Epitaxial α-Ta (110) film on a-plane sapphire substrate for superconducting qubits with long coherence times

  1. Boyi Zhou,
  2. Lina Yang,
  3. Tao Wang,
  4. Yanfu Wu,
  5. Kanglin Xiong,
  6. and Jiagui Feng
Realization of practical superconducting quantum computing requires large-scale integration of qubits with long coherence times. It has been reported that {lpha}-Ta (110) film can
greatly improve the coherence times of qubits. Compared to the commonly used {\alpha}-Ta (110) film deposited on c-plane sapphire, {\alpha}-Ta (110) film can be epitaxially grown on a-plane sapphire because of the atomic relationships at their interface. Here, we demonstrate the growth of a large-scale well-ordered quasi-single crystal {\alpha}-Ta (110) film with a low density of defects on a-plane sapphire. The root mean square of the film with thickness of 200 nm is below 0.7 nm over a 10 {\mu}m \times 10 {\mu}m area and the residual resistance ratio is as high as 15.5. Transmon qubits are also fabricated using this kind of film and show relaxation times exceeding 150 {\mu}s. These results suggest {\alpha}-Ta (110) film grown on a-plane sapphire is an alternative promising choice for large-scale superconducting circuits with long coherence times of qubits.

Fabrication of Al/AlOx/Al junctions with high uniformity and stability on sapphire substrates

  1. Yuzhen Zheng,
  2. Shuming Li,
  3. Zengqian Ding,
  4. Kanglin Xiong,
  5. Jiagui Feng,
  6. and Hui Yang
Tantalum and aluminum on sapphire are widely used platforms for qubits of long coherent time. As quantum chips scale up, the number of Josephson junctions on Sapphire increases. Thus,
both the uniformity and stability of the junctions are crucial to quantum devices, such as scalable superconducting quantum computer circuit, and quantum-limited amplifiers. By optimizing the fabrication process, especially, the conductive layer during the electron beam lithography process, Al/AlOx/Al junctions of sizes ranging from 0.0169 to 0.04 {\mu}m2 on sapphire substrates were prepared. The relative standard deviation of room temperature resistances (RN) of these junctions is better than 1.7% on 15 mmx15 mm chips, and better than 2.66% on 2 inch wafers, which is the highest uniformity on sapphire substrates has been reported. The junctions are robust and stable in resistances as temperature changes. The resistances increase by the ratio of 9.73% relative to RN as the temperature ramp down to 4K, and restore their initial values in the reverse process as the temperature ramps back to RT. After being stored in a nitrogen cabinet for 100 days, the resistance of the junctions changed by1.16% in average. The demonstration of uniform and stable Josephson junctions in large area paves the way for the fabrication of superconducting chip of hundreds of qubits on sapphire substrates.