Hydrogen in tin (di)oxide

• Applicant: Professor Dr. Eduard Lavrov
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2021 to 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 467412802

• Final Report Year: 2025

Final Report Abstract

The project was aimed at investigating the structural and electrical properties of hydrogen-related defects in tin oxides. Tin dioxide (SnO2), a wide band gap (3.6 eV) semiconductor, is widely used in transparent conductive electronics, solar cells, gas sensors, touchscreens, catalysis, and spintronics. It consistently exhibits n-type conductivity, typical of transition metal oxides, with even the purest crystals showing resistivity in the range of 106 to 107 Ω·cm. Despite its wide applicability, the microscopic origin of donor states in as-grown SnO2 remains unresolved. Addressing this question would significantly improve our understanding of the material and potentially enable the realization of p-type conductivity—currently the main limitation in developing SnO2 -based semiconductor devices. In contrast to tin dioxide, tin monoxide (SnO) is a p-type semiconductor—a rare case among transition metal oxides. It has an indirect band gap of 0.7 eV and a direct gap between 2.5 and 3.4 eV, allowing SnO to remain largely transparent in the visible spectrum. As with SnO2 , the nature of native conductivity in SnO is still under debate. Recent theoretical studies suggest that hydrogen-related defects may be responsible for both the ntype conductivity in SnO2 and the p-type conductivity in SnO. This possibility motivated us to undertake a comprehensive study of such defects. As part of this project, we fully characterized interstitial hydrogen—the simplest hydrogen-related defect—in SnO2 , and confirmed that it acts as the principal shallow donor in the pure material. We also investigated hydrogen complexes with transition metal impurities, which act as acceptors when substituting tin atoms in the lattice. Particularly, we identified the spectroscopic signature of an Fe–O–H complex. Additionally, we discovered previously unreported lines in the infrared absorption spectrum. One such line, likely corresponding to a hydrogen complex with Cr or Ir, exhibits Fermi resonance—a phenomenon rarely observed in these materials. Another line, attributed to electronic transitions involving Ir, may serve as a useful probe of the Fermi level position in SnO2. A muon spin rotation (µSR) investigation conducted on a collection of SnO samples suggests that hydrogen is unlikely to act as a shallow acceptor in this material. As a significant by-product of the project, we developed a novel technique for forming ohmic contacts on SnO2 surfaces by treating them with hydrogen plasma at moderate temperatures.

Publications

• Comprehensive study of the interstitial hydrogen donor in SnO₂. Physical Review B, 108(20).
  Herklotz, F.; Lavrov, E. V.; Melnikov, V. V.; Galazka, Z. & Agekyan, V. F.
  
• Ohmic contacts on SnO2 produced by hydrogen plasma treatment. Applied Physics Letters, 125(4).
  Chaplygin, I.; Galazka, Z.; Herklotz, F. & Lavrov, E. V.
  
• “Ohmic contacts on SnO2 produced by hydrogen plasma treatment”, Conference “Transparent Conductive Oxides — Fundamentals and Applications”, Leipzig, Deutschland, 23–27.09.2024
  I. Chaplygin, Z. Galazka, F. Herklotz & E. V. Lavrov
  
• “The interstitial hydrogen donor in SnO2 : A comprehensive spectroscopic study”, Conference “Transparent Conductive Oxides — Fundamentals and Applications”, Leipzig, Deutschland, 23–27.09.2024
  F. Herklotz, E. V. Lavrov, V. V. Melnikov, Z. Galazka & V. F. Agekyan
  
• Transition metal–hydrogen complexes in SnO₂. Physical Review B, 111(19).
  Herklotz, F.; Chaplygin, I.; Lavrov, E. V.; Melnikov, V. V.; Galazka, Z. & Vines, L.
  
• “Transition metalhydrogen complexes in SnO2 ”, International Conference on Defects in Semiconductors 33, Shanghai, China . 13–19.09.2025
  F. Herklotz, I. Chaplygin, E. V. Lavrov, V. V. Melnikov, Z. Galazka & L. Vines