Selected Publications

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BYU Authors: William R. Evans, Michael Clemens, and David D. Allred, published in Proc. SPIE
We used spectroscopic ellipsometry to determine the optical constants of seven thin-film ThO2 samples deposited by radio-frequency sputtering, thickness ranging between 24 and 578 nm, for the spectral range of 1.2 to 6.5. We used a hollow-cathode light source and vacuum monochromator to measure constants at 10.2 eV. None of the deposition parameters studied including DC-bias voltages successfully increase the n of (that is, densify) thoria films. The value of n at 3.0 eV is 1.86 ± 0.04. We find compelling evidence to conclude that the direct band gap is at ~5.9 eV, clarifying the results of others, some of whom observed the absorption edge below 4 eV. The edge in the two thickest films is of a narrow feature (FWHM=0.4 eV) with modest absorption (α~6μm-1, k ~0.1). Absorption may go down briefly with increasing energy (from 6.2 to 6.5 eV). But at 10.2 eV absorption is very high and index low as measured by variable-angle reflectometry, α = 47.3 ± 5.5 μm-1 and k = 0.48 ± 0.05, and n =0.87 ± 0.12.
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BYU Authors: Donovan Chipman, Andrew Ning, and David Allred, published in SpaceOps 2006 Conference, (June 2006, Rome, Italy)
This paper discusses the proposal to use an intermediate Martian atmospheric (IMA) structure as a partial solution to the difficulties associated with current full pressure spacesuit (FPS) designs. An IMA is similar to a regular pressurized space structure, except that its pressurant is carbon dioxide from the Martian atmosphere instead of Earth standard air. Astronauts can work in such a structure needing only breathing gear for EVA equipment. Large volumes for workspace can be created in this manner without having to meet the exacting construction standards of a regular manned space vehicle. Design options for the assembly and pressurization of such large structures are considered. We explain the construction of a small intermediate atmospheric demonstrator that operates inside of a simulated Martian atmosphere.
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BYU Authors: David Allred, published in J. Non-Cryst. Solids
Thin films of Silicon-Germanium (SiGe) were deposited by plasma enhanced chemical vapor deposition (PECVD) for use in high speed devices, Micro-electrical mechanical systems (MEMS) and bolometric infrared detectors. SiGe films grown by PECVD typically have lower stress, lower deposition temperatures and higher growth rates (200 angstrom/min) compared with other deposition techniques. The samples were deposited at temperatures from 500 degrees C to 580 degrees C and doped using either diborane (B2H6) or phosphine (PH3). As-deposited films had predominantly (111) and (220) texture determined by X-ray diffraction (XRD). Annealing produced crystalline material with no evidence of cracking as determined by resistivity measurements. It also produced variations of crystallite orientations with predominantly (111) texture. As-grown films exhibited compressive stresses as low as 18 MPa. Stress in annealed samples increased with increasing annealing temperature and time. (c) 2006 Elsevier B.V. All rights reserved.
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BYU Authors: Jed E. Johnson, David D. Allred, R. Steven Turley, William R. Evans, and Richard L. Sandberg, published in MRS Proc.
As applications for extreme ultraviolet (EUV) radiation have been identified, the demand for better optics has also increased. Thorium and thorium oxide thin films (19 to 61 nm thick) were RF-sputtered and characterized using atomic force microscopy (AFM), spectroscopic ellipsometry, low-angle x-ray diffraction (LAXRD), x-ray photoelectron spectroscopy (XPS), and x-ray absorption near edge structure (XANES) in order to assess their capability as EUV reflectors. Their reflectance and absorption at different energies were also measured and analyzed at the Advanced Light Source in Berkeley. The reflectance of oxidized thorium is reported between 2 and 32 nm at 5, 10, and 15 degrees from grazing. The imaginary component of the complex index of refraction, β, is also reported between 12.5 and 18 nm. Thin films of thorium were found to reflect better between 6.5 and 9.4 nm at 5 degrees from grazing than all other known materials, including iridium, gold, nickel, uranium dioxide, and uranium nitride. The measured reflectance does not coincide with reflectance curves calculated from the Center for X-Ray Optics (CXRO) atomic scattering factor data. We observe large energy shifts of up to 20 eV, suggesting the need for better film characterization and possibly an update of the tabulated optical constants.
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BYU Authors: Guillermo Acosta, David D. Allred, and Robert C. Davis, published in SVC Bulletin
We describe a technique which allows for atomic force microscopy to be used to make a physical measurement of the thickness of thin film samples. When dealing with a film which is ultrathin (<100 nm), standard measurement techniques may become difficult to apply successfully. The technique developed involves the fabrication of a distinct, abrupt step on the film surface, using a device we call the Abruptor. This step can be scanned with an atomic force microscope, revealing the height of the step. Films from 6-15 nm are now routinely measured in this way, though it is possible to apply this measurement technique to thinner and thicker films. The thinnest film we measured was 3.6 nm.
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BYU Authors: Kristi Adamson, Shannon Lunt, Richard Sandberg, Elke Jackson, David Allred, and R. Steven Turley, published in J. Utah Acad. Sci.
We have studied thin films (100-200Ǻ) of uranium oxide created through DC magnetron sputtering. The oxidation of the uranium surface has been examined through x-ray photoelectron spectroscopy (XPS). This work shows that the surface does not oxidize immediately, but over a period of several weeks. By comparison with the work of Teterin [1] (“A Study of Synthetic and Natural Uranium Oxides by X-Ray Photoelectron Spectroscopy.” J. Phys. Chem. Minerals. 1981), our thinner samples are a mixture of UO2 and γ-UO3, with γ-UO3 becoming more prominent as the sample has more time to oxidize. The surface of the sample oxidizes more quickly than the rest of the sample.