Molecular beam epitaxy of superconducting ${\mathrm{Sn}}_{1\text{−}x}{\mathrm{In}}_x\mathrm{Te}$ thin films

We report a systematic study on the growth conditions of ${\mathrm{Sn}}_{1\text{−}x}{\mathrm{In}}_x\mathrm{Te}$ thin films by molecular beam epitaxy for maximization of superconducting transition temperature $T_{\mathrm{c}}$. Careful tuning of the flux ratios of Sn, In, and Te enables us to find an optimum condition for substituting rich In content $(x=0.66)$ into the Sn site in a single phase of ${\mathrm{Sn}}_{1\text{−}x}{\mathrm{In}}_x\mathrm{Te}$ beyond the bulk solubility limit at ambient pressure $(x=0.5)$. $T_{\mathrm{c}}$ shows a dome-shaped dependence on In content $x$ with the highest $T_{\mathrm{c}}=4.20\mathrm{K}$ at $x=0.55$, being consistent to that reported for bulk crystals. The well-regulated ${\mathrm{Sn}}_{1\text{−}x}{\mathrm{In}}_x\mathrm{Te}$ films can be a useful platform to study possible topological superconductivity by integrating them into the state-of-the-art junctions and/or proximity-coupled devices.

Figure 1

(a) X-ray diffraction (XRD) patterns in 2θ-ω scans for ${\mathrm{Sn}}_{1\text{−}x}{\mathrm{In}}_x\mathrm{Te}$ (SIT) thin films with various $x$. The inset schematically draws the sample structure. (b) Normalized rocking curves at (222) diffractions for the samples shown in (a). (c) Cubic lattice constant calculated from the angle of (222) diffractions (red circles) as a function of $x$. The data for bulk SIT taken from Ref. [25] (gray circles) are also plotted for comparison. (d) A map of growth condition as a function of $x$ and the flux ratio $\beta =P_{\mathrm{Te}}/\left(P_{\mathrm{Sn}}+P_{\mathrm{In}}\right)$; $P_{\mathrm{Sn}}$, $P_{\mathrm{In}}$, and $P_{\mathrm{Te}}$ being the equivalent beam pressure of the respective elements. The inset shows the magnified view around the high-$x$ region. Red circles and stars, green triangles, blue squares, and purple diamonds represent the rocksalt SIT, trigonal ${\mathrm{In}}_2{\mathrm{Te}}_3$, cubic ${\mathrm{In}}_2{\mathrm{Te}}_3$, and tetragonal InTe, respectively. In (c) and (d), red star symbols stand for the samples whose XRD patterns are exemplified in (a). (e) XRD patterns in 2θ-ω scans for the films with $x=0.46–0.50$ grown under conditions with different β, which are highlighted in (d) with a gray line. Impurity peaks in XRD data are represented by the same symbols shown in (d).

Figure 2

(a) Temperature dependence of resistivity for SIT thin films with different $x$. The inset shows the resistivity up to 300 K. (b) $T_{\mathrm{c}}$ as a function of $x$ for thin films (red) and bulk samples taken from Ref. [25] (gray). In both cases, $T_{\mathrm{c}}$ is defined as the temperature where the resistivity reaches zero in the linear scale.

Figure 3

(a) and (b) Temperature dependence of resistivity under (a) out-of-plane and (b) in-plane magnetic field for a ${\mathrm{Sn}}_{0.45}{\mathrm{In}}_{0.55}\mathrm{Te}$ ($x=0.55$) thin film. The magnetic field is changed by 0.1 T for (a) and 0.25 T for (b). The insets schematically show the configuration of current I and magnetic field H. An open diamond and dashed lines in (a) represent the definition of ${\mu }_0{H}_{\mathrm{c}2}$ in the case of 1.7 T. (c) ${\mu }_0{H}_{\mathrm{c}2}$ as a function of temperature for out-of-plane (red) and in-plane (blue) magnetic field. The solid lines represent fitting curves (see the text). The closed and open circles are the data measured by the standard PPMS and a ${}^3\mathrm{He}$ cooling system, respectively. Only the former ones are used for the fitting (for details, see Sec. S3 in the Supplemental Material [34]).