Selective area epitaxy of PbTe-Pb hybrid nanowires on a lattice-matched substrate

Topological quantum computing is based on the braiding of Majorana zero modes encoding topological qubits. A promising candidate platform for Majorana zero modes is semiconductor-superconductor hybrid nanowires. The realization of topological qubits and braiding operations requires scalable and disorder-free nanowire networks. Selective area growth of in-plane InAs and InSb nanowires, together with shadow-wall growth of superconductor structures, have demonstrated this scalability by achieving various network structures. However, the noticeable lattice mismatch at the nanowire-substrate interface, acting as a disorder source, imposes a serious obstacle along with this road map. Here, combining selective area and shadow-wall growth, we demonstrate the fabrication of PbTe-Pb hybrid nanowires—another potentially promising Majorana system—on a nearly perfectly lattice-matched substrate CdTe, all done in one molecular beam epitaxy chamber. Transmission electron microscopy shows the single-crystal nature of the PbTe nanowire and its atomically sharp and clean interfaces to the CdTe substrate and Pb overlayer, without noticeable interdiffusion or strain. The nearly ideal interface condition, together with the strong screening of charge impurities due to the large dielectric constant of PbTe, holds promise towards a clean nanowire system to study Majorana zero modes and topological quantum computing.

Figure 1

Schematic crystal structures of PbTe and CdTe and fabrication procedure of prepatterned CdTe substrates and PbTe-Pb hybrid nanowires. (a) Crystal structures of PbTe and CdTe. (b) Preparation of $\mathrm{Si}{\mathrm{O}}_{\mathrm{x}}$ shadow walls (dark grey) on a CdTe substrate (green). (c) ${\mathrm{Al}}_2{\mathrm{O}}_3$ mask (light grey) deposition. (d) Wet etching of ${\mathrm{Al}}_2{\mathrm{O}}_3$ with openings on a CdTe substrate (white). (e) Selective area growth of PbTe nanowires (blue). The arrows represent the PbTe flux direction. (f) Shadow wall growth of Pb overlayer (orange). The arrows represent Pb flux direction. (g) Capping the device with CdTe.

Figure 2

Scanning electron microscope (SEM) images of PbTe and PbTe-Pb nanowires and nanostructures grown on CdTe(001). (a)–(c) SEM images of a PbTe nanowire (a), an Aharonov-Bohm loop (b), and a double cross (Hall bar) composed of PbTe nanowire (c), all selectively grown on CdTe (001). (d)–(f) SEM images of PbTe-Pb hybrid nanowire structures for NS tunneling spectroscopy (d), SS Josephson junction (e), and superconducting island devices (f), respectively. All the scales bars are 1 micron.

Figure 3

Scanning transmission electron microscopy characterizations of the cross section of a PbTe-Pb hybrid nanowire grown on CdTe(001) substrate. (a) High-angle annular dark field (HAADF) image. (b)–(d) Energy-dispersive x-ray spectroscopy (EDX) maps of Pb (b), Cd (c), and Te (d), respectively. (e) Atomically resolved image near the CdTe-PbTe interface. (f) Inverse fast Fourier transformation (IFFT) of (e), resolving the columns of atoms as vertical lines. (g), (h) Atomically resolved images near the PbTe-Pb interface (g) and CdTe capping layer-Pb interface (h).

Figure 4

Basic transport characterization of PbTe nanowires. (a) False-color SEM of a field effect mobility device (device A). For clarity, the upper (lower) panel is the device without (with) dielectric and top gate (orange). Scale bar is 1 micron. (b) Cross section of the device [after measurement, cut along the yellow line in (a)] with each layer labeled. Scale bar is 100 nm. (c) Conductance of the device as a function of top gate voltage $\left(V_{\mathrm{TG}}\right)$. The arrows indicate $V_{\mathrm{TG}}$ sweep directions. The red line is the mobility fitting curve. (d) Magnetoconductance of the device at two $V_{\mathrm{TG}}$ settings, resolving a weak antilocalization peak near $B=0\mathrm{T}$. The magnetic field ($B$) direction is perpendicular to the substrate plane. All measurements are formed in a dilution fridge with a base temperature $\sim 20\mathrm{mK}$.