Effect of Buried-Interface Preparation for Nb Superconducting Resonators on InP

Substrate surface preparation is a key step in the fabrication of electronic devices. In III–V semiconductor platforms, e.g. used in HEMTs and lasers, removal of the native substrate oxide is extremely important, where improper removal will negatively affect end-of-line device performance. However, the impact of substrate preparation in hybrid superconductor–semiconductor (S–Sm) systems relevant to quantum information applications remains poorly understood. This study compares three surface preparations for the deposition of sputtered Nb films on InP: (i) no intentional oxide removal (control), (ii) $\mathit{\text{in-situ}}$ ${\mathrm{A}\mathrm{r}}^{+}$ milling, and (iii) S-passivation. $\mathit{\text{In-situ}}$ ${\mathrm{A}\mathrm{r}}^{+}$ milling reduces the O concentration at the metal–substrate (MS) interface, but also roughens the InP surface, increasing the effective thickness of the Nb–InP interface and promoting O incorporation through extended defects in the Nb film. S-passivation suppresses interfacial O more effectively while preserving a sharper and smoother buried interface. It also yields Nb films with higher superconducting transition temperatures and less structural damage than the ${\mathrm{A}\mathrm{r}}^{+}$-milled samples. Despite these materials improvements, the microwave response is comparable across the three preparations. At single photon powers, the highest internal quality factors, $Q_i$, are approximately $1.30\times {10}^5$ ($130\mathrm{k}$) for the control resonators, $9.7\times {10}^4$ ($97\mathrm{k}$) for the S-passivated resonators, and $8.4\times {10}^4$ ($84\mathrm{k}$) for the ${\mathrm{A}\mathrm{r}}^{+}$-milled resonators. These results suggest that the present devices are not primarily limited by dielectric loss at the buried Nb–InP interface.