Accurately measuring semiconductor properties of materials in small volumes helps engineers determine the range of applications for which these materials may be suitable in the future, particularly as the size of electronic and optical devices continues to shrink. A measurement technique was developed that is capable of achieving this level of sensitivity.
The approach provides quantitative feedback on material quality, with particular applications for the development and manufacturing of optoelectronic devices. The method is capable of measuring many of the materials that one day may be ubiquitous to next-generation optoelectronic devices.
Optoelectronics is the study and application of electronic devices that can source, detect, and control light. Optoelectronic devices that detect light, known as photodetectors, use materials that generate electrical signals from light. Photodetectors are found in smartphone cameras, solar cells, and in the fiber optic communication systems that make up broadband networks. 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 new technique uses 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.
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 is integrated into a photodetector device structure. Materials with 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.
One area that will benefit from the real-world applications of the technology is infrared detection, a vital component in molecular sensing, thermal imaging, and certain defense and security systems. High-speed detectors operating at these frequencies could 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.