An AI-generated depiction of time-resolved CL imaging.
Introduction
Cathodoluminescence (CL) is the emission of light that occurs when a material is excited by a high-energy electron beam. When an electron beam strikes a sample, it transfers energy to the sample’s constituent atoms and molecules, exciting electrons into higher energy states. Ultraviolet or visible photons are released as electrons transition back to the ground state which is collected by a CL detector. CL detectors are often installed in scanning electron microscopes (SEMs) so that high-resolution CL images can be recorded. This data helps scientists gain insight into the fundamental properties and behaviors of semiconductors, dielectric, and photonic materials.
Time-resolved CL goes beyond the conventional CL approach by incorporating precise time-domain measurements by introducing synchronization between a pulsed electron beam and time-correlated single-photon detectors (SPD). This allows the CL signal variation to be recorded over small-time scales, such as over tens of nanoseconds. Time-resolved CL enables the study of fast processes, such as energy transfer, carrier dynamics, and excited state lifetimes, all with nanoscale spatial resolution.
Lifetime Mapping and g(2) Imaging
Time-resolved CL encompasses lifetime mapping and autocorrelation g(2) measurements. Lifetime imaging involves pulsed electron beams to trace the decay of excited states over time, permitting a detailed look at the dynamics of electron beam-induced excitation and deexcitation processes of optoelectronic materials.
g(2) curve courtesy of Dr. Sophie Meuret (AMOLF, Amsterdam)
The second-order autocorrelation function, also known as g(2)(τ), is a pivotal parameter in quantum optics and provides insight into photon emission synchronization. The technique can involve either a continuous or pulsed electron beam and is used to identify and characterize single-photon emissions at the nanoscale. The g(2) signal indicates the coherence and bunching or antibunching of photons, playing a significant role in helping understand fundamental physical processes of quantum systems.
Shedding Light on Nanoscale Dynamics
The advent of time-resolved CL has opened new frontiers in the ability to explore the dynamics of light emission at the nanoscale. It sheds light on the optical properties of materials, quantum phenomena, and interactions between photons and electrons.
Semiconductor Physics and Optoelectronics: Time-resolved CL plays a crucial role in studying semiconductors, where it serves as an effective strategy to assess the optical properties, band structures, and carrier dynamics that underpin their functionality. By examining excited state lifetimes, researchers can investigate carrier diffusion and recombination and review the impact of defects on device performance. The predominant application of time-resolved CL in the semiconductor industry is the optimization of optoelectronic devices such as light-emitting diodes (LEDs), photodetectors, and solar cells.
- Semiconductor quantum wells: Quantum wells are ultra-thin layers in electronic devices where electrons are confined. Excited lifetime states reveal the inner workings of carrier recombination processes and the influence of quantum confinement on radiative and non-radiative decay rates. Knowledge in these areas is valuable in the design and optimization of quantum well structures for applications in lasers, photodetectors, and optoelectronic devices.
- Defect engineering in semiconductors: Defects determine the properties and performance of semiconductor materials. Time-resolved CL precisely localizes and characterizes defects within the material and is used to map the excited state lifetimes associated with defect-related emissions. This knowledge guides engineering strategies aimed at enhancing the material quality and overall device efficiency.
- Perovskite solar cells: Perovskite solar cells have emerged as a promising alternative for efficient and cost-effective photovoltaics. Time-resolved CL imaging has contributed significantly to understanding the charge carrier dynamics and degradation mechanisms in perovskites materials. By mapping the excited state lifetimes and tracking carrier recombination processes, researchers have identified factors affecting device performance, such as trap states and ion migration. These insights guide the optimization of perovskite solar cells and the development of strategies to enhance both their stability and efficiency.
Quantum Dots and Nanostructures: Time-resolved CL offers a window into the world of quantum dots and nanostructures. These nanoscale entities exhibit unique optical properties and quantum confinement effects. Researchers investigate size-dependent emission characteristics, excitonic behaviors, and energy transfer processes within these structures. Such knowledge is essential for developing next-generation quantum technologies and advancing fields like quantum computing and quantum communication.
Plasmonics and Nanophotonics: Plasmonic structures, which manipulate light at the nanoscale using surface plasmons, hold tremendous potential for applications in sensing, imaging, and energy harvesting. Time-resolved CL allows for the visualization and characterization of plasmonic resonances, enabling the exploration of plasmon-enhanced phenomena and the study of localized electromagnetic fields. It aids in understanding radiative and non-radiative decay processes in plasmonic nanostructures, paving the way for advancements in nanophotonics and the design of more efficient optical devices.
