Interface-Sensitive Electron Spin Resonance (iESR) Spectroscopy of Spin-Active Defects in Epitaxially-Grown Materials for Classical and Quantum Devices

We show that Electron Spin Resonance (ESR) spectroscopy can be used to detect and identify surface- and interface- defects in epitaxial materials by using lithographically fabricated coplanar waveguide (CPW) resonators. We will demonstrate the improved sensitivity and wide applicability of chip-scale, interface-sensitive ESR (iESR) spectroscopy by applying it to both classical and quantum material stacks. ESR spectroscopy has been a useful tool in the study of defects in semiconductors owing to its sensitivity to a low density of spin-active defects, its ability to readily differentiate between various charge states of defects, and to discern asymmetries in the defects’ environment by extracting the associated g-tensor. However, commercially available ESR spectrometers utilize bulky cavity microwave resonators which are not sensitive to a low density of spin-defects in sub-micron epitaxial thin-films, at interfaces, or on surfaces due to the resonator’s mode volume being much larger than the region being probed. The two-dimensional nature of our high-Q microwave resonators used in iESR spectroscopy allows us to overcome this limitation, achieving defect detection sensitivities as low as 10⁸ 1/sq-cm. We aim to demonstrate the versatility of iESR spectroscopy by presenting results from our characterization of diverse material systems. We will first present results from using iESR spectroscopy to measure surface-defects in MOCVD grown films of the ultra-wide bandgap semiconductor β-Ga₂O₃, which have been known to limit device performance.[1] Our measurements show the presence of defect signatures in epitaxial β-Ga₂O₃ beyond those that can be observed by commercial X-Band spectrometers. We will discuss the characteristics and possible sources of these additional signatures, as well as techniques to remove the associated defects through appropriate surface treatments. We will then present the applicability of iESR spectroscopy to superconducting qubit platforms. It is known that all interfaces of the superconducting qubit material stack possess defects which are sources of significant loss and decoherence. Although these interface-defects are modeled as two-level-systems (TLS), their microscopic nature is unknown.[2] We will show that superconducting CPW resonators, with their exceptionally high Q-factors (10⁴ - 10⁵) and shallow microwave modes (~1 μm), are ideally suited to measure spin-active defects at the interfaces of the superconducting qubit material stack, and that iESR can provide valuable insight into the possible nature of TLS defects.

[1] Elaheh Ahmadi et al Appl. Phys. Express 10, 071101 (2017)
[2] P. Macha et al Appl. Phys. Lett. 96, 062503 (2010)

This work was supported by AFOSR under grant No. FA9550-23-1-0688 and SUPREME, one of seven centers in SRC JUMP 2.0.