solution. It is suggested that pH of the biosolution is important to effectively physisorb the Si-tagged protein A. These results will open up the fabrication of a sensing device for locally ionized biomolecules.

Passivation of Si Surfaces Investigated by &lt;i&gt;In Situ&lt;/i&gt; Photoluminescence Techniques

We investigated Si surfaces modified by wet-chemical and electrochemical treatments using pulsed photoluminescence (PL) and infrared spectroscopic ellipsometry during and after processing, both also in surface mapping techniques. Etching of oxidized Si surfaces by HF containing solutions lead to an enhancement in PL due to hydrogenation of the surface what improves the surface passivation and reduces the recombination loss of charge carriers via surface/interface states. PL measurements show that the H-terminated surface is attacked soon by HF or H2O species increasing again the recombination loss. Hence, a narrow time window for this type of processing exists. Nitrogen purging or exchanging the etching solution by a non-etching solution under negative bias decelerated the defect formation in HF solutions. Grafting of organic molecules (exchanging the H-Si by a C-Si bond) induces only small amounts of defects at the interface but stabilizes PL on a high level (i.e. surface recombination is low) for much longer times than for H-terminated Si surfaces.