J. K. Simons

Synchrotron radiation induced chemical vapor deposition of thin films from metal hexacarbonyls

We have studied the initial stages of synchrotron radiation [SR] induced chemical vapor deposition [CVD] of metal‐containing thin films from metal hexacarbonyl gases. We have measured the dependence of the initial deposition rate upon gas pressure at room temperature. Substrates were exposed to SR for single fills of the electron storage ring at constant pressure of Mo(CO)6, Cr(CO)6, or W(CO)6 gas. Deposition was monitored in situ by Auger electron spectroscopy using the SR as the excitation source. The presence of metal, carbon, and oxygen in the deposited films was observed, and the results are consistent with previous studies. Deposition was found to be isolated to areas of the substrate exposed to the SR beam. We discuss these results as they relate to the use of SR as a means to induce CVD and the possibility of patterned deposition using a masked SR source.

Synchrotron radiation induced reactions on surfaces: mechanisms and applications

Abstract In this paper we tabulate recent research that delves into the application of synchrotron radiation for inducing surface chemical reactions. We then present results from recent experiments designed to test whether surface photochemical reactions can be brought about by direct core-level excitation of the adsorbate, as opposed to an indirect mechanism where photoabsorption by bulk atoms produces secondary electrons that cause the reaction. Experiments were performed on SiF 4 adsorbed on Ge(100) at 30 K. It was found that direct excitation of Si2p core levels causes the reaction to occur. Finally we show the application of X-ray dependent surface photochemistry in two systems of technological relevance. The first is adamantane (C 10 H 16 ) adsorbed on Si(111) at 85 K. X-rays with energies greater than the CK edge (284 eV) cause significant bond breaking and the formation of SiC. Lower energy X-rays appear to create adamantyl radicals which bond strongly with the substrate and are stable to 625 K. The second is Teflon where irradiation with photons with energies lower than 12 eV induces the desorption of relatively large fluorocarbon fragments, as compared to photolysis with photons of energies greater than 22 eV (C2s excitation).

Synchrotron radiation (5–50 eV) induced degradation of fluorinated polymers

The photochemistry of solid, free‐standing films of poly(tetrafluoroethylene) (PTFE) and poly(hexafluoropropylene–tetrafluoroethylene) (FEP) was investigated by monitoring the neutral photodesorption products produced upon irradiation with synchrotron radiation (5–50 eV). CF3, C2F5, C3F5, C3F7, and C4F7 were the predominant species observed in the mass spectra of the irradiated polymers. Oligomers at least as large as 293 amu (C7F12) were also detected. Distinct differences are observed in the photodesorption yields and time responses of the yields based on the energy of the initial excitation. An edge jump at 22 eV in the photon‐stimulated desorption spectra of both PTFE and FEP films is assigned to the ionization of the C(2s) states of the polymers. The energy dependence of the photodesorption yields is accounted for by considering two separate reaction pathways.

X‐ray energy dependent photochemistry of the adamantane (C10H16)/Si(111)‐7×7 surface

We have studied the synchrotron radiation (SR) induced photochemistry of the adamantane (C10H16) dosed Si(111)‐7×7 surface using photoelectron spectroscopy. The adamantane/Si surfaces were characterized before and after they were exposed to broadband SR of varying energies and then annealed. When annealed to 525 K, the unexposed adamantane layers desorbed leaving only a layer of Si–H and hydrated Si–C species on the surface. When the adamantane adlayer was irradiated with broadband SR that contained energies greater than the C(1s) binding energy (284 eV), the surface reacted to form a layer similar to SiC. In contrast, when the surface was irradiated with broadband SR with energies below 284 eV, only the adamantane in direct contact with the Si surface dissociated. The remaining adamantane adlayer reacted with the surface in a manner which left the molecular structure of the reacted adamantane intact. The reacted adamantane layer was stable up to temperatures of at least 675 K.

The soft x‐ray photochemistry of physisorbed SiF4. I. Reactions of the molecular species through desorption and dissociation

We present evidence that demonstrates photolysis of SiF4 adsorbed on Ge(100) at 30 K. Silicon 2p soft x‐ray photoemission spectroscopy (PES) indicates that upon irradiation, the molecularly adsorbed SiF4 dissociates into SiFn species (where n=0,1,2,3) and desorbs as molecular SiF4. Also, the Si 2p PES from undissociated molecules exhibits a number of distinct kinetic‐energy shifts. These are attributed to anisotropic adsorption in which different molecular sites have different apparent Si 2p binding energies. A structure of the adsorbate layer is proposed to account for the varying core hole screening. Examination of the gas phase during irradiation confirms molecular desorption and shows the system to have a significant neutral molecular desorption yield. Changes in the valence‐level photoemission structure and signal intensity are consistent with the observed fragmentation and desorption, both of which lead to disappearance of the molecularly adsorbed species.

THE SOFT X-RAY PHOTOCHEMISTRY OF PHYSISORBED SIF4. II. MECHANISMS AND KINETICS

We present an analysis of the extensive photolysis of an adsorbate resulting from adsorbate core‐level excitation. The system studied was SiF4 adsorbed on Ge(100) at 30 K. Photolysis fragments and molecular species (identified with Si 2p soft x‐ray photoemission spectroscopy) were measured as a function of monochromatic (140‐eV) photon exposure and adsorbate coverage. The photolysis cross sections for 55–140‐eV photons were determined and the neutral photon‐stimulated desorption cross section for a selected SiF4 excitation is also presented. In the Si 2p absorption region, it was found that the photolysis cross section was one to three times the preedge value and comparable in magnitude to that of gas phase photoabsorption, while the total yield increased at most by a factor of 1.4. Both of these observations indicate that direct core excitation of the adsorbate is a major path by which photolysis occurs as opposed to an indirect, substrate‐driven one.

Synchrotron‐radiation‐induced reactions of tetraethoxysilane on Si studied by photoemission spectroscopy

The synchrotron‐radiation (SR)‐induced chemical reactions of tetraethoxysilane Si(OC2H5)4 (TEOS) adsorbed on Si has been studied using photoemission spectroscopy. It is demonstrated that TEOS adsorbs intact on the Si surface below room temperature. SR in the vacuum ultraviolet region decomposes the TEOS molecules adsorbed on Si to form a silicon‐oxide‐like film on the Si surface, but some carbon remains in the film. Results of irradiation experiments on condensed layers of TEOS and water adsorbed on Si at 85 K indicate that the presence of water reduces the carbon contamination.

X-ray Absorption Near Edge Structure of Single-Crystal Sapphire Using Synchrotron Radiation: The Interatomic-Distance Correlation

The high-resolution Al L-edge X-ray absorption near edge structure (XANES) from a single-crystal sapphire sample has been recorded using the total-electron yield (TEY), X-ray fluorescence yield (XFY), and recombination luminescence excitation spectrum (RLES) utilizing synchrotron radiation. There is a very clear splitting of the Al L2 and L3 components (78.38 eV and 78.80 eV) in those spectra. To our knowledge, this is the first report on the Al L23 doublet splitting of single-crystal sapphire obtained from the total-electron yield measurement. All the spectra show similar structure, while the main feature of the RLES is an anticorrelated behavior relative to the TEY and XFY. The edge structures up to 11 eV from the L23 absorption edge are interpreted as due to transition of electron to empty levels of Al ion according to molecular-orbital calculations on the [AlO6]-9 molecules as determined by Balzarotti et al. The post-edge features between 11-60 eV above the Al L23 absorption edge were found to correlate very well with the interatomic distances from the absorbing atom to a neighboring atom as predicted by multiple scattering model of Bianconi and Natoli.

Synchrotron radiation studies of diamond nucleation and growth on Si

Valence‐band as well as Si(2p) and C(1s) core‐level photoemission, Auger, and near‐edge x‐ray‐absorption fine‐structure spectroscopies were used to follow the surface chemistry associated with diamond film deposition with a filament‐assisted chemical‐vapor‐deposition reactor on atomically clean and diamond polished Si(100) and Si(111) surfaces. Raman spectroscopy and atomic force microscopy (AFM) were also used ex situ to characterize the deposited films. Within 3 min of deposition, a carbon‐rich SiC layer, at least 13 A thick, was observed to develop. At early stages of growth (<10 min of deposition), no differences were observed between the clean and diamond‐polished surfaces. With additional deposition, a 20–30‐A‐thick amorphous carbon overlayer was deposited on the clean Si surfaces: The amorphous carbon layer did not promote diamond nucleation. Deposition of an a‐C:H layer on top of the amorphous carbon layer also did not promote diamond nucleation. In contrast, ∼500 A diamond films were deposited wi...

Photoemission study of synchrotron radiation induced reactions of TEOS adsorbed on silicon surface

Abstract We have investigated the synchrotron radiation (SR) induced chemical reactions of tetraethoxysilane Si(OC 2 H 5 ) 4 (TEOS) adsorbed on Si, using photoemission spectroscopy. TEOS adsorbs on the Si surface with its molecular structure intact, at least below room temperature. SR in the vacuum ultraviolet region decomposes the TEOS molecules adsorbed on Si to form a silicon oxide like film, but some carbon remains in the film. Results of irradiation experiments on condensed layers of TEOS and water adsorbed on Si at 85 K indicate that the addition of water enhances the removal of carbon contamination in the oxide film.