Dihydride Species Research Articles

Reaction of H2S with Si(100)

The dissociative adsorption of H2S on Si(100) and the subsequent desorption of hydrogen from and diffusion of sulfur into the same surface have been investigated using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Desorption of hydrogen occurred at 546 °C from surfaces exposed to H2S at temperatures ranging from −145 to 425 °C; an additional H2 desorption channel at 433 °C was seen for the −145 °C case. A comparison of this behavior with desorption from atomic-hydrogen-dosed Si(100) indicated that these features are due to the desorption of surface monohydride and dihydride species, respectively. In the TPD studies, the surface was saturated by 0.5 ML of H2S for all substrate exposure temperatures. No desorption feature attributable to sulfur-related species was observed for any of the surface conditions. However, AES measurements revealed a sharp decrease in the concentration of sulfur at the surface over the temperature range of 525−625 °C, indicating that H2 desorption is accompanied by diffusion of sulfur into the Si crystal. The exponential decay of the sticking coefficient derived from the coverage dependence of the H2S adsorption at 25 °C is consistent with a two-step model for the adsorption kinetics.

The cluster/surface analogy: the binding of norbornene on a triangulated array of metals

Reactions of [Os3(CO)10(NCMe)2] 1 with norbornene (bicyclo[2.2.1]hept-2-ene, nbe) led to two isomeric triosmium clusters [Os3(μ-H)(CO)10(μ-η1∶η2-C7H9)] 2 and 3 and also the dihydride species [Os3(μ-H)2(CO)9(μ3-η1∶η2∶η1-C7H8)] 4. The molecular structure of compound 4 established by single crystal X-ray analysis shows norbornene–metal interactions similar to the proposed adsorption mode of norbornene found on the Pt(111) surface. The two isomers 2 and 3 exhibit an intermediate bonding mode between the solely π bound ligand and the di-σ,π bonding mode exhibited by cluster 4. These isomers differ in the orientation of the C7H8 moiety with respect to the triosmium triangle. Both 2 and 3 may exist in other isomeric forms which differ in the location of the hydride ligand. Compound 2 converts into 3 which on heating undergoes C–H bond cleavage to produce 4.

Preparation and reactivity of dihydrogen complexes [MX(η2-H2)P4]BF4 (M = Ru or Os; X = halogenide or SEt−; P = phosphite)

Monohydride complexes MHXP4 [M = Ru or Os; X = Cl−, Br−, I−, SEt− or N3−; P = P(OEt)3, PPh(OEt)2 or PPh2OEt] were prepared by treating dihydride species MH2P4 first with CF3SO3Me and then with an excess of the anionic ligand X. In an argon atmosphere, protonation of MHXP4 with HBF4·Et2O gives dihydrogen cations [MX(η2-H2)P4]+, with X = Cl, Br, I or SEt; the classical dihydride [MH2(N3)P4]+ was obtained with the azide ligand. Instead, in a hydrogen atmosphere, protonation of MHXP4 with HBF4·Et2O gives hydride–dihydrogen [MH(η2-H2)P4]+ species, according to a proposed mechanism involving interaction of Bronsted acid with ligand X. Some [MX(η2-H2)P4]+ cations were thermally unstable and fully characterised in solution (1H and 31P NMR, variable temperature T1 measurements), whereas the [OsX(η2-H2){PPh(OEt)2}4]BF4 complexes were stable and isolated as solids. Treatment of [MX(η2-H2)P4]+ cations with alkyne PhCCH gave evolution of H2 and formation of the vinylidene intermediate [MX{CC(H)Ph}P4]+ which, by reaction with base, afforded the final acetylide M(CCPh)XP4 derivatives. Treatment with propargyl alcohols HCCC(OH)RR′ of the [MX(η2-H2)P4]+ cations, instead, gave propadienylidene derivatives [MX(CCCRR′)P4]BPh4 (M = Ru or Os; R = R′ = Ph or R = Ph, R′ = Me). Hydrazine complexes [MX(NH2NH2)P4]BPh4 were also prepared by substitution of the dihydrogen ligand in the new η2-H2 derivatives.

Hydrogen population on Ge-covered Si(001) surfaces

A series of isochronal annealing experiments have been performed using high resolution electron energy loss spectroscopy in order to probe the thermal decomposition of monohydride and dihydride species from Ge-covered Si(001) surfaces. The Ge coverage was varied from submonolayer until completely covering the silicon surface. The films were exposed to atomic hydrogen at room temperature and the intensities of the hydrogen vibrational features were monitored as a function of annealing temperature. Marked steps in the stretching mode intensities are observed, directly connected to monohydride decomposition. The existence of a desorption state specific to surface alloying is thus clearly identified in the Ge-H stretching intensity temperature dependence curve, interpreted as involving the recombinative desorption of two hydrogen atoms from mixed Ge-Si dimers. From the scissors mode intensity variation, it is deduced that both Ge and Si dihydride species decompose simultaneously at all coverages, and that it always occurs before monohydride desorption takes place. Furthermore, we observe the ${\mathrm{G}\mathrm{e}\ensuremath{-}\mathrm{H}}_{2}$ formation enhancement due to the presence of the neighboring silicon atoms. Finally, the desorption of all hydride species is shown to be boosted when increasing the Ge contents at the Si(001) surface. To summarize this study, the complete experimental desorption diagram of the hydrogen population, as functions of both stoichiometry and temperature, is presented.

Crystallization control of the amorphous buffer layer in hydrogenated nanocrystalline silicon

Hydrogenated nanocrystalline silicon film have been deposited by RF sputtering in a pure hydrogen plasma at a substrate temperature of 150°C. The unintentionally grown amorphous buffer layer was almost completely crystallized at an annealing temperature as low as 150°C. By combining infrared absorption measurements and electron microscopy observations, the critical role of dihydride species, SiH 2, in the crystallization process have been clearly evidenced and discussed.

Evidence for a ruthenium dihydride species as the active catalyst in the RuCl2(PPh3)-catalyzed hydrogen transfer reaction in the presence of base

The role of the base in RuCl2(PPh3)3-catalyzed hydrogen transfer reactions is to generate a highly active RuH2(PPh3)3 catalyst from the dichloride via two consecutive alkoxide displacement–β-elimination sequences.

High-resolution electron energy loss spectroscopy evidence for electron beam-induced decomposition of trimethylsilane adsorbed on Si(100)

The effects of incident electrons on trimethylsilane ((CH 3) 3SiH) adsorbed onto Si(100) at 110 K are examined using high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). TPD from trimethylsilane-covered Si(100) show hydrogen desorption from silicon monohydride states at 780 K ( β 1); at high trimethylsilane exposures, two low-temperature (130 K and 155 K) molecular TPD states appear. The absence of silicon dihydride species on the trimethylsilane covered Si(100) surface was demonstrated by comparing the data obtained from this surface with that obtained from disilane-covered Si(100). HREELS and TPD data indicate that carbon deposition on Si(100) is enhanced by irradiating the trimethylsilane covered surface only when the molecular states are present.

Energetics of silicon hydrides on the Si(100)-(2×1) surface

Density functional theory methods are used to calculate the structures and energies of silicon trihydride (SiH3) and dihydride (SiH2) species on the Si(100)-(2×1) surface. These species are intermediates in the growth of silicon films by chemical vapor deposition of silane and disilane. The lowest-energy trihydride species is metastable with respect to the lowest-energy dihydride species, but two surface dangling bonds must be available to affect the transformation to the dihydride. In the lowest-energy configurations, dimers either have both dangling bonds occupied or both unoccupied. While the energy difference between isomers with fully occupied and partially occupied dimers will strongly favor fully occupied dimers at low temperatures, there will be a distribution of dimer occupations at high temperatures. The structures and energies of some other local minima corresponding to tri- and dihydrides are also described. While these species are energetically unfavorable and should only exist transiently, they illustrate the relative energetics of some alternative bonding behavior of the silicon surface.

Hydrogen adsorption and desorption processes on Si(100)

The hydrogen adsorption and desorption processes on the Si(100)(2×1) surface were investigated using in-situ infrared absorption spectroscopy in the multiple internal reflection geometry. It is demonstrated that the distribution of hydride species (SiH, SiH 2, and SiH 3) significantly changes during adsorption of atomic hydrogen and desorption of molecular hydrogen. At the initial stages of hydrogen adsorption, the monohydride Si (Si–H) and dihydride Si (Si–H 2) are populated, with Si–H being dominant. For higher hydrogen exposures the dihydride and trihydride Si are formed. Thermal annealing causes hydrogen to desorb from the hydride species. For annealing temperature up to approximately 400°C, the trihydride Si is etched away, producing a H-terminated surface which consists of monohydride (SiH) and dihydride (SiH 2) species. We demonstrate that the conversion from the monohydride to the dihydride phase occurs during thermal annealing.

Fabrication of SiGe quantum dots: a new approach based on selective growth on chemically prepared H-passivated Si(100) surfaces

We report, in this paper, on a new method to produce SiGe quantum dots on Si(100) surfaces. Starting from the fact that the adsorption of hydride molecules (SiH 4, GeH 4) requires free adsorption sites on the surface, the basic idea of our approach is to limit the number of sites for molecular adsorption. We show that etching of Si(100) surfaces in ammonium fluoride (NH 4F) solution initially produces a flat and dihydride-terminated Si(100) surface and that longer etching leads to the formation of microscopic (111) facets which are regularly distributed along the surface. Hydrogen atoms are found to desorb completely from surface dihydrides at ∼400°C while those from monohydride-terminated (111) facets remain stable up to 650°C. Thus, for growths carried out in the temperature range of 400–650°C, the adsorption of hydride molecules occurs only on the sites that have been previously terminated by dihydride species, i.e. free of hydrogen. Compared with islands formed by the strain-induced growth mode transition, we demonstrate, by using this new approach, that SiGe islands with better uniformity and much smaller sizes (down to ∼200 Å) can be achieved.

A quasi-equilibrium model for the uptake kinetics of hydrogen atoms on Si(100)

A quasi-equilibrium model has been developed to describe the uptake kinetics of atomic hydrogen on Si(100) at 373 and 635 K. A new model is required because a simple consideration of only adsorption at dangling bond sites and Eley-Rideal abstraction cannot be reconciled with a saturation coverage of one monolayer (ML) at 635 K. We argue that although diffusion at low temperatures can be explained by hot precursor dynamics, slow vibrational relaxation in the chemisorption potential does not lead to an almost coverage independent sticking probability. Instead, we propose that a high sticking probability is maintained by reaction with doubly occupied dimers to give dihydride species which then migrate across the surface by an isomerization reaction. Subsequently, dihydride units either react with unoccupied dimer sites or molecular hydrogen is lost by desorption from two dihydride units (the β 2 desorption channel). At 635 K the dihydride species may be regarded as a mobile chemisorbed precursor, although only the bonding arrangement is propagated by isomerization. When the atomic hydrogen source is turned off, the small steady-state dihydride coverage is rapidly lost to leave a saturated monohydride phase. A saturation coverage of 1.5 ML is obtained when abstraction and adsorption reactions are in dynamic equilibrium at 373 K. It is shown that the rate constant for abstraction from monohydride is essentially the same as for abstraction from dihydride. The measured initial rate constant for loss of adsorbed hydrogen during exposure to atomic deuterium is (1.8 ± 0.1) times larger than that for loss of adsorbed deuterium during atomic hydrogen exposure at 635 K. At this temperature, the initial rate constant for loss of deuterium during exposure to atomic hydrogen is found to be the same regardless of initial coverage, but the rate decreases more slowly than expected for first-order kinetics as the reaction proceeds. The uptake model also implies that a small amount of deuterium is lost as D 2 and HD via the β 2 thermal desorption channel at 635 K.

Infrared Spectroscopy of Sub-Surface Defects induced by Remote Hydrogen Plasma exposure of Silicon(100)

ABSTRACTInfrared multiple internal reflection (MIR) spectroscopy was used to investigate the local chemical bonding in sub-surface defects induced by remote hydrogen plasma exposure (RHPE) of Si(100) wafers. Exposure of very lightly doped n-type Si ([P] =; 5 × 1013 cm−3) to a remote hydrogen plasma for 2 min at 200°C results in the formation of Si monohydride species. An intense narrow band at 2078 cm−1 (FWHM = 7 cm−1) and a small shoulder at 2065 cm−1 are observed. The data are consistent with monohydride termination of Si{111} platelet defects with a weak interaction between H atoms on opposing internal surfaces. In contrast, platelet nucleation at 200°C followed by growth at 300°C selectively generates Si dihydride species, as evidenced by a single broad infrared band at 2109 cm−1. The P concentration was found to have a marked influence on the areal density and chemical bonding of sub-surface hydrogen. The MIR spectrum of lightly doped Si ([P] = 2 × 1014 cm−3) after RHPE at 200°C contains broad peaks at 2078 and 2130 cm−1 consistent with Si monohydride and trihydride species. We infer that hydrogen saturates broken bonds along Si{111} Type I glide planes (one bond per Si atom) and along Si{111} Type II glide planes (three bonds per Si atom). The Si-H peak area indicates a H areal density ∼2 times higher than in very lightly doped Si.

Synthesis of the New Chiral (R)- and (S)-Aminodiphosphine Ligands sec-Butylbis(2-(diphenylphosphino)ethyl)amine, sec-Butylbis(2-(dicyclohexylphosphino)ethyl)amine, and (α-Methylbenzyl)bis(2-(dicyclohexylphosphino)ethyl)amine and Their Organometallic Chemistry When Combined with Iridium

The new chiral aminodiphosphine ligands (R)- and (S)-sec-butylbis(2-(diphenylphosphino)ethyl)amine, (R)- and (S)-sec-butylbis(2-(dicyclohexylphosphino)ethyl)amine, and (R)- and (S)-(α-methylbenzyl)bis(2-(dicyclohexylphosphino)ethyl)amine have been synthesized from optically pure (R)-(−)- and (S)-(+)-sec-butylamine or (R)-(+)- and (S)-(−)-(α-methylbenzyl)amine. The reactions between these PNP* ligands and [Ir(COD)(OMe)]2 have been investigated in either aprotic or protic solvents (COD = cycloocta-1,5-diene). Depending on the substituents at either the carbon stereocenter or the phosphorus donors, iridium products with different structures and/or stabilities are obtained. Among the new complexes, there are monohydride, dihydride, and trihydride species. It is observed that (i) cyclohexyl substituents on the phosphorus donors favor the formation of complexes where the PNP* ligand adopts a meridional conformation; (ii) phenyl groups attached to the carbon stereocenter lead to the formation of thermodynamicall...

Mechanism of H 2 desorption from H-terminated Si(001) surfaces

Semiempirical molecular orbital calculations were performed to investigate the mechanism of H 2 desorption from dihydride species on Si(001) surfaces. The lowest energy pathways were calculated with respect to three different mechanisms which have been proposed previously. We performed additional calculations under the different H coverage conditions to examine the dependence of activation energy on the varieties of surrounding hydride species. The new transition state structure was obtained by the calculation of the recombinative desorption of two H atoms from adjacent Si dihydrides. We have found that the activation barrier of the recombinative desorption mechanism was the lowest of all and it was hardly influenced no matter what the surrounding hydride species is.

Ab initio molecular dynamics of H 2 desorption from Si(100)-2 × 1

We present the first ab initio molecular dynamics (AIMD) trajectories of two competing mechanisms proposed for H 2 desorption from Si(100)-2 × 1. We show that silicon dihydride species are the most likely desorption precursors and that surface corrugation is responsible for focusing desorbing trajectories, based on comparisons with experimental dynamical data. The equal roles played by the transition state (TS) structure and post-detachment dynamics is emphasized.

Neutron Diffraction Studies of [Cp(PMe3)2RuH] and [Cp(PMe3)2RuH2]BF4 at 20 K

The crystal and molecular structures of [Cp(PMe3)2RuH] (1) and [Cp(PMe3)2RuH2]BF4 (2) have been determined from neutron diffraction measurements at 20 K. The Ru−H bond lengths in 1 (1.630(4) Å) and 2 (1.599(8), 1.604(9) Å) are the first terminal Ru−H distances to be determined by single-crystal neutron diffraction. The “Cp‘L2Ru” system is now the first to have neutron diffraction studies of the monohydride, dihydride, and dihydrogen species. This is of particular importance in understanding the activation of H2 by this metal−ligand fragment. Thus detailed comparisons with the recent structure determination by neutron diffraction of the dihydrogen complex [Cp*(dppm)Ru(η2-H2)]BF4 (dppm = bis(diphenylphosphino)methane) [Klooster, W. T.; Koetzle, T. F.; Jia, G.; Fong, T. P.; Morris, R. H.; Albinati, A. J. Am. Chem. Soc. 1994, 116, 7677−7681] are presented. A network of C−H···F−BF3 hydrogen bonds links the anion and cation moieties in 2. The C(2)−H(2)···F(4) interaction has an H···F separation of only 2.078(8) Å, the shortest such interaction characterized to date by neutron diffraction. Ru−H stretching frequencies determined from the diffraction data are in good agreement with those from IR measurements.

Aluminum-Selective Chemical Vapor Deposition Induced by Hydrogen Desorption on Silicon

We studied the influence of the chemical structure of silicon-hydride species on Al deposition rate in Al-chemical vapor deposition (CVD) employing dimethylaluminum hydride (DMAlH) on a hydrogen-terminated Si surface. It was revealed that the peak temperature of hydrogen desorption from Si–H2 and Si–H3 in TDS spectra coincides well with the peak temperature of the Al deposition rate. We found that thermal hydrogen desorption from dihydride and/or trihydride species induced the Al-selective deposition reaction. Monohydride is not useful for the deposition at 240–290°C because of its higher desorption temperature of 350°C. In the early stage of Al deposition, a lower density of Al nuclei on the NH4F-treated surface compared with that on the HF-treated surface suggests that Al nucleation occurred at step edges on which di- and trihydride species exit.

Reaction kinetics in synchrotron-radiation-excited Si epitaxy with disilane. I. Atomic layer epitaxy

We investigated the mechanism of silicon crystal growth mediated by a surface photochemical reaction. The growth process consists of reactive sticking of disilane (Si2H6) onto a partially hydrogen covered surface followed by the photon-stimulated desorption of hydrogen atoms and consequent regeneration of dangling bonds. The saturation coverage of Si admolecules resulting from self-limiting chemisorption of disilane was found to be 0.42 monolayer (ML), and the ejection of H+ and H+2 ions was observed by time-of-flight mass spectroscopy. Hydrogen removal by the purely electronic process differs from thermal desorption, however, in that not all of the hydrogen is removed. Analysis of film growth by repetition of the cycle of disilane exposure, evacuation, and synchrotron radiation irradiation showed that the onset temperature of thermal growth (350 °C) is the same as that of H2 desorption from the dihydride species. Below 350 °C a digital growth of 0.18 ML/cycle occurs over a wide range of gas exposure times, irradiation times, substrate temperatures, and the irradiation intensities. If the temperature is raised to facilitate thermal desorption of hydrogen atoms and migration of Si adatoms, the number of Si adatoms delivered in each cycle increases significantly. Photolytic, thermal, and photothermal effects result in growth rates of 0.4 ML/cycle at 430 °C and 1 ML/cycle 480 °C.

TPD and ESD of hydrogen from disilane covered Si(100)

Disilane covered Si(100) was investigated using temperature programmed desorption (TPD) and electron stimulated desorption (ESD) for various disilane exposures. At low coverages, only monohydride species are observed on the surface as seen by TPD. At high coverages, dihydride species appear. Kinetic energy distributions (KEDs) for H + and H − were also obtained from Si(100) covered with varying amounts of disilane. Both H + and H − KEDs exhibited two peaks each, suggesting that there are at least two types of hydrogen bound states on the surface. ESD decay curves were also obtained and exhibited double exponential decay behavior. ESD total cross sections were calculated using H + and H − signal decays and compared with related data from the literature.

Recombinative desorption of hydrogen from the Ge(100)–(2×1) surface: A laser-induced desorption study

The recombinative desorption of H2 from Ge(100)–(2×1) is studied by temperature programed desorption (TPD) and laser-induced desorption (LID). In contrast to what is observed for the Si(100)–(2×1) surface, the TPD spectra for Ge(100) do not appear to show appreciable formation of a stable dihydride species. Both the TPD and LID results are consistent with the first-order recombinative desorption kinetics. Analysis of the LID results yield an activation energy, Ea=40±2 kcal/mol and preexponential factor, ν=4×1013±1 s−1. The results are discussed in terms of several mechanisms that have been proposed for the first-order recombinative desorption of hydrogen from Si(100)–(2×1).