Quantitative scanning microwave microscopy of 2D electron and hole gases in AlN/GaN heterostructures

Although the scanning microwave microscope (SMM) is based on the atomic force microscope (AFM), the SMM differs from the AFM by being able to sense subsurface electromagnetic properties of a sample. This makes the SMM promising for in-depth nondestructive characterization of nanoelectronic structures. However, the SMM raw data are convoluted with the sample topography, making it especially challenging for quantitative characterization of nonplanar structures. In this paper, using the topography information simultaneously obtained by the AFM and the in situ extracted probe geometry, we de-embed from the topography-corrupted SMM data the sheet resistance of 2D electron or hole gas (2DEG or 2DHG) buried at the interface of an AlN/GaN heterostructure, including the lateral depletion of the 2DEG from an etched step. The SMM results are validated by Hall-effect measurements. The limitation and possible improvement in the present technique are discussed. With improved setup, the SMM can be used to nondestructively monitor the local sheet resistance of 2DEG or 2DHG during device manufacture. These studies help to pave the way to 3D microwave tomography on the nanometer scale.