Operando Imaging of Ultrafast Charge Carrier Dynamics at Biased Semiconductor Interfaces

In this project, Professor James Cahoon at the University of North Carolina at Chapel Hill is establishing operando pump-probe microscopy as a new approach for probing ultrafast charge-carrier dynamics in electrically contacted semiconductor nanostructures. Pump-probe microscopy enables direct visualization of how electrons and holes move, separate, and recombine in semiconductor materials with ultrafast time resolution and high spatial resolution. Nearly all previous studies using this technique have examined materials under open-circuit conditions, even though practical optoelectronic devices operate under applied electrical bias. Understanding how applied voltage, internal electric fields, and steady-state current flow influence charge-carrier motion remains a major challenge in semiconductor science. Professor Cahoon and his students will establish the experimental framework needed to directly observe carrier dynamics in electrically active semiconductor devices under controlled bias conditions. Their studies will yield fundamental new understanding of bias-dependent carrier behavior and provide a quantitative bridge between microscopic charge dynamics and macroscopic device function. The project aligns with national priorities to maintain U.S. competitiveness in semiconductors and microelectronics. Students will receive interdisciplinary training in spectroscopy, semiconductor fabrication, materials science, and modeling, while research results will be disseminated through publications, scientific meetings, and public outreach activities. The project will develop operando pump-probe microscopy for imaging biased semiconductor interfaces with femtosecond temporal resolution and diffraction-limited spatial resolution. Microscopy-compatible resistor, diode, and field-effect transistor device architectures will be fabricated to enable systematic variation of applied electrical bias during ultrafast optical measurements. High-sensitivity, low-fluence measurements will be implemented to probe carrier dynamics in regimes where photoexcited carriers minimally perturb the intrinsic electrostatic potential of the semiconductor. Experimental measurements will quantify carrier drift, diffusion, separation, and recombination on picosecond time scales and will test foundational transport relationships, including the fluctuation-dissipation theorem and Einstein relation, under nonequilibrium electrical conditions. The project will also examine how electrically induced population and depopulation of surface and defect states alter carrier motion and relaxation dynamics. Experimental observations will be interpreted using continuum modeling approaches that incorporate realistic device geometries and electrostatic conditions. Studies will focus on two model material systems: quasi-one-dimensional silicon nanowires with controlled dopant distributions and two-dimensional semiconductor flakes. Together, these efforts will establish operando pump-probe microscopy as a new platform for understanding ultrafast carrier behavior in electrically active semiconductor systems. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria. NSF Award ID: 2601722 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: James Cahoon | Institution: University of North Carolina at Chapel Hill, CHAPEL HILL, NC | Award Amount: $550,000 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2601722 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2601722.html