Low Temperature Growth of Superconducting Thin Films for Quantum Computation

The field of superconducting quantum computing uses superconducting circuit elements to store information in qubits. These devices are fabricated using high quality thin film superconductors. Poor quality superconductors lead to a decrease in the critical temperature of the superconductor, making it difficult to use qubit systems. In this work, we explore methods of low temperature (<10 K) molecular beam epitaxy (MBE) growth to increase the critical temperatures of superconducting tantalum (Ta) and niobium (Nb) thin films on sapphire substrates. We compare the critical temperature, T_C, of thin films grown at low temperature (LT) to room temperature (RT). We found that LT MBE growth does result in higher critical temperatures compared to RT growth, most significantly for Ta thin films. Using atomic force microscopy imaging for structural analysis, we saw that LT growth produced smoother Ta films, whereas it produced rougher Nb films. Though we did not see a consistent structural difference between LT and RT growth, the increase in T_C as a result of LT growth was significant in both Ta and Nb.