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William R. Evans Brigham Young University Utah Academy 2006 – Snow College

Determining Optical Constants for ThO 2 Thin Films Sputtered Under Different Bias Voltages from 1.2 to 6.5 eV by Spectroscopic Ellipsometry. William R. Evans Brigham Young University Utah Academy 2006 – Snow College. EUV Astronomy. The Earth’s magnetosphere in the EUV. Overview.

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William R. Evans Brigham Young University Utah Academy 2006 – Snow College

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  1. Determining Optical Constants for ThO2 Thin Films Sputtered Under Different Bias Voltages from 1.2 to 6.5 eV by Spectroscopic Ellipsometry William R. Evans Brigham Young University Utah Academy 2006 – Snow College

  2. EUV Astronomy The Earth’s magnetosphere in the EUV Overview • Our goal is a better understanding of the optical properties of materials in the EUV. • The material we have been studying most recently is ThO2. • My project was to see if we could increase the density of our films by depositing at higher bias voltages. • The films were studied with visible and near UV spectroscopic ellipsometry.

  3. Our Results: n

  4. n not related to Bias Voltage or Thickness

  5. What we Stumbled upon: Absorption

  6. Band Gaps – The Chemistry

  7. Band Gaps – The Physics

  8. Our Results for the Band Gap • This measurement for the band gap of ThO2 came out to be about 5.92 eV. • Our average was 6.11 ± 0.15 eV.

  9. Comparing to the Literature • In reviewing the literature, there seems to be a couple of different band gaps that people detect:

  10. Why the Differences? • Essentially, we find two camps regarding the band gap of ThO2. One camp measuring about 5.9 eV, and the other camp measuring about 4 eV.

  11. Another Look at the Data: • Important to note: • There is a narrow absorption feature at about 6.2 eV, with full width half max of about 0.4 eV. • Also: • We find that the main thin film paper reporting a band gap of about 4 eV is Mahmoud, who deposited his films by spray pyrolysis onto glass.

  12. However... • It would be quite easy to say “You are not responsible for other people’s data,” or “He was obviously using an inferior technique and/or samples.” • Yet the second statement is anything but humble regarding our own imperfect techniques, and the first thoroughly begs the question as to what’s going on here.

  13. However... • Not only that, but Mahmoud’s techniques appear thoroughly valid! • Whatever it is he is measuring really does appear to have a band gap of 3.8 eV!!!

  14. However... • One thing we might note is that Mahmoud’s samples were prepared by spray pyrolysis onto glass. • Depending on the quality of the glass, this could effectively dope the samples introducing extra electrons into the films. • ThO2 is known to be sensitive to oxygen doping due to its fluorite structure. • When annealed in oxygen ThO2 is known to turn opaque. However, when annealed in hydrogen or vacuum, it becomes transparent again.

  15. What we think might be going on... • Rivas-Silva, et. al. report an energy structure as follows: • We note some important features: • The energy spectrum includes a thin middleenergy band due to the 5f electrons in Th. This band has a width of about 0.4 eV, the same as the absorption feature that we detected. • The middle band is in almost the right place if we interpret our absorption feature as a jump from the valence band into the middle band.

  16. What we think might be going on... • If the middle band were centered at about -9.8 eV in stead of -11.8 eV, the ~6 eV band gap reported in the majority of the thin film sources would be explained as a jump from the valence band to the middle band. • Also, if the conduction band started at about -6 eV in stead of about -7 eV, the ~4 eV band gap reported by Mahmoud and others could be explained by a transition from the middle band, which had some electrons in it due to mild doping, transitioning into the conduction band.

  17. “Conclusions” • First of all, we have shown that reactive sputtering cannot be expected to significantly affect the optical constants of ThO2 thin films. • Secondly, exactly what is going on with the band gap of ThO2 is still not really understood. • It appears that there are two fundamental band gaps in ThO2, but more research is needed. • One thing that we might try is creating an intentionally doped sample of ThO2 and measuring it between 1.0 and 10.0 eV to see if we could observe all three hypothesized absorption features.

  18. Acknowledgements • Dr. Allred • Dr. Turley • The BYU EUV Thin Film Optics Group, past and present • BYU Department of Physics and Astronomy, BYU Office of Research and Creative Activities, and Rocky Mountain NASA Space Grant Consortium for support and funding • Kristin Evans

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