Supersonic Molecular Jet Research Articles

Low-temperature epitaxial growth of cubic SiC thin films on Si(111) using supersonic molecular jet of single source precursors

Epitaxial cubic SiC thin films have been deposited on Si(111) by supersonic molecular jet epitaxy of the single source precursors of methylsilane (MS), CH 3SiH 3, and dimethylethylsilane (DMES), (CH 3) 2SiH(CH 2CH 3), at temperatures in the range 780–950 °C. Single crystalline, crack-free epitaxial cubic SiC thin films were successfully grown on carbonized Si(111) substrates at temperatures as low as 830 °C using MS and DMES. Highly oriented cubic SiC thin films in the (111) direction were obtained on uncarbonized Si(111) substrates at 780 °C using MS and at 950 °C from DMES, However, the growth temperature of DMES was lowered to 830 °C on Si(111) when the substrates were initially carbonized with a supersonic jet of acetylene. Below 780 °C, moreover, only polycrystalline cubic SiC thin films were grown on either carbonized or uncarbonized Si(111) surfaces with MS. The advantage of supersonic molecular jets of the single source precursors employed in this study is evident in that the surface carbonization process may not be necessary, and the deposition procedure is quite simple and safe to handle. Real-time, in situ optical reflectivity was used to monitor the film growth. The as-grown films were characterized in situ by Auger electron spectroscopy (AES) and ex situ by ellipsometry, SEM, FTIR, UV/Visible spectroscopy, and XRD (especially omega and phiscan measurements).

Application of Supersonic Molecular Jets in Semiconductor Thin Film Growth

The flux and the incident kinetic energy are the most important deposition variables in thin film growth processes. By changing these variables, one can, in principle, alter the reaction pathways and the rate at which they occur and produce a different material than under thermody-namic equilibrium conditions. The significance of supersonic molecular jets stems from the fact that both, the flux and the incident kinetic energy of neutral growth species, can be varied independently. The number of studies that are exploring these advantages in a wide range of materials systems is growing rapidly. In this article the application of supersonic molecular jets in semiconductor thin film growth is reviewed. The effects of both the superthermal incident kinetic energy and the flux on the growth and properties of elemental and compound semiconductors are examined.

Growth of hexagonal GaN thin films on Si(1 1 1) with cubic SiC buffer layers

The growth of GaN films on silicon or sapphire substrates has not been straightforward due to their large mismatch (>13%). SiC is structurally the closest material to GaN (especially cubic form) and has a small lattice mismatch (3.5%) between SiC and GaN, and therefore can be useful as the substrate for GaN. Single-crystal wafers of cubic SiC and GaN, however, are hard to obtain and have not been produced commercially yet. Therefore, a thin layer of cubic SiC on substrates such as Si and oxide single crystals might solve this problem and will be one of a suitable buffer layers for the cubic GaN film growth. Epitaxial cubic SiC buffer layers were grown on either carbonized or uncarbonized Si(111) substrates as low as temperature of 830°C using a supersonic molecular jet of t-butyldimethylsilane, (CH3)3CSiH(CH3)2, and the polycrystalline hexagonal GaN thin films were subsequently deposited on them at 600°C using a MOMBE system. The buffer layers and the GaN films were characterized by AES, XRD, FTIR, ellipsometry, and X-ray phi(φ)-scan measurements. The growth temperature of t-butyldimethylsilane was much lower than conventional CVD growth temperatures and this is the first report to obtain epitaxial cubic SiC films on Si from t-butyldimethylsilane and new attempt to grow h-GaN thin films on cubic SiC buffer layers.

Nanofabrication by direct epitaxial growth

We describe a novel, all dry approach that uses direct epitaxial growth for nanostructure fabrication. The two major requirements for achieving direct epitaxial growth are the ability to generate and to subsequently maintain and control spatial and chemical selectivity in the film growth process. The spatial selectivity is generated by pattering a surface adsorption layer on Si(100) using scanning electron beam lithography. This artificial lateral variation in surface reactivity is used as a template in subsequent epitaxy. Selective epitaxial growth on the resulting patterns is achieved by supersonic molecular jet epitaxy. Systematic investigation of the effects of various patterning and growth parameters on spatial and chemical selectivity at a sub- 100-nm feature scale using hydrogen terminated and nitrogen terminated growing Si(100) surfaces are presented.

Raman spectroscopy of supersonic jets of CO2: Density, condensation, and translational, rotational, and vibrational temperatures

The links between translational, rotational, and vibrational temperatures of supersonic molecular jets, and their density and degree of condensation, are discussed in terms of quantitative Raman scattering experimental data. Such links are established as the result of energy and momentum conservation laws, and of the collisional regime in the jet. Four representative supersonic expansions of CO2, generated at different stagnation pressures are analyzed, showing the potential of quantitative Raman spectroscopy for a complete characterization of the jet.

Growth of Cubic SiC Thin Films on Silicon from Single Source Precursors by Supersonic Jet Epitaxy

AbstractCubic SiC thin films have been grown by supersonic jet epitaxy of single molecular precursors on Si(100), Si(111) and Separation by IMplanted OXygen (SIMOX) silicon on insulator (SOI) substrates at temperatures in the range 780 - 1000 °C. Real-time, in situ optical reflectivity was used to monitor the film growth. Films were characterized by ellipsometry, x-ray diffraction (XRD), and transmission electron microscopy (TEM). Monocrystalline, crack-free epitaxial cubic SiC thin films were successfully grown at 830 °C on carbonized Si(111) substrates using supersonic molecular jets of dimethylisopropylsilane, (CH3)2CHSiH(CH3)2, and diethylmethylsilane, (CH3CH2)2SiHCH3. Highly oriented cubic SiC thin films in the [100] direction were obtained on SIMOX(100) at 900 °C with dimethylisopropylsilane and on Si(100) at 1000 °C with diethylmethylsilane. A carbonized Si(100) surface was found to enhance SiC deposition from diethylmethylsilane at a growth temperature of 950 °C.

Fluorescence Excitation Spectra of HCO [formula omitted] in a Supersonic Jet

The HCO radical was produced by photolysis of acetaldehyde in a supersonic molecular jet. HCO B ̃ ̃ 2A′− X ̃ 2A′ transitions were detected by laser-induced fluorescence. Eight vibronic bands were observed that showed both a- and b-type transitions, indicating that the observed transitions are from the B ̃ ̃ state, not the C̃ state. The quantum yield of fluorescence abruptly decreased at lines 2770 cm −1 above the zero vibrational state of B̃, indicating the onset of a channel leading to predissociation. Rotational spectra of the lowest six vibrational levels of the B̃ state are analyzed.

超音速分子ジェット発生用バルブの安定化ドライバー

超音速分子ジェット発生用バルブの安定化ドライバー

Anion formation from gaseous and condensed CF3I on low energy electron impact

Anion formation following electron impact to CF3I is studied in the energy range 0–15 eV. The experiments include gas phase CF3I in the effusive molecular beam under single collision conditions, clusters in a supersonic molecular jet and CF3I condensed in the UHV in multilayer amounts onto a cold metallic substrate. In isolated molecules fragment anions are formed via dissociative attachment (DA) and dipolar dissociation (DD). The DA resonances are located at 0.0 and 3.8 eV and are assigned as single particle and two particle resonance, respectively. The low energy resonance exhibits an exceedingly high cross section for I− formation, while the higher energy resonance decomposes into CF3−, F−, and FI− with comparatively low intensity. Both resonances possess significant C–I antibonding character as apparent from their decomposition dynamics. In clusters the stabilized molecular anion CF3I− and larger complexes of the form (CF3I)n− and (CF3I)n⋅I− are observed. At higher energies anion formation is affected by inelastic scattering from one molecule and capture of the slow electron by a second molecule within one cluster (self-scavenging). Scavenging features in clusters and in isolated molecules beyond single collision conditions are compared. Electron stimulated desorption (ESD) is dominated by CF3− which is generated via DA from the core excited resonance with its strong F3C–I antibonding nature.

Supersonic molecular jet Fourier transform interferometry applied to nitric oxides

A supersonic molecular jet has been located inside the sample chamber of a Bruker IFS120HR Fourier transform interferometer. Infrared bands of NO, NO 2 and N 2O 4 are presented. Test spectra with CO 2 are also discussed.

A supersonic molecular jet for a Fourier transform interferometer: The ν3 band of OCCCO

A supersonic molecular jet apparatus has been constructed for use with a Fourier transform infrared spectrometer (Bruker IFS 120 HR) or with the millimeter-wave and diode laser spectrometers available in the Giessen laboratory. Design criteria were chosen to provide optimum conditions for obtaining spectral simplification for relatively large molecules with large amplitude motions. Use with unstable species is planned. Thus, throughput could not be too high, and temperature need not be extremely low. First results with the jet system attached to the interferometer are presented. Test measurements with CO 2 show a rotational temperature of 22 K for a hole nozzle, and are used to contrast the spectra from a hole and a slit nozzle. A trial jet spectrum of OCCCO shows significant cooling of the low-lying ν 7 bending mode and considerable simplification, revealing some new features not detected in an earlier room-temperature study.

Solid state proton spin relaxation in ethylbenzenes: Methyl reorientation barriers and molecular structure

We have investigated the dynamics of the ethyl groups and their constituent methyl groups in polycrystalline ethylbenzene (EB), 1,2-diethylbenzene (1,2-DEB), 1,3-DEB, and 1,4-DEB using the solid state proton spin relaxation (SSPSR) technique. The temperature and Larmor frequency dependence of the Zeeman spin-lattice relaxation rate is reported and interpreted in terms of the molecular dynamics. We determine that only the methyl groups are reorienting on the nuclear magnetic resonance time scale. The observed barrier of about 12 kJ/mol for methyl group reorientation in the solid samples of EB, 1,2-DEB, and 1,3-DEB is consistent with that of the isolated molecule, implying that in the solid state, intermolecular electrostatic interactions play a minor role in determining the barrier. The lower barrier of 9.3±0.2 kJ/mol for the more symmetric 1,4-DEB suggests that the crystal structure is such that the minimum in the anisotropic part of the intramolecular potential is raised by the intermolecular interactions leading to a 3 kJ/mol decrease in the total barrier. We are able to conclude that the methyl group is well away from the plane of the benzene ring (most likely orthogonal to it) in all four molecules, and that in 1,2-DEB, the two ethyl groups are in the anticonfiguration. Our SSPSR results are compared with the results obtained by microwave spectroscopy and supersonic molecular jet laser spectroscopy, both of which determine molecular geometry better than SSPSR, but neither of which can determine ground electronic state barriers for these molecules.

High-resolution hyperfine spectroscopy in real space

A new experimental technique is presented for the observation of high-resolution hyperfine spectra in real space. The method is demonstrated for the case of the hyperfine spectrum of a single rotational line of iodine. Light from a single-mode ring dye laser, which intersects a supersonic molecular jet, excites only those velocity groups for which one of the hyperfine components is properly Doppler shifted into resonance. This new method is useful for spectral measurement and for the characterization of molecular beams.

Observation and geometry assignment of the conformation of benzyl alcohol in the gas phase

Supersonic molecular jet laser spectroscopy is used to establish the perpendicular conformation of benzyl alcohol and a number of sterically unencumbered derivatives in the gas phase.

ChemInform Abstract: Determination of the Minimum‐Energy Conformation of Allylbenzene and Its Clusters with Methane, Ethane, Water, and Ammonia

Supersonic molecular jet laser time-of-flight mass spectroscopy (TOFMS) is employed to determine the minimum-energy conformation of the allyl group with respect to the benzene ring of allylbenzene, 1-allyl-2-methylbenzene, and 1-allyl-3-methylbenzene. The spectra are assigned and conformations are suggested with the aid of molecular orbital-molecular mechanics (MOMM-85) calculations. Based on the experimental and theoretical results, the minimum-energy conformer is found to have {tau}{sub 1} (C{sub ortho}-C{sub ipso}-C{sub {alpha}}-C{sub {beta}}) = ca. 90{degree} (i.e., the allyl group is essentially perpendicular to the plane of the benzene ring) and {tau}{sub 2} (C{sub ipso}-C{sub {alpha}}-C{sub {beta}}-C{sub {gamma}}) = {plus minus}120{degree} (i.e., the olefin C=C bond is eclipsed with the C{sub {alpha}}-H{sub {alpha}} bond). The TOFMS of allylbenzene clustered with methane, ethane, water, and ammonia are also presented. A Lennard-Jones potential energy 6-12-1 atom-atom calculation is used to characterize the structures of these clusters. Experiments and calculations demonstrate that the four different solvent molecules studied can form stable clusters with allylbenzene by coordinating to the {pi}-system of the allyl substituent in addition to that of the aromatic ring.

Diode laser spectroscopy of the hydrogen bond vibration ν2 OC---HF in a continuous wave supersonic jet

The high resolution spectrum of the ν2 (C≡O) stretching vibration in the hydrogen bonded dimer, OC---HF, has been recorded in a continuous wave (cw) supersonic molecular jet using a diode laser spectrometer. Spectroscopic analysis gives the following rovibrational parameters (in cm−1): ν0=2167.69 9 04(11); B0=0.102 200 647(13); D0J =3.244(18)×10−7; B2=0.101 552 5(15); D2J =3.449(36)×10−7. Investigation of observed line profiles allows a lower limit of 0.68 ns to be made for the excited state vibrational predissociative lifetime.

Determination of the minimum-energy conformation of allylbenzene and its clusters with methane, ethane, water, and ammonia

Supersonic molecular jet laser time-of-flight mass spectroscopy (TOFMS) is employed to determine the minimum-energy conformation of the allyl group with respect to the benzene ring of allylbenzene, 1-allyl-2-methylbenzene, and 1-allyl-3-methylbenzene. The spectra are assigned and conformations are suggested with the aid of molecular orbital-molecular mechanics (MOMM-85) calculations. Based on the experimental and theoretical results, the minimum-energy conformer is found to have {tau}{sub 1} (C{sub ortho}-C{sub ipso}-C{sub {alpha}}-C{sub {beta}}) = ca. 90{degree} (i.e., the allyl group is essentially perpendicular to the plane of the benzene ring) and {tau}{sub 2} (C{sub ipso}-C{sub {alpha}}-C{sub {beta}}-C{sub {gamma}}) = {plus minus}120{degree} (i.e., the olefin C=C bond is eclipsed with the C{sub {alpha}}-H{sub {alpha}} bond). The TOFMS of allylbenzene clustered with methane, ethane, water, and ammonia are also presented. A Lennard-Jones potential energy 6-12-1 atom-atom calculation is used to characterize the structures of these clusters. Experiments and calculations demonstrate that the four different solvent molecules studied can form stable clusters with allylbenzene by coordinating to the {pi}-system of the allyl substituent in addition to that of the aromatic ring.

Formation of water clusters in a free molecular jet of binary mixtures

The formation process of water clusters is investigated experimentally with a supersonic free molecular jet of water vapor mixtures of Ar, Xe, N2, CO2 and CO, especially noticing the effect of the solute molecules on the cluster formation. The terminal concentrations of clusters formed through a supersonic molecular expansion are measured with a mass spectrometer by changing the mole fraction of water vapor at the source. It is observed that, in the range of relatively small H2 O mole fraction of binary mixtures at the source, the formation process of water clusters is controlled mainly by the thermodynamical effects of the expansion. On the other hand, at larger H2 O mole fractions, the molecular dynamical behavior of mixture molecules must be taken into account for understanding the formation process of water clusters. In both cases, the binary species play an important role in the formation process of water clusters.

A study of nonrigid aromatic molecules. Observation and spectroscopic analysis of the stable conformations of various alkylbenzenes by supersonic molecular jet laser spectroscopy

A study of nonrigid aromatic molecules. Observation and spectroscopic analysis of the stable conformations of various alkylbenzenes by supersonic molecular jet laser spectroscopy

Benzene clustered with N2, CO2, and CO: Energy levels, vibrational structure, and nucleation

Two-color time-of-flight mass spectroscopy is employed to study the van der Waals (vdW) clusters of benzene(N2)n (n≤8), benzene(CO2)n (n≤7), and benzene(CO)n (n=1,2) created in a supersonic molecular jet. Potential energy calculations of cluster geometries, normal coordinate analysis of vdW vibrational modes, and calculations of the internal rotational transitions are employed for the assignment of the benzene(solvent)1 cluster spectra in the 000 and 610 regions of the benzene 1B2u←1A1g transition. The respective vibronic and rotational selection rules for these clusters are determined based on the appropriate point groups and molecular symmetry groups of the clusters. Good agreement between the calculated and experimental spectra is obtained with regard to the vdW vibrational and internal rotational modes. The solvent molecules rotate nearly freely with respect to benzene about the benzene–solvent bond axis in the benzene(solvent)1 clusters. In the excited state a small ∼20 cm−1 barrier to rotation is encountered. Studies of larger clusters (n>2) reveal a broad red shifted single origin in the 610 spectra. A linearly increasing cluster energy shift is observed as a function of cluster size. The cluster energy shifts are not saturated by one solvent molecule on each side of the aromatic ring; several solvent molecules effectively interact with the solute π electronic cloud. Both homogeneous and inhomogeneous nucleation take place for the clusters studied depending on the ratio of the solvent–solvent binding energy to the cluster binding energy.