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Scanning electron microscopy (SEM) is a widely used technique across many scientific fields, including the chemical, nanotechnology and semiconductor sectors. There are many different variations where the operating principles differ slightly from conventional SEM instruments. In this article, we look at cathodoluminescence SEM instruments.
Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) is a high-energy imaging technique that is often used to determine the topology and composition of a surface. In SEM, electrons are fired from an electron gun towards a sample of interest. To create a high energy electron beam, the electrons are concentrated and focused through a series of lenses, followed by a pair of scanning coils and deflector plates before being directed towards the sample.
When the electrons interact with the sample, their energy quickly diminishes due to elastic scattering and absorption mechanisms; and it is the energy exchange between the electrons and the sample that causes the electrons to reflect by elastic scattering means. Secondary electrons are also released by inelastic scattering and the emission of electromagnetic radiation, both of which are observable by the detector. SEM produces an image using the signal intensity distribution observed by the detector.
Cathodoluminescence is a luminescence technique similar to photoluminescence which is widely used on semiconductor materials. Conventional luminescence is caused by the recombination of an electron and a hole when the molecules are excited. By comparison, cathodoluminescence occurs when a high energy electron beam is used to excite the electrons, but the primary electrons carry too much energy and emit secondary Auger electrons. The secondary electrons then scatter and cause electrons within the valence band to be promoted to the conduction band, where upon it luminesces by releasing cathodoluminescent photons.
How Cathodoluminescence Benefits SEM
Cathodoluminescence principles can be employed to SEM instruments for use on many different types of materials, including low-dimensional semiconductors, rocks and minerals, and solid-state oxides such as zirconia and titania.
Cathodoluminescence SEM is a non-destructive microscopy technique that maps the electronic and optical properties of a material, with a nanospatial resolution. Cathodoluminescence SEM can back out many different properties of a material, including the structural composition, electrical conductivity/insulating properties, and the effects and/or presence of defects, impurities, dopants, vacancies and contamination. It is a technique that can be employed for use in quality control and failure analysis laboratories.
Unlike in conventional SEM, the spatial resolution is strongly influenced by the interaction between the landing electrons and the sample, rather than by the accelerating voltage in the electron column. When the electrons hit the sample, they cause a transfer of energy through inelastic scattering, and this energy is eventually released as either as cathodoluminescent photons, or as X-ray photons (or both). Through this mechanism, it is easier to generate cathodoluminescence photons than it is X-ray photons because the energy required to do so is lower.
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Potential Issues with Using Cathodoluminescence SEM
The operating principles for bulk and thin materials does differ, and in general cathodoluminescence SEM is better suited for thinner materials, i.e. materials that are less than 100 nm in thickness. As such, some issues can arise when using cathodoluminescence SEM on bulk materials. To use cathodoluminescence SEM on bulk materials, the general volume needs to be small. This translates to an operational mode where low voltages and low primary electron energies are used.
However, lower primary energies come with three main consequences. The first is that there is a lower energy transference from the electron to the sample and this results in fewer cathodoluminescent photons being generated, and ultimately, a lower signal.
Another potential issue is the generation of surface effects. The region of a bulk sample being analyzed can be closer to the surface of the electron beam than other part of the material, and this can cause the generation volume to be smaller. Additionally, some regions of a surface can undergo non-radiative recombination, and this creates a “dead layer” where cathodoluminescence will not occur.
For many bulk materials, a higher injection density is required to penetrate the surface. However, this can result in lower cathodoluminescence emittance when the material becomes electronically saturated. High injection densities can also lead to changes in the local equilibrium state, unwanted beam induced changes to the sample, luminescence saturation and the generation of quenching effects.
“CATHODOLUMINESCENCE IN THE SCANNING ELECTRON MICROSCOPE: APPLICATION TO LOW-DIMENSIONAL SEMICONDUCTOR STRUCTURES”- Gustafson A. and Kapon E., Scanning Microscopy International, 1998
“Contrast and decay of cathodoluminescence from phosphor particles in a scanning electron microscope”- den Engelsen D., et al, Ultramicroscopy, 2015, DOI: 10.1016/j.ultramic.2015.05.009
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