1. 7 April 2006 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

2. 7 April 20062 Overview Our goal is a better understanding of the optical properties of materials in the EUV. EUV Astronomy The Earth’s magnetosphere in the EUV The material we have been studying most recently is ThO 2. 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. 7 April 20063 Our Results: n

4. 7 April 20064 n not related to Bias Voltage or Thickness

5. 7 April 20065 What we Stumbled upon: Absorption

6. 7 April 20066 Band Gaps – The Chemistry

7. 7 April 20067 Band Gaps – The Physics

8. 7 April 20068 Our Results for the Band Gap This measurement for the band gap of ThO 2 came out to be about 5.92 eV. Our average was 6.11 ± 0.15 eV.

9. 7 April 20069 Comparing to the Literature In reviewing the literature, there seems to be a couple of different band gaps that people detect:

10. 7 April 200610 Why the Differences? Essentially, we find two camps regarding the band gap of ThO 2. One camp measuring about 5.9 eV, and the other camp measuring about 4 eV.

11. 7 April 200611 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. 7 April 200612 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. 7 April 200613 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. 7 April 200614 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. ThO 2 is known to be sensitive to oxygen doping due to its fluorite structure. When annealed in oxygen ThO 2 is known to turn opaque. However, when annealed in hydrogen or vacuum, it becomes transparent again.

15. 7 April 200615 What we think might be going on... We note some important features: –The energy spectrum includes a thin middle energy 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. Rivas-Silva, et. al. report an energy structure as follows:

16. 7 April 200616 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. 7 April 200617 “Conclusions” First of all, we have shown that reactive sputtering cannot be expected to significantly affect the optical constants of ThO 2 thin films. Secondly, exactly what is going on with the band gap of ThO 2 is still not really understood. –It appears that there are two fundamental band gaps in ThO 2, but more research is needed. –One thing that we might try is creating an intentionally doped sample of ThO 2 and measuring it between 1.0 and 10.0 eV to see if we could observe all three hypothesized absorption features.

18. 7 April 200618 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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