Selected Publications

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.