Solid-State Reactions at Niobium-Germanium Interfaces in Hybrid Superconductor-Semiconductor Devices

Abstract

Hybrid Superconductor-Semiconductor (S-Sm) materials systems are promising candidates for quantum computing applications. Their integration into superconducting electronics has enabled on-demand voltage tunability at millikelvin temperatures. Ge quantum wells (Ge QWs) have been among the semiconducting platforms interfaced with superconducting Al to realize voltage tunable Josephson junctions. Here, we explore Nb as a superconducting material in direct contact with Ge channels by focusing on the solid-state reactions at the Nb/Ge interfaces. We employ Nb evaporation at cryogenic temperatures (100 K) to establish a baseline structure with atomically and chemically abrupt Nb/Ge interfaces. By conducting systematic photoelectron spectroscopy and transport measurements on Nb/Ge samples across varying annealing temperatures, we elucidated the influence of Ge out-diffusion on the ultimate performance of superconducting electronics. This study underlines the need for low-temperature growth to minimize chemical intermixing and band bending at the Nb/Ge interfaces.

Figures (8)

• Figure 1: (a) Substrate temperature as a function of time during a typical cryogenic Nb growth process starting with GeO$_2$ desorption at 500 $^{\circ}$C and cooling the sample to -170 $^{\circ}$C for the Nb e-beam deposition. (b) Cross-sectional TEM image of a cryogenically grown 100 nm thick film of Nb on Ge(001) showing a nanocrystalline structure with grain sizes ranging from 5 to 10 nm. (c) Elemental analysis of the Nb/Ge(001) interface by EDS showing minimal intermixing and a narrow interface layer ( < 4 nm).
• Figure 2: (a) Valence band spectra vs binding energy for different annealing temperatures of a 30 nm Nb film on Ge(001). Higher annealing temperatures show sharper valence bands at the Fermi level edge. (b) Band bending at the Nb/Ge interface as a function of annealing temperature for a 30 nm Nb film. (c) Valence band spectra for 200 nm thick Nb films annealed at 300 $^{\circ}$C, 575 $^{\circ}$C, and 675 $^{\circ}$C.
• Figure 3: (a) Anneal temperature dependence of Nb3p peaks. Peaks shift to higher binding energies as anneal temperature increases. (b) Anneal temperature dependence of Ge2p peaks. Ge2p peaks increase in signal the higher the anneal temperature. (c) Overall elemental analysis of each sample.
• Figure 4: Superconducting transport properties of Nb/Ge heterostructure as grown (blue) and after UHV annealing at 300 $^{\circ}$C (green) and 500 $^{\circ}$C (red). (a) Normalized resistance (R$_\text{N}$) as a function of temperature. (b) Dependence of critical magnetic field (B$_\text{C}$) on temperature. (c) Critical current density (J$_\text{C}$) as a function of temperature.
• Figure S1: The evolution of the work function for an 8 nm thin film of Nb on Ge(001) as a function of annealing temperatures. The increase in the Nb work function with the annealing temperature is consistent with the Ge incorporation into the thin film.