In-SEM photoluminescence

When a material absorbs a photon, one of its electrons get excited to a higher energy state. As it returns to its ground state, sometimes another photon is emitted. An illustration of the principle is depicted in Figure 1 for both semiconductors with a direct band gap, where photon absorption and emission are directly allowed, and semiconductors with and an indirect band gap, where photon absorption and emission are not allowed unless mediated by absorption or emission of a phonon. Other types of samples, such as organic molecules, do not have a band structure where electrons are free to move. They exhibit photoluminescence through excited states present in the electronic orbitals themselves. In biosciences, PL is colloquially known as fluorescence.

By analysing the emitted light, valuable information can be obtained about the material band structure, defects, impurities, and electronic transitions. PL signal is generally at longer wavelengths (lower energies) than the excitation source, as some energy is dissipated in the process.

Typical PL experiments consist of varying the excitation power and recording signal evolution, scanning the excitation source on the sample to perform PL maps, or varying sample temperature while recording PL spectra. In the context of in-SEM photoluminescence, additional insight is obtained from high-resolution SEM imaging. Luminescence can be correlated with structural properties resolved at nm resolution and completed with auxiliary characterization techniques offered by SEM.

PL instruments rely on an excitation light source, a detection system and a dichroic mirror. The dichroic mirror is a special optical component engineered to split incoming light into reflected and transmitted spectral ranges. Laser light gets reflected towards the sample, and PL signal gets transmitted towards the detection system. This way, not only is the PL signal efficiently collected, but the detection system is protected from the excitation light source reflected from the sample, which is often bright enough to saturate or even damage the detection system. This principle is illustrated in Figure 2.  

Often, the excitation light source is insufficiently rejected by the dichroic mirror, and additional filters are placed in the path of the PL signal. Additional components are usually present in the system, for example to control excitation power and polarization, focus it on the sample, or redirect the signal between different detection systems. Excitation sources are usually lasers, which provide much higher brightness than any other light source and are available at wavelengths across the whole UV-VIS-NIR range. High performance PL detection systems typically consist in an imaging spectrometer equipped with a CCD camera specialized for spectroscopy, very similar to a cathodoluminescence spectroscopy system. The installation of the injection line / PL setup on the Allalin platform is sketched out in Figure 3.

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