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Measurement of semiconductor material quality gets more sensitive

A measuring technique that is able to characterise materials at scales much smaller than any current technologies may be able to accelerate the discovery and investigation of 2D, micro- and nanoscale materials.

The Cockrell School of Engineering's design approach was focused on developing the capability to provide quantitative feedback on material quality, with particular applications for the development and manufacturing of optoelectronic devices. The method demonstrated is said to be capable of measuring many of the materials that engineers believe will one day be ubiquitous to next-generation optoelectronic devices.

In an optoelectronic material the amount of time that the electrons remain "photoexcited," or capable of producing an electrical signal, is a reliable indicator of the potential quality of that material for photodetection applications.

The current method used for measuring the carrier dynamics, or lifetimes, of photoexcited electrons is costly and complex and only measures large-scale material samples with limited accuracy. The UT team decided to try using a different method for quantifying these lifetimes by placing small volumes of the materials in specially designed microwave resonator circuits.

Samples are exposed to concentrated microwave fields while inside the resonator. When the sample is hit with light, the microwave circuit signal changes, and the change in the circuit can be read out on a standard oscilloscope. The decay of the microwave signal indicates the lifetimes of photoexcited charge carriers in small volumes of the material placed in the circuit.

"Measuring the decay of the electrical (microwave) signal allows us to measure the materials' carrier lifetime with far greater accuracy," Daniel Wasserman, an associate professor at Cockrell who led the team, explained. "We have discovered it to be a simpler, cheaper and more effective method than current approaches."

Carrier lifetime is a critical material parameter that provides insight into the overall optical quality of a material while also determining the range of applications for which a material could be used when it's integrated into a photodetector device structure. For example, materials that have a very long carrier lifetime may be of high optical quality and therefore very sensitive, but may not be useful for applications that require high-speed.

"Despite the importance of carrier lifetime, there are not many, if any, contact-free options for characterising small-area materials such as infrared pixels or 2D materials, which have gained popularity and technological importance in recent years," Assoc Prof Wasserman said.

One area certain to benefit from the real-world applications of this technology is infrared detection, a vital component in molecular sensing, thermal imaging and certain defence and security systems.

"A better understanding of infrared materials could lead to innovations in night-vision goggles or infrared spectroscopy and sensing systems," Assoc Prof Wasserman added.

High-speed detectors operating at these frequencies could even enable the development of free-space communication in the long wavelength infrared - a technology allowing for wireless communication in difficult conditions, in space or between buildings in urban environments.

Author
Bethan Grylls

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