A Scanning Microwave Microscopy Study of FIB-Induced Impedance Changes in InGaAs/InP and HfO2/InGaAs/InP Heterostructures

Focused ion beam (FIB) milling is a powerful direct-write approach for nanoscale structuring of semiconductor heterostructures, but it inevitably induces structural, compositional, and electrical modifications that require complementary characterization techniques  for proper assessment. In this work, scanning microwave microscopy (SMM) is used as the primary probe to follow the evolution of the local impedance response induced by Ga⁺ FIB milling in two III-V-based stacks relevant to mid-infrared photonics: an uncapped 150 nm heavily doped InGaAs layer on InP and a HfO₂ (35 nm)/InGaAs (150 nm)/InP heterostructure. By combining SMM with atomic force microscopy (AFM) and Raman spectroscopy, the study correlates changes in amplitude and phase of the microwave reflection coefficient S11 with implantation, amorphization, oxide degradation, roughening, and the progressive exposure of the underlying semiconductor layers. The uncapped InGaAs/InP sample shows a relatively direct evolution of the SMM signal with dose, consistent with progressive Ga implantation and the transition from InGaAs-dominated to InP-dominated response. In the HfO₂-capped structure, the microwave response is more complex: the oxide initially acts as a partial buffer, delaying damage transfer to the semiconductor, but its protective effect diminishes at higher doses, leading to a mixed response associated with degradation, semiconductor damage, and multilayer coexistence within the irradiated regions. Raman spectra support these trends by revealing dose-dependent broadening and intensity changes in the vibrational bands, while AFM confirms the onset of roughening and morphological inhomogeneity at high doses. Overall, the results show that SMM is highly sensitive to nanoscale electrical and dielectric gradients generated by FIB milling and provides subsurface information that cannot be captured by morphology alone, making it a valuable tool for the optimization of FIB processing in plasmonic III-V heterostructures, owing to its high sensitivity in detecting implantation at very low doses.