Epitaxial growth of large-gap quantum spin Hall insulator on semiconductor surface (1411.5314v1)

Abstract: Formation of topological quantum phase on conventional semiconductor surface is of both scientific and technological interest. Here, we demonstrate epitaxial growth of 2D topological insulator, i.e. quantum spin Hall (QSH) state, on Si(111) surface with a large energy gap, based on first-principles calculations. We show that Si(111) surface functionalized with 1/3 monolayer of halogen atoms [Si(111)-sqrt(3) x sqrt(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 QSH state with an energy gap as large as 0.8 eV. This surprising phenomenon is originated from an intriguing substrate orbital filtering effect, which critically select the orbital composition around the Fermi level leading to different topological phases. Particularly, the substrate-orbital-filtering effect converts the otherwise topologically trivial freestanding Bi lattice into a nontrivial phase; while 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.

Epitaxial Growth of Large-Gap Quantum Spin Hall Insulators on Semiconductor Surfaces

The research presented by Zhou et al. addresses a pivotal area in quantum materials, focusing on the epitaxial growth of large-gap quantum spin Hall (QSH) insulators on semiconductor surfaces. Using first-principles calculations, the authors demonstrate the successful formation of a two-dimensional topological insulator, specifically a QSH state, on a halogenated Si(111) surface, marking a significant step in the search for stable, large-gap QSH phases.

The paper outlines a procedure wherein heavy metals, chiefly bismuth (Bi), are grown on a Si(111) surface functionalized with a third monolayer of halogen atoms, yielding a trigonal symmetry surface ideal for epitaxial growth. Noteworthy is the Bi overlayer's ability to form a hexagonal lattice with exceptional kinetic and thermodynamic stability. This configuration maintains an atomic linkage to the Si substrate while achieving electronic decoupling, thus preserving isolated QSH states. The findings reveal an unprecedented energy gap approximately 0.8 eV wide, significantly more substantial than most known 2D QSH insulators.

The authors attribute this phenomena primarily to a substrate orbital filtering effect, whereby the Si substrate selectively influences the orbital composition at the Fermi level, converting a typically trivial freestanding Bi lattice into a nontrivial phase. This contrasting behavior is further exemplified with gold (Au), which remains in a topologically trivial phase despite its similar lattice structure without substrate influence. The paper provides a critical insight into utilizing a substrate as a filter for orbital selection, expanding possibilities for novel topological phases.

Through density functional theory (DFT) based first-principles calculations, the authors meticulously explore the geometry, band structure, and band topology of the proposed epitaxial systems. Results exhibit a profound orbital hybridization between Bi and the dangling bonds of exposed Si, effectively isolating desired p orbxtals from quantum interference. The band structure analysis reveals significant SOC-driven splitting and gap formation in Bi on Si surfaces, affirming the topological Z2 invariant.

Implications and Future Prospects

The implications of this research are manifold. Practically, the successful epitaxial growth on a semiconductor substrate opens the door to easier experimental realizations of large-gap QSH insulators, which are crucial for room-temperature applications in spintronics and quantum computing. Theoretically, the concept of substrate orbital filtering provides a novel mechanism for inducing nontrivial topological phases, suggesting potential exploration in various substrate materials and configurations.

This work lays a foundation for future explorations in epitaxial growth techniques, particularly the interface between topological insulators and conventional semiconductor technology. The findings could lead to advancements in Si-based technology through dissipationless interconnects and edge states, thereby enhancing electronic circuit design. Furthermore, the proposed mechanisms can inspire new material combinations harnessing different heavy metals and semiconductor substrates to tailor desirable electronic and topological properties.

In summary, this paper presents a crucial advancement in the field of quantum materials, demonstrating the feasibility of large-gap QSH insulators on semiconductor surfaces and providing an innovative framework for realizing stable topological phases through substrate interactions.