Probing The Low-Resistivity in Sub-5 nm Thin Nanocrystalline NbP and TaP Semimetals

Ultrathin materials with low resistivity are important for energy-efficient nanoelectronics: from interconnects for dense logic and memory to spintronic devices and neuromorphic memristor arrays. However, the electrical resistivity of ultrathin metal films typically increases with decreasing film thickness due to electron-surface scattering, limiting the performance of metal-based interconnects and nanoelectronics [1]. To overcome this bottleneck, novel topological semimetals such as transition metal pnictides (NbP, NbAs, TaP, TaAs) have been suggested in the ultrathin film limit [2, 3]. These materials have been demonstrated to be topological Weyl semimetals in their single crystal forms [2], a class of materials wherein surface conduction is predicted to dominate thin-film transport even in the presence of disorder [3].<br/><br/>Here we uncover an unconventional (compared to metals) reduction of electrical resistivity with decreasing film thickness of thin nanocrystalline NbP and TaP semimetals. Our electrical measurements suggest surface-channel dominated transport with decreasing film thicknesses in these nanocrystalline semimetals.<br/><br/>NbP and TaP thin films are sputtered on both sapphire and MgO substrates at 400 °C, a process compatible with back-end-of-the-line semiconductor fabrication. 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. A thin seed layer of Nb (or Ta) is used to reduce lattice mismatch between the substrate and sputtered NbP (or TaP) thin films, which also helps promote the local short-range order within the NbP films. The compositional homogeneity of the NbP thin films is further confirmed by energy-dispersive X-ray analysis.<br/><br/>The electrical resistivity of NbP and TaP shows a decreasing trend with decreasing film thicknesses (from ~80 nm down to ~1-2 nm) measured across a temperature range from 5 K up to room temperature. The sub-5 nm thin TaP and NbP films show significantly lower resistivity (e.g., ~12 µΩ-cm for 1 nm thin TaP and ~34 μΩ-cm for ~1.5 nm NbP, at room temperature) compared to their bulk single-crystal counterparts, as well as topological insulators and most metals at similar thicknesses [1,2,5,6].<br/><br/>Temperature-dependent measurements of our thin NbP films show increased conductivity with decreasing temperature. In contrast, bulk thick NbP (~80 nm) shows an opposite trend indicating impurity- or disorder-limited transport [5]. Based on a two-channel (surface and bulk) model of the experimental transport data, we deduce that with decreasing film thicknesses, the surface-to-bulk conductance ratio increases. We simultaneously find that the surface conductance increases with decreasing temperature, consistent with metallic behavior [5, 6]. Hall measurements show a decreasing trend of carrier density with decreasing thickness, and thus the estimated mobility increases for thinner films (e.g., ~7.4 cm²/V/s for ~4.3 nm NbP vs. ~0.2 cm²/V/s for bulk NbP). This also allows us to estimate a high surface-area-normalized carrier density of ~10¹⁶cm^-2, and a 2D surface mobility of ~12 cm²/V/s. As film thickness decreases, the surface mobility contribution becomes more pronounced, leading to an increased conductivity in ultrathin NbP.<br/><br/>In summary, we uncovered surface-state-dominated conduction in ultrathin nanocrystalline NbP and TaP films. This leads to an exceptional decrease in the resistivity with decreasing film thickness in such semimetals, promising for next-generation nanoelectronics. This work was supported in part by 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). [5] P. Corbae et al., <i>Nat. Mat</i>. (2023). [6] C. Zhang et al., <i>Nat. Mat. </i>(2019).