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The Medium-l Program of the Michelson Doppler Imager (MDI) instrument on board SOHO provides continuous observations of oscillation modes of angular degree, l, from 0 to ∼ 300. The initial results show that the noise in the Medium-l oscillation power spectrum is substantially lower than in ground-based measurements. This enables us to detect lower amplitude modes and, thus, to extend the range of measured mode frequencies. The MDI observations also reveal the asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in the upper convective boundary layer. The sound-speed profile inferred from the mean frequencies gives evidence for a sharp variation at the edge of the energy-generating core. In a thin layer just beneath the convection zone, helium appears to be less abundant than predicted by theory. Inverting the multiplet frequency splittings from MDI, we detect significant rotational shear in this thin layer.
The influence of higher processing temperatures on the formation reaction of Cu(In,Ga)(Se,S)2 thin films using a three step reactive annealing process and on the device performance has been investigated. High process temperatures generally lead to the formation of larger grains, decrease the amount of void formation and their distribution at the back Mo/Cu(In,Ga)(Se,S)2 interface, and lead to a much faster formation reaction that shortens the overall reaction process. However, high temperature processing also leads to a decrease in device performance. A loss in open circuit voltage and fill factor could be attributed to enhanced interface recombination processes for the samples fabricated at higher process temperatures, which itself may be caused by a lack of Na and subsequent poor passivation of interface defect states. The lack of Na resulted in a decrease in free charge carrier concentration by two orders of magnitude.
The technique of ion beam deposition (IBD) is utilized to investigate low-energy, ion-induced damage on Si and Ge; to study reactive ion cleaning of Si and Ge; to fabricate amorphous isotopic heterostructures; and to fabricate and study the low-temperature epitaxial deposition of 74Ge on Ge(100), 30Si on Si(100), and 74Ge on Si(100). The techniques of ion scattering/channeling and cross-sectional TEM are combined to characterize the deposits.
The reaction rate of sputter deposited Ti films on c-Si as a function of process ambient was studied. Sintering temperatures ranging from 600 to 1100° C, under pure ammonia, forming gas, nitrogen and argon were used. The additional effect of a reactively sputtered TiN cap on the reaction rate was also investigated. Processed films were then analyzed using AES, RBS and four point probe resistivity mapping. It was found that for temperatures below 700° C, an ammonia ambient has the most pronounced effect on reducing the rate of formation of titanium silicide, followed by forming gas (N 2/H2 10% vol), nitrogen and argon. Additionally, the presence of the TiN cap further reduced the reaction rate while exhibiting significant diffusion of nitrogen into the silicide film. For the samples annealed in ambients containing nitrogen, a thin layer of Tix Ny was simultaneously formed on top of the silicide film. The thickness and Stoichiometry of this titanium nitride films were then correlated with the sintering temperature and ambient. The details of these findings and their impact on the formation of the self-aligned titanium silicide (salicide) will be presented.
New results on the ultrafast laser-induced formation of high-quality patterned silicon dioxide (SiO2) layers on silicon substrates at room temperature are presented. Using a pulsed ArF excimer laser (λ = 193 nm) operating at pulse energies near the melting threshold of silicon, SiO2 layers can be grown at a rate of 3–4 Å/pulse with an oxygen pressure of 500 torr. This growth rate exceeds by a factor of 5 the rate found using a XeCI excimer laser (λ = 308 nm) indicating that oxygen atoms produced by the direct photodissociation of oxygen by the ArF excimer laser play a role in achieving this extremely fast oxidation rate. Careful analysis of the laser exposed regions following each laser shot was performed using angle-resolved XPS with 150 micron spatial resolution. After one laser shot, a nonstoichiometric SiOx interface layer (<5Å) is clearly visible. Subsequent laser shots show the oxide peak shifting to higher energy as the oxide thickness increases until the characteristic ∼4.5 eV chemical shift for SiO2 (relative to the bulk Si 2p peak) is realized at a thickness of ∼20Å. The nonstoichiometric SiOx layer is still present at this oxide thickness but is confined to the interface region (<10Å) as observed in thermally grown oxides. Oxide thicknesses obtained by angle-resolved XPS were confirmed using sputter depth-profiling. XPS measurements on samples irradiated at low oxygen pressure (5 torr) show evidence for oxidation of the silicon substrate outside the focused laser beam (2X). At this pressure the recombination time for oxygen atoms is long enough to allow diffusion and apparent reaction with the silicon substrate at reduced temperatures.
We studied the lattice strain induced in the MeV ion bombarded InP crystals and the annealing behaviors of lattice strain, Raman line shift, and linewidth. The lattice spacing for the planes parallel to the surface decreases as a result of irradiation, and amounts to a strain of −0.061% for (100) face, −0.056% for (110) face, and −0.050% for (111) face for 15 MeV Cl bombarded samples to a dose of 1.25E15 ions/cm2. The negative lattice strain, Raman line shift, and line width completely recover at 450°C, and show a major recovery stage at 250°C – 350°C.
Preamorphization by indium of boron implanted silicon layers has been studied as a means of reducing defects in the annealed and activated shallow junctions. The In preamorphized samples after RTP annealing at 950 to 1150°C show an absence of spanning dislocations. A 5 sec. anneal at 1100 °C results in the complete annihilation of residual dislocation loops at the original crystalline/amorphous (c/a) interface. The minimum dose to preamorphize Si with 200keV In was 5×1013/cm2. During annealing the In was found to localize at two peaks, one at the original c/a interface and the other closer to the surface, where In precipitation was observed.
Thin films (1–2μm) of cadmium sulphide, deposited by electrophoresis, consist of a close packed layer of randomly oriented cubic-phase microcrystalline particles with an average diameter of 30nm. A CW CO2 laser, operating at 10.6μm has been used to convert this into a polycrystalline structure with columnar crystals, of hexagonal phase, which extend through the thickness of the film and whose c-axis is perpendicular to the substrate. The recrystallised regions comprise aligned grains up to 300nm in diameter whose cathodoluminescence spectrum exhibits a narrow peak centred at 2.42eV with a half width identical to that for evaporated CdS (0.1eV).
Synthesis of transition metal nitrides induced by excimer laser irradiation of metal foils immersed in liquid nitrogen was studied. Nitrogen incorporation was found by sputtering Auger Electron Spectroscopy in all six investigated metals (Ti, Zr, Hf, V, Nb and Ta). X-ray diffraction was used to determine the compounds formed. The morphologies of the irradiated surfaces often indicated that the surfaces had gone through a molten state. Micronthick mononitrides were grown on Ti, Zr and Hf, whereas Ta and Nb formed Ta2N and Nb2N respectively. The nitrides on V were not satisfactorily identified. The extent of nitridation correlates with the thermodynamic driving force, i.e. the Gibbs free energy for nitride formation.
The phenomenological model of ion mixing based on the concept of a thermal spike and chemically biased diffusion is further developed. Experimental results available to date are compared with the model.