Silicide formation and the interaction of metals with polycrystalline Si

The main factors affecting solid-phase Si-metal interactions are reported in this work. The influence of the orientation of the Si substrates and the presence of impurities in metal films and at the Si-metal interface on the formation of nickel and chromium silicides have been demonstrated. We have observed that the formation and kinetic rate of growth of nickel silicides is strongly dependent on the orientation and crystallinity of the Si substrates; a fact which, up to date, has never been seriously investigated in silicide formation. Impurity contaminations in the Cr film and at the Si-Cr interface are the most dominant influencing factors in the formation and kinetic rate of growth of CrSi₂. The potentiality and use of silicides as a diffusion barrier in metallization on silicon devices were also investigated.

Two phases, Ni₂Si and NiSi, form simultaneously in two distinct sublayers in the reaction of Ni with amorphous Si, while only the former phase was observed on other substrates. On (111) oriented Si substrates the growth rate is about 2 to 3 times less than that on <100> or polycrystalline Si. Transmission electron micrographs establish-·that silicide layers grown on different substrates have different microcrystalline structures. The concept of grain-boundary diffusion is speculated to be an important factor in silicide formation.

The composition and kinetic rate of CrSi₂ formation are not influenced by the underlying Si substrate. While the orientation of the Si substrate does not affect the formation of CrSi₂ , the purity of the Cr film and the state of Si-Cr interface become the predominant factors in the reaction process. With an interposed layer of Pd₂Si between the Cr film and the Si substrate, CrSi₂ starts to form at a much lower temperature (400°C) relative to the Si-Cr system. However, the growth rate of CrSi₂ is observed to be independent of the thickness of the Pd2Si layer. For both Si-Cr and Si-Pd₂Si-Cr samples, the growth rate is linear with time with an activation energy of 1.7 ± 0.1 ev.

A tracer technique using radioactive ³¹Si (T_1/2 = 2.26 h) was used to study the formation of CrSi₂ on Pd₂Si. It is established from this experiment that the growth of CrSi₂ takes place partly by transport of Si directly from the Si substrate and partly by breaking Pd₂Si bonds, making free Si atoms available for the growth process.

The role of CrSi₂ in Pd-Al metallization on Si was studied. It is established that a thin CrSi₂ layer can be used as a diffusion barrier to prevent Al from interacting with Pd₂Si in the Pd-Al metallization on Si.

As a generalization of what has been observed for polycrystalline-Si-Al interaction, the reactions between polycrystalline Si (poly Si) and other metals were studied. The metals investigated include Ni, Cr, Pd, Ag and Au. For Ni, Cr and Pd, annealing results in silicide formation, at temperatures similar to those observed on single crystal Si substrates. For Al, Ag and Au, which form simple eutectics with Si annealing results in erosion of the poly Si layer and growth of Si crystallites in the metal films.

Backscattering spectrometry with 2.0 and 2.3 MeV ⁴He ions was the main analytical tool used in all our investigations. Other experimental techniques include the Read camera glancing angle x-ray diffraction, scanning electron, optical and transmission electron microscopy. Details of these analytical techniques are given in Chapter II.

Discovery of non-zero neutrino mixing angle θ_13 using Daya Bay antineutrino detectors

The Daya Bay Reactor Antineutrino Experiment observed the disappearance of reactor $\bar{\nu}_e$ from six $2.9~GW_{th}$ reactor cores in Daya Bay, China. The Experiment consists of six functionally identical $\bar{\nu}_e$ detectors, which detect $\bar{\nu}_e$ by inverse beta decay using a total of about 120 metric tons of Gd-loaded liquid scintillator as the target volume. These $\bar{\nu}_e$ detectors were installed in three underground experimental halls, two near halls and one far hall, under the mountains near Daya Bay, with overburdens of 250 m.w.e, 265 m.w.e and 860 m.w.e. and flux-weighted baselines of 470 m, 576 m and 1648 m. A total of 90179 $\bar{\nu}_e$ candidates were observed in the six detectors over a period of 55 days, 57549 at the Daya Bay near site, 22169 at the Ling Ao near site and 10461 at the far site. By performing a rate-only analysis, the value of $sin^2 2\theta_{13}$ was determined to be $0.092 \pm 0.017$.

Oxidative chemisorption of SO_2 on γ - Al_2O_3 and deposition of H_2-permselective SiO_2 films

The interaction of SO_2 with γ - Al_2O_3 and the deposition of H_2 permselective SiO_2 films have been investigated. The adsorption and oxidative adsorption of SO_2 on γ - Al_2O_3 have been examined at temperatures 500-700°C by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). At temperatures above 500°C most of SO_2 adsorbed on the strong sites on alumina. The adsorbed SO_2 species was characterized by an IR band at 1065 cm^(-1). The equilibrium coverage and initial rate of adsorption decreased with temperature suggesting a two-step adsorption. When γ - Al_2O_3 was contacted with a mixture of SO_2 and O_2, adsorption of SO_2 and oxidation of the adsorbed SO_2 to a surface sulfate characterized by broad IR bands at 1070 cm^(-1), 1390 cm^(-1) took place. The results of a series of TGA experiments under different atmospheres strongly suggest that surface SO_2 and surface sulfate involve the same active sites such that SO_2 adsorption is inhibited by already formed sulfate. The results also indicate a broad range of site strengths.

The desorption of adsorbed SO_2 and the reductive desorption of oxidatively adsorbed SO_2 have been investigated by microreactor experiments and thermogravimetric analysis (TGA). Temperature programmed reduction (TPR) of adsorbed SO_2 showed that SO_2 was desorbed without significant reaction with H_2 when H_2 concentration was low while considerable reaction occurred when 100% H_2 was used. SO_2 adsorbed on the strong sites on alumina was reduced to sulfur and H_2S. The isothermal reduction experiments of oxidatively adsorbed SO_2 reveal that the rate of reduction is very slow below 550°C even with 100% H_2. The reduction product is mainly composed of SO_2. TPR experiments of oxidatively adsorbed SO_2 showed that H_2S arose from a sulfate strongly chemisorbed on the surface.

Films of amorphous SiO_2 were deposited within the walls of porous Vycor tubes by SiH_4 oxidation in an opposing reactants geometry : SiH_4 was passed inside the tube while O_2 was passed outside the tube. The two reactants diffused opposite to each other and reacted within a narrow front inside the tube wall to form a thin SiO_2 film. Once the pores were plugged the reactants could not reach each other and the reaction stopped. At 450°C and 0.1 and 0.33 atm of SiH_4 and O_2, the reaction was complete within 15 minutes. The thickness of the SiO_2 film was estimated to be about 0.1 µm. Measurements of H_2 and N_2 permeation rates showed that the SiO_2 film was highly selective to H_2 permeation. The H_2:N_2 flux at 450°C varied between 2000-3000.

Thin SiO_2 films were heat treated in different gas mixtures to determine their stability in functioning as high-temperature hydrogen-permselective membranes. The films were heat-treated at 450-700°C in dry N_2, dry O_2, N_2-H_2O, and O_2-H_2O mixtures. The permeation rates of H_2 and N_2 changed depending on the original conditions of film formation as well as on the heat treatment. Heating in dry N_2 slowly reduced the permeation rates of both H_2 and N_2. Heating in a N_2-H_2O atmosphere led to a steeper decline of H_2 permeability. But the permeation rate of N_2 increased or decreased according to whether the film deposition had been carried out in the absence or presence of H_2O vapor, respectively. Thermal treatment in O_2 caused rapid decline of the permeation rates of H_2 and N_2 in films that were deposited under dry conditions. The decline was moderate in films deposited under wet conditions.

I. Geology of the Southwest quarter of the Elsinore Quadrangle. II. Geochemical properties of the waters of the Elsinore Quadrangle [= Geology of the Lake Elsinore Quadrangle, California. Mineral deposits of Lake Elsinore Quadrangle, California]

The Lake Elsinore quadrangle covers about 250 square miles and includes parts of the southwest margin of the Perris Block, the Elsinore trough, the southeastern end of the Santa Ana Mountains, and the Elsinore Mountains.

The oldest rocks consist of an assemblage of metamorphics of igneous effusive and sedimentary origin, probably, for the most part, of Triassic age. They are intruded by diorite and various hypabyssal rocks, then in turn by granitic rocks, which occupy over 40 percent of the area. Following this last igneous activity of probable Lower Cretaceous age, an extended period of sedimentation started with the deposition of the marine Upper Cretaceous Chico formation and continued during the Paloecene under alternating marine and continental conditions on the margins of the blocks. A marine regression towards the north, during the Neocene, accounts for the younger Tertiary strata in the region under consideration.

Outpouring of basalts to the southeast indicates that igneous activity was resumed toward the close of the Tertiary. The fault zone, which characterizes the Elsinor trough, marks one of the major tectonic lines of southem California. It separates the upthrown and tilted block of the Santa Ana Mountains to the south from the Perris Block to the north.

Most of the faults are normal in type and nearly parallel to the general trend of the trough, or intersect each other at an acute angle. Vertical displacements generally exceed the horizontal ones and several periods of activity are recognized.

Tilting of Tertiary and older Quaternary sediments in the trough have produced broad synclinal structures which have been modified by subsequent faulting.

Five old surfaces of erosion are exposed on the highlands.

The mineral resources of the region are mainly high-grade clay deposits and mineral waters.

Variable-angle electron spectroscopic studies of various polyatomic molecular systems

The complementary techniques of low-energy, variable-angle electron-impact spectroscopy and ultraviolet variable-angle photoelectron spectroscopy have been used to study the electronic spectroscopy and structure of several series of molecules. Electron-impact studies were performed at incident beam energies between 25 eV and 100 eV and at scattering angles ranging from 0° to 90°. The energy-loss regions from 0 eV to greater than 15 eV were studied. Photoelectron spectroscopic studies were conducted using a HeI radiation source and spectra were measured at scattering angles from 45° to 90°. The molecules studied were chosen because of their spectroscopic, chemical, and structural interest. The operation of a new electron-impact spectrometer with multiple-mode target source capability is described. This spectrometer has been used to investigate the spin-forbidden transitions in a number of molecular systems.

The electron-impact spectroscopy of the six chloro-substituted ethylenes has been studied over the energy-loss region from 0-15 eV. Spin-forbidden excitations corresponding to the π → π*, N → T transition have been observed at excitation energies ranging from 4.13 eV in vinyl chloride to 3.54 eV in tetrachloroethylene. Symmetry-forbidden transitions of the type π → np have been oberved in trans-dichloroethyene and tetrachlor oethylene. In addition, transitions to many states lying above the first ionization potential were observed for the first time. Many of these bands have been assigned to Rydberg series converging to higher ionization potentials. The trends observed in the measured transition energies for the π → π*, N → T, and N → V as well as the π → 3s excitation are discussed and compared to those observed in the methyl- and fluoro- substituted ethylenes.

The electron energy-loss spectra of the group VIb transition metal hexacarbonyls have been studied in the 0 eV to 15 eV region. The differential cross sections were obtained for several features in the 3-7 eV energy-loss region. The symmetry-forbidden nature of the ¹A_1g → ¹A_1g, 2t_2g(π) → 3t_2g(π*) transition in these compounds was confirmed by the high-energy, low-angle behavior of their relative intensities. Several low lying transitions have been assigned to ligand field transitions on the basis of the energy and angular behavior of the differential cross sections for these transitions. No transitions which could clearly be assigned to singlet → triplet excitations involving metal orbitals were located. A number of states lying above the first ionization potential have been observed for the first time. A number of features in the 6-14 eV energy-loss region of the spectra of these compounds correspond quite well to those observed in free CO.

A number of exploratory studies have been performed. The π → π*, N → T, singlet → triplet excitation has been located in vinyl bromide at 4.05 eV. We have also observed this transition at approximately 3.8 eV in a cis-/trans- mixture of the 1,2-dibromoethylenes. The low-angle spectrum of iron pentacarbonyl was measured over the energy-loss region extending from 2-12 eV. A number of transitions of 8 eV or greater excitation energy were observed for the first time. Cyclopropane was also studied at both high and low angles but no clear evidence for any spin- forbidden transitions was found. The electron-impact spectrum of the methyl radical resulting from the pyrolysis of tetramethyl tin was obtained at 100 eV incident energy and at 0° scattering angle. Transitions observed at 5.70 eV and 8.30 eV agree well with the previous optical results. In addition, a number of bands were observed in the 8-14 eV region which are most likely due to Rydberg transitions converging to the higher ionization potentials of this molecule. This is the first reported electron-impact spectrum of a polyatomic free radical.

Variable-angle photoelectron spectroscopic studies were performed on a series of three-membered-ring heterocyclic compounds. These compounds are of great interest due to their highly unusual structure. Photoelectron angular distributions using HeI radiation have been measured for the first time for ethylene oxide and ethyleneimine. The measured anisotropy parameters, β, along with those measured for cyclopropane were used to confirm the orbital correlations and photoelectron band assignments. No high values of β similar to those expected for alkene π orbitals were observed for the Walsh or Forster-Coulson-Moffit type orbitals.

Structural and clumped-isotope constraints on the mechanisms of displacement along low-angle detachments

Despite years of research on low-angle detachments, much about them remains enigmatic. This thesis addresses some of the uncertainty regarding two particular detachments, the Mormon Peak detachment in Nevada and the Heart Mountain detachment in Wyoming and Montana.

Constraints on the geometry and kinematics of emplacement of the Mormon Peak detachment are provided by detailed geologic mapping of the Meadow Valley Mountains, along with an analysis of structural data within the allochthon in the Mormon Mountains. Identifiable structures well suited to constrain the kinematics of the detachment include a newly mapped, Sevier-age monoclinal flexure in the hanging wall of the detachment. This flexure, including the syncline at its base and the anticline at its top, can be readily matched to the base and top of the frontal Sevier thrust ramp, which is exposed in the footwall of the detachment to the east in the Mormon Mountains and Tule Springs Hills. The ~12 km of offset of these structural markers precludes the radial sliding hypothesis for emplacement of the allochthon.

The role of fluids in the slip along faults is a widely investigated topic, but the use of carbonate clumped-isotope thermometry to investigate these fluids is new. Faults rocks from within ~1 m of the Mormon Peak detachment, including veins, breccias, gouges, and host rocks, were analyzed for carbon, oxygen, and clumped-isotope measurements. The data indicate that much of the carbonate breccia and gouge material along the detachment is comminuted host rock, as expected. Measurements in vein material indicate that the fluid system is dominated by meteoric water, whose temperature indicates circulation to substantial depths (c. 4 km) in the upper crust near the fault zone.

Slip along the subhorizontal Heart Mountain detachment is particularly enigmatic, and many different mechanisms for failure have been proposed, predominantly involving catastrophic failure. Textural evidence of multiple slip events is abundant, and include multiple brecciation events and cross-cutting clastic dikes. Footwall deformation is observed in numerous exposures of the detachment. Stylolitic surfaces and alteration textures within and around “banded grains” previously interpreted to be an indicator of high-temperature fluidization along the fault suggest their formation instead via low-temperature dissolution and alteration processes. There is abundant textural evidence of the significant role of fluids along the detachment via pressure solution. The process of pressure solution creep may be responsible for enabling multiple slip events on the low-angle detachment, via a local rotation of the stress field.

Clumped-isotope thermometry of fault rocks associated with the Heart Mountain detachment indicates that despite its location on the flanks of a volcano that was active during slip, the majority of carbonate along the Heart Mountain detachment does not record significant heating above ambient temperatures (c. 40-70°C). Instead, cold meteoric fluids infiltrated the detachment breccia, and carbonate precipitated under ambient temperatures controlled by structural depth. Locally, fault gouge does preserve hot temperatures (>200°C), as is observed in both the Mormon Peak detachment and Heart Mountain detachment areas. Samples with very hot temperatures attributable to frictional shear heating are present but rare. They appear to be best preserved in hanging wall structures related to the detachment, rather than along the main detachment.

Evidence is presented for the prevalence of relatively cold, meteoric fluids along both shallow crustal detachments studied, and for protracted histories of slip along both detachments. Frictional heating is evident from both areas, but is a minor component of the preserved fault rock record. Pressure solution is evident, and might play a role in initiating slip on the Heart Mountain fault, and possibly other low-angle detachments.

I. Anisotropy and crystal structure of ferromagnetic thin films. II. Investigations into magnetic microstructure by Lorentz microscopy

The induced magnetic uniaxial anisotropy of Ni-Fe alloy films has been shown to be related to the crystal structure of the film. By use of electron diffraction, the crystal structure or vacuum-deposited films was determined over the composition range 5% to 85% Ni, with substrate temperature during deposition at various temperatures in the range 25° to 500° C. The phase diagram determined in this way has boundaries which are in fair agreement with the equilibrium boundaries for bulk material above 400°C. The (α+ ɤ) mixture phase disappears below 100°C.

The measurement of uniaxial anisotropy field for 25% Ni-Fe alloy films deposited at temperatures in the range -80°C to 375°C has been carried out. Comparison of the crystal structure phase diagram with the present data and those published by Wilts indicates that the anisotropy is strongly sensitive to crystal structure. Others have proposed pair ordering as an important source of anisotropy because of an apparent peak in the anisotropy energy at about 50% Ni composition. The present work shows no such peak, and leads to the conclusion that pair ordering cannot be a dominant contributor.

Width of the 180° domain wall in 76% Ni-Fe alloy films as a function of film thickness up to 1800 Å was measured using the defocused mode of Lorentz microscopy. For the thinner films, the measured wall widths are in good agreement with earlier data obtained by Fuchs. For films thicker than 800 Å, the wall width increases with film thickness to about 9000 Å at 1800 Å film thickness. Similar measurements for polycrystalline Co films with thickness from 200 to 1500 Å have been made. The wall width increases from 3000 Å at 400 Å film thickness to about 6000 Å at 1500 Å film thickness. The wall widths for Ni-Fe and Co films are much greater than predicted by present theories. The validity of the classical determination of wall width is discussed, and the comparison of the present data with theoretical results is given.

Finally, an experimental study of ripple by Lorentz microscopy in Ni-Fe alloy films has been carried out. The following should be noted: (1) the only practical way to determine experimentally a meaningful wavelength is to find a well-defined ripple periodicity by visual inspection of a photomicrograph. (2) The average wavelength is of the order of 1µ. This value is in reasonable agreement with the main wavelength predicted by the theories developed by others. The dependence of wavelength on substrate deposition temperature, alloy composition and the external magnetic field has been also studied and the results are compared with theoretical predictions. (3) The experimental fact that the ripple structure could not be observed in completely epitaxial films gives confirmation that the ripple results from the randomness of crystallite orientation. Furthermore, the experimental observation that the ripple disappeared in the range 71 and 75% Ni supports the theory that the ripple amplitude is directly dependent on the crystalline anisotropy. An attempt to experimentally determine the order of magnitude of the ripple angle was carried out. The measured angle was about 0.02 rad. The discrepancy between the experimental data and the theoretical prediction is serious. The accurate experimental determination of ripple angle is an unsolved problem.