Silicon Center Research Articles

Creation of glass-characteristic point defects in crystalline SiO2 by 2.5 MeV electrons and by fast neutrons

Point defects in crystalline SiO2, created by 2.5 MeV electron irradiation at dose below the amorphization threshold or by fast neutrons, were compared by luminescence spectroscopy. Oxygen dangling bonds (“non-bridging oxygen hole centers”, NBOHCs), peculiar to amorphous state of SiO2, were detected for the first time in electron-irradiated non-amorphized α-quartz crystal. Their presence may signal the formation of nucleation centers in crystal structure as the first step to radiation-induced amorphization. Compared to crystal, irradiated by 1019 cm−2 fast neutrons, their concentration was over 100 times lower, and their inhomogeneous broadening was at least 2.5 times smaller. Divalent silicons (“silicon oxygen deficiency centers”, SiODC(II)), inherent to oxygen-deficient or irradiated SiO2 glass, were detected in neutron-irradiated (1019 n/cm2) α-quartz but were not found after the electron irradiation. Radiation-induced interstitial O2 molecules, characteristic to irradiated glassy SiO2 and other oxide glasses, are found in α-quartz only after neutron irradiation. The oxygen atoms, displaced by the 2.5 MeV e− irradiation of α-quartz for fluences up to 1019 e−/cm2 evidently stays entirely in the peroxy linkage (Si-O-O-Si bond) form.

Highly enriched Si28 reveals remarkable optical linewidths and fine structure for well-known damage centers

Luminescence and optical absorption due to radiation damage centers in silicon has been studied exhaustively for decades, but is receiving new interest for applications as emitters for integrated silicon photonic technologies. While a variety of other optical transitions have been found to be much sharper in enriched $^{28}$Si than in natural Si, due to the elimination of inhomogeneous isotopic broadening, this has not yet been investigated for radiation damage centers. We report results for the well-known G, W and C damage centers in highly enriched $^{28}$Si, with optical linewidth improvements in some cases of over two orders of magnitude, revealing previously hidden fine structure in the G center emission and absorption. These results have direct implications for the linewidths to be expected from single center emission, even in natural Si, and for models for the G center structure. The advantages of $^{28}$Si can be readily extended to the study of other radiation damage centers in Si.

Method of Increasing Photosensitivity of Silicon Power Planes for Solar Energy

The solubilities of sulfur are determined by neutron activation analysis and silicon compensation by sulfur with different concentrations of the initial boron. It is shown that the sulfur solubility at a silicon doping temperature of 1250°C is ~1.8 × 1016 cm–3. Joint analysis of the results of studies of photoconductivity (PC) spectra, both in combined illumination with integral and infrared (IR) light and after the integral light is switched off, and the results of a study of the isothermal relaxation spectra of the capacity (DLTS) of Si❬S❭ crystals manufactured at different pressures of the diffusant vapor been established that single sulfur centers in silicon create two deep levels of ES – 0.12 and ES – 0.18 eV. All of the remaining energy levels of ES – 0.27 and ES – 0.53 eV are related to the quasi-molecules S2 and S4, respectively.

(Invited) Deep-Level Analysis of Passivation of Transition Metal Impurities in Silicon

The transition metals (TMs) from 3d and 4d series are very effective recombination centers in silicon. They are harmful for some electronic devices and Si solar cells and should be either neutralized or removed from active areas of the devices. Some of the TMs have low diffusivities, so their gettering is difficult. In this paper we present experimental results on the interactions of transition metals with hydrogen atoms in silicon and on passivation by the displacement of the metals from the interstitial to the substitutional site. The results have been obtained by means of deep level transient spectroscopy. We have studied iron, titanium and vanadium from the 3d series, and Mo from the 4d series. It is found that in the presence of excess vacancies 3d TMs tend to move to substitutional sites and are quite stable in this position with no or much reduced recombination activity and reduced diffusion. Hydrogen reactions with TMs are found to be dependent on the charge states of the reacting species. It is shown that H-TM reactions in p-type Si can be facilitated by manipulating charge states of the reacting species using several techniques.

Simultaneous coherence enhancement of optical and microwave transitions in solid-state electronic spins.

Solid-state electronic spins are extensively studied in quantum information science, as their large magnetic moments offer fast operations for computing1 and communication2-4, and high sensitivity for sensing5. However, electronic spins are more sensitive to magnetic noise, but engineering of their spectroscopic properties, for example, using clock transitions and isotopic engineering, can yield remarkable spin coherence times, as for electronic spins in GaAs6, donors in silicon7-11 and vacancy centres in diamond12,13. Here we demonstrate simultaneously induced clock transitions for both microwave and optical domains in an isotopically purified 171Yb3+:Y2SiO5 crystal, reaching coherence times of greater than 100 μs and 1 ms in the optical and microwave domains, respectively. This effect is due to the highly anisotropic hyperfine interaction, which makes each electronic-nuclear state an entangled Bell state. Our results underline the potential of 171Yb3+:Y2SiO5 for quantum processing applications relying on both optical and spin manipulation, such as optical quantum memories4,14, microwave-to-optical quantum transducers15,16, and single-spin detection17, while they should also be observable in a range of different materials with anisotropic hyperfine interactions.

Comparison of Two Phosphinidenes Binding to Silicon(IV)dichloride as well as to Silylene.

The cyclic alkyl(amino) carbene (cAAC) anchored silylene with two phosphinidenes was isolated as (cAAC)Si{P(cAAC)}2 (3) at room temperature, which was synthesized from the reduction of (Cl2)Si{P(cAAC)}2 (2) using 2 equiv of KC8. Compound 2 resulted from the reaction of 2 equiv of (cAAC)PK (1) with 1 equiv of SiCl4. Compounds 2 and 3 are the first examples where two terminal phosphinidenes are binding each to a silicon center characterized by single crystal X-ray structural analysis. Furthermore, the structure and bonding of compounds 2 and 3 have been investigated by theoretical methods for comparison.

Coumarin Probe for Selective Detection of Fluoride Ions in Aqueous Solution and Its Bioimaging in Live Cells.

We have synthesized novel coumarin-based fluorescent chemosensors for detection of fluoride ions in aqueous solution. The detection mechanism relied on a fluoride-mediated desilylation triggering fluorogenic reaction and a strong interaction between fluoride and the silicon center. In this work, the hydroxyl-decorated coumarins containing oxysilyl moiety have been synthesized through the aldehyde-functionalized coumarins. The optical responses toward fluoride, as well as aqueous stability studies of both aldehyde and hydroxyl functionalized coumarins, have been investigated. Due to the highest fluorescence enhancement upon the addition of fluoride and good stability in aqueous solution, the hydroxyl-decorated coumarin connected with the bulky tert-butyldiphenyloxysilyl group (-OSitBuPh2) has been selected for further investigation of its potential as a fluoride sensor. This hydroxyl-decorated coumarin can selectively sense fluoride ions in aqueous media (contain 0.8% MeCN) with desirable response times (40 min). The limit of detection of this compound was determined as 0.043 ppm, satisfying the standard fluoride level (0.7 ppm) in drinking water recommended by U.S. Department of Health and Human Services. The application of this silyl-capped coumarin derivative for fluoride analysis in collected water samples displayed satisfactory analytical accuracy (<5% error). Finally, this compound was successfully employed in fluorescence bioimaging of fluoride ions in human liver cancer cells, indicating its excellent cell permeability, ability to retain inside the living cells, and good stability under physiological conditions.

Rhodium-catalyzed Enantioselective Synthesis and Properties of Silicon-stereogenic Benzofuranylmethylidene-benzoxasiloles

The enantioselective synthesis of silicon-stereogenic benzofuranylmethylidene-benzoxasiloles was achieved with moderate enantioselectivity by the cationic rhodium(I)/(S)-BINAP complex-catalyzed desymmetrization of symmetrical bis(2-ethynylphenol)silanes, possessing the tert-butyl and methyl groups on the silicon atom. This reaction involves 1,2-silicon/1,3-carbon migrations and oxycyclization, and the 1,2-silicon migration constructs the stereogenic silicon center. The thus obtained benzofuranylmethylidene-benzoxasiloles with the electron-donating methoxy, methyl, and phenyl groups on two benzene rings showed good fluorescence quantum yields in solution state, on the other hand, that with the electron-withdrawing trifluoromethyl groups on two benzene rings showed good fluorescence quantum yield in solid state.

Reduction Rate of 1-Phenyl Phospholane 1-Oxide Enhanced by Silanol Byproducts: Comprehensive DFT Study and Kinetic Modeling Linked to Reagent Design.

Important stoichiometric transformations like Wittig and Appel reactions have been implemented in a catalytic fashion in the past decade. The phosphine oxide generated in situ can be reintroduced as phosphine into the catalytic cycle using mild and selective silane reagents (redox-driven catalysis). While the field of experimental investigation has been fully expanding in the past decade, theoretical studies are still sparse. In this present work, density functional theory (DFT) has been used to characterize the free energy surfaces of the reduction of 1-phenyl phospholane 1-oxide with four different silanes. Found stationary points have been studied in-depth to highlight mechanistic peculiarities, like the effect of substituents at the silicon center and the parallel and competitive reactivity between the precursor silanes and their semioxidized byproducts. Calculated thermodynamic parameters in combination with "real" values for concentrations have been used in the formulation of rate equations for simple bimolecular and monomolecular steps of the mechanism. The deterministic integration concentrations versus time of such rate equations led to a realistic description of the systems under study and paved the way to strategic and rational design of new silanes with increased reactivity.

S–H Bond Activation in Hydrogen Sulfide by NHC-Stabilized Silyliumylidene Ions

Reactivity studies of silyliumylidenes remain scarce with only a handful of publications to date. Herein we report the activation of S–H bonds in hydrogen sulfide by mTer-silyliumylidene ion A (mTer = 2,6-Mes2-C6H3, Mes = 2,4,6-Me3-C6H2) to yield an NHC-stabilized thiosilaaldehyde B. The results of NBO and QTAIM analyses suggest a zwitterionic formulation of the product B as the most appropriate. Detailed mechanistic investigations are performed at the M06-L/6-311+G(d,p)(SMD: acetonitrile/benzene)//M06-L/6-311+G(d,p) level of density functional theory. Several pathways for the formation of thiosilaaldehyde B are examined. The energetically preferred route commences with a stepwise addition of H2S to the nucleophilic silicon center. Subsequent NHC dissociation and proton abstraction yields the thiosilaaldehyde in a strongly exergonic reaction. Intermediacy of a chlorosilylene or a thiosilylene is kinetically precluded. With an overall activation barrier of 15 kcal/mol, the resulting mechanistic picture is fully in line with the experimental observation of an instantaneous reaction at sub-zero temperatures.

Toward Asymmetric Synthesis of Pentaorganosilicates

Introducing chiral silicon centers was explored for the asymmetric Rh-catalyzed cyclization of dihydrosilanes to enantiomerically enriched spirosilanes as targets to enable access to enantiostable pentacoordinate silicates. The steric rigidity required in such systems demands the presence of two naphthyl or benzo[b]thiophene groups. The synthetic approach to the expanded spirosilanes extends Takai’s method (Kuninobu et al. in Angew Chem Int Ed 52(5):1520–1522, 2013) for the synthesis of spirosilabifluorenes in which both a Si–H and a C–H bond of a dihydrosilane are activated by a rhodium catalyst. The expanded dihydrosilanes were obtained from halogenated aromatic precursors. Their asymmetric cyclization to the spirosilanes were conducted with [Rh(cod)Cl]2 in the presence of the chiral bidentate phosphane ligands (R)-BINAP, (R)-MeO-BIPHEP, and (R)-SEGPHOS, including derivatives with P-(3,5-t-Bu-4-MeO)-phenyl (DTBM) groups. The highest enantiomeric excess of 84% was obtained for 11,11′-spirobi[benzo[b]-naphtho[2,1-d]silole] with the DTBM-SEGPHOS ligand.

Hard mask and lithographic capabilities improvement by amorphous carbon step coverage optimization in high aspect ratio device pattern

Due to the complexity of micro control unit (MCU), the fabrication process of MCU has become very challenging, and in particular the hard mask (HM) and lithographic technologies. Therefore, it is very important to improve the performance of the two processes. The performances of the HM and lithographic processes strongly depend on the step coverage (S/C) of HM. Poor HM S/C will directly impact the protection provided by HM on device patterning during the etching process and overlay marks during photoresist (PR) rework. The application of C2H2 base amorphous carbon (a-C) to form the HM instead of C3H6 base a-C can achieve better sidewall and bottom S/C due to the fact that C2H2 base a-C has superior sticking coefficient (higher carbon to hydrogen (C/H) ratio). Furthermore, the silicon (Si) center damaging problem in device patterning is alleviated by the good S/C performance of C2H2 base a-C. Experimental results have demonstrated that the overlay mark is not damaged with five PR reworks with the application of C2H2 base a-C. Furthermore, a process parameter design of experiment (DOE) for C2H2 base a-C to obtain the trends of sidewall and bottom S/C is proposed. The presented experimental results show that bad sidewall S/C of a-C film induces the Si edge damage in high aspect ratio device patterning. The proposed DOE will allow us to optimize the C2H2 base a-C process to improve the S/C by 49%–91% better than the unoptimized process. This helps to reduce the amount of device pattern damage significantly.

Enhancing Diamond Fluorescence via Optimized Nanorod Dimer Configurations

Light extraction from silicon (SiV) and nitrogen (NV) vacancy diamond color centers coupled to plasmonic silver and gold nanorod dimers was numerically improved. Numerical optimization of the coupled dipolar emitter—plasmonic nanorod dimer configurations was realized to attain the highest possible fluorescence enhancement by simultaneously improving the color centers excitation and emission through antenna resonances. Conditional optimization was performed by setting a criterion regarding the minimum quantum efficiency of the coupled system (cQE) to minimize losses. By comparing restricted symmetric and allowed asymmetric dimers, the advantages of larger degrees of freedom achievable in asymmetric configurations was proven. The highest 2.59 × 108 fluorescence enhancement was achieved with 46.08% cQE via NV color center coupled to an asymmetric silver dimer. This is 3.17-times larger than the 8.19 × 107 enhancement in corresponding symmetric silver dimer configuration, which has larger 68.52% cQE. Among coupled SiV color centers the highest 1.04 × 108 fluorescence enhancement was achieved via asymmetric silver dimer with 37.83% cQE. This is 1.06-times larger than the 9.83 × 107 enhancement in corresponding symmetric silver dimer configuration, which has larger 57.46% cQE. Among gold nanorod coupled configurations the highest fluorescence enhancement of 4.75 × 104 was shown for SiV color center coupled to an asymmetric dimer with 21.8% cQE. The attained enhancement is 8.48- (92.42-) times larger than the 5.6 × 103 (5.14 × 102) fluorescence enhancement achievable via symmetric (asymmetric) gold nanorod dimer coupled to SiV (NV) color center, which is accompanied by 16.01% (7.66%) cQE.

Electron spin resonance identification di-carbon-related centers in irradiated silicon

A previously unreported electron spin resonance (ESR) spectrum was found in γ-ray irradiated silicon by the detection of the change in microwave photoconductivity arising from spin-dependent recombination (SDR). In the specially prepared silicon crystals doped by 13C isotope, a well resolved hyperfine structure of SDR-ESR lines due to the interaction between electrons and two equivalent carbon atoms having nuclear spin I = 1/2 was observed. The Si-KU4 spectrum is described by spin Hamiltonian for spin S = 1 and of g and D tensors of orthorhombic symmetry with principal values g1 = 2.008, g2 = 2.002, and g3 =2.007; and D1 = ± 103 MHz, D2 = ∓170 MHz, and D3 = ± 67 MHz where axes 1, 2, and 3 are parallel to the [11¯0], [110], and [001] crystal axes, respectively. The hyperfine splitting arising from 13C nuclei is about 0.35 mT. A possible microstructure of the detect leading to the Si-KU4 spectrum is discussed.

Self-assembly of a Si-based cage by the formation of 24 equivalent covalent bonds.

A robust, nano-sized covalent cage, of composition, [(PhSi)6(ctc)4]6- (H6ctc = cyclotricatechylene) has been prepared in a simple reaction in good yield. The tetrahedral anionic cage is stable in both the solid and solution state and exhibits an affinity for Cs+ ions which bind to the internal surface of the cage.

Versatile multicolor nanodiamond probes for intracellular imaging and targeted labeling.

We report on the sizable production of fluorescent nanodiamonds (FNDs) containing a near infrared (NIR) color center - namely the silicon vacancy (SiV) defect, and their first demonstration inside cells for bio-imaging. We further demonstrate a concept of multi-color bio-imaging using FNDs to investigate intercellular processes using two types of FNDs. Due to their specific spectral properties, SiV FNDs can be distinguished from common nitrogen-vacancy (NV) FNDs and show a distinct initial spreading throughout the cell interior. The reported results are the first demonstration of multi-color labeling with FNDs that are especially interesting for in vivo bio-imaging due to their stable fluorescence.

Pyridine Functionalized N-Heterocyclic Silane Complexes of Iridium and Rhodium–An Unexpected Change in Coordination

A new pyridine functionalized N-heterocyclic silane with ambident reactivity as a ligand has been synthesized and characterized by NMR spectroscopy (1H, 13C{1H}, 29Si), mass spectrometry, elemental analysis, and X-ray crystallography. This ligand reacts with iridium and rhodium cod precursors (cod = 1,5-cyclooctadiene) to yield two new complexes that possess divergent, and unexpected, binding properties. In particular, no oxidative addition occurs at the intraligand Si-H unit. With the iridium(I) center, the ligand acts as a tertiary amino-pyridine chelator, whereas coordination of rhodium(I) to the ligand occurs with arene π-complexation on an opposite side. The latter interaction yields an unprecedented, electronically induced, coordination change at the silicon center over a long spatial distance. The new iridium and rhodium compounds are of high interest as they provide two potentially different reactions sites in one complex and as these sorts of complexes are known to activate small molecules like d...

N-Heterocyclic Silylene Coordinated Dialkyl Borenium Equivalent

In an attempt to prepare the heavier analogue of NHC-stabilized borenium cation, we have prepared a silylene-coordinated borenium equivalent (1a), which can be viewed as a contact ion pair of borenium and triflate. The triflate anion coordinates to the oxophilic silicon center, leaving the boron center existing in a trigonal-planar geometry in the solid state and in solution. Nucleophilic substitution reactions of 1a all result in the release of free silylene ligand, demonstrating the borenium-ion-like reactivity of 1a. Chemical reduction of 1a leads to a neutral radical possessing a partial Si–B double bond.

Two-stage permanent deactivation of the boron-oxygen-related recombination center in crystalline silicon

We analyze the lifetime evolution during permanent deactivation of the boron-oxygen-related defect center (BO defect) in boron-doped, oxygen-rich Czochralski-grown silicon (Cz-Si). In particular, we examine the impact of the samples’ states prior to the permanent deactivation process. Samples that were initially fully degraded show a two-stage deactivation process consisting of a fast and a slow deactivation component, which can be fitted by two exponential functions with their respective rate constants. For both components, we find a pronounced increase of the rate constants with illumination intensity. In addition, we observe that the rate constant describing the slow deactivation component of samples deactivated after complete degradation is identical to the rate constant determined on samples, which were deactivated immediately after annealing in darkness. In the latter case, a purely mono-exponential deactivation behavior was observed. Our study clearly demonstrates that the asymptotic deactivation behavior does not depend on the initial state of the lifetime sample. We prove that the same is valid for initially degraded and dark-annealed PERC solar cells. Hence, it is not necessary to first degrade the sample to realize a fast BO deactivation.

Synthesis, characterization and catalytic activity of melamine immobilized MCM-41 for condensation reactions

In the present study, melamine immobilized MCM-41 is synthesized by grafting on modified MCM-41. The surface area, pore size and pore volume of MCM-41-Mela were found to be decreased after immobilization of melamine. FTIR and Raman spectroscopy results revealed the successful grafting of melamine on the surface of MCM-41. The 29Si CP/MAS NMR of MCM-41-Mela showed the existence of T2, T3, Q3 and Q4 silicon centres. The catalytic activities were investigated through a liquid phase Knoevenagel condensation reaction between furfural and acetylacetone. A conversion of 93.1% was achieved with 100% selectivity towards 3-(2-furylmethylene)-2,4-pentanedione (FMP) under solvent-free condition. The catalytic activity of MCM-41-Mela was then successfully carried out for Knoevenagel condensation with different substrates, giving excellent yields of the corresponding products. The catalyst was easily regenerated and could be reused for three times without loss of catalytic activity. Simple preparation methods, high efficiency and reusability of the heterogeneous MCM-41-Mela catalyst demonstrate a great potential for future catalysis application.