“Measuring the decay of the electrical (microwave) signal allows us to measure the materials' carrier lifetime with far greater accuracy. We have discovered it to be a simpler, cheaper and more effective method than current approaches.”Daniel Wasserman
A collaborative effort led by the University of Texas at Austin has demonstrated a new technique for measuring the quality of semiconductor materials much more sensitively than currently possible, providing a means to probe their properties in small volumes and offering a characterization window into new materials and structures otherwise unavailable. The approach could also offer a better understanding of infrared materials, benefiting innovations in infrared detection, molecular sensing, thermal imaging and defense and security systems.
As reported in Nature Communications[Dev et al. Nat. Commun.(2019) DOI: 10.1038/s41467-019-09602-2],the enhanced power of the new method to characterize materials at much smaller scales increases our understanding of optical and electronic materials, as well as the discovery and investigation of 2D, micro- and nanoscale materials.The approach could be developed into a rapid scanning system that allows for spatial mapping or rapid feedthrough measurements of material quality.
The system and the measurement technique were based on identifying the capability of providing quantitative feedback on material quality, especially applications for the development and manufacturing of optoelectronic devices. In optoelectronic materials, the amount of time that the electrons remain “photoexcited”, or capable of producing an electrical signal, informs on its potential for photodetection.
Measuring the lifetime of photoexcited electrons tends to be an expensive and difficult process, with limited accuracy for large-scale material samples. However, here they used a different method for quantifying these lifetimes by positioning small amounts of the materials in designed microwave resonator circuits. When the samples were exposed to concentrated microwave fields and hit with light, the microwave circuit signal changes, which can be read on an oscilloscope. The decay of the signal shows the lifetimes of photoexcited charge carriers in small volumes of the material.
In leveraging microwave technologies for measuring vital optical properties, the spatial scale of microwaves doesn't match well with optical materials and devices, so this approach allowed them to use RF techniques for investigating micro-, nano- or even 2D materials. As team leader Daniel Wasserman said “Measuring the decay of the electrical (microwave) signal allows us to measure the materials' carrier lifetime with far greater accuracy. We have discovered it to be a simpler, cheaper and more effective method than current approaches.”
The team now want to investigate designs allowing for a single probe measurement of material properties for scanning and high feedthrough applications, integrating the optical and RF. They are also interested in using the technique to measure the lifetimes of a number of different, particularly 2D, materials, as well as exploring the integration of new materials to achieve higher speed and sensitivity detection for IR applications.
A schematic of the microwave circuit with the microwave signal (the blue waveform) travelling along the bus line of the circuit, and being modulated by the infrared pulse (red) incident upon the infrared pixel. The inset shows a micrograph of the pixel sitting on the gap of the microwave resonator.