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Ultrahigh-Temperature Semiconductors Made from Polymer-Derived Ceramics

Authors

  • Hee-Yeon Ryu,

    1. Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, Colorado 80309–0427
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    • Present address: Senior Research Engineer, Environment and Energy Research Team, Hyundai Motor Company, 460–30 Sam–Dong, Uwang–Si, Gyeonggi–Do, 437–041 Korea.

  • Qi Wang,

    1. National Renewable Energy Laboratory, National Center for Photovoltaics, Golden, Colorado 80401
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  • Rishi Raj

    Corresponding author
    1. Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, Colorado 80309–0427
      †Author to whom correspondence should be addressed. e-mail: qjayaram@materials.iisc.ernet.in
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    • Present address: R. Raj is affiliated with PDC-Energy, LLC, based in Lousville, CO.

Errata

This article is corrected by:

  1. Errata: Correction Volume 97, Issue 4, 1326, Article first published online: 26 March 2014

  • E. I. C. Johnson—contributing editor

  • This paper derives from the doctoral thesis of H. –Y. Ryu at the University of Colorado (2005).

  • This research was supported by the Air Force Office of Scientific Research, and in part by the Ceramics Program in the Division of Materials Research at the National Science Foundation under Grant No. 0502781.

†Author to whom correspondence should be addressed. e-mail: qjayaram@materials.iisc.ernet.in

Abstract

We report the semiconductor behavior of polymer-derived ceramics at high temperatures extending up to 1300°C, far above that of any known material. The conductivity depends strongly on the N/O molar ratio, reaching its highest value when the ratio is approximately unity. The temperature dependence of the conductivity for these specimens, σ, shows good agreement with the Mott's variable range hopping (VRH) mechanism for three–dimensional conduction in amorphous materials as described byinline image. The comparison yields the following range of values for the density of states, N(E)=4.9 × 1017–5.9 × 1018 (eV·cm3)−1, hopping energy, W=0.017–0.047 eV, and hopping distance, R=13.4–21.8 nm. The charge carrier mobilities predicted by the VRH model are in excellent agreement with the values measured in the Hall experiment. The long hopping distances are an unusual feature of this ceramic, suggesting long-range wave functions that may arise from clusters of SiCNO atoms that can exist in the form of a nanodomain network. Specimens that are either rich in oxygen (at the expense of nitrogen) or rich in nitrogen, have conductivities that are four to eight orders of magnitude lower than the ∼equimolar compositions. One oxygen-rich specimen shows band-gap controlled semiconductivity with an activation energy of 1.1 eV. Taken together, these results suggest that the electronic properties of the SiCNO ceramics are controlled by complex interactions between C and other atoms (Si, N, and O). These results are at variance with the simple picture where “free carbon” is assumed to determine the electronic behavior.

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