Scanning microwave microscope algorithm offers robust calibration method for doped semiconductors

As electronic nanodevices become more widespread, it becomes increasingly important to characterize the electric properties of doped semiconductors on such a small scale. The scanning microwave microscope (SMM), an atomic force microscope interfaced with a vector network analyzer (VNA), is a versatile tool to investigate materials at microwave frequencies on the nanoscale, including dielectrics, semiconductors and 2-D materials.

In Review of Scientific Instruments, researchers present a comprehensive outline for calibrating these microscopes using a one-port algorithm that relates the measured scattering parameter (S-parameter) S₁₁ to samples’ carrier densities.

To suss out usable microwave scattering data received at the tip-sample interface, apart from extraneous signals originating elsewhere in the microscope, the algorithm draws from classical VNA one-port calibration to quantify the scattering parameter S₁₁. In the case of doping density determination, the report advises to choose maximum, minimum and intermediate doped standards. Using error coefficients determined through calibration, subsequent measurements then provide the sample intrinsic S₁₁ values. In such calibration methods, the authors note, a suitable choice of calibration standards is crucial in order to properly determine the error coefficiencts.

Testing samples of MBE-grown, Si-doped GaAs multilayers proved that while tuning fork and cantilever setups can differ in terms of conditions for which they are best suited, the algorithm worked comparably well for both setups over a wide frequency range up to 27.5 gigahertz. The calibration method can predictably work well with silicon structures, too.

After having proven the general suitability of the calibration when the same substrate is used, the team will now look to find ways of making the calibrations transferrable between substrates.

Source: “Scanning microwave microscopy applied to semiconducting GaAs structures,” by Arne Buchter, Johannes Hoffmann, Alexandra Delvallée, Enrico Brinciotti, Dimitri Hapiuk, Christophe Licitra, Kevin Louarn, Alexandre Arnoult, Guilhem Almuneau, François Piquemal, Markus Zeier, and Ferry Kienberger, Review of Scientific Instruments (2018). The article can be accessed at https://doi.org/10.1063/1.5015966 .