Reversible Tuning of Superconductivity in Ion-Gated NbN Ultrathin Films by Self-Encapsulation with a High-$\kappa$ Dielectric Layer

Ionic gating is a powerful technique for tuning the physical properties of a material via electric-field-induced charge doping, but is prone to introduce extrinsic disorder and undesired electrochemical modifications in the gated material beyond pure electrostatics. Conversely, reversible, volatile, and electrostatic modulation is pivotal in the reliable design and operation of novel device concepts enabled by the ultrahigh induced charge densities attainable via ionic gating. Here we demonstrate a simple and effective method to achieve reversible and volatile gating of surface-sensitive ultrathin niobium nitride films via controlled oxidation of their surface. The resulting niobium oxide encapsulation layer exhibits a capacitance comparable to that of nonencapsulated ionic transistors, withstands gate voltages beyond the electrochemical stability window of the gate electrolyte, and enables a fully reversible tunability of both the normal-state resistivity and the superconducting transition temperature of the encapsulated films. Our approach should be transferable to other materials and device geometries where more standard encapsulation techniques are not readily applicable.

Figure 1

(a) Sketch of the multiple-Hall-bar structure and of the measurement configuration. Each channel is 1.15 mm long and $100\mu \mathrm{m}$ wide. The typical size of the Au side gate is $0.8\times 1.2{\mathrm{mm}}^2$. The ionic liquid droplet is drop casted so as to cover the Au gate and the gated channel only. (b) Sketch of the cross section of the gated channel. (c) Sheet resistance $R{}_s$ as a function of temperature $T$ of the reference channel. Inset shows a magnification around the superconducting transition.

Figure 2

(a) Sheet resistance $R{}_s$ as a function of the gate voltage $V{}_G$ applied as a triangular wave at $T=220$ K for different values of the sweep rate. (b) Typical response of $R{}_s$ (blue line, bottom panel) to the steplike application and removal of positive and negative values of $V{}_G$ (red line, top panel) at $T=220$ K. Dashed line is a guide to the eye.

Figure 3

Normalized resistance variation, $\mathrm{\Delta }R/R^{\prime }=[R{}_s(\mathrm{\Delta }n{}_{2\mathrm{D}})-R{}_s(0)]/R{}_s(\mathrm{\Delta }n{}_{2\mathrm{D}})$ as a function of the induced charge density $\mathrm{\Delta }n{}_{2\mathrm{D}}$ obtained upon the steplike application of $V{}_G$. Inset: the induced charge density $\mathrm{\Delta }n{}_{2\mathrm{D}}$ as a function of the applied gate voltage $V{}_G$ determined via double-step chronocoulometry in the same measurements. Dashed lines are linear fits to the data and allow us to estimate the gate capacitance $C{}_G$.

Figure 4

(a) Normalized resistance $R/R(15\mathrm{K})$ as a function of the referenced temperature $T^{\ast }=[T-T{}_c{}^{\mathrm{ref}}{]}_{V{}_G}-[T{}_c{}^{\mathrm{act}}-T{}_c{}^{\mathrm{ref}}{]}_0$ for different values of the applied gate voltage $V{}_G$. Dashed lines highlight the criteria used to obtain $T{{}_c}^{10}$, $T{{}_c}^{50}$, and $T{{}_c}^{90}$ from the resistive transitions. Inset: enlargement of the same data close to the midpoint of the transition ($T{{}_c}^{50}$). (b) The $T{}_c$ shift $\mathrm{\Delta }T{}_c$ as a function of the induced charge density $\mathrm{\Delta }n{}_{2\mathrm{D}}$ determined for $T{{}_c}^{10}$ (blue up triangles), $T{{}_c}^{50}$ (black diamonds), and $T{{}_c}^{90}$ (red down triangles). Inset: maximum $T{}_c$ tunability $\mathrm{\Delta }T{}_c{}^{50}/T{}_c{}^{50}$ as a function of the NbN film thickness. Black dots are calculated from the data of Ref. [10]. The red dot is the maximum tunability achieved in this work.

Figure 5

High-resolution x-ray photoelectron spectroscopy spectra of the Nb3d region in (a) an ungated NbN film, (b) a NbN film gated at $V{}_G=+6$ V, and (c) a NbN film gated at $V{}_G=-6$ V. Filled circles are the experimental data; solid lines are the fitted signals and relative components.

Figure 6

(a) Two examples of $I(V)$ characteristics of N/I/N junctions made through the Nb oxide layer, in a region of the reference channel (blue symbols) and in a region of the active channel (red symbols). The curves have already been corrected to eliminate the contribution of the spreading resistance. The blue and the red lines represent their fit with the Simmons model [92]. The values of $\varphi$ (barrier height) and $s$ (barrier thickness) extracted from the fit are indicated. For these particular fits, the area of the junctions is fixed to $1.2\times {10}^{-8}\mathrm{m}$, based on the geometric estimation. The inset shows a picture of the setup. (b) Effective thickness $s$ and (c) effective height $\varphi$ of the potential barrier, as extracted from the Simmon’s fit of some $I(V)$ curves in the ungated (blue symbols) and gated (red symbols) channels. Panels (d) and (e) summarize in a schematic picture the evolution of the potential barrier upon gating, as it results from XPS and tunnel measurements.