Nitrogen surface passivation of the Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$

The influence of surface treatments on the transport properties of epitaxial films of the three-dimensional Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ is investigated. We show that exposure of the surface to a low-energy nitrogen plasma improves carrier mobilities, their temperature dependence, and allows for the observation of the quantum Hall effect in scaled thin films. The results provide insights into surface band bending as a function of the surface chemistry and the relative contributions of surface and bulk states to the measured transport properties.

Figure 1

Transport properties of $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ thin films with two different film thicknesses, 24 and 280 nm, before and after ${\mathrm{N}}^{*}$ surface treatment, respectively. (a) Inverse of the magnitude of the low-field Hall coefficient, $|{\left(e{R}_H\right)}^{-1}|$; (b) the carrier mobility, estimated from the low-field Hall effect and the sheet resistance, which is shown in (c). Measurements were carried out on a Hall bar structure.

Figure 2

Longitudinal and transverse magnetoresistances measured using a Hall bar structure before and after ${\mathrm{N}}^{*}$ surface treatment of the $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ film surfaces. The data shown in panels (a) and (b) is from a 24-nm film, while that in panels (c) and (d) is from a 280-nm-thick film. To reduce the effects from intermixing between $R_{xx}$ and $R_{\mathrm{xy}}$ the curves have been symmetrized ($R_{xx}$) and antisymmetrized ($R_{\mathrm{xy}}$), respectively.

Figure 3

Comparison of the transport properties of $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ thin films (50-nm thickness) before and after thermal annealing in the PPMS vacuum chamber and ${\mathrm{N}}^{*}$ surface treatment, respectively. The sheet resistance is shown in (a), the inverse of the magnitude of the low-field Hall coefficient, $|{\left(e{R}_H\right)}^{-1}|$ in (b) and the extracted mobility in (c). The samples were measured in van der Pauw geometry. For same sample was measured, then annealed at 80 °C for 10 min, remeasured, then annealed to 100 °C for 10 min and remeasured. The ${\mathrm{N}}^{*}$ treated film is from a different piece of the same sample.

Figure 4

XPS of the same $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ thin film (50-nm thickness) with and without ${\mathrm{N}}^{*}$ surface treatment. Panel (a) shows a survey scan, while panels (b) and (c) show high resolution spectra around the O 1s peak (b) and the nitrogen 1s peak (c), respectively. The lines show the components of fits to the peaks.