Search

Plasma-enhanced chemical vapor (PECVD) codeposition of benzene and 1,2-dicarbadodecaborane, 1,2-B10C2H12 (orthocarborane) and benzene, 1,7 dicarbadodecaborane, and 1,7-B10C2H12 (metacarborane) results in semiconducting boron carbide composite films with significantly longer drift carrier lifetimes than plasma-enhanced chemical vapor deposited semiconducting boron carbide synthesized from orthocarborane or metacarborane alone. Capacitance versus voltage C ( V ) and current versus voltage I ( V ) measurements indicate the hole carrier lifetimes for PECVD benzene/orthocarborane based semiconducting boron carbide composites increase to 2.5 ms from values of ≤35 μs for the PECVD semiconducting boron carbide films fabricated without benzene. For PECVD benzene/metacarborane based semiconducting boron carbide composites, there is an increase in the hole carrier lifetime to roughly 300 ns from values of 50 ns for those films fabricated without benzene.

The surface of the complex oxides, in particular the manganese perovskites, is dominated by a different free enthalpy at the surface. This can be demonstrated by surface segregation after annealing the samples. By comparing relative x-ray photoelectron spectroscopy intensities of the manganese perovskite La1−xCaxMnO3 (x=0.1, 0.35) for different emission angles, the profile of elemental concentration was determined. Analysis shows evidence for pronounced surface segregation of calcium. Our results suggest that there are strong differences between surface and bulk enthalpies without segregation or surface relaxation. This is consistent with the previously observed surface resonance in the electronic band structure.

We have grown nickel doped boron–carbon alloy films by the technique of plasma enhanced chemical vapor deposition. The source gas closo-1,2-dicarbadodecaborane (orthocarborane) was used to grow the boron–carbon alloy, while nickelocene [Ni(C5H5)2] was used as the dopant source for nickel. With sufficient levels of Ni doping, diodes with characteristic tunnel diode behavior can be fabricated. The doping of nickel transformed a B5C p-type material, relative to lightly doped n-type silicon, to a strongly n-type material. In order to gain insight into the shift of the Fermi level of the Ni-doped material, we have examined the changes in the electronic structure of sodium doped films of the precursor molecule orthocarborane which has an icosahedral structure similar to that of boron–carbon materials. The establishment of unoccupied states at the Fermi level with Na doping of the orthocarborane films is consistent with the transformation of the p-type B5C to an n-type material with Ni doping.

Using photoelectron spectroscopy, the overlayer electronic structure was observed to be modulated by the Hg growth mode for Hg adsorbed on W(110) at 200 K. The Hg layer‐by‐layer growth was also characterized by variations in the ratio between the Hg 5d shallow core level and W 4f photoemission intensities. This layer‐by‐layer growth occurs in spite of surprisingly weak electronic interactions between the adatoms and the substrate.

Vapor deposited short chain polyaniline thin films were studied using angle-resolved photoemission spectroscopy. Films grown on two different substrates, oxidized Si surfaces and Au, show distinctly different properties. Some indication of preferential molecular orientation was observed for ultrathin films deposited on Au substrates, but not on the Si substrates with the native oxide surface. Clear indication of metallic character was observed for some of the polyaniline ultrathin films, consistent with recent theory suggesting that alignment of chains parallel with a metal substrate enhance metallicity.

We compare the valence band photoemission resulting from molecular adsorption of nido‐decaborane(14) (B10H14) and nido‐2,3‐diethyl‐2,3‐dicarbahexaborane on Si(111) at 100 K. The relative shift of the apparent valence band edge with increasing molecular coverage is consistent with the relative energy difference in the highest occupied molecular orbital for these two molecules as inferred from modified neglect of differential overlap calculations. The results suggest that the screening is very similar, despite the very different molecular structures of these two nido cage molecules.

Nido‐pentaborane(9) (B5H9), 1, nido‐2,3‐diethyl‐2,3‐dicarbahexaborane(8) [(C2H5)2C2B4H6], 2, nido‐decaborane(14) (B10H14), 3, closo‐1,2‐dicarbadodecaborane(12) (H2C2B10H10), 4, can be used as possible source compounds for boron and boron carbide thin film deposition. Inner shell electron energy‐loss spectroscopy (ISEELS) studies of the boron 1s and carbon 1s core excitations of gas phase species have been undertaken so as to characterize these molecular precursors at solid surfaces. The near edge structure of ISEELS provides a good ‘‘fingerprint’’ for the identification of these molecular species. A comparison is made between calculated [modified neglect of differential overlap (MNDO)] bond lengths for molecular clusters and the x‐ray or electron diffraction bond lengths, bond lengths determined from extended energy‐loss fine structure.

From spin-polarized inverse photoemission, the experimental spin-resolved unoccupied band structure of gadolinium on Mo(112) has been constructed. The spin-dependent unoccupied electronic structure near the Fermi level is dominated by shallow dispersion of a spin-minority band. A spin-majority and -minority bulk-band pair also exhibits some dispersion across the Brillouin zone.

We will illustrate how atom beam scattering (ABS) can be used to study the growth and ordering of adsorption of Hg on Cu(001). At low coverage (less than 2% of a monolayer), we obtain that the scattering cross section of He with (adsorbed) Hg is about 50 A2. We find that mercury forms two stable phases on Cu(001) in the temperature range between 200 and 330 K, a c(2×2) and a high density c(4×4) phase. From ABS diffraction data, we can deduce the corrugation of the surface electron charge density of the mercury layer in the two ordered phases. From the analysis of our data, we obtain a Debye temperature for the Hg overlayer of about 115 K. We will report preliminary calculations on the adsorption energy of Hg on Cu(001) and on the interaction of He with a Hg preplated Cu(001) surface.

Gadolinium overlayers on Ni(111) have been studied by angle resolved photoemission, angle resolved AES, LEED, and RHEED. We have observed pronounced interdiffusion of nickel with the gadolinium overlayer at temperatures as low as 150 K. This is in marked contrast with gadolinium overlayers on Cu(100) where substantial interdiffusion is not observed until 360 K, but is consistent with studies of ytterbium on nickel. [A. Nilsson, B Eriksson, N. Martenssom, J. N. Andersen, and J. Onsgaard, Phys. Rev. B 38, 10357, (1988) and I. Chorkendorff, J. Onsgaard, J. Schmidt–May and R. Nyholm, Surf. Sci. 160, 587, (1985).] There is a strong interfacial heat of interaction observed with gadolinium on both copper and nickel resulting in pronounced binding energy shifts observed in photoemission. An extremely small kinetic barrier to rare earth diffusion through nickel has been measured. The results are compared to transition metal overlayers on transition metal substrates.

We have investigated the desorption of N2 from Ni(100) using thermal desorption spectroscopy (TDS). A modified Polanyi–Wigner equation has been used to obtain the desorption energy and the preexponential factor, both of which depend on the coverage of the adsorbate. We show that there is a large lateral interaction among the adlayer molecules when N2 goes down as ordered c(2×2) on Ni. In addition, the overlayer ordering, in the thermal desorption process, is observed to affect the thermal desorption spectra.

The ionic and neutral decomposition of gaseous ferrocene, 1, 1’‐dichloroferrocene and 1, 1’, 2, 2’‐tetrachloroferrocene, has been investigated via photon and electron induced ionization mass spectroscopy. The thermodynamic cycle incorporating the ionic and neutral fragments of ferrocene has been constructed. Elimination reactions in the decomposition of the dichloroferrocene and tetrachloroferrocenes have been identified.

Molecular bromine dissociatively adsorbs to form a chemisorbed overlayer on Ni(100). The bromine induced 4px, y and 4pz orbitals have been identified at Γ with binding energies of 6.6±0.2 eV and 5.3±0.2 eV below the Fermi energy, respectively. The relative ionization cross‐section variations of the two bromine levels 4px and 4pz are observed to differ with changing photon energy.

We have investigated photoassisted, plasma enhanced chemical vapor deposition and pyrolytic deposition of boron from decaborane (B10H14) and boron nitride from decaborane combined with nitrogen or ammonia. The use of decaborane for depositing boron and boron nitride thin films is seen as a viable alternative to diborane or boron halides.

Thin (1–7 monolayer) well‐ordered, disordered, and liquid Hg overlayers on Ag(100) have been investigated with angle resolved photoemission and low‐energy electron diffraction. The well‐ordered overlayers exhibit new electronic states as a result of the strained, cubic, overlayer crystallographic structure. The new electronic states depend upon the long‐range crystallographic order of the overlayer, not simply the local crystallographic symmetry. The photoemission cross section and branching ratio of the 5d mercury states depends on the overlayer long‐range crystallographic order and cannot be explained by photoelectron diffraction.

The prospect of using chemical vapor deposition to deposit mixed metals and silicides from a single source compound is attractive but largely uninvestigated. Studies of decomposition energies of such compounds are nearly nonexistent. One such compound which has successfully been used to make a silicide coating is [Mn(CO)5 ]2 (μ‐SiH2 ). We have used electron impact mass spectroscopy, photoionization mass spectroscopy, and photoabsorption to determine bond energies within this compound. The combination of methods allows a high degree of confidence in the resultant ionization and fragment appearance potentials. Some possible mechanisms of decomposition are discussed. A complete ionic decomposition thermodynamic cycle has been generated, and the results are used to illuminate the coating processes previously observed.

The ionization potentials for CCl4, CCl3Br, CBr2Cl2, CBr4, CCl3, and CBr3 have been measured using electron impact techniques. These numbers have been combined with previously published thermodynamic values in order to construct thermodynamic cycles for ionic and neutral decomposition of the bromochloromethanes. The n to σ*, n to s, and n to p electronic transition energies have also been determined on the basis of photoabsorption spectra.

The interaction of iodine vapor on an Fe(110) single crystal surface at room temperature has been investigated primarily through the use of LEED and UPS. Both a series of chemisorbed overlayers and an epitaxial iodide layer are observed. Iodide formation proceeds through an island growth mechanism with the iodide basal plane parallel to the Fe(110) surface. Evidence is presented that the defects introduced on the surface during argon ion sputtering may be important as nucleation sites for iodide growth. The results are compared with the results of previous studies.

A semiempirical calculation is outlined that allows analysis of experimental results for the apparent surface concentration of binary alloys, obtained by x‐ray photoelectron spectroscopy, (XPS). A segregation profile giving the enrichment of the segregating element at and near the surface is obtained from the analysis. Using previously published data for Cu17 Ni83(100) and Cu17 Ni83(111), it is shown that copper segregation is not restricted to the first few layers, but instead extends significantly into the selvedge (near‐surface) region. This occurrence is not explainable by the use of present ideal solution models. An extended ideal solution model is presented, in which the bond strengths vary gradually from top‐layer values to bulk values. This model is consistent with the observed penetration of copper enrichment into the selvedge. The parameters describing the gradual change of bond strengths are determined via comparison of the results of the model with XPS data.