Cathodoluminescence - physical principles and detection systems

Cathodoluminescence (CL) is the emission of light by a solid subject to electron bombardment (cathode ray). It was widely used in everyday life by the television industry until the advent of LCD flat screens. The operation of cathode-ray displays is based on the emission of light by phosphors emitting different colors (red, green, blue) under the impact of three electron beams, i.e. cathodoluminescence. In the field of materials science, the phenomenon of cathodoluminescence makes it possible to highlight spatial variations in the local optical properties of a non-metallic material, and leads to a characterization technique commonly used to study the local properties of semiconductor materials and insulators. In principle, this method is non-destructive, in the sense that the material is not destroyed by the electron beam. However, the optical and electronic properties of materials may be permanently or temporarily altered by the impact of the beam, which may ionize or create defects, or cause them to diffuse.

Cathodoluminescence characterization techniques require little or no sample preparation (polished and metallized surfaces in the case of insulators). Light emission can be observed with an optical microscope, but experiments are very frequently carried out with a scanning electron microscope (SEM). This makes it very easy to obtain a two-dimensional image of the luminescence properties of the sample under study. Some scanning transmission electron microscopes (STEM) are equipped with a cathodoluminescence detection system.

The spatial resolution of cathodoluminescence is mainly determined by the photon emission volume. This depends on a number of parameters, and can vary from the cubic nanometer (nm ³ ) to the cubic micrometer (μm ³ ).

Cathodoluminescence is a spectroscopic method for identifying, on a local scale, the point defects and impurities responsible for a material's luminescent properties. The photons detected have wavelengths in the infrared (IR), visible and ultraviolet (UV) regions of the electromagnetic spectrum.

The sensitivity of cathodoluminescence is such that we can detect variations in impurity concentrations of the order of 10 ¹⁴ atoms · cm ^–3 , which is far superior to what can be obtained with X-ray microanalysis. In geology, it is therefore possible to detect the presence of traces. On the other hand, there is no universal theory of cathodoluminescence that can quantitatively determine the concentration of impurities in a material; this is because the mechanisms giving rise to luminescence are not only affected...