Unconventional Resistivity Scaling in Nanocrystalline NbP and TaP sub-5 nm Thin Film

The electrical resistivity of ultrathin metal films typically increases with decreasing film thickness due to electron scattering from the film surfaces. This behavior limits the performance of metal-based interconnects in all modern nanoelectronics [1]. To overcome this bottleneck, novel quantum materials like topological semimetals with disorder-tolerant conductive surface states have been suggested in the ultrathin film limit [2, 3].<br/><br/>Here we uncover the reduction of electrical resistivity with decreasing film thickness of ultrathin nanocrystalline NbP and TaP films, probed by detailed transport measurements, material characterization, and modeling. These materials have been demonstrated to be topological Weyl semimetals in the bulk [2], a class of materials wherein surface conduction is predicted to dominate thin-film transport even in the presence of disorder [3].<br/><br/>NbP and TaP thin films are sputtered on both r-plane sapphire and MgO substrates at 400 °C, a process compatible with back-end-of-the-line (BEOL) semiconductor fabrication. A thin seed layer of Nb (or Ta) is used to reduce lattice mismatch between the substrate and sputtered NbP (or TaP) thin films. High-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) imaging reveals nano-crystallinity of the ultrathin NbP films with short-range ordering at the NbP surface, irrespective of the film thickness. The compositional homogeneity of the NbP thin films is further confirmed by energy-dispersive X-ray analysis.<br/><br/>Our measured resistivity of NbP at room temperature decreases from 207 to 149 μΩ-cm (from ~18 nm down to ~2.3 nm film thickness), unlike conventional metals. We further confirm the decreasing resistivity trend with decreasing thicknesses of NbP film irrespective of the thickness of the seed layer. For example, with a 4 nm Nb (Ta) seed layer, we find that the resistivity of NbP thin films decreases from 135 μΩ-cm (18 nm thick film) to ~34 μΩ-cm (~1.5 nm thin film). On the other hand, the measured resistivity for ~1 nm thin TaP film is ~12 μΩ-cm. For both cases, the resistivity is significantly lower than the bulk single-crystal resistivity. We also note that the resistivity of NbP (TaP) thin films with a 4 nm seed layer is lower compared to their thinner seed layer counterparts due to possible charge transfers from the thicker metal seed layer to the NbP (TaP) thin films.<br/><br/>Temperature-dependent electrical resistivity measurements of NbP show a significantly weaker temperature dependence of resistivity in the thin NbP compared to control metal Nb. This behavior suggests a disorder-driven localization of conduction instead of phonon-dominated transport. To understand our measurements, we present a transport model in terms of a surface channel and bulk conduction in all films with surface-dominated conduction in the thinner NbP films leading to a decrease in effective electrical resistivity. Our measured Hall resistance is nearly linear with the magnetic field. This is a signature of transport dominated by single carriers (holes for NbP).<br/><br/>In summary, we demonstrated an unconventional resistivity scaling trend with the film thickness in sub-5 nm NbP and TaP thin films. Our transport measurements suggest surface-dominated conduction in such materials even at room temperature, which is promising for next-generation nanoelectronics. This work was supported in part by the Stanford Graduate Fellowship (A.I.K.) and the Stanford SystemX Alliance.<br/><br/>Refs: [1] D. Gall et al., <i>MRS Bulletin</i> (2021). [2] C. Shekhar et al., <i>Nat. Phys. </i>(2015). [3] N. Lanzillo et al., <i>Phys. Rev. Appl.</i> (2022). [4] Y-B. Yang et al., <i>Phys. Rev. Lett.</i> (2019).