Monopole antenna in quantitative near-field microwave microscopy of planar structures

We have developed an analytical model of a near-field microwave microscope based on a coaxial resonator with a sharpened tip probe. The probe interacts with a layered sample that features an arbitrary depth distribution of permittivity. The microscopic tip end with the accumulated charge is regarded as a monopole antenna radiating an electric field in near zone. The impedance of such an antenna is determined within a quasi-static approximation. The proposed model is used for calculating the sample-sensitive parameters of the microscope, specifically, resonance frequency f(0) and quality factor Q(0), as a function of probe-sample distance h. The theory has been verified experimentally in studies of semiconductor structures, both bulk and thin films. For measurements, we built a similar to 2.1 GHz microscope with an effective tip radius of about 100 mu m. The theoretical and experimental dependences f(0)(h) and Q(0)(h) were found to be in a good agreement. The developed theory underlies the method for determining sheet resistance R-sh of a semiconductor film on a dielectric substrate proposed in this article. Studies were performed on doped n-GaN films on an Al2O3 substrate. The effective radius and height of the probe determined from calibration measurements of etalon samples were used as the model fitting parameters. For etalon samples, we employed homogeneous sapphire and doped silicon plates. We also performed four-probe dc measurements of Rsh. The corresponding values for samples with R-sh > 1k Omega were found to be 50% to 100% higher than the microwave results, which are attributed to the presence of microdefects in semiconductor films. (C) 2016 AIP Publishing LLC.