Unconventional Resistivity Scaling in Polycrystalline NbP Thin Films

The electrical resistivity of ultrathin metal films increases with decreasing thickness due to electron scattering from the film surfaces. This behavior limits the performance of all modern nanoelectronics which include metal-based interconnects [1]. Here, topological Weyl semimetals (TWS) with their disorder-tolerant conductive surface charge states can be useful at the ultrathin film limit [2, 3]. However, for nanoelectronics applications, ultrathin TWS films need to be explored using industry-compatible deposition methods and temperatures.

Here we uncover, for the first time, a decreasing trend in the electrical resistivity of NbP (a TWS candidate) with decreasing film thicknesses. The NbP thin films are sputtered on sapphire substrates at 400°C. Our measured room temperature resistivity of NbP decreases from 220 to 168 μΩ-cm (from 80 nm down to 2.5 nm film thickness), unlike conventional metals. Material characterizations reveal the poly/nano-crystallinity of our NbP ultrathin films. These indicate the possibility of topological protection even in poly/nanocrystalline TWS films [4], enabling the unconventional resistivity scaling. Temperature- and magnetic-field-dependent transport are being evaluated to probe this possibility and tune the TWS properties for energy-efficient nanoelectronics.

Refs: [1] D. Gall et al., MRS Bulletin (2021). [2] C. Shekhar et al., Nat. Phys. (2015). [3] N. Lanzillo et al., Phys. Rev. Appl. (2022). [4] Y-B. Yang et al., Phys. Rev. Lett. (2019).