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 f0 and quality factor Q0, 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 ∼2.1 GHz microscope with an effective tip radius of about 100 μm. The theoretical and experimental dependences f0(h) and Q0(h) were found to be in a good agreement. The developed theory underlies the method for determining sheet resistance Rsh 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 Rsh > 1 kΩ were found to be 50% to 100% higher than the microwave results, which are attributed to the presence of microdefects in semiconductor films.
A method for determining the parameters of a layered semiconductor structure, using the data obtained by near-field microwave probing with a micron-size lateral resolution, was developed and tested experimentally. We have measured a frequency spectrum of the impedance of a coaxial antenna formed on a test structure surface. The corresponding inverse problem has been solved based on the quasistatic theory for the impedance of a monopole antenna interacting with a layered medium, which was proposed earlier [A. N. Reznik and S. A. Korolyov, J. Appl. Phys. 119, 094504 (2016)]. This method was applied to a low-barrier Mott diode structure with a nearly 100 nm thick undoped layer grown on a conducting substrate GaAs. Computer simulation allowed us to establish the optimal frequency intervals and estimate the accuracy of determining the structure parameters. Measurements were taken in the frequency range of 0.1–67 GHz on commercially available equipment. Three antennas with a radius of the central conductor of 5.5, 11, and 25 μm, respectively, were used. The accuracy of the experimental evaluation of the layer thickness d and conductivity σ was ∼1–3%, and for the substrate conductivity, it came to about 15%. As an example, we also present the parameters σ and d in four points of the sample surface image. These data show strong lateral inhomogeneity of the structure under study.
In this work, our earlier method for measuring resistance R of semiconductor films with a near-field scanning microwave microscope [A. N. Reznik and S. A. Korolyov, J. Appl. Phys. 119, 094504 (2016)] is studied in a 0.1 kΩ/sq < R < 15 kΩ/sq range. The method is based on a microscope model in the form of a monopole or dipole antenna interacting with an arbitrary layered structure. The model fitting parameters are determined from the data yielded by calibration measurements on a system of etalon samples. The performance of the method was analyzed experimentally, using strip-probe and coaxial-probe microscopes in the frequency range of 1-3 GHz. For test structures, we used doped GaN films on the AlO substrate and also transistor structures based on the AlGaN/GaN heterojunction and AlGaAs/GaAs/InGaAs/GaAs/AlGaAs quantum well with a conducting channel. The obtained microwave microscope data were compared with the results of measurements by the van der Pauw method. At the first stage of the experiment, the calibration etalons were bulk homogeneous samples with different permittivity/conductivity values. In this case, satisfactory agreement between the microscope and the van der Pauw data was obtained with a strip probe on all tested samples in the entire range of R. With a coaxial probe, such accordance was observed only in high-ohmic samples with R > 1 kΩ/sq. The use of GaN film structures as a calibration system helped to increase the accuracy of the coaxial-probe-aided measurement of R to a level of ∼10%.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.