Hydrogen-terminated Si Research Articles

Hot carrier transfer from plasmon decay in Ag20at H-Si(111) surface: Real-time TDDFT simulation in Wannier gauge.

Plasmon decay is believed to play an essential role in inducing hot carrier transfer at the interfaces between plasmonic nanoparticles and semiconductor surfaces. In this work, we employ real-time time- dependent density functional theory (RT-TDDFT) simulation in the Wannier gauge to gain quantum- mechanical insights into the nonlinear dynamics of the plasmon decay in the Ag20 nanoparticle at a semiconductor surface. The first-principles simulations show that the plasmon decay is more than two times faster when the Ag20 nanoparticle is adsorbed on a hydrogen-terminated Si(111) surface, taking place within 100 femtoseconds of the plasmon excitation. Hot carrier transfer across the interface is observed as the plasmon decay takes place, and nearly 30% of holes are generated deep in the valence band of the semiconductor surface. The use of Wannier gauge in RT-TDDFT simulation is particularly convenient for gaining quantum-mechanical insights into non-equilibrium electron dynamics in complex heterogeneous systems.

Two-dimensional analysis of the interfacial solvation structure of an ionic liquid electrolyte on a hydrogen-terminated Si electrode by atomic force microscopy

Ionic liquids (ILs) have been intensively studied as new electrolytes for lithium-ion batteries (LIBs). Structural analysis of interfaces between an IL-based electrolyte and an LIB electrode would provide beneficial information for improving LIBs. In this study, we investigated the interfacial structures between an IL, 1-methyl-1-propyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide, and a H-terminated Si(111) electrode in the presence and absence of Li salt by frequency modulation atomic force microscopy utilizing a quartz tuning fork sensor. Two-dimensional frequency shift mapping imaging of the solvation structure at the interface showed that the layered solvation structure was only observed in the absence of Li salts in the ILs, which was in good agreement with our previous studies performed on IL/lithium titanate interfaces. Combined with electrochemical measurements, the partial disappearance of the layered solvation structure in the Li salt-doped IL was strongly suggested to be due to the Li-ion insertion/extraction at the IL/Si interface.

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.

High Energy Limit of the Size-Tunable Photoluminescence of Hydrogen-Terminated Porous Silicon Nanostructures in HF

The photoluminescence (PL) of various porous silicon (PSi) layers was studied during chemical dissolution in HF. The relative PL quantum efficiency of some layers was also monitored. Typically, the PL increased, reached a maximum and then dropped down to complete extinction, accompanied with a PL blueshift. During PL fall, both the PL intensity and layer quantum efficiency fell sharply, accompanied by a decrease in full width at half maximum and a slowing blueshift. In the final stage, the PL intensity decreased without any further blueshift, the saturated PL peak wavelength being ∼515 nm (∼2.4 eV) for most layers, identifying a high energy limit for the achievable PL of hydrogen-terminated Si nanostructures. Our results show that sudden catastrophic mechanical failure of nanostructure cannot explain the sharp PL drop and saturation of PL blueshift. Rather, they support the idea of a critical size (∼1.5–2 nm) below which the PL quantum efficiency vanishes. The possible reasons were discussed, privileging the emergence of structural non-radiative defects below a certain size, though the decreasing intrinsic quantum efficiency of Si nanocrystals with decreasing size could also play an important role. Maximum PL intensity was generally obtained for a peak wavelength of ∼565 nm (∼2.2 eV).

Photoelectrochemical and Nanogravimetric Study of Electrolytic Transformation of Silicon-Oxide Interface

While silica is chemically stable in most acid electrolytes, its properties could significantly change due to electrolyte absorption. We demonstrate the photoelectrochemical and nanogravimetric features that allow identification of electrolytic transformation of Si-SiO2 interface. To that end, photoresponsivity of p-type silicon with ultrathin thermal oxide layer (10 nm) was studied. The responses were compared with those of hydrogen-terminated Si surface. The quartz crystal nanogravimetry (QCN) characterized the transformation process with nanogram resolution in situ and in real time. In neutral solution (pH 7), electrolyte absorption along with some dissolution of the silica layer has been detected by the QCN. No silica dissolution indications were observed in an acid electrolyte; its absorption was about one third of the dry oxide mass. Other discussed phenomena included photoelectron generation, recombination, and charging of the oxide film. The observed effects could be generalized to other Si/oxide systems and this has been demonstrated with HfO2 and Al2O3 ultrathin layers produced by the atomic layer deposition (ALD). The obtained results enable us to properly understand photoresponsivity, passivity, and degradation of Si/oxide electrodes for applications in devices of solar energy conversion.

Hot Carrier Dynamics at Ligated Silicon(111) Surfaces: A Computational Study.

We provide a case-study for thermal grafting of benzenediazonium bromide onto a hydrogenated Si(111) surface using ab initio molecular dynamics (AIMD) calculations. A sequence of reaction steps is identified in the AIMD trajectory, including the loss of N2 from the diazonium salt, proton transfer from the surface to the bromide ion that eliminates HBr, and deposition of the phenyl group onto the surface. We next assess the influence of the phenyl groups on photophysics of hydrogen-terminated Si(111) slabs. The nonadiabatic couplings necessary for a description of the excited-state dynamics are calculated by combining ab initio electronic structures and reduced density matrix formalism with Redfield theory. The phenyl-terminated slab shows reduced nonradiative relaxation and recombination rates of hot charge carriers in comparison with the hydrogen-terminated slab. Altogether, our results provide atomistic insights revealing that (i) the diazonium salt thermally decomposes at the surface allowing the formation of covalently bonded phenyl group, and (ii) the coverage of phenyl groups on the surface slows down charge carrier cooling driven by electron-phonon interactions, which increases photoluminescence efficiency at the near-infrared spectral region.

Model-free capacitance analysis of electrodes with a 2D+1D dispersion of time constants

A constant phase element (CPE) is often used in electrochemical impedance spectroscopy to describe the non-ideal impedance of solid/electrolyte interfaces. The generalization of its mathematical description leads to the concept of a variable or generalized phase element αgpe, which, in association with Brug's expression, can be used to determine the interfacial capacitance of metallic electrodes, whose time-constant distribution is restricted to a bidimensional plane (C2D). In this work, a heuristic approach is used to extend the generalization principle even further, in order to arrive at an expression of capacitance, Cgpe, that is valid for electrodes with time-constant dispersions that also spread into the electrode bulk, causing a 2D+1D time-constant distribution. As a proof of concept, the proposed method is applied in the analysis of the electrochemical impedance of n-doped hydrogen-terminated Si(100) electrode in 100 mM Na2SO4, in a potential range that extends from accumulation to depletion. The single crystalline structure and polished surface of the electrode grants a quasi-ideal capacitive behavior to the system that allows comparison of the proposed analysis with the traditional representation of effective capacitance Ceff. Both methods yield similar results in the depletion region, where the impedance response is predominantly defined by the ideal behavior of the silicon space charge region. In accumulation, however, when impedance is influenced by the somewhat larger distribution of relaxation times of the electric double layer, Ceff shows frequency dispersion, whereas Cgpe does not. A dimensional analysis of the functional elements in Cgpe reveals the underlying reason for this intriguing behavior and also shows that C2D is, in fact, just an approximate expression of the more general Cgpe. It is also shown that the generalized capacitance representation allows for a clear detection of the onset of size border effects, and these appear at a higher frequency than what is observed in the Ceff representation.

Effect of reactant dosing on selectivity during area-selective deposition of TiO2 via integrated atomic layer deposition and atomic layer etching

A key hallmark of atomic layer deposition (ALD) is that it proceeds via self-limiting reactions. For a good ALD process, long reactant exposure times beyond that required for saturation on planar substrates can be useful, for example, to achieve conformal growth on high aspect ratio nanoscale trenches, while maintaining consistent deposition across large-area surfaces. Area-selective deposition (ASD) is becoming an enabling process for nanoscale pattern modification on advanced nanoelectronic devices. Herein, we demonstrate that during area-selective ALD, achieved by direct coupling of ALD and thermal atomic layer etching (ALE), excess reactant exposure can have a substantially detrimental influence on the extent of selectivity. As an example system, we study ASD of TiO2 on hydroxylated SiO2 (Si–OH) vs hydrogen-terminated (100) Si (Si–H) using TiCl4/H2O for ALD and WF6/BCl3 for ALE. Using in situ spectroscopic ellipsometry and ex situ x-ray photoelectron spectroscopy, we show that unwanted nucleation can be minimized by limiting the water exposure during the ALD steps. Longer exposures markedly increased the rate of nucleation and growth on the desired non-growth region, thereby degrading selectivity. Specifically, transmission electron microscopy analysis demonstrated that near-saturated H2O doses enabled 32.7 nm thick TiO2 patterns at selectivity threshold S > 0.9 on patterned Si/SiO2 substrates. The correlation between selectivity and reactant exposure serves to increase fundamental insights into the effects of sub-saturated self-limiting surface reactions on the quality and effectiveness of ASD processes and methods.

Electronic band structure of layers within meta generalized gradient approximation of density functionals

Semilocal exchange functionals are propitious in describing several properties of solids due to computational competence. For calculating band gaps of solids, the underestimation of generalized gradient approximation (GGA) functionals is known. The next rung of GGA in Jacob's ladder, i.e., meta-GGA, is expected to improve the band-gap calculation. In this regard, exchange-only potentials are very effective in producing accurate band gaps of solids. But, due to the presence of lattice-dependent parameters, these methods are incompetent for monolayers or slabs. Here, we show the potency of advance meta-GGA energy functionals in elucidating the band gaps of different layered structures. The recently proposed MGGAC meta-GGA functional by Patra et al. Phys. Rev. B 100, 155140 (2019) has proved to be very promising for band gaps within the semilocal treatment of density-functional theory. This conclusion is acquired by applying these functionals to encouraging examples of doped graphene, graphane, and halogenated graphene having insulating band gaps. Also, the improved values of energies of hydrogen-terminated Si(111) surface states are demonstrated with this meta-GGA functional.

Fast room-temperature functionalization of silicon nanoparticles using alkyl silanols.

Silicon nanoparticles (Si NPs) are a good alternative to conventional heavy metal-containing quantum dots in many applications, due to their low toxicity, low cost, and the high natural abundance of the starting material. Recently, much synthetic progress has been made, and crystalline Si NPs can now be prepared in a matter of hours. However, the passivation of these particles is still a time-consuming and difficult process, usually requiring high temperatures and/or harsh reaction conditions. In this paper, we report an easy method for the room-temperature functionalization of hydrogen-terminated Si NPs. Using silanol compounds, a range of functionalized Si NPs could be produced in only 1 h reaction time at room temperature. The coated NPs were fully characterized to determine the efficiency of binding and the effects of coating on the optical properties of the NPs. It was found that Si NPs were effectively functionalized, and that coated NPs could be extracted from the reaction mixture in a straightforward manner. The silanol coating increases the quantum yield of fluorescence, decreases the spectral width and causes a small (∼50 nm) blue-shift in both the excitation and emission spectra of the Si NPs, compared to unfunctionalized particles.

Initial growth analysis of ALD Al2O3 film on hydrogen-terminated Si substrate via in situ XPS

The initial stage of Al2O3 films deposited by atomic layer deposition (ALD) on a H-terminated Si(001) wafer was investigated with in situ X-ray photoelectron spectroscopy. At deposition temperatures of 250 °C and 300 °C, no Al peak was detected for the first 10 ALD cycles. An Al peak was first observed at ALD cycle 12, and its intensity increased with increasing ALD cycle number. The 10 initial ALD cycles were defined as an incubation period. The Si 2p and Al 2p narrow spectra indicated that Si sub-oxide formed during the incubation period and trimethylaluminum (TMA) adsorbed on it after the incubation period. The incubation period at a growth temperature of 350 °C decreased to five ALD cycles because the H2O exposure at a higher temperature promoted the oxidation of the H-terminated Si surface and formed Si sub-oxide.

Molecular Strategies to Modulate the Electrochemical Properties of P-Type Si(111) Surfaces Covalently Functionalized with Ferrocene and Naphthalene Diimide.

The surface coverage and molecular composition of redox-active molecules anchored on conductive surfaces regulate the kinetic and thermodynamic parameters of charge transfer reactions, providing a means to tune the electrochemical properties of hybrid materials. Herein, anchoring strategies and structural properties of redox-active probes, derived from ferrocene (Fc) and naphthalene diimide (NDI), are shown to regulate the electrochemical properties of functionalized p-doped Si(111) surfaces. Covalent functionalization of hydrogen-terminated Si(111) surfaces with Fc and NDI affords redox-active hybrid interfaces characterized through microscopy, spectroscopy, and voltammetry methods. Molecular design and synthetic grafting strategies modulate the electrochemical properties of the Fc-functionalized Si surfaces with a much higher (ca. 25 times) surface coverage (1.25 × 10-10 mol cm-2) for one-step photografting compared to divergent synthetic routes. Interestingly, the thermal grafting of an alkadiyne followed by "click" reaction with ferrocenyl-azide leads to one of the highest surface coverages (9.97 × 10-10 mol cm-2) of organo-iron reported and a significant anodic shift of the half-potential (>350 mV) compared to photografting methods. Similar experiments with NDI units exhibited electrochemical properties that diverge from those recorded for NDI in solution. The results presented herein offer access to novel redox-active Si interfaces that evidence tunable electrochemical properties of potential interest for microelectronic applications.

Estimation of Molecular Orientation Using Water Drop Angle

In self-assembled monolayer, SAM, molecules are attracted each other to be closely packed. Such a structural feature causes the stable mechanical, chemical, and thermo-dynamical properties. In addition, recent preparation techniques allowed the direct electronic coupling between organic functionality and Si substrate. As the results, SAMs possess great potential in the fields of electrochemical sensors and molecular electronics. From the viewpoint of application, it is desirable to precisely estimate the molecular orientation in SAMs. In the present study, we propose the structural analysis using water drop angle. To form hexadecyl-SAMs, hydrogen-terminated Si substrate was soaked in 1-hexadecene under UV light for 2 hours. UV light is capable of breaking Si-H bond homolytically, which yields the Si dangling bonds that can act as the starting point for radical chain propagation. When a droplet is attached to SAM and the SAM surface is tilted little by little, the droplet will lunge forward and finally slide downward. The tilting angle when the droplet starts sliding is called ‘water drop angle’, WDA. We measured WDA by an original system, in which sample surface could be simultaneously tilted and rotated from 0 to 90 and from 0 to 360 degrees, respectively. Figure 1(a) shows the typical measurement results for the hexadecyl-SAMs. WDAs were found to considerably change with the rotation angles. In addition, the period in the WDA change was nearly equal to 180 degrees. Here in-plane rotation angle was defined the orientation flat of the wafer, Si(111)<110>, as 0 degree. We believe that the periodic change is in relation to the molecular orientation. Namely, when a sample is tilted in the direction as shown in Fig. 1(b), a droplet is hooked by the SAM molecules, resulting in the large WDAs. On the other hand, in the case of Fig. 1(c), the friction between a droplet and the SAM surface is expected to be weak, corresponding to the small WDAs. The maximum WDAs for the hexadecyl-SAMs were observed at the rotation angles about 210 degrees. To justify these experimental results, it is extremely important to notice that the sterific hindrance between the (Si-C-)C-H bonds of the SAM molecules and the surrounding unreacted Si-H bonds should be minimum in six molecular orientation. As a result, the WDA reasonably exhibited the maximum value at the rotation angles of 210 degrees. In the ECS meeting, we will indicate that our measurement method is useful to estimate the orientation direction of the molecules covalently-bonded to the solid surface. Figure 1

Morphological evolution of Ag nanoclusters grown on hydrogen-terminated si(111)-(1 × 1) surface: Appearance of quantum size effect at room-temperature

We have observed the early stages of Ag nanocluster evolution on a hydrogen-terminated Si(111)-(1 × 1) surface at room temperature (RT) by using scanning tunneling microscopy and low energy electron diffraction, and examined changes in number density, height, base diameter, volume and shapes of the nanocluster with a deposition amount from 0.2 to 8 ML. It is found that Ag nanoclusters evolve by repeating the stages of nucleation/growth and coalescence, and change the growth morphology and the growth mode after the coalescence. Transition from initially formed dome-like three dimensional clusters to flat-top two dimensional (2D) islands evidently shows that quantum size effect emerges during the growth process of Ag nanoclusters even at RT. The characteristic stability of 8 atomic layer height in the 2D islands is well interpreted based on a free-electron gas model for free-standing nanofilms.

Atomistic investigation on the initial stage of growth and interface formation of Fe on H-terminated Si(111)-(1 × 1) surface

The initial growth process of a Fe thin film on a hydrogen-terminated Si(111)-(1 × 1) surface is investigated through direct atomic scale measurements by using scanning tunneling microscopy (STM) and spectroscopy (STS). By depositing Fe onto the substrate at room temperature, nanometer-size Fe clusters having a (111)-oriented body-centered-cubic structure are formed on the substrate surface with maintaining the (1 × 1) structure. As the clusters grow, hill-like structures composed of {110} nano-facets appear on the surface of the Fe clusters. In the empty-state STM image, a depression of the substrate surface is observed at the periphery of a Fe nanocluster. The depression indicates the formation of a Schottky junction between the Fe nanocluster and the substrate surface.

Epitaxial growth of a deposited amorphous Si layer formed on hydrogen-terminated Si(001) surfaces by ion beam induced epitaxial crystallization combined with ion beam mixing

We have investigated the crystallization process of an amorphous Si layer deposited on hydrogen-terminated Si(001) surfaces by ion beam induced epitaxial crystallization (IBIEC) combined with the ion beam mixing (IBMX) method, aiming at the clarification of the essential IBMX effect on crystallization. In this study, we deposited a 10−15 nm thick amorphous Si layer on clean and two kinds of H-terminated Si(001) surfaces of dihydride (DH) and monohydride (MH), and then carried out the IBMX by 10-keV Si+ ion irradiation with a fluence of 1 × 1015 ions cm−2 at RT near the amorphous/crystalline interface followed by the IBIEC treatment by 180-keV Ar+ ion irradiation with fluences of 5 × 1015 and 1 × 1016 ions cm−2 at 300 °C–500 °C. The thickness of the crystallized layer was quantitatively measured by the Rutherford backscattering channeling method. We found that the epitaxial crystallization of the amorphous Si layer formed on both DH and MH surfaces can be realized by IBMX followed by IBIEC treatments at significantly lower temperatures than 500 °C in all processes. This fact suggests that the epitaxial crystallization of the amorphous layer formed on the crystalline substrate surface terminated by the obstructive layer is effectively enhanced, regardless of the structure and coverage of the interfacial layers. It should be emphasized that the IBIEC combined with IBMX treatments make it possible to epitaxially crystallize the amorphous layers formed on obstructive layers covering crystalline substrates at significantly low temperatures.

Growth and chemical modification of silicon nanostructures templated in molecule corrals: Parallels with the surface chemistry of single crystalline silicon

Molecule corrals having diameters of 30–50 nm were created on highly oriented pyrolytic graphite (HOPG) using cesium ion bombardment. The molecule corrals were used as templates to grow silicon nanostructures by physical vapor deposition (PVD). The nanostructures could be grown with control over geometry (rings and mesas), and size distribution. In addition, transmission electron microscopy (TEM) results suggest that the silicon nanostructures are most likely polycrystalline.The chemical modification of these silicon nanostructures with nitrobenzene was compared to that of clean and hydrogen-terminated single crystalline silicon. X-ray photoelectron spectroscopy (XPS) of the modified nanostructures showed peaks located at 398.9 eV, 400.4 eV, and 402.1 eV for the N 1s region, which are consistent with those observed on a Si(100) single crystal. The chemical modification was further characterized by the presence of nitrogen-containing peaks in TOF-SIMS spectra. We conclude that the reaction of nitrobenzene on silicon nanostructures provides evidence that the reactivity of the nanostructures is similar to that of hydrogen-terminated Si(111) and Si(100).

&amp;lt;i&amp;gt;In Situ&amp;lt;/i&amp;gt; Imposing Bias ATR-FTIR Observation at Hydrogen Terminated Si(111) Electrode Surface-Modified with Adsorbed Monolayer

Since hydrogen-terminated Si surface has hydrophobicity, it is expected that adsorbed monomolecular film of surfactant will be formed on the Si surface in aqueous solution containing the surfactant. Such an adsorbed monolayer film is very effective for the development of a functional electrode. In this study, we have investigated the state of adsorption about an aerosol OT as the monolayer on the electrode surface and its orientation with hydrogen-terminated Si(111) surface by in situ ATR-FTIR spectroscopy. At this time, in situ observation performed while imposing bias to the electrode. The results suggested that the aerosol OT were desorbed by the oxidation of back-bonds in the Si atoms on the electrode surface under the imposing noble potential, although no change was observed especially when imposing less-noble potential.

Dual Liquid Junction Photoelectrochemistry: Part II. Open-Circuit Photovoltage Variations Due to Surface Chemistry, Interfacial Dipoles, and Non-Ohmic Junctions at Back Contacts

Dual-liquid-junction photoelectrochemistry and finite-element computational modeling quantified the effect on open-circuit photovoltage, Voc, of varying barrier heights at the back, traditionally ohmic contact to a semiconductor. Variations in experimental back-contact barrier heights included changes in the redox potential energy of the contacting phase afforded by a series of nonaqueous, metallocene-based redox couples that demonstrate facile, one-electron transfer and dipole-based band edge shifts due changes in the chemical species at the semiconductor surface. Variation in semiconductor surface chemistry included hydrogen-terminated Si(111) as well as methyl-terminated Si(111) that yields a shift in band-edge alignment of ∼0.3 eV relative to hydrogen termination. While methylation of n-Si improves Voc values at rectifying contacts, methylation at an ohmic contact has a deleterious impact on Voc values. We discuss the present experimental and computational results in the context of non-ideal semiconductor contacts.

The growth of metastable fcc Fe78Ni22 thin films on H-Si(1 0 0) substrates suitable for focused ion beam direct magnetic patterning

We have studied the growth of metastable face-centered-cubic, non-magnetic Fe78Ni22 thin films on silicon substrates. These films undergo a magnetic (paramagnetic to ferromagnetic) and structural (fcc to bcc) phase transformation upon ion beam irradiation and thus can serve as a material for direct writing of magnetic nanostructures by the focused ion beam. So far, these films were prepared only on single-crystal Cu(1 0 0) substrates. We show that transformable Fe78Ni22 thin films can also be prepared on a hydrogen-terminated Si(1 0 0) with a 130-nm-thick Cu(1 0 0) buffer layer. The H-Si(1 0 0) substrates can be prepared by hydrofluoric acid etching or by annealing at 1200 °C followed by adsorption of atomic hydrogen. The Cu(1 0 0) buffer layer and Fe78Ni22 fcc metastable thin film were deposited by thermal evaporation in ultra-high vacuum. The films were consequently transformed in-situ by 4 keV Ar+ ion irradiation and ex-situ by a 30 keV Ga+ focused ion beam, and their magnetic properties were studied by magneto-optical Kerr effect magnetometry. The substitution of expensive copper single crystal substrate by standard silicon wafers dramatically expands application possibilities of metastable paramagnetic thin films for focused-ion-beam direct magnetic patterning.