Why Topological Materials?
Topological semimetals represent a fundamentally different approach to the interconnect scaling problem. Unlike conventional metals where surface scattering degrades conductivity, topological materials possess topologically protected surface states — electronic states that exist at surfaces and interfaces and are robust against backscattering from non-magnetic impurities and surface roughness. This protection arises from the bulk band topology and is guaranteed by fundamental symmetry principles.
If these properties persist in thin film form, topological conductors could potentially maintain high conductivity at nanometer dimensions where conventional metals fail. This makes them a high-risk, high-reward candidate for beyond-Cu interconnects.
NbAs: A Type-I Weyl Semimetal
Niobium arsenide (NbAs) crystallizes in the body-centered tetragonal structure (space group I4₁md) and hosts 24 Weyl nodes in its Brillouin zone. Bulk single crystals exhibit ultra-high electron mobility exceeding 5 × 10⁶ cm²/V·s at low temperatures, extremely large magnetoresistance, and clear Shubnikov-de Haas oscillations confirming the Weyl fermion nature of its charge carriers.
The surface of NbAs hosts Fermi arc states connecting projections of bulk Weyl nodes of opposite chirality. These surface states have been directly observed by angle-resolved photoemission spectroscopy (ARPES) and are predicted to contribute to surface-dominated transport in thin films and nanostructures.
Thin Film Growth Challenges
Growing high-quality NbAs thin films is significantly more challenging than depositing elemental metals. The compound has a narrow thermodynamic stability window, As is volatile and tends to desorb at growth temperatures, and maintaining stoichiometry requires precise flux control. My approach uses co-sputtering from separate Nb and As targets in the UHV system, with systematic optimization of substrate temperature, As overpressure, and deposition rate to achieve oriented films.
Bulk Mobility (cm²/V·s)
Ultra-high carrier mobility in NbAs single crystals at cryogenic temperatures.
Weyl Nodes
Number of Weyl points in the NbAs Brillouin zone, connected by surface Fermi arcs.
Research Questions
The central questions driving this research are: (1) Can topologically protected surface states survive in thin films with nanometer thickness? (2) Do they contribute measurably to electrical transport? (3) Is the resistivity size effect fundamentally different in topological vs. trivial metals? Answering these questions requires both high-quality thin films and careful transport measurements designed to distinguish surface and bulk contributions to conductivity.
Impact
While topological interconnect materials are at an earlier technology readiness level than Ru or Cu alternatives, they represent a potentially transformative approach that could break the fundamental scaling limitations imposed by surface scattering in conventional metals. This research contributes to the emerging field of "topological electronics" and helps assess whether the exotic quantum properties of Weyl semimetals can be harnessed for practical semiconductor applications.