Modulation improves photoconductivity measurements in microwave impedance microscopy

Microwave impedance microscopy (MIM) is a technique that uses a scanning microwave probe to measure the photoconductivity of a variety of light-sensitive systems and materials, including carbon nanotubes, graphene, perovskites and 2-D lateral heterostructures. However, current MIM methods require the sample to be scanned many times at different optical powers, which can introduce errors.

A research team now reports in Review of Scientific Instruments new MIM methods that use probes with modulated optical sources to measure local photoconductivity in a single scan. Demonstrations of their optically modulated MIM, energy-resolved MIM, and time-resolved MIM methods show that their approach could lead to powerful tools for studying and developing photosensitive materials.

In their optically modulated MIM demonstration, the authors used the change in the scanning tip’s microwave impedance under modulated illumination to measure local photoconductivity of a sample of aluminum on gallium arsenide (GaAs). In the energy-resolved optical MIM demonstrations, they used a broadband light source with a tunable monochrometer to measure the sample’s variations in band-gap energy as a function of position by varying photon energy. For time-resolved optical MIM demonstrations, they calculated local photo-carrier lifetimes of a sample material when applying or removing a pulsed optical source from its surface.

All three modulated MIM methods provided clear and consistent 50-nanometer spatial resolution data for local photoconductivity in a single scan, without the need for an electrical contact on the sample material. The modulated MIM methods also removed most topographical artifacts and enhanced signal-to-noise ratios compared to measurements done with unmodulated MIM methods.

Source: “Optically coupled methods for microwave impedance microscopy,” by Scott R. Johnston, Eric Yue Ma, and Zhi-Xun Shen, Review of Scientific Instruments (2018). The article can be accessed at https://doi.org/10.1063/1.5011391 .