Kinetics of the decomposition of disilane molecules on a silicon growth surface in vacuum chemical epitaxy

In the framework of the kinetic approach based on data of technological experiments, the range of characteristic rates of decomposition of disilane radical molecules adsorbed on the surface during the growth of a silicon layer is determined. The relationship between the rate of incorporation of silicon atoms into a growing crystal and the characteristic rate of pyrolysis of hydride molecules on the growing surface is established. The temperature dependences of the decomposition rate of disilane molecules exhibit an unusual activationless behavior in the growth temperature range. The form of the observed dependences is determined by the pyrolysis model, conditions of transferred of hydrogen from an adsorbed molecule onto the surface of the growing layer, being a function of the gas pressure and temperature in the reactor. It is demonstrated that the basic features of the behavior of the decomposition rate of disilane molecules are controlled by the specifics of the interaction of the silicon dihydride molecular beam with the growth surface under conditions of low and high degrees of bonding of hydrogen to free surface bonds. The temperature dependences are qualitatively described by a relation composed of two activation curves with different activation energies at low and high temperatures and preexponential factors depending on the surface coverage by hydrogen atoms.     

The special features of adsorption and the kinetics of decomposition of monosilane molecules on the epitaxial surface of silicon

Analytic equations relating the rate of the incorporation of silicon atoms into a growing crystal to the characteristic frequency of the pyrolysis of silane molecules on the surface of silicon were obtained over the temperature range corresponding to the epitaxial growth of silicon films. As distinct from the earlier works, it was assumed that adsorbed silicon atoms and monosilane molecules formed double bonds with the surface. The data of technological experiments for the most extensively used pyrolysis models obtained thus far were used to determine the region of the characteristic frequencies of the decomposition of hydride molecule radicals adsorbed on the surface of a silicon plate over the temperature range 450–700°C. The temperature dependence of the frequency of monosilane molecule decomposition was shown to be to a great extent determined by the form of the temperature dependence of the $ \tilde v_{SiH_2 }^0 $ \tilde v_{SiH_2 }^0 preexponential factor. It was also found that the characteristic frequency of the decomposition of silane molecules was sensitive to the stage of pyrolysis at which hydrogen atoms released from silane molecules were captured by the surface. Decomposition occurred at the highest rate if hydrogen molecules were adsorbed at the stage of the adsorption of monosilane. The lowest rate of decomposition was observed if hydrogen molecules were adsorbed at the stage of the decomposition of radicals already captured by the surface. The temperature dependence of the coefficient of adsorption of monosilane molecules was characterized by a negative activation energy of the process for almost all the most important system models over the temperature range of growth. At elevated growth temperatures, the adsorption of monosilane molecules by the surface of silicon proceeded via an intermediate state characterized by the difference of desorption and chemisorption energies on the order of 0.28 eV.     

Effect of prior mechanical and thermal treatment on the thermal decomposition and sublimation of cubic ammonium perchlorate

The thermal decomposition of ammonium perchlorate (AP) in its cubic modification has been studied in the temperature range 300–390°C. Two distinct regions of temperature dependence are observed for the rate constants of the decomposition. The activation energies in the two regions are found to be 20 ± 2 kcals/mole and 60 ± 2 kcals/mole. Prior mechanical and thermal treatment of the materials is seen to result in a marked increase in the thermal reactivity of AP in the range 300–370°C. Differential thermal analysis of pre-compressed AP shows pronounced changes in the exothermic characteristics of the material. Prior compression of AP in addition enhances the sublimation of the material. The activation energies are not altered by the pretreatment. The results are explained in terms of crystal imperfections and the role of dislocations in the thermal decomposition of AP.     

Silicon carbide nanotubes (SiCNTs) serving for catalytic decomposition of toxic diazomethane (DAZM) gas: a DFT study

In the present study, the adsorption and decomposition of diazomethane (DAZM) on the surface of (6,0) zigzag silicon carbide nanotube (SiCNT) are investigated using density functional theory calculations. The geometry structures of the three stable configurations, adsorption energies and electronic properties of DAZM adsorption on the surface of SiCNT are investigated. It was found that the DAZM molecule is decomposed over the surface of (6,0) SiCNT with activation energy (E_act) of 0.523 eV. The curvature effect on the adsorption energies of the DAZM molecule is also considered by studying (5,0) and (7,0) SiCNTs. The results display that DAZM adsorption over smaller diameter of SiCNT is thermodynamically more favourable than larger one.     

Existence and thermal stability of mono and dihydride phases on Si(1 1 1) and Ge(1 1 1) surfaces: A photoemission study

Based on UPS and XPS investigations, it is concluded that a monohydride phase forms at first on the Si(1 1 1)7 × 7 surface. Upon further hydrogen dosing at room temperature, a dihydride phase develops and superposes to the previously formed monohydride phase. The dihydride phase desorbs completely around 250°C and the monohydride phase at about 550°C. A pure dihydride phase obtained by H adsorption cannot be observed on a silicon surface. Silane or disilane adsorption at room temperature exhibits the characteristic features of the dihydride phase without the associated monohydride phase. The obtained phase desorbs at the same temperature as the H induced dihydride phase. That is to our mind the only possibility to obtain a pure dihydride phase.

For germanium in careful conditions we observe only a monohydride phase which desorbs at 150°C. For high hydrogen exposures, we obtain a new phase but XPS measurements indicate oxygen contamination. This place desorbs at 225°C and allows clear distinction between H adsorption and contamination. It is concluded that Ge and Si surfaces have different reactivities for hydrogen adsorption. These conclusions are extended to all Ge and Si surfaces either crystallized or amorphous.     

Interaction between surface migrating pentacene molecules and chemically modified surfaces of silicon oxides studied by pulsed molecular beam scattering

The interaction between pentacene molecules and organic self-assembling monolayers formed on silicon oxides (SiO₂) was studied by measuring the surface scattering time profile of the pulsed molecular beam of pentacene. It was found that the surface residence time (SRT) of pentacene was significantly reduced on a surface treated with hexamethyl silazarane (HMDS) compared with that on a bare SiO₂ surface. The activation energies derived from the temperature dependence of the SRT were 24 kJ/mol and 100 kJ/mol for HMDS-SiO₂ and the bare SiO₂, respectively. A surface treated with octadecyltrichlorosilane (OTS) showed SRT values almost the same as those on the bare SiO₂ surface, which was due to exposed SiO₂ regions on the thermally-degraded OTS-SiO₂. The growth mechanism with improved quality is due to the shallower adsorption potential and enhanced migration of pentacene by the surface alkylation.     

Growth dynamics studied by RHEED during Si/Ge epitaxy from gaseous hydrides

Si and Ge epitaxial growth from disilane and germane in a gas-source molecular beam epitaxy (GSMBE) system is followed in situ by reflection high-energy electron diffraction (RHEED) intensity oscillations. During Ge homoepitaxy, the growth rate on a Ge(100) substrate is found to be limited by surface hydrogen desorption below 350°C and by hydride adsorption above this temperature. Ge heteroepitaxy on Si results in incomplete layer growth leaving exposed Si at the surface during the initial stages of growth. Therefore, a gradual change in the observed Si surface concentration is seen as growth proceeds. Si heteroepitaxy on Ge follows the Volmer-Weber growth mode and proceeds via island formation. This, combined with Ge surface segregation, results in a slow decrease of the Ge surface population at the growth front. During heteroepitaxial growth, hydride reaction rates differ on Si and Ge surfaces, and therefore a changing concentration of the surface species is manifest as a gradual change in the observed oscillation frequency. This effect, observed during the early stages of growth, shows strong temperature dependence, consistent with previous observations on SiGe alloys. Following several layers of growth, however, the surfaces become rough. The influence of this roughness on the oscillation frequency is also discussed.     

Evaluation of desorption activation energy of SiCl2 molecules

The chemical etching of silicon in Cl₂ ambient was considered. The desorption activation energy for an SiCl₂ molecule was evaluated using an experimentally measured dependence of etching rate on concentration of Cl₂ molecules. It was found that the desorption activation energy of SiCl₂ molecules is equal to E_d=(1.605±0.010) eV. This corresponds to a value of the mean lifetime of adsorbed molecules on the surface of τ=46 ms at temperature T=724 K.     

Molecular orbital study of the chemisorption of carbon monoxide on a tungsten (100) surface

The adsorption energies of carbon monoxide chemisorbed at various sites on a tungsten (100) surface have been calculated by extended Hückel molecular orbital theory (EHMO). The concept of a “surface molecule” in which CO is bonded to an array of tungsten atoms W_n has been employed. Dissociative adsorption in which C occupies a four-fold, five-coordination site and O occupies either a four- or two-fold site has been found to be the most stable form for CO on a W surface. Stable one-fold and two-fold sites of molecularly adsorbed CO have also been found in which the CO group is normal to the surface plane and the C atom is nearest the surface. Adsorption energies and molecular orbitals for the stable molecularly and dissociatively adsobred CO sites are compared with the experimental data on various types of adsorbed CO, i.e., virgin-, α-, and β-CO. Models are suggested for each of these adsorption types. The strongest bonding interactions occur between the CO 5σ orbital and the totally symmetric 5d and 6s orbitals of the W_n cluster. Possible mechanisms for conversion of molecularly adsorbed CO to dissociatively adsorbed CO are proposed and the corresponding activation energies are estimated.     

Active oxidation: Silicon etching and oxide decomposition basic mechanisms using density functional theory

The etching of silicon atom from the Si(1 0 0)-p(2 × 2) surface, i.e. the desorption of SiO molecules from this surface, either clean or pre-oxidized, is investigated at the density functional theory level. The reaction paths for desorption are given as a function of the initial oxidation state of the extracted silicon atom. The associated activation energies and the atomic configurations are discussed. Particularly, it is shown that desorption of SiO molecules takes place during conventional thermal oxide growth (∼2 eV activation) via non-oxidized silicon atoms. Further SiO extraction mechanisms of higher silicon oxidation states required higher temperatures. In particular, doubly oxidized silicon atoms (Si²⁺) are able to decompose with an activation of ∼4 eV which corresponds to the actual temperature where decomposition of oxides is observed. This confirms the statement that decomposition of oxide layer nucleates at the interface with silicon where Si²⁺ has been detected thanks to XPS experiment.     

Simulation of the thermal decomposition of the C9H10 methylcubane molecule

The thermal stability of the C₉H₁₀ methylcubane molecule was studied over the temperature range 1100–2100 K by molecular dynamics simulation with the tight binding potential. The temperature dependence of its lifetime to the decomposition moment was determined. The activation energy E _a = 1.7 ± 0.2 eV and frequency factor A = 10^{15.63 ± 0.53} s^?1 of the Arrhenius equation were found. Possible channels and final products of molecule decomposition were studied.     

Thin PTCDA films on Si(0 0 1): 2. Electronic structure

We have studied the thin film formation and the electronic structure of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA), on clean and on hydrogen-passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), angle-resolved photoelectron spectroscopy (ARPES), near edge X-ray absorption fine structure (NEXAFS) and low energy electron diffraction (LEED). On the H passivated surface the changes in the electronic structure of the substrate and the molecules with increasing film thickness are very small. The molecular orbitals show a dispersive behavior, indicating that the PTCDA layers are ordered. On the reactive clean surface the anhydride groups of the molecule interact with the substrate as indicated by changes in the core level binding energies. This results in a much lower ordering in the film compared to PTCDA on a passivated silicon surface. There is no sign of decomposition of the molecule because of the more reactive substrate.     

The dependence of accommodation coefficient on surface temperature

The dependence of energy accommodation coefficients of inert gases on clean metal surfaces is analyzed in the framework of classical lattice theory. It is found that a Knudsen layer must exist next to the surface to adjust the gas molecular velocity distrivution to surface conditions. At energies below the well-depth, the accommodation coefficient slope is inversely proportional to the square root of the temperature; at energies above the welldepth, the accommodation coefficient is independent of surface temperature.     

Low energy electron attachment to C<Subscript>60</Subscript>

Low energy electron attachment to the fullerene molecule (C₆₀) and its temperature dependence are studied in a crossed electron beam–molecular beam experiment. We observe the strongest relative signal of C₆₀ anion near 0 eV electron energy with respect to higher energy resonant peaks confirming the contribution of s-wave capture to the electron attachment process and hence the absence of threshold behavior or activation barrier near zero electron energy. While we find no temperature dependence for the cross-section near zero energy, we observe a reduction in the cross-sections at higher electron energies as the temperature is increased, indicating a decrease in lifetime of the resonances at higher energies with increase in temperature.     

CO在Pu(100)表面吸附的研究

采用密度泛函理论(DFT)研究了CO分子在Pu (100)面上的吸附. 计算结果表明：CO在Pu (100)表面的C端吸附比O端吸附更为有利，属于强化学吸附. CO吸附态的稳定性为穴位倾斜>穴位垂直>桥位>顶位. CO分子与表面Pu原子的相互作用主要源于CO分子的杂化轨道和Pu原子的杂化轨道的贡献. 穴位倾斜吸附的CO分子的离解能垒较小(0.280eV)，表明在较低温度下，CO分子在Pu (100)表面会发生离解吸附，离解的C，O原子将占据能量最低的穴位.     

CO在Pu(100)表面吸附的研究

采用密度泛函理论(DFT)研究了CO分子在Pu (100)面上的吸附. 计算结果表明：CO在Pu (100)表面的C端吸附比O端吸附更为有利,属于强化学吸附. CO吸附态的稳定性为穴位倾斜>穴位垂直>桥位>顶位. CO分子与表面Pu原子的相互作用主要源于CO分子的杂化轨道和Pu原子的杂化轨道的贡献. 穴位倾斜吸附的CO分子的离解能垒较小(0.280eV),表明在较低温度下,CO分子在Pu (100)表面会发生离解吸附,离解的C,O原子将占据能量最低的穴位. 关键词： 密度泛函理论 Pu (100) CO 分子和离解吸附     

Diameter-independent kinetics in the vapor-liquid-solid growth of Si nanowires

We examine individual Si nanowires grown by the vapor-liquid-solid mechanism, using real-time in situ ultra high vacuum transmission electron microscopy. By directly observing Au-catalyzed growth of Si wires from disilane, we show that the growth rate is independent of wire diameter, contrary to the expected behavior. Our measurements show that the unique rate-limiting step here is the irreversible, kinetically limited, dissociative adsorption of disilane directly on the catalyst surface. We also identify a novel dependence of growth rate on wire taper.     

DC conductivity and dielectric constant of cellulose nitrate detector

The temperature dependence of DC conductivity as well as the temperature and frequency dependence of dielectric constant of cellulose nitrate (CN) track detector was undertaken for better understanding of the track response of the detector. In the temperature range −100° to + 100°C, CN is found to be an insulating semiconductor with activation energies 0.04 eV, 0.78 eV and 1.24 eV. The transition of the activation processes takes place at (−10±1)°C and (60±1)°C. The high activation energy transition temperature coincides with the glass transition temperature of CN. The dielectric constant is found to increase with temperature and decrease with frequency. These results reveal the physical basis of the Ion Explosion Spike model of Fleischer et al. (1975). A possible explanation of the registration temperature effect is also given on the basis of these results.