Measurement of electron carrier lifetimes in semiconductor materials typically uses time-resolved photoluminescence or direct-current photoconductive-decay methods. The former method suffers from poor accuracy for small samples and requires bulky collection optics and a detector tailored to the sample’s emission wavelength, whereas the latter technique requires fabrication of custom ohmic contacts and is not feasible for certain organic or 2D materials.
An alternative method spearheaded by the University of Texas at Austin uses a variation of time-resolved microwave reflectance (TMR) to instead probe the change in RF conductivity over time upon optical excitation (rather than depending on PL emission and, hence, eliminating the need for optics/detectors or ohmic contacts). The team designed a microstrip split-ring resonator (SRR) that strongly localizes the RF field interacting with a photoexcited material under test, enabling electrical readout of carrier dynamics with five orders of magnitude better measurement sensitivity and with incident optical intensity values as low as 35 fJ on material samples as small as 24 × 24 µm.
By placing a small volume of material in the microwave SRR circuit, the microwave signal is amplified when illuminated and easily read on an oscilloscope with higher accuracy and at lower cost than other methods. Carrier lifetime measurements define the overall optical quality of a semiconductor or other optoelectronic material, dictating the range of applications for which that material is suited. Reference: S. Dev et al., Nat. Commun., 10, 1625 (Apr. 9, 2019).
