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Epitaxial growth of large-gap quantum spin Hall insulator on semiconductor surface

  1. Feng Liua,c,1
  1. aDepartment of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112;
  2. bSchool of Physics and Technology, University of Jinan, Jinan, Shangdong 250022, China; and
  3. cCollaborative Innovation Center of Quantum Matter, Beijing 100871, China

  1. Edited by David Vanderbilt, Rutgers, The State University of New Jersey, Piscataway, NJ, and approved August 27, 2014 (received for review May 28, 2014)

Significance

Quantum phase of matter is of great scientific and technological interest. The quantum spin Hall (QSH) insulator is a newly discovered two-dimensional material that exhibits topological edge state residing inside bulk energy gap, so that its edge is metallic with quantized conductance and its bulk is insulating. For its potential applications in spintronics and quantum computing, a large energy gap is desirable, e.g., for room-temperature application. So far, large-gap QSH insulators have been predicted only in freestanding films. Here we demonstrate the formation of a large-gap QSH state on a semiconductor substrate through epitaxial growth of heavy metal atoms on halogenated Si surface. Our findings not only reveal a new formation mechanism of large-gap QSH insulator, but may also pave the way for its experimental realization.

Abstract

Formation of topological quantum phase on a conventional semiconductor surface is of both scientific and technological interest. Here, we demonstrate epitaxial growth of 2D topological insulator, i.e., quantum spin Hall state, on Si(111) surface with a large energy gap, based on first-principles calculations. We show that the Si(111) surface functionalized with one-third monolayer of halogen atoms [Si(111)-<mml:math><mml:mrow><mml:msqrt><mml:mn>3</mml:mn></mml:msqrt></mml:mrow></mml:math>3×<mml:math><mml:mrow><mml:msqrt><mml:mn>3</mml:mn></mml:msqrt></mml:mrow></mml:math>3-X (X = Cl, Br, I)] exhibiting a trigonal superstructure provides an ideal template for epitaxial growth of heavy metals, such as Bi, which self-assemble into a hexagonal lattice with high kinetic and thermodynamic stability. Most remarkably, the Bi overlayer is atomically bonded to but electronically decoupled from the underlying Si substrate, exhibiting isolated quantum spin Hall state with an energy gap as large as ∼0.8 eV. This surprising phenomenon originates from an intriguing substrate-orbital-filtering effect, which critically selects the orbital composition around the Fermi level, leading to different topological phases. In particular, the substrate-orbital-filtering effect converts the otherwise topologically trivial freestanding Bi lattice into a nontrivial phase; and the reverse is true for Au lattice. The underlying physical mechanism is generally applicable, opening a new and exciting avenue for exploration of large-gap topological surface/interface states.

Footnotes

  • ↵1To whom correspondence should be addressed. Email: fliu{at}eng.utah.edu.

  • Author contributions: F.L. designed research; M.Z. performed research; W.M., Z.L., Z.W., and P.L. contributed new reagents/analytic tools; M.Z., Z.L., and F.L. analyzed data; and M.Z. and F.L. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1409701111/-/DCSupplemental.

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