in situ Growth of and Proximity-induced Superconductivity in Al-Ge Quantum Well Heterostructures

Technical Summary: In situ Growth of and Proximity-induced Superconductivity in Al-Ge Quantum Well Heterostructures

Problem Statement
Planar Ge/SiGe heterostructures are a promising platform for engineering topological superconductivity due to their strong spin-orbit interaction, tunable Landé g-factors, and high hole mobility. However, a critical bottleneck has been the lack of a high-quality, in situ epitaxial superconductor–semiconductor architecture within this material system. Previous approaches relied on post-growth annealing or ex situ deposition of superconductors (e.g., Al or Nb), which often resulted in interface disorder, atomic interdiffusion, and oxidation. These imperfections degrade the quality of the proximity-induced superconducting gap, hindering the realization of advanced quantum devices such as topological qubits and Majorana zero modes, which require extremely low disorder and sharp interfaces. While in situ growth has proven successful in III–V systems, it had not yet been established for Ge/SiGe heterostructures.

Methodology
The authors addressed this challenge by utilizing a multi-chamber molecular beam epitaxy (MBE) system to grow high-quality Al-Ge quantum well (QW) heterostructures in situ.

• Growth Strategy: The process began with the growth of a 16-nm-thick Ge QW on a strain-relaxed Si$_{0.2}$Ge$_{0.8}$ virtual substrate. Crucially, a 1-nm-thick Si capping layer was grown in situ on top of the Ge QW before transferring the sample to a III–V chamber. This Si layer serves to prevent atomic Al-Ge interdiffusion and protect the Ge from oxidation and etching damage.
• Deposition: Aluminum films (>5 nm) were deposited in situ at temperatures below 0 °C to ensure optimal interface sharpness and surface flatness.
• Characterization: Structural and elemental analyses were performed using cross-sectional scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX).
• Device Fabrication: Planar Josephson junctions (JJs) and a superconducting quantum interference device (SQUID) were fabricated by selectively etching the Al film using a transene type-D etchant, which stops at the Si capping layer, preserving the underlying Ge QW.
• Measurements: Low-temperature transport measurements were conducted in a dilution refrigerator (base temperature ~12 mK) using both current-bias and voltage-bias configurations (including lock-in techniques for voltage-bias spectroscopy).

Key Contributions and Results

1. Material Quality: Structural analysis confirmed that the in situ grown Al film forms an atomically sharp interface with the Si-capped Ge QW, with no discernible atomic interdiffusion. The Al film is polycrystalline with a flat top surface, and the Si layer effectively separates the diamond-structured Ge lattice from the FCC Al lattice.
2. Proximity-Induced Superconductivity: Transport measurements on the fabricated JJs demonstrated robust, gate-tunable proximity-induced superconductivity in the Ge hole gases.
   • Gate Control: The switching current ($I_{sw}$) and retrapping current ($I_{rt}$) increased with more negative gate voltages, confirming the ability to tune the supercurrent.
   • Interface Transparency: Analysis of the excess current ($I_{exc}$) and normal state resistance ($R_n$) yielded a peak interface transparency of approximately 79% for one device and 63% for another, indicating high-quality junctions.
   • Overdamped Regime: The devices were found to operate in the overdamped regime (Stewart-McCumber parameter $\beta_c \approx 0.006$).
3. Spectroscopic Features:
   • Andreev Reflections: Voltage-bias measurements resolved multiple Andreev reflections (MARs) up to the fourth order ($n=4$).
   • Superconducting Gap: The superconducting gap was extracted to be $\Delta \approx 180$ $\mu$eV.
   • Critical Parameters: The critical magnetic field ($B_c$) was determined to be ~30 mT, and the critical temperature ($T_c$) was ~1.35 K.
4. SQUID Performance: A SQUID device exhibited clear flux-tuning of the supercurrent. The interference pattern followed the standard asymmetric SQUID model, with an effective loop area consistent with theoretical expectations accounting for flux focusing and Meissner effects.

Significance
The paper claims that this work establishes a vital experimental foundation for advancing heterostructure engineering in Ge-based systems. By demonstrating the feasibility of in situ MBE growth of Al on Si-capped Ge QWs, the authors provide a pristine material platform capable of hosting robust proximity-induced superconductivity. This approach overcomes previous limitations regarding interface quality and disorder. The authors anticipate that this platform will be instrumental for studying emerging superconducting phenomena, such as novel pairing mechanisms in hole gases, and for constructing topological quantum devices, including Majorana zero modes and superior superconductor-semiconductor hybrid qubits. The work does not claim to have observed topological states yet but positions the developed material and fabrication technology as a necessary prerequisite for such future investigations.