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) \textit{in-situ} $\mathrm{Ar}^{+}$ milling, and (iii) S-passivation. \textit{In-situ} $\mathrm{Ar}^{+}$ 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 $\mathrm{Ar}^{+}$-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 $1.30\times10^{5}$ ($130\mathrm{k}$) for the control resonators, $9.7\times10^{4}$ ($97\mathrm{k}$) for the S-passivated resonators, and $8.4\times10^{4}$ ($84\mathrm{k}$) for the $\mathrm{Ar}^{+}$-milled resonators. These results suggest that the present devices are not primarily limited by dielectric loss at the buried Nb--InP interface.