Publications

2020

  1. Advances in X-Ray/EUV Optics and Components XV, (August 2020). Proc. SPIE 11491, 1149104-1 (2020).
    Proceedings
  2. Erik B. Watkins, Izabela Kruk, Jaroslaw Majewski, and David D. Allred
    J. Nucl. Mater. 539, 152356 (2020).
    Journal

2019

  1. J. Colter Stewart, Micah N. Shelley, Nathan R. Schwartz, Spencer K. King, Daniel W. Boyce, James W. Erikson, David D. Allred, and John S. Colton
    Opt. Mater. Express 9 (12), 4677-4687 (2019).
    Journal
  2. Brian I. Johnson, Tahereh G. Avval, Grant T. Hodges, Victoria Carver, Karen Membreno, David D. Allred, and Matthew R. Linford
    Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems II, 111160O (San Diego, CA, September 2019). Proc. SPIE 11116, (2019).
    Proceedings
  3. Brian I. Johnson, Tahereh Gholian Avval, Grant P. Hodges, Karen Membreno, David D. Allred, and Matthew R. Linford
    Society of Vacuum Coaters, 2019 Technical Conference Proceedings, Optical Coatings (August 22, 2019).
    Proceedings
  4. Expanding the Far UV Range of Aluminum-Coated Mirrors for Space-Based Observations to Reflect Hydrogen Lyman Lines via Fluoride Multilayers
    David D. Allred, Leoul E. Tilahun, J. Gabriel Richardson, and R. Steven Turley
    Optical Interference Coatings 2019, (Santa Ana Pueblo, NM, June 2019).
    Proceedings
  5. Joseph B. Muhlestein, Benjamin D. Smith, Margaret Miles, Stephanie M. Thomas, Anthony Willey, David D. Allred, and R. Steven Turley
    Opt. Express 27 (3), 3324-3336 (2019).
    Journal
  6. 2018 Conference on Laboratory Instruction Beyond the First Year of College, Part of the BFY Conference series, (Baltimore, MD, July, 2018).
    Proceedings

2018

  1. Michael Greenburg, David D. Allred, and R. Steven Turley
    Annual Conference of the Utah Academy of Sciences, Arts, & Letters (Orem, UT, April 2017). J. Utah Acad. Sci. 94, 317-325 (2018).
    Proceedings

2017

  1. David D. Allred, R. Steven Turley, Stephanie M. Thomas, Spencer G. Willett, Michael J. Greenburg, and Spencer B. Perry
    UV/Optical/IR Space Telescopes and Instruments: Innovative Technologies and Concepts VIII Proc. SPIE 10398, 34-01 (2017).
    Proceedings
  2. Journal
  3. Travis J. Moore, Matthew R. Jones, Dale R. Tree, and David D. Allred
    J. Quant. Spectrosc. Radiat. Transf. 187, 358-363 (2017).
    Journal

2015

  1. Lawrence K. Barrett, Dallin J. Barton, Steven G. Noyce, David D. Allred, Richard R. Vanfleet, and Robert C. Davis
    J. Microelectromech. Syst. 24 (5), 1331-1337 (2015).
    Journal

2013

  1. Heming He, David A. Andersson, David D. Allred, and Kirk D. Rector
    J. Phys. Chem. C 117 (32), 16540-16551 (2013).
    Journal

2012

  1. Heming He, P. Wang, D. D. Allred, Jaroslaw Majewski, M. P. Wilkerson, and Kirk D. Rector
    Anal. Chem. 84 (23), 10380-10387 (2012).
    Journal
  2. Kellen Moulton, Nicholas B. Morrill, Adam M. Konneker, Brian D. Jensen, Richard R. Vanfleet, David D. Allred, and Robert C. Davis
    J. Micromech. Microeng. 22 (5), 055004 (2012).
    Journal
  3. J. A. Brame, J. E. Goodsell, S. A. Getty, Y. Zheng, and D. D. Allred
    J. Utah Acad. Sci. 88, 444-454 (2012).
    Proceedings

2011

  1. Proceedings
  2. Lei Pei, Amy Balls, Cary Tippets, Jonnathan Abbott, Matthew R. Linford, Jian Hu, … David D. Allred, Richard R. Vanfleet, and Robert C. Davis (1 other authors)
    J. Vac. Sci. Technol. A 29, 021017 (2011).
    Journal

2010

  1. Bryan Hicks, Stephanie Getty, and David Allred
    Thin Solid Films 518 (17), 5014-5017 (2010).
    Journal
  2. Nicole Brimhall, Nicholas Herrick, David D. Allred, R. Steven Turley, Michael Ware, and Justin Peatross
    Appl. Optics 49 (9), 1581-1585 (2010).
    Journal

2009

  1. Nicole Brimhall, Nicholas Herrick, David D. Allred, R. Steven Turley, Michael Ware, and Justin Peatross
    Opt. Express 17 (26), 23873-23879 (2009).
    Journal
  2. R. Steven Turley, David Allred, Anthony Willey, Joseph Muhlestein, and Zephne Larsen
    J. Utah Acad. Sci. 86, 273-286 (2009).
    Proceedings

2008

  1. Elisabeth (Liz) Strein and David Allred
    Thin Solid Films 517 (3), 1011-1015 (2008).
    Journal
  2. E. Strein, D. Allred, and R. S. Turley
    Society of Vacuum Coaters, 2008 Technical Conference, (April 2008, Chicago, IL), 448-442 (2008). 
    Proceedings
  3. Guillermo Acosta, Richard Vanfleet, David D. Allred, and R. S. Turley
    Society of Vacuum Coaters, 2008 Technical Conference, (April 2008, Chicago, IL), 448-442 (2008). 
    Proceedings
  4. Nicole Brimhall, Matthew Turner, Nicholas Herrick, David D. Allred, R. Steven Turley, Michael Ware, and Justin Peatross
    Rev. Sci. Instrum. 79 (10), 103108 (2008).
    Journal
  5. Proceedings

2007

  1. Jonathan Goodsell, Stephanie A. Getty, Jon Brame, and David D. Allred
    J. Utah Acad. Sci. 84, 130-140 (2007).
    Proceedings
  2. Bounds and Conditions, A Kolob In Our Solar System
    Sunstone, (2007).
    News/Outreach
  3. Jon Brame, Stephanie Getty, Johnathan Goodsell, and David D. Allred
    2006 Materials Research Society Fall Meeting, (November 2006, Boston, MA). MRS Proc. 963, 0893-JJ05-09 (2007).
    Proceedings

2006

  1. Journal
  2. D. D. Allred, G. A. Acosta, N. Farnsworth-Brimhall, and R. S. Turly
    Society of Vacuum Coaters, 2006 Technical Conference Proceedings, Optical Coating (April 2006, Washington, DC). Proc. SVC 2006, 314-325 (2006).
    Proceedings
  3. Nicole F. Brimhall, Amy B. Grigg, R. Steven Turley, and David D. Allred
    Proc. SPIE 6317, 631711 (2006).
    Proceedings
  4. William R. Evans, Sarah C. Barton, Michael Clemens, and David D. Allred
    Advances in X-Ray/EUV Optics, Components, and Applications, (August 2006, San Diego, CA).  Proc. SPIE 6317, 631711 (2006).
    Proceedings
  5. Donovan Chipman, Andrew Ning, and David Allred
    SpaceOps 2006 Conference, (June 2006, Rome, Italy)
    Proceedings
  6. Srinivasan Kannan, Craig Taylor, and David Allred
    J. Non-Cryst. Solids 352 (9-20), 1272-1274 (2006).
    Journal
  7. Jed E. Johnson, David D. Allred, R. Steven Turley, William R. Evans, and Richard L. Sandberg
    2005 Materials Research Society Fall Meeting, Symposium JJ – Actinides 2005 – Basic Science, Applications and Technology, (November 2005, Boston, MA). MRS Proc. 893, 207-214 (2006).
    Proceedings

2005

  1. Trade Magazine
  2. Determining Composition of Thin Films of Uranium Oxide by X-Ray Photoelectron Spectroscopy
    Kristi Adamson, Shannon Lunt, Richard Sandberg, Elke Jackson, David Allred, and R. Steven Turley
    J. Utah Acad. Sci. 82, 176-186 (2005).
    Proceedings

2004

  1. Luke J. Bissel, David d. Allred, R. Steven Turley, William R. Evans, and Jed. E. Johnson
    Proc. SPIE 5538, 84-91 (2004).
    Proceedings
  2. Proceedings
  3. Richard L. Sandberg, David D. Allred, Luke J. Bissell, Jed E. Johnson, and R. Steven Turley
    AIP Conf. Proc. 705, 796-799 (2004).
    Proceedings
  4. R. E. Robinson, R. L. Sandberg, D. D. Allred, A. L. Jackson, J. E. Johnson, W. Evans, T. Doughty, … K. Adamson, and A. Jacquier (1 other authors)
    Society of Vacuum Coaters, 2004 Technical Conference
    Proceedings
  5. Proceedings

2003

  1. What the Universe Means to People Like Me
    Dialogue 36, 110-125 (2003).
    Book/Chapter

2002

  1. David D. Allred, Matthew B. Squires, R. Steven Turley, Webster Cash, and Ann F. Shipley
    Proc. SPIE 4782, 212-223 (2002).
    Proceedings
  2. Mars Jars
    Fifth International Mars Society Conference, (August 2002, Boulder, CO).
    Self-Published
  3. The Optical Constants of Sputtered U and a Si at 30.4 and 58.4 nm
    M. B. Squires, David D. Allred, and R. Steven Turley
    J. Utah Acad. Sci. 76, 74-87 (2002).
    Proceedings

2001

  1. Journal

2000

  1. B. R. Sandel, A. L. Broadfoot, C. C. Curtis, R. A. King, T. C. Stone, R. H. Hill, … D. D. Allred, R. S. Turley, … (4 other authors)
    Space Sci. Rev. 91 (1-2), 197-242 (2000).
    Journal

1999

  1. David D. Allred, R. Steven Turley, and Matthew B. Squires
    Proc. SPIE 3767, 280-287 (1999).
    Proceedings
  2. Matthew B. Squires, David D. Allred, and R. Steven Turley
    Proc. SPIE 3767, 288-294 (1999).
    Proceedings
  3. R. Osorio-Saucedo, C. Vázquez-López, W. Calleja, D. D. Allred, and C. Falcony
    Thin Solid Films 338 (1-2), 100-104 (1999).
    Journal

1997

  1. G. B. Thompson and D. D. Allred
    J. X-ray Sci. Technol. 7 (2), 159-170 (1997).
    Journal

1996

  1. Journal
  2. J. Gonzalez-Hernandez, B. S. Chao, S. R. Ovshinsky, and D. D. Allred
    J. X-ray Sci. Technol. 6 (1), 1-31 (1996).
    Journal
  3. Journal

1995

  1. Qi Wang, D. D. Allred, and L. V. Knight
    J. Raman Spectrosc. 26 (12), 1039-1043 (1995).
    Journal
  2. Douglas L. Harper, Royal G. Albridge, Norman H. Tolk, Wang Qi, David D. Allred, and Larry V. Knight
    Phys. Rev. A 52 (6), 4631-4639 (1995).
    Journal
  3. Fang Yuan, John A. Johnson, David D. Allred, and Robert H. Todd
    J. Vac. Sci. Technol. A 13 (1), 136 (1995).
    Journal

1994

  1. Journal
  2. M. Cai, D. D. Allred, and A. Reyes-Mena
    J. Vac. Sci. Technol. A 12 (4), 1535 (1994).
    Journal
  3. F. Ruiz, C. Vázquez-López, J. González-Hernández, David D. Allred, G. Romero-Paredes, R. Peña-Sierra, and G. Torres-Delgado
    J. Vac. Sci. Technol. A 12 (4), 2565 (1994).
    Journal

1993

  1. Ming Cai, Qi Wang, David D. Allred, Larry V. Knight, Dorian M. Hatch, Arturo Reyes-Mena, and Guizhong Zhang
    24th Annual Boulder Damage Symposium Proceedings -- Laser-Induced Damage in Optical Materials, (September 1992, Boulder, CO). Proc. SPIE 1848, 521 (1993).
    Proceedings
  2. W. I. Karain, L. V. Knight, D. D. Allred, and A. Reyes-Mena
    Nanostruct. Mat. 3 (1-6), 419-424 (1993).
    Journal
  3. Cheryl Barnett Davis, David D. Allred, A. Reyes-Mena, J. González-Hernández, Ovidio González, Bret C. Hess, and Worth P. Allred
    Phys. Rev. B 47 (20), 13363-13369 (1993).
    Journal
  4. Qi Wang, D. D. Allred, and Jesus González-Hernández
    Phys. Rev. B 47 (10), 6119-6121 (1993).
    Journal

1992

  1. F. Yuan, Y. Shi, L. V. Knight, R. T. Perkins, and D. D. Allred
    Thin Solid Films 220 (1-2), 284-288 (1992).
    Journal
  2. David D. Allred, Ming Cai, Qi Wang, Dorian M. Hatch, and A. Reyes-Mena
    J. X-ray Sci. Technol. 3 (3), 222-228 (1992).
    Journal
  3. Douglas P. Hansen , Arturo Reyes-Mena , John R. Colton, Larry V. Knight, and David D. Allred
    Multilayer and Grazing Incidence X-Ray/EUV Optics for Astronomy and Projection Lithography, (July 1992, San Diego, CA). Proc. SPIE 1742, 476 (1992).
    Proceedings
  4. Wael I. Karain, Larry V. Knight, David D. Allred, and Arturo Reyes-Mena
    Soft X-Ray Microscopy, (July 1992, San Diego, CA). Proc. SPIE 1741, 12-18 (1992).
    Proceedings
  5. Manufacturing of atomically sharp silicon tips and their use as photocathodes
    W. I. Karain, Larry V. Knight, David D. Allred, and A. Reyes-Mena
    J. Utah Acad. Sci. 69, 257-272 (1992).
    Proceedings
  6. J. Gonzalez-Hernandez, B. S. Chao, D. A. Pawlik, D. D. Allred, and Qi Wang
    J. Vac. Sci. Technol. A 10 (1), 145-151 (1992).
    Journal

1991

  1. Memorie K. Williams, Evan Hansen , Arturo Reyes-Mena, and David D. Allred
    EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy II, (July 1991, San Diego, CA). Proc. SPIE 1549, 147-154 (1991).
    Proceedings
  2. Roland D. Seals, Charles M. Egert, and David D. Allred
    Optical Surfaces Resistant to Severe Environments, (July 1991, San Diego, CA). Proc. SPIE 1330, 164-167 (1991).
    Proceedings
  3. S. S. Chao, D. A. Pawlik, J. González-Hernández, Qi Wang, and D. D. Allred
    Solid State Commun. 79 (3), 205-207 (1991).
    Journal
  4. J. González-Hernández, O. Zelaya, J. G. Mendoza-Alverez, E. López-Cruz, D. A. Pawlik, and D. D. Allred
    J. Vac. Sci. Technol. A 9 (3), 550-554 (1991).
    Journal

1990

  1. Proceedings
  2. J. González-Hernández, Elías López-Cruz, D. D. Allred, and Worth P. Allred
    J. Vac. Sci. Technol. A 8 (4), 3255-3259 (1990).
    Journal
  3. Elías López-Cruz, J. González-Hernández, D. D. Allred, and W. P. Allred
    J. Vac. Sci. Technol. A 8 (3), 1934-1938 (1990).
    Journal
  4. J. K. Shurtleff, D. D. Allred, R. T. Perkins, and J. M. Thorne
    Materials Research Society
    Proceedings
  5. J. Gonzalez-Hernandez, A. Reyes-Mena, Elias Lopez-Cruz, D. D. Allred, and Worth P. Allred
    Materials Research Society
    Proceedings
  6. David D. Allred, Qi Wang, and Jesus Gonzalez-Hernandez
    Materials Research Society
    Proceedings

1989

  1. Raymond T. Perkins, David D. Allred, Larry V. Knight, and James M. Thorne
    X-Ray/EUV Optics for Astronomy and Microscopy, (July 1989, San Diego, CA).  Proc. SPIE 1160, 56-65 (1989).
    Proceedings
  2. Qi Wang, David D. Allred, L. V. Knight, and Jesus Gonzalez-Hernandez
    Proc. SPIE 1160, 229-234 (1989).
    Proceedings
  3. J. K .Shurtleff, D. D. Allred, R. T. Perkins, and J. M. Thorne
    Proc. SPIE 1159, 664 (1989).
    Proceedings

1988

  1. Accelerated Life Test for Thermoelectric Junctions: Solder Element Interactions
    David D. Allred and On Van Nbuyen
    Proceedings of the Seventh International Conference on Thermoelectric Energy Conversion
    Proceedings

1987

  1. S. R. Ovshinsky, R. T. Young, D. D. Allred, G. DeMaggio, and G. A. Van der Leeden
    Phys. Rev. Lett. 58 (24), 2579-2581 (1987).
    Journal
  2. J. Gonzalez-Hernandez, D. D. Allred, and O. V. Nguyen
    Materials Research Society
    Proceedings

1986

  1. Proceedings
  2. A New Multiphase Thermoelectric Material for Power Generation
    O. V. Nguyen, David D. Allred, and S. R. Ovshinsky
    Sixth Internional Conference on Thermoelectric Energy Conversion, (March 1986, Arlington, TX).
    Proceedings
  3. D. D. Allred, J. Gonzalez-Hernandez, O. V. Nguyen, D. Martin, and D. Pawlik
    J. Mat. Res. 1, 468-475 (1986).
    Journal

1985

  1. David D. Allred, Michael R. Jacobson, and Elizabeth E. Chain
    Sol. Energ. Mater. 12 (2), 87-129 (1985).
    Journal

1984

  1. Optical Properties of Amorphous Germanium Films Prepared by CVD
    David D. Allred and J. A. Piontkowski
    Fifth International Conference on Thermoelectric Energy Conversion, (March 1984, Arlington, TX), 546-557 (1984).
    Proceedings
  2. Thermal Diffusivity by Modified Angstrom Technique
    N. Jackett, David D. Allred, T. H. Sein, and J. U. Trefny
    Fifth International Conference on Thermoelectric Energy Conversion, (March 1984, Arlington, TX), p 116-119 (1984).
    Proceedings

1983

  1. CVD a-Ge and a-Ge:X Films: Preparation and Properties
    David D. Allred and J. A. Piontkowski
    Fourth European Conference on Chemical Vapour Deposition
    Proceedings

1981

  1. E. Randich and D. D. Allred
    Thin Solid Films 83 (4), 393-398 (1981).
    Journal
  2. D. D. Allred, D. C. Booth, B. R. Appleton, P. D. Miller, C. D. Moak, J. P. F. Sellschop, C. W. White, and A. L. Wintenberg
    IEEE Trans. Nucl. Sci. 28 (2), 1838-1840 (1981).
    Journal

1980

  1. D.C. Booth, D. D. Allred, and B. O. Seraphin
    J. Non-Cryst. Solids 35-36 (1), 213-218 (1980).
    Journal

1979

  1. D. C. Booth, D. D. Allred, and B. O. Seraphin
    Sol. Energ. Mater. 2 (1), 107-124 (1979).
    Journal
  2. Selective Surfaces by Chemical Vapor Deposition
    Second Annual Coating for Solar Collectors Symposium
    Proceedings
  3. M. Janai, D. D. Allred, D. C. Booth, and B. O. Seraphin
    Sol. Energ. Mater. 1, 11-27 (1979).
    Journal
  4. D.C. Booth, D.D. Allred, and B. O. Seraphin
    Sol. Energ. Mater. 2 (1), 107-124 (1979).
    Journal

1978

  1. G. E. Carver, D. D. Allred, and B. O. Seraphin
    Proc. SPIE 0161, 66-71 (1978).
    Proceedings
  2. D. C. Booth, M. Janai, G. Weiser, D. D. Allred, and B. O. Seraphin
    Proc. SPIE 0161, 72-77 (1978).
    Proceedings
  3. G. J. Clark, C. W. White, D. D. Allred, B. R. Appleton, F. B. Koch, and C. W. Magee
    Nucl. Instr. Meth. 149 (1-3), 9-18 (1978).
    Journal
  4. J.F. Ziegler, C.P. Wu, P. Williams, C. W. White, B. Terreault, B. M. U. Scherzer, … and D. D. Allred (20 other authors)
    Nucl. Instr. Meth. 149 (1-3), 19-39 (1978).
    Journal
  5. G. J. Clark, C. W. White, D. D. Allred, B. R. Appleton, and I. S. T. Tsong
    Phys. Chem. Miner. 3 (2), 199-211 (1978).
    Journal

1977

  1. G. J. Clark, C. W. White, D. D. Allred, B. R. Appleton, C. W. Magee, and D. E. Carlson
    Appl. Phys. Lett. 31, 582-585 (1977).
    Journal

1972

  1. John E. Esterl, David Allred, J. C. Hardy, R. G. Sextro, and Joseph Cerny
    Phys. Rev. C 6 (1), 373-375 (1972).
    Journal

Undergraduate Student Author

Graduate Student Author

Selected Publications

BYU Authors: Raymond T. Perkins, David D. Allred, Larry V. Knight, and James M. Thorne, published in Proc. SPIE
Methods of designing strong, high transmission soft x-ray windows are discussed. A material which contains several elements, most notably 0, N, and C, produce the most spectrally neutral window. It is noted that a predominantly single element material such as diamond is in reality an edge filter. A structure to support very thin films and to provide exceptional mechanical strength is discussed. Pressure cycling data for such a supported window are presented.
BYU Authors: Qi Wang, David D. Allred, and L. V. Knight, published in Proc. SPIE
Laser Raman spectroscopy has been found to he useful for characterizing amorphous semi-conductor niultilayers especially the interfaces of multilayers. We have extended this technique to the characterization of W/C niultilayers used in soft x-ray optics and ultrathin sputtered carbon films. Unlike the multilayers previously studied which contained only semiconductors and di-electrics, these are semiconductor/metal multilayers. The dominate Raman feature is due to a-C and consists of a broad peak at about 1580 cm-1 (G-peak) and a shoulder at about 1400 cm -1 (D-peak). This was deconvoluted with Gaussians to yield two peaks (one at 1570 cm' and the other at 1420 cim 1). Among the multilayer samples peak positions and relative magnitudes changed little. The intensity ratio of the D-peak to G-peak was much larger for multilayer sample, however , than for the single layer pair samples. This may due to different. forms (amorphous or crystalline) of tungsten layer in the samples and indicate the differences in the structure of carbon layers and tor bonding structure at interfaces.
BYU Authors: J. K .Shurtleff, D. D. Allred, R. T. Perkins, and J. M. Thorne, published in Proc. SPIE
Thin film deposition techniques currently being used to produce multilayer x-ray optics (MXOs) have difficulty producing smooth, uniform multilayers with d-spacings less than about twelve angstroms. We are investigating atomic layer epitaxy (ALE) as an alternative to these techniques. ALE is a relatively new thin film deposition technique which we believe can produce MXOs with very small d-spacings. ALE accomplishes this by depositing a single layer of atoms during each cycle of the deposition process. Multilayers deposited by ALE should have sharp interfaces and smooth, uniform layers with precise d-spacings.
BYU Authors: David D. Allred, published in Proceedings of the Seventh International Conference on Thermoelectric Energy Conversion
BYU Authors: D. D. Allred, published in Phys. Rev. Lett.
Transition to a superconducting zero-resistance state at 155 K is observed for the first time in bulk material. A new five-element compound has been synthesized with nominal composition Y1Ba2Cu3F2Oy@B. Fluorine plays a critical role in achieving this effect. X-ray diffraction and electron microprobe analysis indicate that the samples are multiphasic. Evidence is presented that the samples contain superconducting phases with onset temperatures considerably above 155 K. Magnetic measurements suggest a flux-trapping effect below 260 K, and diamagnetic deviations from Curie-Weiss behavior in the range 250 K≤T≤100 K indicate a Meissner effect in a small superconducting volume fraction.
BYU Authors: D. D. Allred, published in Materials Research Society
We have prepared, heat treated and characterized various amorphous semiconductor periodic multilayers and ultrathin films. These were prepared by several vapor deposition techniques at substrate temperatures ranging from 25°C to 300°C and possessed periodicities from 22 to 400Å. Films were subjected to isochronal thermal treatments at progressively higher temperatures. Two effects were observed: enhanced diffusion and retarded crystallization. Interdiffusion, at rates which are many orders of magnitude higher than those anticipated from crystalline data, was observed in a-Si/a-Ge multilayers. Crystallization of germanium, the more readily crystallized member of the couple, is retarded; the extent depends on the thickness of the layer. The thinner the layer, the greater the retardation. Where intermixing is thermodynamically unfavorable as in a-Si/a-SiNx or a-Ge/a-SiNx multilayers, and ultrathin germanium layers on SiO2, interdiffusion does not occur, however, crystallization of silicon or germanium is again substantially retarded.
BYU Authors: David D. Allred, published in Proc. SPIE
This study addresses the question, "How can the optical properties of matter in ultrathin amorphous nonmetallic films in multilayers best be determined from reflectance (R) and transmission (T) measurements." A blue shift in the band gap of plasma CVD a-Si:H/a-SiN.:H multilayers was reported sev-eral years ago. It was suggested that the shift in the band gap, Eg, relative to bulk a-Si:H as given by the Tauc plot was due to quantum confinement effects. The purpose of this study is to evaluate the usefulness of various effective media theories (EMT) for determining the optical constants of materials in a multilayer and to explore to what extent a shift in band gap to higher energy may be an artifact of the method of optical analysis. Incoherent approaches are the most common methods of determining band gap from R and T. These do not require iteration to obtain optical constants from the optical data. The band gap determined by such methods was, however, generally 8% higher than the actual band gap when a suitable hypothetical case was investigated. Coherent effective media theory provides a noteworthy alternative to both incoherent EMT and fully coherent multilayer modeling, (which is accurate but is excessively com-plicated and poorly convergent). The accuracy of the band gap is at the limit, 2-3%, of what can be expected for graphical methods. A previously unappreciated source of optical artifacts was also identified. Dispersion, which is commonly ignored when Eg is determined graphically, is shown to distort, in certain cases, the anticipated straight line behavior of the aE vs. E plot.
BYU Authors: David D. Allred, published in Sixth Internional Conference on Thermoelectric Energy Conversion, (March 1986, Arlington, TX).
BYU Authors: D. D. Allred, published in J. Mat. Res.
Raman spectroscopy (RS) and low-angle x-ray diffraction (LAXRD) have been used to characterize semiconductor multilayer interfaces. In the present study a model for Raman spectra of multilayers is developed and applied to the specific case of the interfaces of a-Si/a-Ge multilayers. Quantification of the “blurring” of interfaces is possible because peak heights in the Raman spectra of thin films are proportional to the number of scatterers, thus RS is capable of directly “counting” the total number of chemical bonds of a given type in the film. Multilayers, prepared by various deposition techniques, are compared. The relative roles of LAXRD and RS in investigating interfaces are contrasted. Several a-Si/a-Ge multilayers deposited by ultra-high vacuum (UHV) evaporation (MBD) are found to exhibit very regular periodicities and exceptionally sharp interfaces (<1.0 Å intermixing).
BYU Authors: David D. Allred, published in Sol. Energ. Mater.
Chemical Vapor Deposition (CVD) is a versatile and efficient technique for thin film deposition in the microelectronics, electro-optics, tool and protective coatings industries. This technology has been adapted by several groups to the preparation of optical thin films. We report on several CVD thin films stacks for photothermal solar energy conversion which combine promising spectral selectivity and durability at elevated operating temperatures.
BYU Authors: David D. Allred, published in Fifth International Conference on Thermoelectric Energy Conversion, (March 1984, Arlington, TX), 546-557 (1984).
BYU Authors: David D. Allred, published in Fifth International Conference on Thermoelectric Energy Conversion, (March 1984, Arlington, TX), p 116-119 (1984).
BYU Authors: David D. Allred, published in Fourth European Conference on Chemical Vapour Deposition
BYU Authors: D. D. Allred, published in Thin Solid Films
Coatings of ZrB2 and TiB2 for photothermal solar absorber applications were prepared using chemical vapor deposition (CVD) techniques. Oxidation tests suggest a maximum temperature limit for air exposure of 600 K for TiB2 and 800 K for ZrB2. Both materials exhibit innate spectral selectivity with an emittance at 375 K ranging from 0.06 to 0.09, a solar absorptance for ZrB2 ranging from 0.67 to 0.77 and a solar absorptance for TiB2 ranging from 0.46 to 0.59 ZrB2 has better solar selectivity and more desirable oxidation behavior than TiB2. A 0.071 μm antireflection coating of Si3N4 deposited onto the ZrB2 coating leads to an increase in absorptance from 0.77 to 0.93, while the emittance remains unchanged.
BYU Authors: D. D. Allred, published in IEEE Trans. Nucl. Sci.
Chemical vapor deposited (CVD) amorphous silicon alloyed with carbon or nitrogen (¿-Si:X, X=C or N) to retard high temperature crystallization is a promising absorber material for photothermal solar energy conversion. Films are prepared by decomposing silane containing gas mixures, a technique which is known to incorporate hydrogen into ¿-Si in some cases. Using the 16.45 MeV resonance of the 1H(19F,¿¿)16O reaction we made the first measurements of the hydrogen incorporation in CVD a-Si:X films (X=C,N). We have made three observations. First, the incorporation efficiency of hydrogen into CVD a-Si increases by a factor of twenty as the carbon content increases from 0 to 35 atomic percent which indicates that previous studies of multicomponent systems may need to be reevaluated since this enhancement in incorporation efficiency involves hydrogen--a key alloyant in a-Si. Second, the quantity of hydrogen incorporated increases at a greater than linear rate as a function of carbon content which implies that the presence of hydrogen in the films is not accidental but is a necessary part of film growth. Third, the hydrogen content of a-Si decreases to almost zero after high temperature anneal which may help explain reported shift in optical constants.
BYU Authors: D. D. Allred, published in J. Non-Cryst. Solids
Amorphous silicon holds considerable promise as a photothermal absorber, but high-temperature-induced crystallization limits its usefulness. To attempt to retard the crystallization, we produced CVD a-Si films alloyed with C, N, B, or Ge. These films crystallized differently than did the non-intentionally doped amorphous material. The crystallization temperature was increased from 680 C to 950 C for 18 at.% C-alloyed a-Si, and even then more than 10 hours were required for crystallization. This retardation of crystallization gives alloyed a-Si absorbers sufficient life expectancy for converters operating at temperatures up to 700 C.
BYU Authors: D. D. Allred, published in Sol. Energ. Mater.
By pyrolytic decomposition of silane in the presence of dopant gases, a set of amorphous silicon films was prepared that contains various concentrations of carbon, nitrogen, boron or germanium. The effect of these dopants on the crystallization process and the optical properties is investigated. Films containing aböut 18 at % carbon show the properties most favorable for solar absorbers. The crystallization is retarded to temperatures near 1000°C, and the solar absorptance is greater than that of non-intentionally doped CVD amorphous silicon. From the experimentally determined activation energy of crystallization, the structural lifetime for such absorber films is extrapolated to be in excess of several decades for continuous operation at 700°C. For identical thicknesses of absorber layers, spectrally selective stacks of stabilized amorphous silicon deposited on top of a molybdenum reflector have higher solar absorptance than stacks composed of polycrystalline silicon on a silver reflector, amorphous silicon on molybdenum having been tested at temperatures in excess of 500°C.
BYU Authors: David D. Allred, published in Second Annual Coating for Solar Collectors Symposium
BYU Authors: D. D. Allred, published in Sol. Energ. Mater.
Silicon films were deposited by pyrolytic decomposition of silane on substrates held at various temperatures, View the MathML source, in the range 550 to 800°C. The absorption coefficient, refractive index, anf X-ray diffraction pattern of these films were determined. The films deposited at temperatures, View the MathML source are amorphous, and their absorption profile resembles that reported in the literature for sputtered or evaporated amorphous films after long-time anneal. Films deposited on substrates at or above 670°C are partially crystallized, with particle size increasing gradually with substrate temperature. When the amorphous films are annealed, the resulting changes depend on length and temperature of the anneal. After a temperature-dependent induction period, the samples crystallize rapidly. The volume shrinks by ≈3% as determined from the decrease in film thickness. The onset of crystallization is indicated first by a red shift of the absorption edge, which after further anneal is overcompensated by a blue shift. The results demonstrate that the superior solar absorptance of amorphous silicon can be utilized in photothermal solar energy converters of sufficient stability without sacrificing the advantages of CVD fabrication.
BYU Authors: D.D. Allred, published in Sol. Energ. Mater.
By pyrolytic decomposition of silane in the presence of dopant gases, a set of amorphous silicon films was prepared that contains various concentrations of carbon, nitrogen, boron or germanium. The effect of these dopants on the crystallization process and the optical properties is investigated. Films containing aböut 18 at % carbon show the properties most favorable for solar absorbers. The crystallization is retarded to temperatures near 1000°C, and the solar absorptance is greater than that of non-intentionally doped CVD amorphous silicon. From the experimentally determined activation energy of crystallization, the structural lifetime for such absorber films is extrapolated to be in excess of several decades for continuous operation at 700°C. For identical thicknesses of absorber layers, spectrally selective stacks of stabilized amorphous silicon deposited on top of a molybdenum reflector have higher solar absorptance than stacks composed of polycrystalline silicon on a silver reflector, amorphous silicon on molybdenum having been tested at temperatures in excess of 500°C.
BYU Authors: D. D. Allred, published in Proc. SPIE
High infrared reflectance, coupled with high solar absorptance, is required for efficient photothermal conversion. Converters can be fabricated by depositing an absorber on a highly reflecting metal. The absorber functions in the visible, yet becomes transparent in the near infrared, allowing the metal to suppress the thermal emittance. Economic considerations demand the use of thin films, rather than bulk materials. The thin film reflector must be capable of withstanding high temperatures of operation. Compatibility of the re-flector with the substrate below, and the absorber above, is required for long-time service. Highly reflective silver films suffer reflectance losses by agglomeration, and require stabilization layers. Refractory materials such as molybdenum avoid agglomeration at temperatures of operation of photothermal converters. Unlike other deposition methods, chemical vapor deposition (CVD) can produce molybdenum films with an infrared reflectance rivaling that of bulk molybdenum. CVD is a non-vacuum based technology with potential for sequential throughput fabrication. Studies are being undertaken to determine how sensitively the reflectance reacts to inclusions of impurities into the molybdenum. Thin film passivators deposited on the molybdenum prevent reflectance losses induced by oxidation, and insure high temperature survival of optimal reflectance. Complete converter stacks have been annealed at 550°C for over 1000 hours in air.
BYU Authors: D. D. Allred, published in Proc. SPIE
Efficient photothermal conversion requires surfaces of high solar absorptance and low thermal emittance. This can be accomplished by the tandem action of a good infrared reflector overlaid by a film of sufficient solar absorptance that is transparent in the infrared. Crystalline silicon is a suitable candidate for the absorber layer. Its indirect band gap, however, results in a shallow absorption edge that extends too far into the visible. In contrast, the absorption edge of amorphous silicon is steeper and located farther into the infrared, resulting in a larger solar absorptance. We report on the fabrication of amorphous silicon absorbers by chemical vapor deposition (CVD). Their optical and structural properties are determined as a function of the deposition temperature. We describe the effects of a progressive crystallization during anneal above 650 C and report the performance of converter stacks that are identical "twins" except for the use of a polycrystalline silicon absorber in one and an amorphous absorber in the other.
BYU Authors: D. D. Allred, published in Nucl. Instr. Meth.
The application of nuclear reaction techniques to hydrogen analysis problems in metallurgical, mineralogical and semiconductor areas is described. Hydrogen analyses and profiles obtained with both the 1H(19F, αγ)16O and 1H(15N, αγ)12C reactions are presented. The advantages and disadvantages of the two techniques are discussed. Particular emphasis will be given to interpretive problems associated with analyzing the data. Various corrections to the data will be discussed, including off-resonance cross-section corrections and lower energy resonance corrections. Both crystalline and amorphous materials are examined. The hydrogen content of electrodeposited hard gold films has been determined as a function of plating conditions. Hydrogen contents as high as 9 atom % have been measured. The hydrogen profile of natural and synthetic SiO2 samples was determined. Hydrogen was found to be quite stable in amorphous silica samples but highly mobile in crystalline quartz samples under the analysis conditions. A hydrogen depth profile for a film of glow discharge deposited amorphous silicon () has been obtained and will be compared with a profile measured by secondary ion mass spectrometry (SIMS) on the same sample.
BYU Authors: D. D. Allred, published in Nucl. Instr. Meth.
Over the last few years many ion beam techniques have been reported for the profiling of hydrogen in materials. We have evaluated nine of these using similar samples of hydrogen ion-implanted into silicon. When possible the samples were analysed using two or more techniques to confirm the ion-implanted accuracy. We report the results of this work which has produced a consensus profile of H in silicon which is useful as a calibration standard. The analytical techniques used have capabilities ranging from very high depth resolution () and high sensitivity (< 1 ppm) to deep probes for hydrogen which can sample throughout thin sheets (up to 0.2 mm thick).
BYU Authors: D. D. Allred, published in Phys. Chem. Miner.
The nuclear reaction 19F(1H, αγ) 16O has been used to determine the hydrogen concentration in natural and synthetic quartz samples. The depth-profile of the hydrogen concentration in these samples has been determined in detail for the smoky and X o quartzes. These profiles exhibit a region of high hydrogen concentration in the near surface region (down to a depth of ∼2000Å), with a lower concentration in the bulk of the sample. The results provide a plausible explanation for the substantial disagreement between previous hydrogen analysis in these quartzes by other techniques. Evidence for hydrogen mobility in crystalline quartz under ion beam bombardment is presented and discussed.
BYU Authors: D. D. Allred, published in Appl. Phys. Lett.
Depth profiles for hydrogen in amorphous silicon have been determined by the use of resonantnuclear reactions [1H(15N,αγ)12C and 1H(19F,αγ)16O] and by secondary ion mass spectroscopy(SIMS). Independent calibration procedures were used for the two techniques. Measurements were made on the same amorphous silicon film to provide a direct comparison of the two hydrogen analysis techniques. The hydrogen concentration in the bulk of the film was determined to be about 9 at.% H. The SIMS results agree with the resonantnuclear reaction results to within 10%, which demonstrates that quantitative hydrogen depth profiles can be obtained by SIMSanalysis for materials such as amorphous silicon.
BYU Authors: David Allred, published in Phys. Rev. C
The delayed-proton spectrum following the β decay of C9 (τ12=126.5±1.0 msec) was found to consist primarily of a continuum extending from 13 to 1.5 MeV, the latter being the lowest energy observed. In addition to the previously observed peaks at 9.28 and 12.30 MeV (c.m.), possible peaks between 3 and 7 MeV have been tentatively identified.
BYU Authors: Scott Olsen, Richard Vanfleet, David Allred, Steve Turley, and Robert Davis, published in Proc. SPIE

We report on a large-area, high-aspect-ratio, carbon nanotube (CNT) forest structure produced at BYU acting as a window/separator for a hollow cathode EUV lamp. The structure has large-surface-area, high light trans-mission, and differential pumping. CNT fabrication allows for variable dimensions, which allows various EUV distributions and pressure gradients to be possible. Theory is presented for predicting such distributions and gradients. Several structures have been fabricated; their dimensions, properties, and predicted distributions and gradients are given.

BYU Authors: David D. Allred, published in J. Nucl. Mater.

U-6Nb is a uranium alloy containing 6 wt% niobium that possesses high corrosion resistance. The structure and composition of the passivating oxide layer formed on air-aged U-6Nb, which gives the material its corrosion resistant properties, was characterized using surface scattering techniques. Stable oxide layers formed on the surface of a set of U-6Nb alloy thin films under ambient conditions were investigated using neutron reflectometry (NR), x-ray reflectometry (XRR) and grazing incidence x-ray diffraction (GIXD). The passivating oxide was composed of approximately 27% U, 5% Nb, and 68% O, primarily consisting of a thin niobium oxide layer (5.5 ± 0.4 nm) separating a thicker UO2 layer (27.1 ± 2.3 nm) from the underlying U-6Nb alloy. A critical density of enriched niobium oxide at the metal-oxide interface is hypothesized to limit oxygen diffusion and confer high corrosion resistance to the alloy.

BYU Authors: J. Colter Stewart, Micah N. Shelley, Nathan R. Schwartz, Spencer K. King, Daniel W. Boyce, James W. Erikson, David D. Allred, and John S. Colton, published in Opt. Mater. Express

We have used spectroscopic ellipsometry to measure the optical constants of evaporated amorphous zinc arsenide (Zn3As2). A five parameter model using a Tauc-Lorentz oscillator was found to fit well each of six amorphous samples deposited on Si3N4/silicon, allowing the layer thicknesses and optical constants to be deduced. Layer thicknesses varied from 20 to 70 nm. The fitted value of the optical gap (Tauc gap) is 0.95 eV, close to the 1.0 eV band gap for crystalline bulk zinc arsenide. A single set of parameters from an ensemble Tauc-Lorentz model can be used to determine the thicknesses of amorphous Zn3As2 layers as long as the layers are \&\#x2273; 25 nm thick. Measured film thicknesses do not correlate with targeted thicknesses, likely due to low sticking coefficients of evaporated zinc arsenide.

BYU Authors: David D. Allred and Matthew R. Linford, published in Proc. SPIE

To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF2, are used to protect the of aluminum mirrors against oxidation since aluminum oxide absorbs short wavelength light. In this study, we present, for the first time, combined X-ray photoelectron spectroscopy (XPS) and ellipsometric (SE) studies of aluminum oxidation as a function of MgF2 over a range of layer thickness (0-6 nm). We also show for the first time, dynamic SE data which, with appropriate modeling, tracks the extent of oxide growth every few seconds over a period of several hours after the evaporated Al + MgF2 bilayer is removed from the deposition chamber, exposing it to the air. For each SE data set, because the optical constants of ultrathin metals films depend strongly on deposition conditions and their thickness, the optical constants for Al, as well as the Al and Al2O3 thicknesses, were fit. SE trends were confirmed by X-ray photoelectron spectroscopy. There is a chemical shift in the Al 2s electron emission peak toward higher binding energy as the metal oxidizes to Al+3. The extent of oxide growth can be modeled from the relative area of each peak once they are corrected for the attenuation through MgF2 layer. This generates an empirical formula: oxide thickness= k*log(t) +b, for the time-dependent aluminum-oxide thickness on aluminum surfaces protected by MgF2 as a function of MgF2 layer thickness. Here, k is a factor which depends only on MgF2 thickness, and decreases with increasing MgF2 thickness. The techniques developed can illuminate other protected mirror systems.

BYU Authors: David D. Allred and Matthew R. Linford, published in Society of Vacuum Coaters, 2019 Technical Conference Proceedings, Optical Coatings (August 22, 2019).

Aluminum enjoys broad band reflectivity and is widely used as an astronomical reflector. However, it oxides rapidly, and this oxide absorbs very short wavelength light, which limits the performance of aluminum mirrors. Accordingly, thin transparent layers, such as films of MgF2, are used to protect aluminum. In this study, we present an X-ray photoelectron pectroscopy (XPS) study of the chemical changes in MgF2 - protected aluminum that take place as it oxidizes (is exposed to the air). XPS reveals the rate of Al oxidation for different MgF2 thicknesses as determined from measurements obtained from 5 min to 8 months of air exposure. The degree of Al oxidation depends on the MgF2 over layer thickness.

BYU Authors: David D. Allred, J. Gabriel Richardson, and R. Steven Turley, published in Optical Interference Coatings 2019, (Santa Ana Pueblo, NM, June 2019).
While no solid barrier layer is transparent below ~103nm, simulations show that ~9.5nm LiF on 8.5nm MgF2 on Al could reflect some hydrogen Lyman lines better than a single fluoride layer does. Experiments are promising.
BYU Authors: Joseph B. Muhlestein, Benjamin D. Smith, Margaret Miles, Stephanie M. Thomas, Anthony Willey, David D. Allred, and R. Steven Turley, published in Opt. Express
We report optical constants of e-beam evaporated yttrium oxide Y2O3 thin films as determined from angle-dependent reflectance measurements at wavelengths from 5 to 50 nm. Samples were measured using synchrotron radiation at the Advanced Light Source. The experimental reflectance data were fit to obtain values for the index of refraction and thin film roughness. We compare our computed constants with those of previous researchers and those computed using the independent atom approximation from the CXRO website. We found that the index of refraction near 36 nm is much lower than previous data from Tomiki as reported by Palik. The real part of the optical constants is about 10% to 15% below CXRO values for wavelengths between 17 nm and 30 nm. Films were also characterized chemically, structurally, and optically by ellipsometry and atomic force microscopy.
BYU Authors: Nathan D. Powers, Dallin S. Durfee, and David D. Allred, published in 2018 Conference on Laboratory Instruction Beyond the First Year of College, Part of the BFY Conference series, (Baltimore, MD, July, 2018).
There is a natural tendency for students to act first (e.g. - build and conduct experiments) and think later (e.g. - outline goals, identify challenges, predict outcomes, etc.). This is often apparent in labs that include student design components. We have developed a lab course structure that teaches students how to develop their ideas and make plans before beginning an experiment by providing multiple opportunities for peer and instructor feedback. As a result, we have seen significant improvements in the success rate and quality of student-designed experiments and presentations. We provide a detailed explanation of the course structure and rubrics and evidence of the impacts of this course structure.
BYU Authors: Michael Greenburg, David D. Allred, and R. Steven Turley, published in J. Utah Acad. Sci.

We report on the results of computationally designing and optimizing

multilayer mirrors for broadband reflectivity in a region spanning from

the vacuum ultraviolet to the infrared. Such a mirror would open up

new possibilities for future space observatories. Because of the

immense number of possible layer combinations and thicknesses for a

multilayer mirror, we automated the mirror selection process with a

genetic algorithm. Starting with a random object population within the

simulation, a genetic algorithm iteratively selects and mutates the best

portion of a population of objects that fit given design criteria to create

a new population; this can be repeated as many times as desired. Our

genetic algorithm yielded a high broadband reflectance mirror, which

was then optimized via gradient search within the program. We found

that placing a few layers under an aluminum coating can significantly

increase extreme ultraviolet reflectivity, which would give access to

important spectral lines such as that of the dominant He-II transition.

BYU Authors: David D. Allred, R. Steven Turley, Stephanie M. Thomas, Spencer G. Willett, Michael J. Greenburg, and Spencer B. Perry, published in Proc. SPIE
Protective layers on aluminum mirror surfaces which can be removed via the use of atomic hydrogen or hydrogen plasmas at the point of use in space may allow an expansion of broad-band mirrors into the EUV. LUVOIR (large, UV-optical-IR telescope) is a potential NASA flagship space-based observatory of the 2020’s or 30’s. It would utilize the largest mirrors ever flown1 . Their reflective coating will almost certainly be aluminum, since such telescopes would profit from truly broad-band mirrors. To achieve reflectance over the broadest band, the top surface of such aluminum mirrors, however, needs to be bare, without the oxide layers that naturally form in air. This will open the 11 to 15 eV band. Since thin aluminum films are largely transparent between 15 and 70 eV an EUV mirror under the aluminum could make EUV bands such as 30.4 nm available for space-based astrophysics without sacrificing mirror IR, visible and UV reflectance. The local space environment for the observatory is sufficiently oxygen-free that the surface should remain bare for decades. We discuss protecting as-deposited aluminum mirrors with robust, oxygenimpenetrable, barrier layers applied in vacuo to the aluminum immediately after deposition and before air contact. The goal is that the barrier could also be cleanly, and relatively easily, removed once the mirror is in space. We propose hydrogen atoms as the means for removing the overcoat, since they can be expected to meet the criteria that the means is gentle enough to not roughen the mirror surface, and does not redeposit material on the mirror or other spacecraft components. We have investigated both organic and inorganic (such as, a-Si) hydrogen-removable films that can be applied to the aluminum immediately after its deposition have been investigated. We also examined the REVAP technique, using Cd and Zn. Agglomeration limited their effectiveness as barrier layers. That and dealing with the reevaporated atoms may limit their utility as barrier materials.
BYU Authors: David D. Allred, published in J. Nucl. Mater.

Oxidation of a 1000 Å sputter-deposited thorium thin film at 150 °C in 100 ppm of flowing oxygen in argon produces the long-sought solid form of thorium monoxide. Changes in the scattering length density (SLD) distribution in the film over the 700-min experiment measured by in-situ, dynamic neutron reflectometry (NR) shows the densities, compositions and thickness of the various thorium oxides layers formed. Screened, hybrid density-functional theory calculations of potential thorium oxides aid interpretation, providing atomic-level picture and energetics for understanding oxygen migration. NR provided evidence of the formation of substoichiometric thorium oxide, ThOy (y < 1) at the interface between the unreacted thorium metal and its dioxide overcoat which grows inward, consuming the thorium at a rate of 2.1 Å/min while y increases until reaching 1:1 oxygen-to-thorium. Its presence indicates that kinetically-favored solid-phase ThO can be preferentially generated as a majority phase under the thermodynamically-favored ThO2 top layer at conditions close to ambient.

BYU Authors: Matthew R. Jones, Dale R. Tree, and David D. Allred, published in J. Quant. Spectrosc. Radiat. Transf.
An optical fiber thermometer consists of an optical fiber whose tip is coated with a highly conductive, opaque material. When heated, this sensing tip becomes an isothermal cavity that emits like a blackbody. This emission is used to predict the sensing tip temperature. In this work, analytical and experimental research has been conducted to further advance the development of optical fiber thermometry. An inexpensive optical fiber thermometer is developed by applying a thin coating of a high-temperature cement onto the tip of a silica optical fiber. An FTIR spectrometer is used to detect the spectral radiance exiting the fiber. A rigorous mathematical model of the irradiation incident on the detection system is developed. The optical fiber thermometer is calibrated using a blackbody radiator and inverse methods are used to predict the sensing tip temperature when exposed to various heat sources.
BYU Authors: Lawrence K. Barrett, Dallin J. Barton, Steven G. Noyce, David D. Allred, Richard R. Vanfleet, and Robert C. Davis, published in J. Microelectromech. Syst.
High-aspect-ratio metallic microstructures have a variety of potential applications in sensing and actuation. However, fabrication remains a challenge. We have fabricated nickel microstructures with over 20:1 aspect ratios by electroplating patterned carbon-coated carbon-nanotube forests using a nickel chloride bath. Pulse plating allows nickel ions to diffuse into the interior of the forest during off portions of the cycle. Done properly, this solves the problem of the formation of an external crust, which otherwise blocks nickel deposition in the interior of the structures. Thus, densities of 86 ± 3% of bulk Ni for the composite structures are achieved. Cantilever structures do not yield under load, but break. Measurements of the material properties of this composite material indicate an elastic modulus of ~42 GPa and a strength of 400 MPa. We demonstrate the utility of this method with an external field magnetic actuator consisting of a proof mass and two flexures. We achieved 1-mN actuation forces. [2014-0274]
BYU Authors: David D. Allred, published in J. Phys. Chem. C
Detecting and understanding the complex signatures of species for attribution of highly enriched uranium, HEU, is challenging even though these compounds have been intensively studied for 65 years. Attempts to obtain, for example, chemical speciation signatures on uranium oxides are frustrated by the presence of extremely diverse phases, complex structures, and their tendency to form solid solutions with the coexistence of many nonstoichiometric oxides. More importantly, the spectroscopic signatures of many of these oxides, using common techniques such as X-ray diffraction or Raman scattering, are remarkably similar with each other. On the other hand, the effort to understand the U-O system also exhibits some of the most intriguing and challenging properties in theoretical and computational chemistry. This is due to the spatial extent between localization and delocalization of the 5f orbitals of the uranium atom. In this article, spectroscopic ellipsometry (SE) measurements and a comparison of six fitting methods as well as theoretical calculations are combined to examine the intrinsic electronic structure and the corresponding band gap of uranium oxides to determine the chemical speciation in a,102 nm thick reactively sputtered uranium oxide film. The SE results reveal that the UOx film exhibits two absorption edges, a primary absorption edge slightly above 2.6 eV and a secondary absorption at 1.7-1.8 eV. The optical band gaps compared with the theoretical calculations performed on UO2, U4O9, U3O7, alpha-U3O8, alpha-UO3, delta-UO3, and gamma-UO3 suggest that the UOx film is composed of at least two components; the primary absorption is caused by the alpha-UO3 sublayer, which is superimposed on top of an adjacent alpha-U3O8 sublayer that is hypothesized to be heteroepitaxial growth of alpha-U3O8 along the UOx/substrate interface. Comparison to the ellipsometry measurements shows that the DFT+U and hybrid (HSE) calculations predict the correct trend for band gaps as a function of oxidation state and crystallography but they fail to capture the exact gaps. However, they provide important information for interpretation of the experimental results and highlight some of the structural complexity that prevails in the UOx compounds. The combination of theoretical and experimental methods to examine the intrinsic electronic structure and the band gap of the corresponding uranium oxides could benefit from the development of new methods for better distinguishing chemical speciation in uranium oxides. In addition, the experimental measurement of the indirect band gap of alpha-U3O8, is, to our knowledge, reported for the first time.
BYU Authors: D. D. Allred, published in Anal. Chem.
A unique approach to detect chemical speciation and distribution on nanometer-scale nuclear materials has been achieved by the combination of neutron reflectometry and shell-isolated surface-enhanced Raman spectroscopy. Both surface and underlying layers of the uranium oxide materials were determined with angstrom-level resolution. Our results reveal that the UOx film is composed of three sublayers: an, similar to 38 angstrom thick layer of U3O8 formed along the UOx/substrate interface; the adjacent sublayer consists of an similar to 900 angstrom thick single phase of alpha-UO3, and the top layer is gamma-UO3 with a thickness of similar to 115 angstrom.
BYU Authors: Kellen Moulton, Nicholas B. Morrill, Adam M. Konneker, Brian D. Jensen, Richard R. Vanfleet, David D. Allred, and Robert C. Davis, published in J. Micromech. Microeng.
This paper examines the effect of iron catalyst thickness on the straightness of growth of carbon nanotubes (CNTs) for microelectromechanical systems fabricated using the CNT-templated-microfabrication (CNT-M) process. SEM images of samples grown using various iron catalyst thicknesses show that both straight sidewalls and good edge definition are achieved using an iron thickness between 7 and 8 nm. Below this thickness, individual CNTs are well aligned, but the sidewalls of CNT forests formed into posts and long walls are not always straight. Above this thickness, the CNT forest sidewalls are relatively straight, but edge definition is poor, with significantly increased sidewall roughness. The proximity of a device or feature to other regions of iron catalyst also affects CNT growth. By using an iron catalyst thickness appropriate for straight growth, and by adding borders of iron around features or devices, a designer can greatly improve straightness of growth for CNT-MEMS.
BYU Authors: J. A. Brame, J. E. Goodsell, and D. D. Allred, published in J. Utah Acad. Sci.
A test structure for a prototype nanoscale magnetometer exploiting the strain sensitivity of single-walled carbon nanotubes (SWCNTs) has been fabricated. The nanotube magnetometer would boast reduced dimensions, mass, and power requirements compared with a Fluxgate magnetometer. Dramatic resistance increase with strain has been previously reported for individual nanotubes, and this magnetometer design concept seeks to extend this strainresistance property to an "asgrown" ensemble of SWCNTs. Measurements of a test structure show a correlation between applied magnetic field and device conductivity. This correlation indicates an increase in conductivity with strain to the network of nanotubes; candidate mechanisms for this behavior are discussed.
BYU Authors: Heidi M. Dumais, R. Steven Turley, and David D. Allred, published in J. Utah Acad. Sci.
We measured the transmittance and reflectance of two reactively sputtered diodes prepared with approximately 20 nm of UOx in the extreme ultraviolet (XUV) at the Advanced Light Source at Lawrence Berkeley National Laboratory. Fitting the reflectance data to the Parratt model yielded the thickness of the UOx film. This thickness combined with a simple analysis of the transmission measurements provides estimates for the imaginary part of the index of refraction for UOx at approximately every tenth of a nanometer from about 3 nm to 30 nm with emphasis in the 12- to 13-nm range. The analysis discussed in this paper yields only the imaginary part of the complex index but will lead to a more robust analysis to find both the real and imaginary parts of the index of refraction. These values provide researchers with information for modeling, design, and fabrication of optical systems in the extreme ultraviolet.
BYU Authors: Lei Pei, Amy Balls, Cary Tippets, Jonnathan Abbott, Matthew R. Linford, David D. Allred, Richard R. Vanfleet, and Robert C. Davis, published in J. Vac. Sci. Technol. A
Here, the authors report the fabrication of transparent polymer templates for nanostructured amorphous siliconphotovoltaics using low-cost nanoimprint lithography of polydimethylsiloxane. The template contains a square two-dimensional array of high-aspect-ratio nanoholes (300 nm diameter by 1 μm deep holes) on a 500×500 nm2 pitch. A 100 nm thick layer of a-Si:H was deposited on the template surface resulting in a periodically nanostructured film. The optical characterization of the nanopatterned film showed lower light transmission at 600–850 nm wavelengths and lower light reflection at 400–650 nm wavelengths, resulting in 20% higher optical absorbance at AM 1.5 spectral irradiance versus a nonpatterned film.
BYU Authors: Bryan Hicks and David Allred, published in Thin Solid Films
Single-walled carbon nanotubes (SWCNT) were prepared using iron catalysts deposited by indirect evaporation on silicon substrate covered with 500 nm-thick thermal oxide. Diode SWCNT devices have been fabricated using Au and Al, as the asymmetric metal contacts, and a random network of metallic and semiconducting nanotubes as the device channel. No effort was made to align the SWCNTs or to eliminate metallic nanotubes in our devices. Asymmetric voltage-current behavior was seen. Current rectification was observed in the source-drain bias range of -3V to +3 V. Rectification was somewhat surprising since, although metallic tubes are in the minority (similar to 1/3), they could potentially act as shunts and mask the electric properties of the semiconducting majority. No correlation between electrode spacing and current rectification was observed. The lowest leakage current measured was 1% of the maximum current carrying capacity. Maximum forward-biased current capacities range between 8 mu A and 841 mu A. (C) 2009 Elsevier B.V. All rights reserved.
BYU Authors: Nicole Brimhall, Nicholas Herrick, David D. Allred, R. Steven Turley, Michael Ware, and Justin Peatross, published in Appl. Optics

We use a laser high-harmonics-based extreme-ultraviolet (EUV) polarimeter to determine the optical constants of elemental uranium in the wavelength range from 10 to 47 nm. The constants are extracted from the measured ratio of p-polarized to s-polarized reflectance from a thin uranium film deposited in situ. The film thickness is inferred from a spectroscopic ellipsometry measurement of the sample after complete oxidation in room air. Uranium has been used as a high-reflectance material in the EUV. However, difficulties with oxidation prevented its careful characterization previous to this study. We find that measured optical constants for uranium vary significantly from previous estimates. (C) 2010 Optical Society of America

BYU Authors: Nicole Brimhall, Nicholas Herrick, David D. Allred, R. Steven Turley, Michael Ware, and Justin Peatross, published in Opt. Express
We use laser high-order harmonics and a polarization-ratio-reflectance technique to determine the optical constants of copper and oxidized copper in the wavelength range 10-35 nm. This measurement resolves previously conflicting data sets, where disagreement on optical constants of copper in the extreme ultraviolet most likely arises from inadvertent oxidation of samples before measurement. (C) 2009 Optical Society of America
BYU Authors: R. Steven Turley, David Allred, Anthony Willey, Joseph Muhlestein, and Zephne Larsen, published in J. Utah Acad. Sci.
Optics in the extreme ultraviolet (XUV) have important applications in microelectronics, microscopy, space physics, and in imaging plasmas. Because of the short wavelengths involved in these applications, it is critical to account for interfacial roughness to accurately predict the reflection and absorption of XUV optics. This paper examines two possible effects of roughness on optical absorption, non-specular reflection and enhanced transmission and compares these to measured experimental data on a rough Y2O3 thin film.
BYU Authors: Elisabeth (Liz) Strein and David Allred, published in Thin Solid Films
In the VUV spectrum we see a significant decrease in reflection due to organic contamination on the surface of mirrors. To study VUV mirrors it is requisite to have calibration standards. Such standards are useless as calibration tools if the surface has organic contamination. For our standard, we use a thermally oxidized silicon wafer with a 27 nm oxide overlayer. We found that silicon wafer samples capped with native oxide acquire 0.1 to 0.2 nm of organic contamination within two hours of being cleaned with stored in closed, but nonvacuum, conditions. After a week there is an additional 0.2 to 0,5 run deposition after which no further significant deposition is measured Lip to 90 days. We place the samples in air within one cm of a xenon excimer lamp that radiates 7.2 eV photons which remove half of the remaining contamination every minute. Five minutes exposure is sufficient to clean both fresh and stored samples. Data are determined using spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS). Additionally this paper addresses the need to ensure that these characterization tools are not a source of organic contamination. We determined that the antechamber of our XPS was contaminating samples at a rate of 0.6 nm/30 min as they waited for transfer to the analysis chamber. This contamination was virtually eliminated by attaching an oxygen radical source (ORS) device (Evactron (R) C De-Contaminator RF Plasma Cleaning System) directly to the antechamber. (C) 2008 Elsevier B.V. All rights reserved.
BYU Authors: E. Strein, D. Allred, and R. S. Turley, published in Society of Vacuum Coaters, 2008 Technical Conference, (April 2008, Chicago, IL), 448-442 (2008). 
We studied the cleaning of the native oxide surfaces of silicon wafers using variable-angle spectroscopic (multiwavelength) ellipsometry (SE) and x-ray photoelectron spectroscopy (XPS). We focused on removing surface contamination, while preserving the oxide layer and minimizing surface roughness. Five minute under a xenon excimer lamp in air was adequate to render “carbon free” (<0.05nm overlayer) oxide surfaces previously cleaned with detergents and/or solvents. We further investigated different ways of storing samples and how quickly carbon-containing contamination returns. With nonvacuum storage conditions, two hours after being cleaned an overlayer of 0.1 to 0.2nm reappeared on the surface as measured by SE. After a week in closed storage conditions an additional 0.2 to 0.4nm was deposited. Between the time span of a week and 90 days, we saw no further significant adventitious overlayer deposition. XPS indicates that the overcoat is most probably hydrocarbon/organic with significant oxygen content. We observed that (impure) vacuum storage can contaminate samples more than air. We traced instrumental hydrocarbon contamination to our XPS antechamber and show how attaching a commercial low-pressure oxygen radical source removes the bulk of the contamination.
BYU Authors: Guillermo Acosta, Richard Vanfleet, David D. Allred, and R. S. Turley, published in Society of Vacuum Coaters, 2008 Technical Conference, (April 2008, Chicago, IL), 448-442 (2008). 
When considering the optical performance of thin films in the Extreme Ultraviolet (EUV), developing an accurate physical description of a thin film coating is necessary to be able to successfully model optical performance. With the short wavelengths of the EUV, film interfaces and sample roughness warrant special attention and care. The surfaces of thin film samples are routinely measured by Atomic Force Microscopy, from which roughness can be determined. However, characterizing the quality of interfaces below the surface is much more challenging. In a recent study of scandium oxide thin films, High Resolution Transmission Electron Microscopy and Annular Dark Field Scanning Transmission Electron Microscopy (ADF STEM) were used to study the cross section of the samples. ADF STEM data analyzed along a path into the volume of the sample (normal to the interfaces) reveals information of sample density versus depth. This density-depth profile reflects the presence of subsurface film interfaces in the volume of the sample. Additionally, information from the ADF STEM profile can be used to gauge the roughness of the subsurface interfaces, which is used to refine the sample description during modeling. We believe this is the first use of ADF STEM in this capacity. This characterization technique may provide key insight to subsurface interface quality, which is particularly important when optimizing the performance of multilayer coatings in the EUV.
BYU Authors: Nicole Brimhall, Matthew Turner, Nicholas Herrick, David D. Allred, R. Steven Turley, Michael Ware, and Justin Peatross, published in Rev. Sci. Instrum.
We describe an extreme-ultraviolet (EUV) polarimeter that employs laser-generated high-order harmonics as the light source. The polarimeter is designed to characterize materials and thin films for use with EUV light. Laser high harmonics are highly directional with easily rotatable linear polarization, not typically available with other EUV sources. The harmonics have good wavelength coverage, potentially spanning the entire EUV from a few to a hundred nanometers. Our instrument is configured to measure reflectances from 14 to 30 nm and has similar to 180 spectral resolution (lambda/Delta lambda). The reflection from a sample surface can be measured over a continuous range of incident angles (5 degrees-75 degrees). A secondary 14 cm gas cell attenuates the harmonics in a controlled way to keep signals within the linear dynamic range of the detector, comprised of a microchannel plate coupled to a phosphorous screen and charge coupled device camera. The harmonics are produced using similar to 10 mJ, similar to 35 fs, and similar to 800 nm laser pulses with a repetition rate of 10 Hz. Per-shot energy monitoring of the laser discriminates against fluctuations. The polarimeter reflectance data agree well with data obtained at the Advanced Light Source Synchrotron (Beamline 6.3.2).
BYU Authors: Jacqualine Jackson Butterfield and David D. Allred, published in J. Utah Acad. Sci.
Whereas the real part of the refractive index is dependent on both transmittance and reflectance, the imaginary part can be determined from transmittance data alone. It is possible to use Kramers-Kronig analysis to calculate the real part if the imaginary part is known over a sufficiently broad range. We show that the delta calculated from reflection and transmission data without taking into account roughness may underestimate the real part of the refractive index of the scandium oxide samples we are studying by up to 40% near 270 eV.
BYU Authors: Jonathan Goodsell, Jon Brame, and David D. Allred, published in J. Utah Acad. Sci.
We report on the iron-catalyzed, CVD growth of carbon nanotubes on a variety of substrates and their subsequent analysis. We discuss the use of “indirect evaporation” to prepare the required iron catalyst. The optimum temperature for growth of nanotubes mats was between 925 and 950 degrees C. These were shown by transmission electron microscopy to contain single-walled carbon nanotubes (SWCNTs). We examined the results of the deposition in scanning electron microscopy and saw what appeared to be bright spots randomly arrayed along the carbon nanotube fibers. We termed this pattern ‘beads on a string’. We present evidence that the bright features are not associated with the nanotubes directly but are iron/carbon clusters that happen to lie on the surface of the substrate near where the tubes fell during or after deposition. We have shown that the deposition procedure is fairly robust and growth results have been reproduced using Goddard Space Flight Center (GSFC)-deposited catalyst and CVD facilities at Brigham Young University (BYU).
BYU Authors: Jon Brame, Johnathan Goodsell, and David D. Allred, published in MRS Proc.
We have fabricated flexible electronic devices to test the strain-based change in resistance of a network of single-walled carbon nanotubes (SWCNTs) for use in microscale, high resolution magnetometry. To do this, we first develop a simple, reliable method to obtain catalyst nanoparticles for carbon nanotube growth through indirect, thin-film evaporation. Next we fabricate a two-terminal SWCNT device on a rigid substrate. We then transfer the device, intact, to a flexible substrate for strain testing. Herein, we report progress in growth and measurement techniques.
BYU Authors: William R. Evans and David D. Allred, published in Thin Solid Films
We used spectroscopic ellipsometry to determine the optical constants of seven ThO2 thin-film samples, thickness ranging between 28 and 578 nm, for the spectral range of 1.2 to 6.5 eV. The samples were deposited by biased radio-frequency sputtering at DC bias voltages between 0 and - 68 V. The index of refraction, it, does not depend on bias voltage, sputter pressure, deposition rate, or thickness. Specifically, the value of 11 at 3 eV is 1.86 +/- 0.04 for the unbiased samples and 1.86 +/- 0.04 for the biased samples. The average value of n at 3 eV for the thicker samples (d >= 50 nm) was 1.87 +/- 0.05, and 1.85 +/- 0.02 for the thinner samples (d <= 50 nm). (c) 2006 Elsevier B.V. All rights reserved.
BYU Authors: D. D. Allred, G. A. Acosta, N. Farnsworth-Brimhall, and R. S. Turly, published in Proc. SVC
A technique for analysis of reflection and transmission data from thin films in the Extreme Ultraviolet (EUV) has been developed. Efforts to deposit films on polyimide membranes to allow for transmission measurements are discussed. Later experiments use thin films of interest deposited directly onto photodiode detectors. This coated photodiode is used for making transmission measurements, and use of a second, uncoated detector allows for the simultaneous collection of reflection data as well. The development of this technique and the analysis of data from thorium thin films and scandium oxide thin films are presented here. EUV measurements were made at the Advanced Light Source (ALS), Beamline 6.3.2, in Berkeley, California.
BYU Authors: Nicole F. Brimhall, Amy B. Grigg, R. Steven Turley, and David D. Allred, published in Proc. SPIE
We have measured the reflectance and transmittance of thorium dioxide thin films from 50-280 eV. We have developed several methods for fitting this data that gives the most reliable values for the complex index of refraction, n = 1 - δ + iβ. These fitting methods included fitting film thickness using interference fringes in highly transmissive areas of the spectrum and fitting reflectance and transmittance data simultaneously. These techniques give more consistent optical constants than solitary unconstrained fitting of reflectance as a function of angle. Using these techniques, we have found approximate optical constants for thorium dioxide in this energy range. We found that the absorption edges of thoria were shifted 4 eV and 2 eV to lower energies from those of thorium. We also found that the peak in δ was shifted by 3 eV to lower energy from that of thorium.
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.
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.
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.
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.
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.
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.
BYU Authors: Luke J. Bissel, David d. Allred, R. Steven Turley, William R. Evans, and Jed. E. Johnson, published in Proc. SPIE
Ruthenium is one material that has been suggested for use in preventing the oxidation of Mo/Si mirrors used in extreme ultraviolet (EUV) lithography. The optical constants of Ru have not been extensively studied in the EUV. We report the complex index of refraction, 1 - δ + iβ, of sputtered Ru thin films from 11-14 nm as measured via reflectance and transmission measurements at the Advanced Light Source at Lawrence Berkley National Laboratory. Constants were extracted from reflectance data using the reflectance vs. incidence angle method and from the transmission data by Lambert"s law. We compare the measured indices to previously measured values. Our measured values for delta are between 14-18% less than those calculated from the atomic scattering factors (ASF) available from the Center for X-ray Optics (CXRO). Our measured values of beta are between 5-20% greater than the ASF values.
BYU Authors: Richard L. Sandberg, David d. Allred, Shannon Lunt, Marie K. Urry, and R. Steven Turley, published in Proc. SPIE
Uranium oxide and uranium nitride thin films reflect significantly more than all previously known/standard reflectors (e.g., nickel, gold, and iridium) for most of the 4-10 nm range at low angles of incidence. This work includes measurements of the EUV/soft x-ray (2-20 nm) reflectance of uranium-based thin films (~20 nm thick) and extraction of their optical constants (δ and β). We report the reflectances at 5, 10, and 15 degrees grazing incidence of air-oxidized sputtered uranium, reactively sputtered (O2) uranium oxide, and reactively sputtered (N2) uranium nitride thin films measured at Beamline 6.3.2 at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL). Additionally, we report optical constants of reactively sputtered uranium oxide at nine wavelengths from 4.6 to 17.5 nm derived from ALS angle-scan reflectance measurements. We also report optical constants of uranium nitride at 13 and 14 nm. We compare the reflectance of these uranium-compound thin films to gold, nickel (and nickel oxide), and iridium thin films from 2.5 to 11.6 nm. These metal thin films were chosen for comparison due to their wide use in EUV/soft x-ray applications as low-angle, thin-film reflectors. The uranium compounds can exhibit some surface oxidation in ambient air. There are important discrepancies between UO2"s and UN"s actual thin-film reflectance with those predicted from tabulated optical constants of the elemental constituents of the compounds. These differences are also demonstrated in the optical constants we report. Uranium-based optics applications have important advantages for zone plates, thin-film reflectors, and filters.
BYU Authors: Richard L. Sandberg, David D. Allred, Luke J. Bissell, Jed E. Johnson, and R. Steven Turley, published in AIP Conf. Proc.
We present the measured reflectances (Beamline 6.3.2, ALS at LBNL) of naturally oxidized uranium and naturally oxidized nickel thin films from 100-460 eV (2.7 to 11.6 nm) at 5 and 15 degrees grazing incidence. These show that uranium, as UO2, can fulfill its promise as the highest known single surface reflector for this portion of the soft x-ray region, being nearly twice as reflective as nickel in the 124-250 eV (5-10 nm) region. This is due to its large index of refraction coupled with low absorption. Nickel is commonly used in soft x-ray applications in astronomy and synchrotrons. (Its reflectance at 10° exceeds that of Au and Ir for most of this range.) We prepared uranium and nickel thin films via DC-magnetron sputtering of a depleted U target and resistive heating evaporation respectively. Ambient oxidation quickly brought the U sample to UO2 (total thickness about 30 nm). The nickel sample (50 nm) also acquired a thin native oxide coating (<2nm). Though the density of U in UO2 is only half of the metal, its reflectance is high and it is relatively stable against further changes.
BYU Authors: R. E. Robinson, R. L. Sandberg, D. D. Allred, J. E. Johnson, W. Evans, T. Doughty, K. Adamson, and A. Jacquier, published in Society of Vacuum Coaters, 2004 Technical Conference
The extreme ultraviolet (EUV) is becoming increasingly important. Principal applications include orbital space-based astronomy and lithography for integrated circuit computer chips. A main impediment to further development of efficient mirrors is the lack of reliable optical constants for various materials in this region of the electromagnetic spectrum. One reason for the unreliability of the optical constants is that the sample surfaces are often contaminated with foreign material, especially organic compounds, when exposed to laboratory air. Several cleaning techniques were evaluated, namely: 1) strippable solid optical cleaner (Opticlean®); 2) oxygen plasma etch; 3) high energy UV light/ozone; 4) strippable coating followed by oxygen plasma etch, 5) strippable coating followed by high intensity UV light and, 6) exposure to cold pressurized carbon dioxide (CO2 snow). These processes are compared experimentally based on effectiveness, cleaning time and ease of use. DADMAC (polydiallyldimethyl-ammonium chloride), which forms a layer of known, uniform thickness on silicon wafers is used as a “stand in” for organic contamination. Effectiveness is judged on how well the surface is cleaned. Ellipsometry is used to determine the thicknesses of surface layers. XPS (X-ray Photoelectron Spectroscopy) is used to look for trace contaminants, particularly carbon from the DADMAC. We find that the strippable cleaning coat leaves a residue. Oxygen plasma rapidly removes contaminants, but can quickly oxidize the silicon surface. Exposure to the UV light/ozone for five minutes leaves the surface clean with little additional oxidation. Oxygen plasma or UV light effectively removes the strippable coat residue. Exposure to cold pressurized carbon dioxide has reduced oxide level in one case. The recommended procedure for cleaning bare silicon wafers is strippable coat application followed by 2.5 minutes of exposure to high intensity UV light.
BYU Authors: Richard L. Sandberg, David D. Allred, Jed E. Johnson, and R. Steven Turley, published in Proc. SPIE
We present the measured reflectances (Beamline 6.3.2, ALS at LBNL) of naturally oxidized uranium and naturally oxidized nickel thin films from 2.7 to 11.6 nm at 5°, 10°, and 15° grazing incidence. These show that uranium, as UO2, can fulfill its promise as the highest known single surface reflector for this portion of the soft x-ray region, being nearly twice as reflective as nickel in the 5-10 nm region. This is due to its large index of refraction coupled with low absorption. Nickel is commonly used in soft x-ray applications in astronomy and synchrotrons. (Its reflectance at 10° exceeds that of Au and Ir for most of this range.) We prepared uranium and nickel thin films via DC-magnetron sputtering of a depleted U target and resistive heating evaporation respectively. Ambient oxidation quickly brought the U sample to UO2 (total thickness about 30 nm). The nickel sample (50 nm) also acquired a thin native oxide coating (<2nm). Though the density of U in UO2 is only half of the metal, its reflectance is high and it is relatively stable against further changes. There are important discrepancies between UO2"s actual reflectance with those predicted by the atomic scattering factor model indicative of the need to determine the actual constants of UO2.
BYU Authors: David D. Allred, published in Dialogue
BYU Authors: David D. Allred, Matthew B. Squires, and R. Steven Turley, published in Proc. SPIE
The reported optical constants of uranium differ from that of vacuum significantly more than other elements do over the range of about 150 to 350 eV. This suggests that uranium could be used to produce high reflectance imaging mirrors for many soft x-ray applications. Elemental uranium is too chemically active to be used as a front surface mirror without protection. We computed the expected reflectance of carbon-coated uranium films and of uranium-nickel alloys for low-angle reflectors. Carbon is mostly transparent below its K absorption edge at about 283 eV. The reflectance at 10 degrees from grazing is computed to be greater than 50% at 277 eV (C Kα). For comparison, about 5 degrees is the maximum grazing incidence angle for which conventional materials are computed to have comparable reflectance. We sputter deposited and measured the reflectance of carbon-coated uranium layers at 44.7 Å (C Kα). Sample reflectance was a factor of two greater than that of nickel, the material used for low-angle mirrors. The initial oxidation behavior of sputtered uranium-nickel alloys is similar to pure U so their reflectance was not determined. Coatings based on uranium should be considered for all applications where high-reflectance, broadband, low-angle soft x-ray mirrors are required
BYU Authors: David Allred, published in Fifth International Mars Society Conference, (August 2002, Boulder, CO).
We have prepared our own very small, Martian test tubes and flasks which possess many of the conditions of Mars’ surface (up to the first three of five conditions listed below) and at a cost well below $100**. We did several experiments of interest to us, namely: lightening on Mars in a jar (12-13 year olds) and what might happen to ice cream on Mars (an 8 year old). Student, amateur, and professional researchers alike need Mars-like conditions in which to test their ideas and to help answer their questions. It has been proposed that vacuum stations like those used in industry and university labs could be employed to produce Mars-like conditions to test student ideas for whole classrooms of children.1 It is our goal to find simpler and lower cost ways to help researchers set up their own small test stations where they can test out their ideas. The test stands need only meet those characteristics of Mars which are pertinent for testing the ideas.
BYU Authors: M. B. Squires, David D. Allred, and R. Steven Turley, published in J. Utah Acad. Sci.
BYU Authors: Shannon Lunt, R. Steven Turley, and David D. Allred, published in J. X-ray Sci. Technol.
We designed multilayer mirrors for the IMAGE/Explorer mission that were intended to reflect well at 304 * and poorly at 584 *. The best designs utilized the novel materials U and Y_2O_3with Al or Si as spacer layers. The highest design reflectivities were obtained with aperiodic multilayers, although these were too hard to grow in practice. We found these novel designs using a genetic algorithm calling on a database of 43 promising materials with the freedom of aperiodic multilayer stacks.
BYU Authors: D. D. Allred and R. S. Turley, published in Space Sci. Rev.
The Extreme Ultraviolet Imager (EUV) of the IMAGE Mission will study the distribution of He+ in Earth's plasmasphere by detecting its resonantly-scattered emission at 30.4 nm. It will record the structure and dynamics of the cold plasma in Earth's plasmasphere on a global scale. The 30.4-nm feature is relatively easy to measure because it is the brightest ion emission from the plasmasphere, it is spectrally isolated, and the background at that wavelength is negligible. Measurements are easy to interpret because the plasmaspheric He+ emission is optically thin, so its brightness is directly proportional to the He+ column abundance. Effective imaging of the plasmaspheric He+ requires global 'snapshots' in which the high apogee and the wide field of view of EUV provide in a single exposure a map of the entire plasmasphere. EUV consists of three identical sensor heads, each having a field of view 30 degrees in diameter. These sensors are tilted relative to one another to cover a fan-shaped field of 84 degrees x30 degrees, which is swept across the plasmasphere by the spin of the satellite. EUV's spatial resolution is 0.6 degrees or similar to 0.1 R-E in the equatorial plane seen from apogee. The sensitivity is 1.9 count s(-1) Rayleigh(-1), sufficient to map the position of the plasmapause with a time resolution of 10 min.
BYU Authors: David D. Allred, R. Steven Turley, and Matthew B. Squires, published in Proc. SPIE
We have developed a new family of EUV multilayer mirror coatings using uranium. Using this approach we have coated a set of six mirrors for the EUV Imager, a component of the IMAGE mission. This mission is a Medium Explorer (MIDEX) program, which is scheduled for launch early in 2000. The EUV Imager will study the distribution of He+ in the Earth's plasmasphere by detecting its resonantly scattered emission at 30.4 nm (41 eV) and will produce images of the structure and dynamics of the cold plasma on a global scale. There is, however, a bright emission at 58.4 nm (21 eV), which comes from neutral helium in the earth's ionosphere which also must be blocked. These photons are at too high an energy to filter with aluminum but at too low an energy to have negligible reflectance from most materials commonly used in EUV mirrors. Thus, a multilayer system which satisfied two optical functions, high reflectance (greater than 20%) at 41 eV and low reflectance (less than 2%) at 21 eV, were designed and successfully fabricated. Such mirrors with dual optical functions in the soft x-ray/EUV had not previously been designed or built. These specifications were particularly challenging because many materials have higher single layer reflectances at 58.4 nm than at 30.4 nm. Essentially, the mirror must have low reflectance at 21 eV without loss of reflection at 30.4 nm. This was accomplished. The top part of the multilayer, which reflects well at 30.4 nm, also acts as antireflection layers at 58.4 nm. In the past, multilayers usually have consisted of periodic bilayers. We have explored the use of a periodic mirrors in place of the standard periodic designs. Along the way we have created the computational tools, which include genetic algorithms, to optimize selection of materials and thicknesses. We are currently in the process of building up an EUV characterization system and developing a general way of measuring the optical constants of air-sensitive thin films. We discuss the other material and fabrication challenges faced, which include: (1) The high absorption of almost everything in the EUV. This means that only a few interfaces in a multilayer will contribute to its reflectance. (2) Surface contamination and corrosion. (3) The deposition on flight mirrors that are highly curved (f equals 0.8).
BYU Authors: Matthew B. Squires, David D. Allred, and R. Steven Turley, published in Proc. SPIE
This paper is a report on our effort to use reflectance measurements of a set of amorphous silicon (a-Si) and uranium (U) multilayer mirrors with an uranium oxide overcoat to obtain the optical constants of a-Si and uranium. The optical constants of U, its oxides, and Si, whether crystalline or amorphous, at 30.4 and 58.4 nm in the extreme ultraviolet (EUV) are a source of uncertainty in the design of multilayer optics. Measured reflectances of multilayer mirror coatings do not agree with calculated reflectances using existing optical constants at all wavelengths. We have calculated the magnitude and the direction of the shift in the optical constants of U and a-Si from reflectivity measurements of DC magnetron sputtered a-Si/U multilayers at 30.4 and 58.4 nm. The reflectivity of the multilayers were measured using a UV hollow cathode plasma light source, a 1 meter VUV monochromator, a back-thinned CCD camera, and a channeltron detector. These reflectance measurements were verified by measurements made at LBNL. The reflectances of the multilayer coatings were measured at 14.5 degrees from normal to the mirror surface. The optical constants were calculated using IMD which uses CURVEFIT to fit the optical constants to reflectivity measurements of a range of multilayer mirrors that varied over a span of 150 - 25.0 nm bilayer thickness. The effects of surface oxide and roughness, interdiffusion, and interfacial roughness were numerically subtracted in fitting the optical constants. The (delta) , (beta) determined at 30.4 nm does not well match the values of c-Si published in the literature (HBOC1), but do approach those of a-Si as reported in literature (HBOC). The difference in the optical constants of c-Si and a-Si are larger than can be attributed to differences in density. Why the optical constants of these two materials vary at 30.4 remains an open question.
BYU Authors: D. D. Allred, published in Thin Solid Films
The design and performance of an electrochemical apparatus and, the process for the preparation of porous silicon with different controlled surface structures is described. The apparatus includes controlled rotation of the electrolyte vessel, which is in contact with a thermal bath. This permits the etching electrolyte to react with the silicon substrate at different temperatures and at different rates of renewal of the solution. Atomic force microscopy images show at least three different classes of samples according to the topographical features: samples with surface hillocks, samples with surface holes, and samples with a mixture of holes and hillocks. The different morphologies are important for various applications. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.
BYU Authors: G. B. Thompson and D. D. Allred, published in J. X-ray Sci. Technol.
We report the preparation and structural characterization of lithium hydride and lithium fluoride thin films. These materials, due to their low absorption in the soft x-ray range, may have a role as spacer layers in multilayer mirrors. Theoretical reflection calculations suggest that an epitaxial crystalline multilayer stack of a nitride and a lithium compound spacer layer could produce respectable reflectance for short soft x-ray wavelengths (λ < 10 nm). Lithium targets were magnetron sputtered in the presence of hydrogen or ammonia to prepare the LiH films and nitrogen trifluoride to prepare the LiF films. The films were deposited on room temperature Si (100) or MgO (100) substrates. A near IR-Visible-UV spectrometer indicated a drop in reflectance at ~250 nm for a 100-nm-thick LiH film. This corresponds to a 5-eV band gap (characteristic of LiH). UV fluorescence indicated characteristic LiH defect bands at 2.5, 3.5, and 4.4 eV. The UV fluorescence characterization also indicated a possible lithium oxide (Li2O) contamination peak at 3.1 eV in some of our thin films. Film surface morphology, examined by scanning electron microscopy, appeared extremely rough. The roughness size varied with reactive gas pressure and the type of substrate surface. A LiH/MoN multilayer was constructed, but no significant d spacing peak was seen in a low angle CuKα XRD scan. It is believed that the roughness of the LiH film prevented smooth, uniform planar growth of the multilayer stack. Possible reasons of rough growth are briefly discussed.
BYU Authors: William E. Evenson and David Allred, published in Phys. Rev. B
Using perturbed-angular-correlation (PAC) spectroscopy, via the Hf-181-->Ta-181 probe, we have measured Mn-site electric-field gradients (EFG's) at Ta nuclei in ceramic samples of LaMnO3. Two crystallographic phases coexist over a temperature interval of approximate to 16 K near the orthorhombic-to-rhombohedral transition at approximate to 724 K, which shows a thermal hysteresis of approximate to 1.7+/-0.2 K. Concurrently, in the two phases, we determined the temperature dependence of the EFG parameters, V-zz, eta, and delta, and the ratio of the probe concentrations A(1)/A(2) To explain the apparent coexistence of two phases in this weakly first-order transition, we present a model that assumes a spatial distribution of T-c values. This distribution could arise from a spatially nonuniform distribution of Mn4+ ions. We show the PAC technique to be a uniquely powerful probe of local symmetries that reflect the effects of a local-distribution of valences, which drive the phase transition.
BYU Authors: D. D. Allred, published in J. X-ray Sci. Technol.
We report the effect that thermal annealing in inert and oxidizing atmospheres, and with and without encapsulating layers, has on the structure of tungsten/carbon [W/C] multilayer thin films. This study focuses on the tungsten component and deals mainly with multilayers where the ratio of thickness of tungsten layers is equal to or greater than for the carbon layers (that is, γ ≤ 0.5). This is in contrast to prior studies where the tungsten layer thickness was generally held constant and the carbon layer was varied. Thermal annealing in inert atmospheres produces reactions and other structural changes in the tungsten and carbide layers which depend on the as-deposited multilayer structure which depends, in turn, on the thickness of the tungsten layer. In samples where both the tungsten and carbide fractions of the multilayer are completely amorphous as deposited, which is the case for thin tungsten layers (thickness of tungsten (tw) < 4 nm/period), the reactions in the tungsten layer forming crystalline tungsten and tungsten carbide occur at annealing temperatures above 900°C. The layer pair spacing, or period, (d), in this group shows an expansion of up to 10–15% of the original value as has been reported in the past. Changes in both the tungsten and carbide layers, and their interfaces, contribute to changes in d spacing and relative thickness of the high and low Z components. When the tungsten layer thickness exceeds 4 nm per period the tungsten is partially crystallized in as-prepared samples. In such multilayers interfacial reactions, producing an oriented partially crystalline W2C/C superlattice, occur at temperatures of 600°C and below. The fact that W2C crystallites in one period can form a structure which is correlated to W2C crystallites in neighboring layers is somewhat surprising, since layers are presumably still separated by amorphous carbon which is still visible via Raman. The expansion of the layer pair spacing is relatively small (<5%) in this group and, more importantly, mostly involves increases in the thickness of the high Z components. Samples annealed in air at temperatures below 300°C are progressively destroyed by the oxidation of both tungsten and carbide layers. Encapsulation of similar multilayers with a thin (30 nm) dielectric layer of any of several types can retard oxidation to 600°C. The silicon-containing encapsulants generally perform better. Failure at this temperature is seen to occur from pinhole formation.
BYU Authors: Steven E. Jones, David S. Shelton, R. Steven Turley, M. Jeannette Lawler, and David D. Allred, published in Hyperfine Interact.
We show that Raman spectral lines from H-2, D-2, T-2, HD, HT and DT are readily resolved, permitting an effective means to analyze isotopic hydrogen mixtures used in muon-catalyzed fusion experiments. We propose a Raman spectrographic system to allow for real-time analysis of targets involving all three isotopes of hydrogen.
BYU Authors: Qi Wang, D. D. Allred, and L. V. Knight, published in J. Raman Spectrosc.
A constrained non-linear deconvolution method was used to analyse the Raman spectrum of amorphous carbon. The method was tested by applying it to a model spectrum. Before the deconvolution the observed Raman spectrum was smoothed by a least-squares convoluting procedure. The proper width of the spread function used in the deconvolution was determined by examining the reversibility of the deconvolution process. The deconvolution result for the Raman spectrum of amorphous carbon has a form very similar to the phonon density of states of graphite. This is taken to be additional evidence for Shuker and Gamon's model for Raman spectra of amorphous materials. Potentially, the deconvolution method can also be used for other aspects of Raman spectrum analysis of amorphous materials, such as finding the relative intensity of each peak of study the structure of the material.
BYU Authors: Wang Qi, David D. Allred, and Larry V. Knight, published in Phys. Rev. A
Measurements of the circular polarization of Balmer-α radiation emitted by excited hydrogen atoms, following the transmission of (20-50)-keV protons through thin, tilted amorphous carbon foils, exhibit markedly unexpected behavior asa function of exposure of the foil to the proton beam. Specifically, the circular polarization changes from an initially well understood tilt-angle dependence to a behavior which, for low tilt angles, gives the opposite handedness of circular polarization from that predicted. In addition, the degree of alignment, indicated by the linear Stokes parameter M/I, is enhanced also as a function of dose. These changes in the tilt-angle dependence of the Stokes parameters have been systematically correlated with beam-induced graphitization of the foil, which is observed to occur from Raman measurements.
BYU Authors: Fang Yuan, John A. Johnson, David D. Allred, and Robert H. Todd, published in J. Vac. Sci. Technol. A
The cutting of cross‐linked glasses such as silica and Corning 7059 can be difficult. We conducted an experimental study to determine the feasibility of using a high‐speed waterjet to cut thin Corning 7059 glass. Cutting using either pure de‐ionized high pressure water at 380 MPa (55 000 psi) or de‐ionized water with entrained garnet abrasive was studied. The roughness of the cut surfaces was measured and compared. Photomicrographs were taken of glass examples cut at different traversing rates with pure water and with the abrasive entrained waterjet. Comparative studies of cutting with and without the entrained abrasive material showed that a cutting rate of 127 mm/min with abrasive could achieve a smoothness of about 9 μm rms. The abrasive waterjet can cut Corning 7059 glass into any desired shape. The process is safe, inexpensive, fast, and amenable to computer operation.
BYU Authors: W. I. Karain, Larry V. Knight, and David D. Allred, published in J. Vac. Sci. Technol. A
We have fabricated arrays of siliconfield emitters using semiconductorlithography techniques. The density of the tips was 105/cm2. The maximum current that can be extracted from each emitter is limited by resistive heating. We have investigated how the electron current emitted changes under constant applied voltage. We found that the current is very sensitive to the vacuum conditions. We attribute this to sputtering of the emitters due to ionized residual gas molecules. The poorer the vacuum, the higher the instability in the current. We studied this phenomenon at 10−6 and 10−8 Torr. The model of two concentric spherical shells is used to obtain the ion energy distribution. This is then used to calculate the rate of ion bombardment and the rate of atoms sputtered. A lifetime of the tip can be deduced from these calculations.  
BYU Authors: M. Cai and D. D. Allred, published in J. Vac. Sci. Technol. A
We have used Raman spectroscopy, large‐ and small‐angle x‐ray diffraction spectroscopy of sputter‐deposited, vacuum‐annealed, soft x‐ray Mo/Si thin‐film multilayers to study the physics of silicide formation. Two sets of multilayer samples with d‐spacing 8.4 and 2.0 nm have been studied. Annealing at temperatures above 800 °C causes a gradual formation of amorphous MoSi2 interfaces between the Si and Mo layers. The transition from amorphous to crystalline MoSi2 is abrupt. The experimental results indicate that nucleation is the dominant process for the early stage and crystallization is the dominant process after nucleation is well advanced. In the thicker multilayer, a portion of the siliconcrystallizes during annealing and a strong Raman signal is observed. An advantage of Raman spectroscopy is that the Raman signal of the silicide is observed even before the presence of MoSi2 can be seen using x‐ray diffraction. This study indicates that Raman spectroscopy is an effective technique for characterizing the formation of crystalline silicides.
BYU Authors: David D. Allred, published in J. Vac. Sci. Technol. A
Scanning electron microscopy, atomic force microscopy, and Raman spectroscopy were used to characterize the microstructure of photoluminescent porous silicon (PS) layers formed by the anodic etching (HF:H2O:ethanol), at various current densities, of p‐type (100) silicon wafers possessing resistivity in the range 1–2 Ω cm. Existing models for the origin of luminescence in PS are not supported by our observations. Cross‐sectional as well as surface atomic force micrographs show the material to be clumpy rather than columnar; rodlike structures are not observed down to a scale of 40 nm. A three‐dimensional model of the mesostructure of porous silicon is discussed. Room‐temperature Raman scattering measurements show no evidence for a‐Si:H or polysilanes and the material reported here is composed of 10 nm roughly spherical Si nanocrytallites rather than 3 nm wires postulated in standard quantum confinement models.
BYU Authors: Ming Cai, Qi Wang, David D. Allred, Larry V. Knight, and Dorian M. Hatch, published in Proc. SPIE
Our group is studying the structure and interfaces of soft x-ray multilayers by various techniques including x-ray diffraction and Raman spectroscopy. Raman spectroscopy is particularly useful since it is sensitive to the identity of individual bonds and thus can potentially characterize the abruptness of interfaces in multilayers. Blocking interfacial mixing is very important in achieving and maintaining high reflectivity. We report our studies of the as-deposited and postannealed structure of Mo/Si and W/C multilayers. A high normal- incidence, peak reflectance is mandatory for imaging applications that involve many reflections. The reported theoretical and achieved reflectances of the Mo/Si system are 80% and 65%, respectively. This loss of 15% can bring about a six-fold loss in system throughput in the eight-reflection system contemplated. The interfaces in the Mo/Si system are thought to play a significant role in the degrading reflectance so characterization techniques which have interfacial sensitivity are particularly important. The Mo/Si multilayer system is susceptible to Raman characterization since both the a-Si spacer layer and the MoSi2 compound which forms at the interface have Raman active modes. In this paper we report the first Raman studies, to the best of our knowledge, of the a-Si layers and their crystallization and the crystallization of the Mo/Si interface of the multilayer brought about by a one-hour 1000 degree(s)C anneal. These changes are apparent in the Raman spectra before they can be unambiguously detected by x-ray diffraction.
BYU Authors: W. I. Karain, L. V. Knight, and D. D. Allred, published in Nanostruct. Mat.
We have produced arrays of 10,000 sharp p-type silicon points using an etch plus oxidation method. These points were used as electron emitters. No high vacuum cesiation or high temperature cleaning was needed to observe the electron emission. These are seen to be photosensitive sources of electrons at 200 K and 300 K. They were also used to produce AlKα x-rays. This constitutes the first use of etched, point arrays for generating electrons for x-ray sources.
BYU Authors: Cheryl Barnett Davis, David D. Allred, and Bret C. Hess, published in Phys. Rev. B
We have studied at cryogenic temperatures photoluminescence features which lie more than 0.15 eV below the band edge in ZnxCd1-xTe (0 less-than-or-equal-to x less-than-or-equal-to 0.09) crystals. The same features, namely a defect band which lies at about 0.13-0.20 eV below the band-gap energy and a peak at 1.1 eV, that are observed in pure CdTe samples are observed in these alloy materials. In annealed samples we observe that the 1.1-eV feature, which has been attributed to tellurium vacancies, increases with fast cooling. Increased concentrations of tellurium vacancies can be understood in terms of the phase diagram of CdTe which indicates that higher concentrations of excess Cd appear in CdTe quenched from high temperatures. We also observe an absorption transition near 1.1 eV by photothermal deflection spectroscopy (PTDS). The PTDS phase shifts show that the deep defect is a bulk effect rather than a surface effect. The well-defined absorption peak suggests that the states contributing to the 1.1-eV transition are both localized. Our results also suggest that the defect band which lies 0.13 eV below the band gap (1.48 eV in CdTe) may also be related to tellurium vacancies. However, the fact that the ratio of intensities between this defect band and the 1.1-eV feature is highly variable suggests that the relationship is not simple. The origin of the defect band and its phonon replicas remains controversial.
BYU Authors: Qi Wang and D. D. Allred, published in Phys. Rev. B
In the first-order Raman spectrum of amorphous carbon (a-C) there is a low-frequency feature in the 200-900-cm-1 region. This feature is characteristic of the highly disordered amorphous-carbon materials. We note that the intensity of this feature is very sensitive to the thermal history of samples, thus suggesting that it is an important measure of the degree of disorder of the a-C materials. We also discuss the relationship between this feature and the phonon density of states of graphite.
BYU Authors: F. Yuan, Y. Shi, L. V. Knight, R. T. Perkins, and D. D. Allred, published in Thin Solid Films
We are studying the possibility of producing precision, aspherical mirrors for X-rays and visible light. Our study examines the use of ultrastructure processing to replace mechanical methods of material removal. The method starts with a chemically-mechanically polished, flat silicon wafer. The aim is to preserve atomic scale smoothness of the surface wafer while the wafer is bent to a desired figure. We report measurements of the mechanical properties of various stressing layers. This involves measuring the deformation of several thin silicon wafers coated with chemically vapor deposited nickel and boron films of known thickness. We have found that, under normal conditions, the film does not add to the microroughness of the substrate on either the front or the back surfaces. Film and substrate thicknesses, however, vary by as much as 10%. This is the present limit on figure accuracy. We have developed a model that describes bending of B/Si and Ni/Si structures. The model relates stress and Young's modulus to the measured thickness of the film, and the thickness and curvature of the substrate. This approach is used to measure the stress and Young's modulus for boron and nickel films. The Young's modulus E(f) was 3.05 x 10(12) Pa for the boron films and 1.4 x 10(10) Pa for the nickel films. From the relationship developed and verified for predicting the radii of curvature of the substrate, it may be possible to define a film thickness pattern which would provide a desired optical figure.
BYU Authors: David D. Allred, Ming Cai, Qi Wang, and Dorian M. Hatch, published in J. X-ray Sci. Technol.
Raman spectra are reported from MoSi2 polycrystalline powder and soft x-ray Mo/Si multilayers. The sharp lines at 323 and 438 cm−1 are all due to crystalline MoSi2. These lines in the powder sample intensify with annealing. The Raman spectra of as-deposited multilayers shows a broad asymmetric peak, highest at about 480 cm−1. We attribute this to α-Si which is highly disordered. In contrast to α-Si in semiconductor/semiconductor and semiconductor/dielectric multilayers, in the Mo/Si samples the Raman signal can vanish after modest heating. This provides evidence that the composition of the silicon component of the multilayer changes even with 200°C annealing. Further annealing also produces the signature for crystalline MoSi2 in the multilayer samples. This is the first report of the characterization of Mo/Si soft x-ray multilayers by Raman spectroscopy, and it indicates that Raman spectroscopy may be an effective technique for characterizing these soft x-ray multilayers and may be useful in studying their interfaces.
BYU Authors: Douglas P. Hansen , John R. Colton, Larry V. Knight, and David D. Allred, published in Proc. SPIE
X-ray phase diffraction gratings can be designed to behave in a fashion similar to blazed gratings, directing the majority of the energy into certain desired orders. They should be easy to fabricate using conventional semiconductor production technology, and offer advantages in design flexibility and efficiency over conventional amplitude grating or blazed grating structures. As a multilayered structure, a phase grating has structure in depth as well as across the surface. Most theoretical analyses in the literature treat the embedded structure through simplifying approximations or assumptions. We will discuss a model which treats the embedded structure explicitly using the Fresnel-Kirchhoff integral in the Fraunhofer diffraction limit. This approach produces a set of equations which are identical to the result for an amplitude diffraction grating except for an additional factor which depends on the phase relationships of the various surfaces in the multilayer stack.
BYU Authors: Wael I. Karain, Larry V. Knight, and David D. Allred, published in Proc. SPIE
We have produced arrays of 10,000 sharp p-type silicon points using an etch plus oxidation method. These points were used as electron emitters. No high vacuum caseation or high temperature cleaning was needed to observe the electron emission. These are seen to be photosensitive sources of electrons at 200 K and 300 K. They were also used to produce AlK(alpha ) x rays. This constitutes the first use of etched, point arrays for generating electrons for x-ray sources.
BYU Authors: D. D. Allred and Qi Wang, published in J. Vac. Sci. Technol. A
Tungsten/carbon (W/C) multilayer thin films were prepared by dc magnetron sputtering. All samples consisted of 30 layer pairs with a nominal d spacing varying from 2.5 to 14 nm, the W layer thickness was kept at 2 nm in all samples. The W/C multilayers were subjected to isochronal anneals in a quartz tube furnace at the temperature range from 500 to 950 °C under a flow of high purity Ar gas. X‐ray diffraction, Raman scattering, and Auger depth profile were used to characterize the structure of the as‐prepared and annealedmultilayerfilms. Both the W and C layers appear to be amorphous as‐prepared. An overcoat of 30 nm of plasma enhanced chemical vapor deposited silicon nitride was found to inhibit oxidation during annealing. For those multilayers containing thinner carbon layers (<1 nm), the formation of crystalline W2C occurs at annealing temperature as low as 500 °C and a very small expansion (<2%) in the layer d spacing is observed. On the other hand, for all multilayers with carbon layer thickness equal or greater than 2 nm, crystallization occurs at much higher annealing temperatures and the crystalline phases observed were alpha‐W and WC. It is also observed that in the latter group the period increases monotonically with increasing annealing temperature, the total expansion is about 10% and affects both W and C layers approximately equally. The expansion stops at the crystallization temperature which occurs at 900 °C or higher. The expansion is under investigation but may be interpreted as due to the structural ordering processes in the amorphous W and C layers.
BYU Authors: Memorie K. Williams, Evan Hansen , and David D. Allred, published in Proc. SPIE
Soft x-ray filter designs for the Brigham Young University `Goldhelox Project' are discussed. Three polymers intended for use as a supportive substrate for a soft x-ray solar filter having a passband centered at 171 angstroms are examined. The use of polymer substrates is examined because of vibrational and mechanical stresses associated with the shuttle launch, preventing the use of a free standing filter, and because of Goldhelox's special need to locate the filter near the imaging plane. The uniform consistency of a polymer support prevents any imaging of the filter support structure, as would occur if a traditional mesh support were used. The polymer substrates investigated are: AP-1, Formvar, and polypropylene. Their transmissive characteristics of the polymers are examined along with the feasibility of their use. Transmission as a function of energy for each polymer is given over an energy range of 10 to 180 eV.
BYU Authors: David D. Allred, published in Proc. SPIE
Infrared optically black baffle surfaces are an essential component of many advanced optical systems. All internal surfaces in advanced infrared optical sensors that require stray light management to achieve resolution are of primary concern in baffle design. Current industrial materials need improvements to meet advanced optical sensor systems requirements for optical, survivability, and endurability. Baffles are required to survive and operate in potentially severe environments. Robust diffuse-absorptive black surfaces, which are (1) thermally and mechanically stable to threats of X-ray, launch, and in-flight maneuver conditions, with specific densities to allow an acceptable weight load, (2) handleable during assembly, (3) cleanable, and (4) adaptive to affordable manufacturing, are required as optical baffle materials. An overview of recently developed advanced infrared optical baffle materials, requirements, manufacturing strategies, and the Optics MODIL (Manufacturing Operations Development and Integration Laboratory) Advanced Baffle Program is discussed.
BYU Authors: Qi Wang and D. D. Allred, published in Solid State Commun.
W/C multilayers were prepared on unheated Si substrates by RF sputtering method. Raman scattering, x-ray diffraction and Auger depth profile were used to characterize the structure of the as-prepared and annealed multilayers in the range of 300 to 800 C. The results were compared in samples subjected to three different annealing conditions: 1) in air, 2) in high purity Ar atmosphere and 3) in evacuated and sealed (approximately 10(-4) torr) ampules. The result of Auger profiles indicates that the penetration depth of oxygen in the films depends on the annealing conditions, annealing temperature and layering structure. It also shows the loss of compositional modulation and the oxidation of W in the region reached by the oxygen. Raman scattering from the oxidized top layer(s) displays a spectrum with lines associated with crystalline tungsten oxide and segregated microcrystalline graphite particles
BYU Authors: D. D. Allred, published in J. Vac. Sci. Technol. A
Zinccadmium telluride (Zn x Cd1−x Te ) solid solutionfilms with 0≤x≤0.12 were deposited by the close spaced vapor transport method and characterized using photoluminescence, x‐ray diffraction, and scanning electron microscopy. The two former techniques indicate that films with high crystalline quality can be prepared with moderate substrate temperatures and low argon pressures. Under these conditions deposition rates of up to 1000 Å/s are achieved and Zn concentration in the film is the same as that of the source. The electron micrographs show grain sizes comparable to the film thickness.
BYU Authors: David D. Allred , published in Proc. SPIE
Beryllium coatings with varying thicknesses and columnar grain sizes were deposited by low temperature magnetron sputtering and wet chemically etched to enhance diffuse absorption of light. After etching these coatings exhibited a matte black surface finish and low specular reflectance (below 2%) in the IR up to a critical wavelength dependent upon the original grain size of the coating. Extremely thick coatings (350 j.tm) with original grain sizes of 10 to 12 j.m were produced which exhibited specular reflectances below 0.5% up to 50 p.m wavelength and a Lambertian BRDF at 10.6 p.m averaging 4.3x103 ster1. Scanning electron micrographs are presented for etched and unetched beryllium coatings which showed the etching process produces roughness and porosity over several size scales simultaneously with the maximum size scale limited by the initial coating grain size and thickness. This technique for producing diffuse absorbing baffle materials has great versatility in choice of coating material and substrate and can be expected to provide optical system designers with a variety of material options for stray light management.
BYU Authors: D. D. Allred, published in J. Vac. Sci. Technol. A
The crystalline quality of Zn x Cd1−x Te single crystals prepared by a modified Bridgman method with 0≤x≤0.05 has been analyzed using photoluminescence. The spectrum of a typical sample is dominated by lines originating from the recombination of free and bound excitons. Lines due to free excitons in their ground and first excited states are observed in both the pure CdTe and the mixed crystals. Excitons bound to Cd vacancies are observed in the pure CdTe crystal but not in the mixed crystal. Weaker and broader features appearing at energies below the exciton emission range were associated with transitions involving free‐to‐bound and bound‐to‐bound levels. The origin of the various lines in the spectra was deduced from the detailed measurements of the dependence of the spectrum on temperature and excitation intensity.
BYU Authors: D. D. Allred, published in J. Vac. Sci. Technol. A
Large grain polycrystalline and single crystals of Zn x Cd1−x Te grown by a modified Bridgman method were studied using the photoluminescence and photoconductivity techniques. The temperature dependence of the band gap, as determined by photoluminescence, follows the Varshni equation for measuring temperature in the range of 15–290 K. One of the fitted parameters, the Debye temperature, monotonically decreases with the increase of the atomic zinc concentrations. A a close correlation between the photoluminescence and photoconductivitymeasurements is also found. Samples in which the photoluminescence spectra exhibit emission bands associated to cadmium vacancies and other structural defects, show a photoresponse curve which includes, in addition to the intrinsic band, another broad band at lower energies. Using the ionization energies of the defect related bands in the photoluminescence spectra we have identified the second band in the photoresponse curve due to the photoexcitation of trapped carriers at levels related with the structural defects.
BYU Authors: J. K. Shurtleff, D. D. Allred, R. T. Perkins, and J. M. Thorne, published in Materials Research Society
Thin film deposition techniques currently being used to produce multilayer x-ray optics (MXOs) have difficulty producing smooth, uniform multilayers with d-spacings less than about twelve angstroms. We are investigating atomic layer epitaxy (ALE) as an alternative to these techniques. ALE is a chemical vapor deposition technique which deposits an atomic layer of material during each cycle of the deposition process. The thickness of a film deposited by ALE depends only on the number of cycles. Multilayers deposited by ALE should be smooth and uniform with precise d-spacings which makes ALE an excellent technique for producing multilayer x-ray optics. We have designed and built an ALE system and we have used this system to deposit ZnSe using diethyl zinc and hydrogen selenide.
BYU Authors: D. D. Allred, published in Materials Research Society
The main lines in the photoluminescence spectra of Zn1Cd1−xTe single crystals grown by a modified Bridgman method in the compositional range of 0 ≤ X ≤ 0.25 have been identified. All crystals show only near-band-edge emission. To assist in the identification, various samples with different compositions were annealed under a Cd atmosphere. In the pure crystals, the prominent (A°,X) bound exciton line, as well as the doublet at longer wavelengths, disappear after the annealing. In contrast, the treatments do not change significantly the PL spectra of the mixed crystals.
BYU Authors: David D. Allred and Qi Wang, published in Materials Research Society
Laser Raman spectroscopy has been found to be useful for characterizing amorphous semiconductor multilayers, especially the interfaces of multilayers. Recently, we have extended this technique to the characterization of magnetron sputtered multilayers commonly used as reflectors in soft x-ray optics. Unlike the multilayers previously studied which contained only semiconductors and dielectrics, these are generally semiconductor/metal multilayers. We report here on the Raman characterization of the most common class of multilayers used in soft x-ray optics, those that contain a high density metal like tungsten interspersed with layers of carbon. In all of the metal/carbon multilayers the dominate feature in the Raman spectra is due to a-C. The a-C spectra consists of a broad peak at about 1560 cm-1 (G-peak) and a shoulder at about 1400 cm-1 (D-peak). This can be deconvoluted with Gaussian line shapes to yield two peaks (one at about 1560 to 1570 cm-1 and the other at about 1380 to 1420 cm-1). Among the W/C multilayer samples peak positions and relative magnitudes changed little with carbon thickness over the range of 1 to 12 nm. Significant differences are, however, seen as the identity of the metal component is altered or, especially, as the preparations are varied. For example, the intensity ratio of the D-peak to G-peak was much larger for multilayer samples prepared under conditions of good plasma confinement.