Dangling-bond Research Articles

The reason why thin-film silicon grows layer by layer in plasma-enhanced chemical vapor deposition.

Thin-film Si grows layer by layer on Si(001)-(2 × 1):H in plasma-enhanced chemical vapor deposition. Here we investigate the reason why this occurs by using quantum chemical molecular dynamics and density functional theory calculations. We propose a dangling bond (DB) diffusion model as an alternative to the SiH3 diffusion model, which is in conflict with first-principles calculation results and does not match the experimental evidence. In our model, DBs diffuse rapidly along an upper layer consisting of Si-H3 sites, and then migrate from the upper layer to a lower layer consisting of Si-H sites. The subsequently incident SiH3 radical is then adsorbed onto the DB in the lower layer, producing two-dimensional growth. We find that DB diffusion appears analogous to H diffusion and can explain the reason why the layer-by-layer growth occurs.

Contact effects and quantum interference in engineered dangling bond loops on silicon surfaces.

Dangling bond structures created on H-passivated silicon surfaces offer a novel platform for engineering planar nanoscale circuits, compatible with conventional semiconductor technologies. In this investigation we focus on the electronic structure and quantum transport signatures of dangling bond loops built on H-passivated Si(100) surfaces contacted by carbon nanoribbons, thus leading to a two-terminal planar, nanoscale setup. The computational studies were carried out to rationalize the influence of the local atomic-scale contacts of the dangling bond system to the mesoscopic electrodes as well as the possibility of revealing quantum interference effects in the dangling bond loops. Our results reveal a strong sensitivity of the low-energy quantum transmission to the loop topology and to the atomistic details of the electrode-loop contact. Varying the length of the loop or the spatial position of at least one of the electrodes has a drastic impact on the quantum interference pattern; depending on whether constructive or destructive interference within the loop takes place, the conductance of the system can be tuned over several orders of magnitude, thus suggesting the possibility of exploiting such quantum mechanical effects in the design of two-dimensional, atomic-scale electronic devices such as logic gates.

Hydrogen dangling bonds induce ferromagnetism in two-dimensional metal-free graphitic-C3N4 nanosheets.

Ferromagnetic two-dimensional (2D) ultrathin nanosheets hold great promise for next generation electronics. Ferromagnetic metal-free materials that usually possess only an s/p electronic configuration with weak spin-orbit coupling and a large spin relaxation time, would play an important role in constructing future spintronic devices. However, the absence of an intrinsic spin ordering structure in most metal-free materials greatly hampers the widening scope of ferromagnetic 2D nanostructures as well as in-depth understanding of their ferromagnetic nature. Herein, the induction of intrinsic ferromagnetism in 2D metal-free g-C3N4 ultrathin nanosheets has been achieved through a new effective strategy whereby hydrogen dangling bonds are introduced. In our case, g-C3N4 ultrathin nanosheets with hydrogen dangling bonds showed obvious room temperature ferromagnetic behavior that could even be tuned by the concentration of hydrogen. This work will pave a new pathway to engineer the properties of 2D nanomaterial systems.

Engineering the Effective p-Type Dopant in GaAs/InAs Core–Shell Nanowires with Surface Dangling Bonds

Using first-principles calculation based on density-functional theory, we investigated the effect of surface dangling bond on p-type doping mechanism and the electronic structures in wurtzite (WZ) and zinc blende (ZB) GaAs/InAs core–shell nanowires (NWs) along the [0001] and [111] directions, respectively. The results of the formation energies show that the surface dangling bond of the In atom is a kind of stable defect. Both in WZ and ZB core–shell NWs, we found it is easier and more stable to realize dopant in the GaAs core. Moreover, the position of Cd impurity plays a key role in the formation of p-type nanowires. The farther the distance between the impurity and the surface dangling In atom, the easier it is to form the p-type characteristic of the nanowires. In particular, it shows an intrinsic behavior when doping the Cd impurity near the surface dangling bond. The surface dangling bonds have an ability to capture the holes from the neighbor doping impurity, resulting in the deactivation of dopants. Meanwhile, the transfer of hole moves the valence band down to the lower energy levels and even can lead to a band anticrossing phenomenon in the conduction band. Our results highlight a new physical coupling between the doped state and surface dangling bonds in GaAs/InAs core–shell NWs, and open a new opportunity for the development of tailoring nanoscale electronic properties.

Hole Dangling Bond Capture Cross-Sections in a-Si:H

It is demonstrated that the occupation statistics for a Gaussian distribution of dangling bond states may account for the measured variation of hole mob ility-lifetime values in hydrogenated amorphous silicon as the Fermi energy is systematically varied by doping fro m about 0.55 eV to 1.05 eV belo w the conduction band edge. An assessment of how the deduced dangling bond parameters may be influenced by underlying doping effects suggests that the min imu m cross-section ratio for hole capture into charged (σh - ) and neutral (σh 0 ) dangling bond states requires that σh - /σh 0 ≥ 5. The capture of holes is consequently dominated by charged dangling bonds provided the Fermi energy lies within the upper half of the band-gap. Both σh - and σh 0 are observed to depend upon temperature (T) as σh ∝ T -β wh ich may indicate the presence of tunnelling

Charge transition level of GePb1 centers at interfaces of SiO2/GexSi1−x/SiO2 heterostructures investigated by positron annihilation spectroscopy

In this work, we address the charge trapping properties of Ge dangling bond (DB) defects – GePb1 centers as typified by electron spin resonance spectroscopy (ESR) – found at the interfaces between condensation‐grown Si1−xGex (0.28 < x < 0.8) alloys and insulating SiO2. The ESR observation of singly‐occupied paramagnetic GePb1 centers, carried out at 4.3 K, is complemented by temperature‐dependent positron annihilation spectroscopy (PAS) in the Doppler broadening mode, which enables observation of the neutral‐to‐negative defect transitions as the temperature increases from 50 to 300 K. Through correlation of this re‐charging behavior with the temperature‐induced shift of the Fermi energy, the energy of the Ge DB −/0 transition in the Si0.27Ge0.73 alloy is inferred to be energetically distributed in a ∼0.1 eV interval above the top of the semiconductor valence band. This result refines previous estimates from capacitance–voltage measurements, thus providing independent affirmation that the energy levels of Ge DBs lie inside the band gap of the Si1−xGex alloys.

Antiferromagnetic superexchange mediated by a resonant surface state in Sn/Si(111)

The Sn overlayer on the Si(111) surface has been considered as a prototypical system for exploring two-dimensional (2D) correlated physics on the triangular lattice. Most of the previous theoretical studies were based on the presumption that the surface state dominantly originates from Sn dangling-bond (DB) electrons, leading to a strongly correlated 2D electronic system. By contrast, our density-functional theory calculations show that the Sn DB state significantly hybridizes with Si substrate states to form a resonant state. The strong resonance between the $\mathrm{Sn}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}5{p}_{z}$ and $\mathrm{Si}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}3{p}_{z}$ orbitals facilitates the recently observed antiferromagnetic order through superexchange interactions, giving rise to a band-gap opening. It is thus demonstrated that the insulating ground state of Sn/Si(111) can be characterized as a Slater-type insulator via band magnetism.

Evaluation of the characteristic curves of a-Si:H based devices with the Simmons-Taylor approximation when the defect pool model is used

The performance of a-Si:H devices is highly sensitive to the density of gap states: tail states are distributed in two exponentials and defect states are generated by dangling bonds (DB). The density of DB in a-Si:H can be evaluated with the defect pool model (DPM). Charge trapping and recombination of electron–hole pairs through tail states are described by the Shockley–Read–Hall (SRH) formalism while defect states behave as amphoteric. Equations derived with the SRH formalism can be simplified with the Simmons–Taylor's approximation (STA), especially with the “0 K” approximation (0KSTA). Amphoteric-like defect states were approximated by donor- and acceptor-like decoupled states (DSA). The accuracy of STA was tested in a-Si:H based devices when the density of DB is evaluated with the DPM for different illumination conditions, voltages, temperatures, and some key electrical parameters. Our code was modified to include both the STA and the DSA. Our results indicate that the STA is very accurate under illuminated conditions. Under dark conditions, the STA is acceptable for forward voltages but overestimates the dark current at reverse voltages. The 0KSTA can be used under illuminated conditions for any applied voltage and under dark conditions for forward voltages.

Invited; Nature of Point Defects at High-Mobility Semiconductor/Interfaces Probed by Electron Spin Resonance: Thermal GaAs/GaAs-Oxide Structures

The introduction of III-V semiconductors, such as GaAs, as high-mobility channels in field effect transistors has been hindered by the presence of a large density of electron states (defects) at the GaAs/oxide interface1, leading to strong Fermi-level (Ef) pinning, the origin of which is not fully atomically identified and quantified. This lack of adequate interface (surface) passivation has been a main obstacle on the route to development of high-performance III-V MOS technology. As to the atomic origin of the interface states causing Fermi level pinning, on theoretical grounds, various types of native point defects such as interfacial As-As dimers (like-atom bonds), AsGa antisites, Ga dangling bonds (DBs), and elemental As, have been projected with levels in the GaAs band gap2. X-ray photoelectron spectroscopy has concluded that elimination of interfacial As-oxides is a key requisite for Ef unpinning3. Aiming atomic identification of electrically detrimental interface traps, we here present results of a first experimental extensive low-temperature multi-frequency (K-and Q-band) study by conventional ESR -the tool of choice when it comes to atomically identify paramagnetic point defects- of thermally grown c-GaAs/GaAs-oxide entities. In an exploring approach, for ESR sensitivity reasons, two groups of samples were prepared: (1) GaAs powders, thermally oxidized (O2; 1.1 atm) in the temperature range Tox=350 - 615 °C for various times (22-390 min). (2) Oxidized (430–555 °C) slices (2 ´ 10 mm2), cut with their long edge along a á1-10ñ direction. As a major GaAs-related result, the study reveals the generation of AsGa + antisites upon thermal oxidation in registry with the GaAs substrate. Importantly, no such signal could be observed on a stack of slices of the as-received (100)GaAs substrates. Evaluated from Tox~350 °C onward, the generation is observed in densities increasing with Tox, reaching alarmingly high levels (~1×1013 cm-2), thus providing solid independent evidence of substantial interfacial As enrichment, appearing as endemic for oxidation of GaAs. Comparison with literature data, based on salient ESR parameters, would indicate the observed AsGa+ antisite to concern the electrically well identified EL2 deep double donor center. It thus provides an answer on atomic scale of how a major part of excess As gets incorporated at the interface. Besides AsGa +, four more ESR signals have been observed. An eye catching one concerns an anisotropic behaving signal, which is only observed on oxidized c-GaAs slices after additional vacuum-UV irradiation (10.02 eV). The spectrum is composed of a quartet centered at g~2.27 with average hyperfine splitting constant A= 87 G for the applied magnetic field along the (100)GaAs sample normal; it might stem from a VGa center. Other single-component isotropic behaving spectra are observed at g~2.06 and g~1.937. Also observed is the substitutional Fe dopant (quintet ESR spectrum) in the parent GaAs substrate, centered at g~ 2.07. The atomic nature of the originating point defects is addressed starting from a comparative analysis of the data obtained on oxidized c-GaAs powders and (100)GaAs slices. In summary, the current ESR study on thermally oxidized c-GaAs has revealed several types of point defects. This includes the AsGa + (EL2) antisite as a predominant defect, the work thus providing convincing first atomic identification of an inherent detrimental interface defect. As to realization of device-grade semiconductor/insulator interfaces, it is clear that oxidation of the GaAs substrate interface (surface) is to be strictly avoided. 1 C. W. Wilmsen, in Physics and Chemistry of III-V Compound Semiconductor Interfaces (Plenum, New York, 1985). 2 J. Robertson, Microelectron. Eng. 86, 1558 (2009; L. Lin and J. Robertson, Appl. Phys. Lett. 98, 082903 (2011). 3 W. Wang, C. L. Hinkle, E. M. Vogel, K. Cho, and R. M. Wallace, Microelectron. Eng. 88, 1061 (2011).

5 MeV Proton and 15 MeV Electron Radiation Effects Study on 4H-SiC nMOSFET Electrical Parameters

The impact of proton and electron irradiations on the electrical parameters of 4H-SiC nMOSFETs has been investigated by the time bias stress instability method. This study has allowed observing the effect of holes trapped in the gate oxide together with the generated interface traps. Improvements of important electrical parameters, such as the threshold voltage, the effective mobility and the maximum drain current were observed. These improvements could be connected with the Nitrogen and residual Hydrogen atoms diffusion from SiO 2 /SiC the interface toward the epilayer during irradiation. These atoms are likely to create other bonds by occupying the Silicon and Carbon's dangling bond vacancies. This way, the number of passivated Carbon atoms is increased, hence improving the SiO 2 /SiC interface quality.

Hydrogen interaction kinetics of Ge dangling bonds at the Si0.25Ge0.75/SiO2 interface

The hydrogen interaction kinetics of the GePb1 defect, previously identified by electron spin resonance (ESR) as an interfacial Ge dangling bond (DB) defect occurring in densities ∼7 × 1012 cm−2 at the SiGe/SiO2 interfaces of condensation grown (100)Si/a-SiO2/Ge0.75Si0.25/a-SiO2 structures, has been studied as function of temperature. This has been carried out, both in the isothermal and isochronal mode, through defect monitoring by capacitance-voltage measurements in conjunction with ESR probing, where it has previously been demonstrated the defects to operate as negative charge traps. The work entails a full interaction cycle study, comprised of analysis of both defect passivation (pictured as GePb1-H formation) in molecular hydrogen (∼1 atm) and reactivation (GePb1-H dissociation) in vacuum. It is found that both processes can be suitably described separately by the generalized simple thermal (GST) model, embodying a first order interaction kinetics description based on the basic chemical reactions GePb1 + H2 → GePb1H + H and GePb1H → GePb1 + H, which are found to be characterized by the average activation energies Ef = 1.44 ± 0.04 eV and Ed = 2.23 ± 0.04 eV, and attendant, assumedly Gaussian, spreads σEf = 0.20 ± 0.02 eV and σEd = 0.15 ± 0.02 eV, respectively. The substantial spreads refer to enhanced interfacial disorder. Combination of the separately inferred kinetic parameters for passivation and dissociation results in the unified realistic GST description that incorporates the simultaneous competing action of passivation and dissociation, and which is found to excellently account for the full cycle data. For process times ta ∼ 35 min, it is found that even for the optimum treatment temperature ∼380 °C, only ∼60% of the GePb1 system can be electrically silenced, still far remote from device grade level. This ineffectiveness is concluded, for the major part, to be a direct consequence of the excessive spreads in the activation energies, ∼2–3 times larger than for the Si DB Pb defects at the standard thermal (111)Si/SiO2 interface which may be easily passivated to device grade levels, strengthened by the reduced difference between the average Ef and Ed values. Exploring the guidelines of the GST model indicates that passivation can be improved by decreasing Tan and attendant enlarging of ta, however, at best still leaving ∼2% defects unpassivated even for unrealistically extended anneal times. The average dissociation energy Ed ∼ 2.23 eV, concluded as representing the GePb1-H bond strength, is found to be smaller than the SiPb-H one, characterized by Ed ∼ 2.83 eV. An energy deficiency is encountered regarding the energy sum rule inherent to the GST-model, the origin of which is substantiated to lie with a more complex nature of the forward passivation process than basically depicted in the GST model. The results are discussed within the context of theoretical considerations on the passivation of interfacial Ge DBs by hydrogen.

Competing magnetism inπ-electrons in graphene with a single carbon vacancy

One intriguing finding in graphene is the vacancy-induced magnetism that highlights the interesting interaction between local magnetic moments and conduction electrons. Within density functional theory, the current understanding of the ground state is that a Stoner instability gives rise to ferromagnetism of $\pi$ electrons aligned with the localized moment of a $\sigma$ dangling bond and the induced $\pi$ magnetic moments vanish at low vacancy concentrations. However, the observed Kondo effect suggests that $\pi$ electrons around the vacancy should antiferromagnetically couple to the local moment and carry non-vanishing moments. Here we propose that a phase possessing both significant out-of-plane displacements and $\pi$ bands with antiferromagnetic coupling to the localized $\sigma$ moment is the ground state. With the features we provide, it is possible for spin-resolved STM, STS, and ARPES measurements to verify the newly proposed phase.

Single-Electron Dynamics of an Atomic Silicon Quantum Dot on theH−Si(100)−(2×1)Surface

Here we report the direct observation of single electron charging of a single atomic dangling bond (DB) on the H-Si(100)-2×1 surface. The tip of a scanning tunneling microscope is placed adjacent to the DB to serve as a single-electron sensitive charge detector. Three distinct charge states of the dangling bond--positive, neutral, and negative--are discerned. Charge state probabilities are extracted from the data, and analysis of current traces reveals the characteristic single-electron charging dynamics. Filling rates are found to decay exponentially with increasing tip-DB separation, but are not a function of sample bias, while emptying rates show a very weak dependence on tip position, but a strong dependence on sample bias, consistent with the notion of an atomic quantum dot tunnel coupled to the tip on one side and the bulk silicon on the other.

Electronic effects of isolated halogen atoms on the Ge(001) surface

The asymmetric slab approach and density functional theory (DFT) are used to discuss the interaction of isolated halogen atoms (F, Cl, Br and I) with the Ge(001)c(4×2) surface. For each adsorbate on thick slabs we find two stable adatom configurations separated by ~0.1eV. In both configurations, the unsaturated dangling bonds (DB) of the chemisorbed dimers are fully charged due to the interaction of the DB-derived surface states with the bulk states. This behavior is found to be insensitive to whether the calculations are performed using standard ground-state DFT, or DFT supplemented by MBJLDA and spin-orbit corrections to produce an accurate surface electronic structure and energy gap. On chemisorbed ultrathin Ge slabs, however, these unsaturated DB's are empty for the lowest energy configurations. This results in different geometries and energetics for the chemisorbed dimers. A detailed analysis of the predicted scanning tunneling microscopy images of halogen-chemisorbed surfaces is also presented. It is predicted that the halogen atoms may image darkly, yielding filled-state images similar to that of the clean surface, while the empty-state images should be noticeably different.

Benzaldehyde on Water-Saturated Si(001): Reaction with Isolated Silicon Dangling Bonds versus Concerted Hydrosilylation

Despite strong similarities due to the common presence of silicon monohydrides and isolated silicon dangling bonds (silicon radicals), the water-saturated Si(001)-2 × 1 surface and the hydrogen-ter...

A low thermal impact annealing process for SiO2-embedded Si nanocrystals with optimized interface quality

Silicon nanocrystals (Si NCs) for 3rd generation photovoltaics or optoelectronic applications can be produced by several industrially compatible physical or chemical vapor deposition technologies. A major obstacle for the integration into a fabrication process is the typical annealing to form and crystallize these Si quantum dots (QDs) which involves temperatures ≥1100 °C for 1 h. This standard annealing procedure allows for interface qualities that correspond to more than 95% dangling bond defect free Si NCs. We study the possibilities to use rapid thermal annealing (RTA) and flash lamp annealing to crystallize the Si QDs within seconds or milliseconds at high temperatures. The Si NC interface of such samples exhibits huge dangling bond defect densities which makes them inapplicable for photovoltaics or optoelectronics. However, if the RTA high temperature annealing is combined with a medium temperature inert gas post-annealing and a H2 passivation, luminescent Si NC fractions of up to 90% can be achieved with a significantly reduced thermal load. A new figure or merit, the relative dopant diffusion length, is introduced as a measure for the impact of a Si NC annealing procedure on doping profiles of device structures.

Investigation of hydrogen plasma treatment for reducing defects in silicon quantum dot superlattice structure with amorphous silicon carbide matrix

We investigate the effects of hydrogen plasma treatment (HPT) on the properties of silicon quantum dot superlattice films. Hydrogen introduced in the films efficiently passivates silicon and carbon dangling bonds at a treatment temperature of approximately 400°C. The total dangling bond density decreases from 1.1 × 1019 cm-3 to 3.7 × 1017 cm-3, which is comparable to the defect density of typical hydrogenated amorphous silicon carbide films. A damaged layer is found to form on the surface by HPT; this layer can be easily removed by reactive ion etching.

Spurious dangling bond formation during atomically precise hydrogen depassivation lithography on Si(100): The role of liberated hydrogen

The production of spurious dangling bonds during the hydrogen depassivation lithography process on Si(100)-H is studied. It is shown that the number of spurious dangling bonds produced depends on the size of the primary pattern on the surface, not on the electron dose, indicating that the spurious dangling bonds are formed via an interaction of the liberated hydrogen with the surface. It is also shown that repassivation may occur if hydrogen depassivation lithography is performed near an already patterned area. Finally, it is argued that the product of the interaction is a single dangling bond next to a monohydride silicon on a silicon dimer, with a reaction probability much in excess of that previously observed.

Characterization of 14C in neutron irradiated NBG-25 nuclear graphite

Recent studies suggest that the highest concentration of 14C contamination present in reactor-irradiated graphite exists on the surfaces and within near-surface layers. Surface-sensitive analysis techniques (XPS, ToF-SIMS, SEM/EDS and Raman) were employed to determine the chemical nature of 14C on irradiated NBG-25 (nuclear grade) graphite surfaces. Several 14C precursor species are identified on the surfaces of irradiated NBG-25; the quantities of these species decrease at sub-surface depths, which further suggests that 14C formation is predominantly a surface-concentrated phenomenon. The elevated presence of several surface oxide complexes on irradiated NBG-25 surfaces are attributed directly to neutron irradiation. Larger numbers of oxide bonds were found on irradiated NBG-25 surfaces (when compared to unirradiated samples) in the form of interlattice (e.g. ether) and dangling (e.g. carboxylate and ketone) bonds; the quantities of these bond types also decrease with increasing sub-surface depths.

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

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