Β-hydride Elimination Mechanism Research Articles

Mechanism and Origins of Enantioselectivity in the Nickel-catalyzed Asymmetric Synthesis of Silicon-Stereogenic Benzosiloles.

As important π-skeletons, benzosiloles often possess unique electronic and optical properties and have been widely used in semiconductor materials. Therefore, great attention has been drawn to the area of developing novel synthetic methods for various benzosiloles. However, the synthesis of enantioenriched silicon-stereogenic benzosiloles is still at an early stage and remains to be explored. Herein, we performed systematic density functional theory studies on the recently reported nickel-catalyzed asymmetric synthesis of silicon-stereogenic benosiloles, which was enabled by an enantioselective desymmetrization of (2-alkenyl)aryl-substituted silacyclobutanes. Our computational study shows that the reaction mechanism involves ligand exchange, oxidative addition, alkene insertion, and hydrogen-transfer coupled reductive-demetalation steps. The proposed transmetalation and β-hydride elimination mechanism was not found, which might be due to the unfavorable ring strain of the multicyclic intermediates. The novel hydrogen-transfer coupled reductive-demetalation mechanism was shown to be reasonable for the generation of the silicon-stereogenic benzosilole. Noncovalent interactions (including C-H···π and hydrogen bonding) in the rate-determining alkene insertion transition state account for the origins of the enantioselectivity. Our computational study sheds light on the detailed reaction mechanism and also provides insights for the development of novel approaches for synthesis of high-value silicon-stereogenic compounds.

Out of Thin Air? Catalytic Oxidation of Trace Aqueous Aldehydes with Ambient Dissolved Oxygen.

Water reuse is expanding due to increased water scarcity. Water reuse facilities treat wastewater effluent to a very high purity level, typically resulting in a product water that is essentially deionized water, often containing less than 100 μg/L organic carbon. However, recent research has found that low-molecular-weight aldehydes, which are toxic electrophiles, comprise a significant fraction of the final organic carbon pool in recycled wastewater in certain treatment configurations. In this manuscript, we demonstrate oxidation of trace aqueous aldehydes to their corresponding acids using a heterogeneous catalyst (5% Pt on C), with ambient dissolved oxygen serving as the terminal electron acceptor. Mass balances are essentially quantitative across a range of aldehydes, and pseudo-first-order reaction kinetics are observed in batch reactors, with kobs varying from 0.6 h-1 for acetaldehyde to 4.6 h-1 for hexanal, while they are low for unsaturated aldehydes. Through kinetic and isotopic labeling experiments, we demonstrate that while oxygen is essential for the reaction to proceed, it is not involved in the rate-limiting step, and the reaction appears to proceed primarily through a base-promoted β-hydride elimination mechanism from the hydrated gem-diol form of the corresponding aldehyde. This is the first report we are aware of that demonstrates useful abiotic oxidation of a trace organic contaminant using dissolved oxygen.

Catalytic Dehydrogenation of Dimethylamine Borane by Highly Active Thorium and Uranium Metallocene Complexes

The thorium and uranium complexes (C5Me5)2AnMe2, [(C5Me5)2An(H)(μ-H)]2 (An = Th, U) and [(C5Me5)2U(H)]2 dehydrogenate dimethylamine borane (Me2NH·BH3) at room temperature. Upon mild heating at 45 °C, turnover frequencies (TOFs) of 400 h–1 are obtained, which is comparable to some of the fastest Me2NH·BH3 dehydrogenation catalysts known in the literature. A β-hydride elimination mechanism for dehydrogenation is proposed because of the observation of Me2N═BH2, Me2N═BMe2, and Me2N═BHMe in the 11B NMR spectra of catalytic and stoichiometric reactions. The similar catalytic metrics between the actinide dimethyl and hydride complexes with Me2NH·BH3 indicate that the actinide hydride complexes are the active catalysts in this chemistry.

ChemInform Abstract: Iridium(I)‐Catalyzed C—H Borylation of α,β‐Unsaturated Esters with Bis(pinacolato)diboron.

A new process has been developed for the iridium(I)-catalyzed vinylic C-H borylation of α,β-unsaturated esters with bis(pinacolato)diboron (B2 pin2 ). These reactions proceeded in octane at temperatures in the range of 80-120 °C to afford the corresponding alkenylboronic compounds in high yields with excellent regio- and stereoselectivities. The presence of an aryl ester led to significant improvements in the yields of the acyclic alkenylboronates. Crossover experiments involving deuterated substrates as well as a mixture of stereoisomers confirmed that this reaction proceeds via a 1,4-addition/β-hydride elimination mechanism. Notably, this reaction was also used to develop a one-pot borylation/Suzuki-Miyaura cross-coupling procedure.

Iridium(I)-catalyzed C-H Borylation of α,β-Unsaturated Esters with Bis(pinacolato)diboron.

A new process has been developed for the iridium(I)-catalyzed vinylic C-H borylation of α,β-unsaturated esters with bis(pinacolato)diboron (B2 pin2 ). These reactions proceeded in octane at temperatures in the range of 80-120 °C to afford the corresponding alkenylboronic compounds in high yields with excellent regio- and stereoselectivities. The presence of an aryl ester led to significant improvements in the yields of the acyclic alkenylboronates. Crossover experiments involving deuterated substrates as well as a mixture of stereoisomers confirmed that this reaction proceeds via a 1,4-addition/β-hydride elimination mechanism. Notably, this reaction was also used to develop a one-pot borylation/Suzuki-Miyaura cross-coupling procedure.

Digermane Deposition on Si(100) and Ge(100): from Adsorption Mechanism to Epitaxial Growth

Controlled fabrication of nanometer-scale devices such as quantum dots and nanowires requires an understanding of the initial chemisorption mechanisms involved in epitaxial growth. Vapor phase epitaxy can provide controlled deposition when using precursors that are not reactive with the H-terminated surfaces at ambient temperatures. For instance, digermane (Ge2H6) has potential as such a precursor for Ge ALE on Si(100) surfaces at moderate temperatures; yet, its adsorption configuration and subsequent decomposition pathways are not well understood. In situ Fourier transform infrared spectroscopy and first principles calculations reveal that Ge2H6 chemisorbs through a β-hydride elimination mechanism, forming Ge2H5 and H on both Si(100)-(2 × 1) and Ge(100)-(2 × 1) surfaces, instead of the previously proposed Ge–Ge bond breaking mechanism, and subsequently decomposes into an ad-dimer. The resulting coverage of Ge after a saturation exposure is estimated to be about 0.3 monolayers. Interestingly, the decomposition of adsorbed Ge2H5 on Si(100) is faster than Si2H5 on Ge(100) at 173 K. The desorption temperature of hydrogen on Si(100) is shown to depend on the Ge coverage, falling from 698 K for ∼1/4 ML Ge on Si(100) to 573 K for a nearly full Ge coverage, consistent with H desorption on Ge(100). Furthermore, hydrogen is observed to migrate from Ge to Si, prior to desorption. This property opens the door for selective growth of Ge on patterned H-terminated Si surfaces.

Mechanism of the Aerobic Oxidation of Alcohols by Palladium Complexes of N-Heterocyclic Carbenes

Quantum mechanics (B3LYP density functional theory) combined with solvation (Poisson-Boltzmann polarizable continuum solvent model) was used to investigate six mechanisms for the aerobic oxidation of alcohols catalyzed by (NHC)Pd(carboxylate)(2)(H(2)O) complexes (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Of these, we find that "reductive beta-hydride elimination", in which the beta-hydrogen of a palladium-bound alkoxide is transferred directly to the free oxygen of the bound carboxylate, provides the lowest-energy route and explains the published kinetic isotope effect, activation enthalpy, reaction orders, and dependence of rate on carboxylate pK(a). The traditional beta-hydride elimination mechanism cannot be responsible for the experimentally observed kinetic parameters, which we find could result from the subsequent reductive elimination of acetic acid, which yields a slightly higher calculated activation barrier. Reversible beta-hydride elimination may provide a mechanism for the racemization of chiral alcohols, which would undermine attempts at an enantioselective oxidation. Competition among these pathways can be influenced by changing the electronic properties of the carboxylate and substrate.

Novel catalysts for advanced hydroprocessing: transition metal phosphides

The field of catalysis is continuously invigorated by the discovery of new catalytic materials. Transition metal phosphides represent one such group of compounds that have recently been shown to have excellent activity for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). This is an important area, as continuous decline in the quality of petroleum feedstocks and increasingly stringent environmental regulations have made the removal of sulfur a paramount problem in the refining industry. The structure of the metal-rich phosphides is based on trigonal prisms, which can well accommodate the relatively large phosphorus atoms. The prisms are similar to those in sulfides, but phosphides do not take on layered structures and are metal conductors, not insulators or semiconductors. Lack of layers leads to a more isotropic crystal morphology and potentially better exposure of surface metal atoms to fluid phase reactants. Phosphides can be prepared readily in bulk and supported forms by reduction of phosphate precursors. The catalytic activity of the phosphides for dibenzothiophene HDS and quinoline HDN at 643 K and 3.1 MPa followed the order Fe2P<CoP<MoP<WP<Ni2P. The best catalyst, Ni2P/SiO2, had better activity in hydroprocessing than a commercial Ni–Mo–S/Al2O3 (HDS 98% vs 78% and HDN 80% vs 43%), based on equal sites loaded in the reactor. The sites were titrated by CO chemisorption at room temperature for the phosphides and by pulse O2 chemisorption at dry-ice acetone temperatures for the sulfide. The Ni2P/SiO2 also compared favorably with a commercial Co–Mo–S/Al2O3 catalyst (Ketjenfine 756) using a real feed at 593 K and 3.9 MPa. On the basis of equal weights loaded in the reactor, the Ni2P was again found to have higher performance than the sulfide (HDS 85% vs 80%). Studies of Ni2P/SiO2 catalysts with varying Ni/P ratios indicated that the principal phase remained Ni2P, but that the crystallite size decreased with increasing P content as a result of facilitated contact on the surface between the Ni component and the phosphorus reagent. Activity measurements on these samples indicated that HDS was structure-insensitive, but that HDN was structure-sensitive. Extended X-ray absorption fine structure measurements indicated that the surface formed a phospho sulfide after reaction. The mechanism of HDN was investigated using a series of molecular probes of varying structure. The reactivity order for pentylamines indicated that CNH2 bond scission proceeded by a β-hydride elimination mechanism, similar to that occurring on sulfides. The reactivity order of substituted piperidines indicated that ring-opening proceeded by CN bond scission and involved an α-carbon, and thus was different from the pathway on sulfides, which again appeared to involve a β-hydride elimination. This difference may account for the higher activity of phosphides over sulfides.

The surface chemistry of alkyl and arylphosphate vapor phase lubricants on Fe foil

The surface chemistry of tributylphosphate (TBP) and tricresylphosphate (TCP) on a polycrystalline Fe surface was studied using temperature programmed reaction spectroscopy and Auger electron spectroscopy to illustrate some of the initial steps in the reaction mechanisms of alkyl and arylphosphate vapor phase lubricants. During heating, TBP [(C 4H 9O) 3P=O] adsorbed on the Fe surface decomposes via C–O bond scission to give butyl surface intermediates [C 4H 9–] that react via β-hydride elimination to desorb as 1-butene [CH 3CH 2CH=CH 2] and H 2 without appreciable carbon deposition onto the surface. The thermal decomposition of 1-iodobutane [I-C 4H 9] on Fe was observed to proceed via the same β-hydride elimination mechanism. In contrast to tributylphosphate, meta-tricresylphosphate ( m-TCP) [(CH 3–C 6H 4O) 3P=O] decomposes on Fe via P–O bond scission to produce methylphenoxy intermediates [CH 3–C 6H 4O–]. During heating to 800 K, methylphenoxy intermediates either desorb as m-cresol [CH 3–C 6H 4–OH] via hydrogenation or decompose further to generate tolyl intermediates [CH 3–C 6H 4–]. Some of the tolyl intermediates desorb as toluene [CH 3–C 6H 5] via hydrogenation but the majority decompose resulting in H 2 and CO desorption and carbon deposition onto the Fe surface. The P–O bond scission mechanism of m-TCP was verified by showing that the temperature programmed reaction spectra of m-cresol yield products that are almost identical to those of m-TCP. These results provide insight into the origin of the differences in the performance of alkyl and arylphosphates as vapor phase lubricants. The alkylphosphates decompose via alkyl intermediates that readily undergo β-hydride elimination and desorb into the gas phase as olefins, thus removing carbon from the surface. In contrast, the arylphosphates generate aryloxy intermediates by P–O bond scission and aryl intermediates by further C–O bond scission. Neither of these intermediates can undergo β-hydride elimination and thus they decompose to deposit carbon onto the Fe surface. The higher efficiency for carbon deposition may be the primary reason for the superior performance of the arylphosphates over alkylphosphates as vapor phase lubricants.

Tailoring the Material and Electronic Properties of Ultra-Thin ZrO2 Films for Microelectronics Application

ZrO2 thin film was investigated in this work to replace SiO2 as the gate dielectric material in future metal-oxide-semiconductor field effect transistors (MOSFET). ZrO2 thin film was deposited on planar Si (100) wafers by an atomic layer chemical vapor deposition (AL-CVD) process using a zirconium (IV) t-butoxide Zr(OC4H9)4 precursor and oxygen. Atomic layer controlled deposition of ZrO2 was achieved at 300°C to 400°C where the reaction was thermally activated with an activation energy of 29 kcal/mol, consistent with a β-hydride elimination mechanism leading to ZrO2 deposition. The deposited ZrO2 thin film was stoichiometric, amorphous, uniform, smooth, and conformal. An interfacial zirconium silicate formation was observed by the high-resolution transmission electron microscopy. The characterized electrical properties of ZrO2 including its high dielectric constant, small C- V hysteresis, and low interfacial density, are ideal for the MOSFET application.

Highly conformal ZrO2 deposition for dynamic random access memory application

ZrO 2 is investigated in this work to replace SiO2 as the dielectric material in metal–oxide–metal capacitors in dynamic random access memory (DRAM) devices for its high dielectric constant, good thermal stability, excellent conformality, and large band gap. ZrO2 films were deposited on planar Si (100) wafers and patterned amorphous silicon cylinders by rapid thermal chemical vapor deposition process using a zirconium (IV) t-butoxide Zr(OC4H9)4 precursor and oxygen. At substrate temperature below 300 °C, no significant deposition was observed. At temperatures between 300 and 400 °C, the reaction is thermally activated with an activation energy of 29 kcal/mol, consistent with a β-hydride elimination mechanism leading to ZrO2 deposition. In this regime, one atomic layer of ZrO2 can be deposited after each alternating exposure to the precursor and oxygen, ideal for achieving conformal coverage of ZrO2 over high aspect ratio features. At temperatures above 400 °C, the deposition rate is less sensitive to temperature with an apparent activation energy of 4 kcal/mol due to decomposition and desorption of the precursors. Stoichiometric, uniform, and amorphous ZrO2 was obtained at all temperatures investigated. Highly conformal step coverage of the deposited ZrO2 was achieved on 300 nm×600 nm cylindrical features with an aspect ratio of 4. The dielectric constant of ZrO2 achieved in this work ranges from 15 to 18 depending upon process conditions and small capacitance–voltage hysteresis is observed, ideal for DRAM application.

Rapid thermal chemical vapor deposition of zirconium oxide for metal-oxide-semiconductor field effect transistor application

ZrO 2 is investigated in this work to replace SiO2 as the gate dielectric material in metal-oxide-semiconductor devices for its high dielectric constant, good thermal stability on silicon, and large band gap. ZrO2 films were deposited on p-Si(100) wafers by a rapid thermal chemical vapor deposition process using a zirconium (IV) t-butoxide Zr(OC4H9)4 precursor and oxygen. At temperatures between 300 and 400 °C, the reaction was thermally activated with an activation energy of 29 kcal/mol, consistent with a β-hydride elimination mechanism leading to ZrO2 deposition. In this regime at substrate temperatures below 350 °C, one atomic layer of ZrO2 can be deposited after each alternating exposure to the precursor and oxygen, ideal for achieving conformal coverage of ZrO2 over high aspect ratio features. Stoichiometric, uniform, and amorphous ZrO2 was obtained, and highly conformal step coverage of the deposited ZrO2 was observed on 300 nm features with an aspect ratio of 4. The dielectric constant of ZrO2 achieved in this work ranged from 15 to 18 depending upon process conditions and small C–V hysteresis and low interfacial state density were observed, ideal for metal-oxide-semiconductor field effect transistor application.

ZrO2 film growth by chemical vapor deposition using zirconium tetra-tert-butoxide

ZrO2 films were grown by chemical vapor deposition at T>573 K with the precursor zirconium tetra-tert-butoxide (ZTB) Zr(OC(CH3)3)4. The ZrO2 films were deposited on a Si(100) surface and film thicknesses were measured after growth at various temperatures using ellipsometry. ZrO2 film growth was negligible at T<573 K, increased rapidly with temperature between T=623–723 K, and then decreased at T>773 K. Maximum ZrO2 growth rates of ∼76 Å/min were observed at 723 K with a ZTB pressure of 0.05 Torr. This ZrO2 growth rate is consistent with a reactive sticking coefficient of γ=6.8×10−5. Auger depth-profile analysis measured low carbon levels of <3%. Mass spectrometric investigations observed isobutylene (2-methyl propene) as the dominant reaction product. The isobutylene appeared with a magnitude that correlated with the temperature-dependent ZrO2 growth rates. This correlation indicates that ZTB decomposes to deposit ZrO2 by a β-hydride elimination mechanism. Auger depth-profile analysis revealed higher carbon levels of ≥10% where the ZrO2 growth rate decreased at T>773 K. ZrO2 deposition may be partially poisoned by carbon at these higher temperatures. Fourier transform infrared (FTIR) studies examined the stability of hydroxyl (ZrOH*) and butoxide (ZrOC(CH3)3*) species on ZrO2. Both hydroxyl and butoxide species decreased roughly linearly with temperature from 300–800 K. The comparison between butoxide stability and ZrO2 growth rate argues that the butoxide species decomposition via β-hydride elimination is the rate-limiting step for ZrO2 film growth.

Direct evidence for the β-hydride elimination mechanism in the decomposition of triethylgallium on GaAs(100)

Decomposition of the surface ethyl group formed by dissociative adsorption of triethylgallium (TEGa) on GaAs(100) is investigated using temperature programmed desorption. Deuterium labeling indicates that decomposition of the surface ethyl group proceeds exclusively through a β-hydride elimination reaction mechanism without any scrambling between α- and β-hydrogens. For undeuterated TEGa (TEGa-d 0), the surface ethyl group decomposes and desorbs simultaneously as C 2H 4 and H 2 at 600 K. For perdeuterated TEGa (TEGa-d 15), the desorption of C 2D 4 and D 2 occurs at a slightly higher temperature (630 K). Partially deuterated TEGa (TEGa-d 6), with deuterium on the α-carbons, results in H 2 and C 2H 2D 2 as desorption products at 600 K. A kinetic isotope effect of 8 ± 5 kJ mol −1 for hydride versus deuteride transfer is determined, which is consistent with cleavage of the C βH bond in the transition state, and with tunneling.

Trisneopentylgallium as a precursor for atomic layer epitaxy of GaAs

We report the use of a new precursor, trisneopentylgallium (NPG) for the growth of GaAs by atomic layer epitaxy (ALE). In contrast to most other alkyl gallium precursors such as triethylgallium, which decompose via a β-hydride elimination mechanism, this compound undergoes homolysis similar to that of trimethylgallium (TMGa), the normal choice as an ALE precursor. Clear self-limiting growth behavior similar to that of TMGa was observed over a reasonably wide range of growth conditions (430–500°C). Carbon incorporation was not significantly reduced compared with TMGa suggesting that the adsorbed neopentyl radicals undergo decomposition to result in a methyl terminated surface identical to that obtained for growth with TMGa.

Adsorption and decomposition of diethyldiethoxysilane on silicon surfaces: New possibilities for SiO2 deposition

Diethyldiethoxysilane is an organosilicate that may offer new possibilities for SiO2 deposition. In this study, the adsorption and decomposition of diethyldiethoxysilane (DEDEOS) was examined on Si(100)2×1 and porous silicon surfaces using laser-induced thermal desorption (LITD), temperature-programmed desorption (TPD), Fourier-transform infrared (FTIR), and Auger electron spectroscopy techniques. The FTIR studies revealed that DEDEOS dissociatively adsorbs on porous silicon and deposits ethyl and ethoxy species. These species are observed to decompose via a β-hydride elimination mechanism at ∼700 K. In agreement with this mechanism, TPD studies on Si(100)2×1 observed ethylene (C2H4) at ∼700 K and H2 desorption at ∼800 K. Additionally, the controlled deposition of SiO2 was achieved on Si(100)2×1 using repetitive cycles of DEDEOS adsorption at 300 K followed by thermal annealing at 820 K for 300 s. After the rapid deposition of an oxygen coverage of θO∼2.4 ML, the oxygen deposition rate decreased and reached a constant deposition rate of ∼0.04 ML oxygen per cycle after ten cycles. The constant reactivity of the growing SiO2 film was attributed to dangling bond reactive sites resulting from β-hydride elimination of the ethyl groups (SiCH2CH3→SiH+CH2=CH2) and subsequent H2 desorption (2SiH→2Si+H2). DEDEOS was also utilized to grow SiO2 films under high-pressure chemical vapor deposition conditions. The rate of SiO2 deposition displayed Arrhenius behavior with an activation barrier of Eact=49±6 kcal/mol. This activation barrier is similar to activation barriers measured earlier for SiO2 growth using tetraethoxysilane (TEOS). The deposition rate of SiO2 by DEDEOS was approximately 60 times slower than the deposition rate by TEOS at 943 K and 0.5 Torr.

Ethyl group decomposition kinetics following adsorption of diethylsilane, diethylgermane, and ethylsilane on Si(111)7 × 7

The kinetics of ethyl group decomposition on Si(111)7 × 7 were measured using laser-induced thermal desorption techniques following the adsorption of diethylsilane (DES), diethylgermane (DEG), and ethylsilane (ES). Earlier infrared spectroscopic studies have shown that ethyl species are present after the dissociative chemisorption of DES, DEG, and ES on silicon surfaces. As the ethyl groups decompose via a β-hydride elimination mechanism, the hydrogen coverage increases and ethylene desorbs into the gas phase, i.e. Si- CH 2 CH 3( ad) → Si- H( ad) + CH 2 = CH 2( g). LITD measurements determined that the ethyl group decomposition kinetics consisted of a fast initial step, followed by a slower second step. Decomposition kinetics for DES, DEG, and ES could be described by a model consisting of two concurrent first-order decomposition processes. The similar ethyl group decomposition rate constants for DES, DEG, and ES suggested that both hydrogen and ethyl groups transfer upon adsorption to free dangling bonds on Si(111)7 × 7. Measured activation barriers for the β-hydride elimination reaction were slightly larger than the thermodynamic barrier, but much smaller than the calculated gas-phase barrier. The dependence of the β-hydride elimination mechanism on the availability of free dangling bonds was studied by adsorption of additional atomic hydrogen following ES and DES exposures. The presence of additional hydrogen adsorption did not significantly affect the ethyl group decomposition kinetics. These results suggested that β-hydride transfer occurs through a four-center transition state and is not dependent on adjacent free dangling bond sites.

Adsorption and decomposition of diethylgermane on porous silicon surfaces

Diethylgermane [(CH 3CH 2) 2GeH 2], (DEG), is a potential molecular precursor for germanium deposition on silicon surfaces. In this study, Fourier transform infrared (FTIR) transmission spectroscopy was employed to examine the adsorption and decomposition of DEG on high surface area porous silicon surfaces. The FTIR spectra revealed that DEG dissociatively adsorbs on porous silicon surfaces at 200 K to form SiH, GeH and SiCH 2CH 3 surface species. No spectral features were observed for GeCH 2CH 3 surface species. The C-H stretching modes between 2959 and 2882 cm −1, the Si-H stretch at 2082 cm −1 and the Ge-H stretch at 1999 cm −1 were then employed to monitor the decomposition of the surface species during thermal annealing. Between 300 and 640 K the GeH species transferred hydrogen to the silicon surface and formed SiH species. The SiCH 2CH 3 species decomposed to yield gas phase CH 2CH 2 and additional SiH surface species between 400 and 700 K. These reaction products were consistent with a β-hydride elimination mechanism, i.e. SiCH 2CH 3 → SiH + CH 2CH 2. From 660–800 K the SiH surface species decreased concurrently with H 2 desorption. The decomposition pathways of the surface species following DEG adsorption indicate that DEG may be useful for germanium deposition on silicon surfaces.

Adsorption and decomposition of diethylgermane on porous silicon surfaces

Diethylgermane [(CH 3CH 2) 2GeH 2], (DEG), is a potential molecular precursor for germanium deposition on silicon surfaces. In this study, Fourier transform infrared (FTIR) transmission spectroscopy was employed to examine the adsorption and decomposition of DEG on high surface area porous silicon surfaces. The FTIR spectra revealed that DEG dissociatively adsorbs on porous silicon surfaces at 200 K to form SiH, GeH and SiCH 2CH 3 surface species. No spectral features were observed for GeCH 2CH 3 surface species. The C-H stretching modes between 2959 and 2882 cm −1, the Si-H stretch at 2082 cm −1 and the Ge-H stretch at 1999 cm −1 were then employed to monitor the decomposition of the surface species during thermal annealing. Between 300 and 640 K the GeH species transferred hydrogen to the silicon surface and formed SiH species. The SiCH 2CH 3 species decomposed to yield gas phase CH 2=CH 2 and additional SiH surface species between 400 and 700 K. These reaction products were consistent with a β-hydride elimination mechanism, i.e. SiCH 2CH 3→SiH+CH 2=CH 2. From 660–800 K the SiH surface species decreased concurrently with H 2 desorption. The decomposition pathways of the surface species following DEG adsorption indicate that DEG may be useful for germanium deposition on silicon surfaces.

Diethyldiethoxysilane as a New Precursor for SiO2 Growth on Silicon

AbstractDiethyldiethoxysilane (DEDEOS) is a potential single molecular precursor for SiO2 growth. Additionally, DEDEOS may be applicable in a deposition scheme that will allow precise atomic layer control of SiO2 film thicknesses. Deposition of SiO2 on silicon using DEDEOS was studied in three regimes. In the first regime, the adsorption and decomposition of DEDEOS were studied using temperature-programmed desorption (TPD) and Fourier-transform infrared spectroscopy (FTIR) under UHV conditions. TPD and FTIR results indicated that DEDEOS deposits ethyl and ethoxy species on the surface which decompose by a β-hydride elimination mechanism. In the second regime, step-wise SiO2 growth using DEDEOS was studied by laser-induced thermal desorption (LITD) and TPD as a function of repetitive growth cycles composed of DEDEOS deposition at 300 K followed by thermal annealing to 820 K. The amount of SiO2 growth per cycle initially decreased as a function of cycle until reaching a constant value after approximately 10 cycles. Finally, DEDEOS deposition of SiO2 was studied in a high pressure chamber at surface temperatures between 873 K and 1003 K and at a pressure of 0.5 Torr. The deposition of SiO2 as a function of exposure was found to display a fast initial growth step followed by a slower step which continued indefinitely. The activation energy for SiO2 growth by DEDEOS in the slower growth step was found to be 49 ± 6 kcal/mol. This activation energy is very similar to the activation energy of 45 kcal/mol observed for SiO2 growth by tetraethoxysilane (TEOS).