Using H I absorption alone, we attempt to separate H I absorption lines in quasar spectra into two categories; HDLs (Higher Density Lines) and LDLs (Lower Density Lines), and we discuss the difference in their physical properties. We deblend and fit all H I lines with Voigt profiles, and make an unbiased sample of H I lines covering a wide column density range ($12 < \log N_{HI} < 19$ cm$^{-2}$). To reduce the influence of line blending, we simultaneously fit several Lyman series lines. As a result of a two-point correlation analysis, we found that higher column density H I lines are clustering at $\Delta v < 200$ km s$^{-1}$, while lower ones at $\Delta v < 100$ km s$^{-1}$. We define HDLs as H I lines with $15<\log N_{HI} < 19$ cm$^{-2}$ and all H I lines within $\pm$200 km s$^{-1}$ of a line with $\log N_{HI} > 15$ cm$^{-2}$, and LDLs as others with $12 < \log N_{HI} < 15$ cm$^{-2}$. We found that the HDLs have smaller minimum $b$-values for a given column density than the LDLs. This difference is successfully reproduced by our hydrodynamic simulation. The LDLs seem to be cool or shock-heated diffuse IGM gas, while the HDLs are likely to be cooler dense gas near to galaxies.

Big Bang Nucleosynthesis (BBN) is the synthesis of the light nuclei, Deuterium (D or 2H), 3He, 4He and 7Li during the first few minutes of the universe. In this review we concentrate on recent data which give the primordial deuterium (D) abundance.

We have measured the primordial D/H in gas with very nearly primordial abundances. We use the Lyman series absorption lines seen in the spectra of quasars. We have measured D/H towards three QSOs, while a fourth gives a consistent upper limit. All QSO spectra are consistent with a single value for D/H: 3.325+0.22−0.25X10−5. From about 1994 − 1996, there was much discussion of the possibility that some QSOs show much higher D/H, but the best such example was shown to be contaminated by H, and no other no convincing examples have been found. Since high D/H should be much easier to detect, and hence it must be extremely rare or non-existent.

The new D/H measurements give the most accurate value for the baryon to photon ratio, η, and hence the cosmological baryon density: ωb = 0.0190 ± 0.0009 (1σ) A similar density is required to explain the amount of Lyα absorption from neutral Hydrogen in the intergalactic medium (IGM) at redshift z ≃ 3, and to explain the fraction of baryons in local clusters of galaxies. The D/H measurements lead to predictions for the abundances of the other light nuclei, which generally agree with measurements. The remaining differences with some measurements can be explained by a combination of measurement and analysis errors or changes in the abundances after BBN. The measurements do not require physics beyond the standard BBN model. Instead, the agreement between the abundances is used to limit the non-standard physics.

Silicon nanocrystals have made a recent appearance in the literature because of their potential importance in microelectronic[1] and optoelectronic [2] devices. For example, replacing the traditional Si floating gate in nonvolatile memory field effect transistors with nanocrystals has been shown to produce more reliable and lower power memory devices than traditional transistors [1]. Controlled oxidation of Si nanocrystals is desirable in the processing of Si quantum dots for these devices. If controlled oxidation is attainable, then it could be used to control nanocrystal size, achieve high nanocrystal density, and achieve a high quality interface between Si dot and SiO2 gate oxide layer in nanocrystal based memory. In this work, we report the oxidation properties of various sized silicon nanocrystals, deposited by low-pressure chemical vapor deposition, in different oxidizing environments. In the literature, when a Si column or dot is oxidized below the viscoelastic temperature of SiO2 (950 oC), the oxidation will self-limit to a Si core size that is dependent on the oxidizing conditions and the initial particle size. This self-limiting phenomenon is said to occur because of the presence of a compressive stress in the oxide layer that limits the diffusion of the oxidizing agent through the SiO2 to the Si- SiO2 interface. This compressive stress is present at the interface because of the difference in the density of the oxide and the Si and is enhanced by the radius of curvature of the dot. This self- limiting oxidation phenomenon will be studied experimentally using microscopy techniques and the effect of the constrained structure will be characterized.

Raman scattering studies were carried out on epi Si/Si1-xGex (x = 0.1 to 0.3) heterostructures consisting of a thin Si cap layer (100 - 400 A˚), a grade-down Si1-xGex layer, a constant Si1-xGex, buffer layer and a grade-up graded Si1-xGex layer on (100) oriented Si substrates. Different Ge composition, Si1-xGex layer thicknesses and thermal treatment were used to achieve different relaxation in the Si1-xGex layers. It has been revealed that, to a very good approximation, the absolute strains in the cap Si and constant Si1-xGex layers follow a simple sum-rule that is imposed by the lattice mismatch between unstrained Si and completely relaxed Si1-x Gex. This sum rule can be used to determine the Ge composition and stresses in both cap Si and constant Si1-xGex layers. Excellent agreement was found between the theoretical curve obtained with LO phonon strain coefficient b=−930cm−1 and the experimental total strain for all samples, regardless of the degree of the relaxation of the grade-up Si1-xGex layer.

Single wafer amorphous silicon deposition was characterized through process modeling and film characterization for application in semiconductor production. DOE methodology was used to determine the main deposition parameters, and the responses were limited to device production requirement properties of surface roughness, deposition rate and degree of crystallinity of the as-deposited film. The data trends and models show that deposition temperature and silane flow are the main factors. Increasing either or both factor increases the deposition rate and the surface roughness. The surface morphology, evaluated by AFM, SEM and TEM, was found to be rougher at extreme growth conditions than the poly crystalline film formed after anneal. The as-deposited surface morphology was not a result of pre-anneal crystal formations as determined by TEM cross sections of samples before and after anneal. Lack of crystalinity is important for impurity diffusion considerations. Device application of the single wafer a-Si process will be a compromise between growth rate (and associated throughput) and surface roughness that can be tolerated.

The factors affecting the second layer grain structure of a two-aluminum-layer sputtered film were investigated with the use of a computer-aided experimental design and analysis program. A cold aluminum layer followed by a hotter aluminum layer has been described in the literature as a means of improving metal step coverage. Several factors were screened to determine the most important parameters by which the first layer influences the second. The two-layer interactions were determined by using scanning electron microscope (SEM) analysis to measure final grain structures. A model was generated from the experimental results that shows the influence of the first layer grain size on the second layer if the second sputter deposition is made above ∼300°C. The influence increases at higher temperatures. The results will be compared to grain growth models found in the literature.

Borophosphosilicate glass (BPSG) films used in integrated circuit processing are grown from combinations of toxic and pyrophoric silane, diborane, and phosphine at atmospheric and reduced pressures. A safe, alternative chemical system has been developed with liquid source replacements for each dangerous gas. 2,4,6,8 tetramethylcyclotetrasiloxane (TMCTS) and oxygen are used as the silicon oxide sources, and trimethylphosphite (TMP) and trimethylborate (TMB) are the phosphorous and boron sources, respectively. Films grown in a hot wall LPCVD reactor with standard diffusion boats have thickness uniformities of ± 2 % and doping uniformities < ± 5 %.

The series of azidosilanes, SiEtn(N3)4-n where n = 1,2,3 and Si(t-butyl)(N3)3 were evaluated for the LPCVD of silicon nitride thin films. Both SiEt(N3)3 and Si(t-butyl)(N3)3 gave deposition rates of approximately 100 Å/min at temperatures of 450–500°C but films appear to be porous and air sensitive. Film properties improved as deposition temperatures were increased to 600°C. The polyazides must be handled with extreme caution. An unexplained detonation of one sample of SiEt(N3)3 occurred during the course of this study.

A purified diethylsilane (DES), LTO-410TM, has been studied as a silicon precursor for the deposition of silicon nitride and silicon oxynitride films by LPCVD. In the temperature range from 650 to 725°C mixtures of DES and NH3 produce uniform, oxidation resistant silicon nitride layers. Auger and RBS analyses show that the carbon concentrations in the films are correlated with the DES partial pressures. The film refractive indices also correlate with the carbon levels in the films for a fixed deposition temperature. Mixtures of DES, NH3, and N2O deposit silicon oxynitride films in the temperature range from 650 to 700°C with compositions controlled by the NH3:N2O ratio for a fixed DES flow. These films have carbon levels below the minimum detectable limit of Auger and RBS analyses.