Cathodoluminescence

Cathodoluminescence is an optical and electrical phenomenon whereby a beam of electrons is generated by an electron gun (e.g. cathode ray tube) and then impacts on a luminescent material such as a phosphor, causing the material to emit visible light. The most common example is the screen of a television. In geology, mineralogy and materials science a scanning electron microscope with specialized optical detectors, or an optical cathodoluminescence microscope, is used to examine internal structures of semiconductors, rocks, ceramic, glass etc. in order to get information on the composition, growth and quality of the material.

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Cathodoluminescence occurs because the impingement of a high energy electron beam onto a semiconductor will result in the promotion of electrons from the valence band into the conduction band, leaving behind a hole. When an electron and a hole recombine, it is possible for a photon to be emitted. The energy (color) of the photon, and the probability that a photon and not a phonon will be emitted, depends on the material, its purity, and its defect state. In this case, the "semiconductor" examined can, in fact, be almost any non-metallic material. In terms of band structure, classical semiconductors, insulators, ceramics, gemstones, minerals, and glasses can be treated the same way.

In materials science and semiconductor engineering, cathodoluminescence will mostly be performed in either a scanning electron microscope or a scanning transmission electron microscope. In these cases, the highly focused beam of electrons impinges on a sample and induces it to emit light from a localized area. This light will be collected by an optical system, such as an elliptical mirror. From there, a fiber optic will transfer the light out of the microscope where it will be separated by a monochromator and then detected with a photomultiplier tube. By scanning the microscope's beam in an X-Y pattern and measuring the light emitted with the beam at each point, a map of the optical activity of the specimen can be obtained. The primary advantages to the electron microscope based technique is the ability to resolve features down to 10-20 nanometers, the ability to measure an entire spectrum at each point (hyperspectral imaging) if the photomultiplier tube is replaced with a CCD camera, and the ability to perform nanosecond- to picosecond-level time-resolved measurements if the electron beam can be "chopped" into nano- or pico-second pulses. However, as the abilities are improved, the cost of the electron-microscope based techniques becomes very high. These advanced techniques are useful for examining low-dimensional semiconductor structures, such a quantum wells or quantum dots.

Although direct bandgap semiconductors such as GaAs or GaN are most easily examined by these techniques, indirect semiconductors such as silicon also emit weak levels of light, and can be examined as well. In particular, the luminescence of dislocated silicon is different from intrinsic silicon, and can be used to map defects in integrated circuits.

Except of the much higher magnification and good versatility, an electron microscope with a cathodoluminescence detector will be more complicated and more expensive compared to an easy to use optical cathodoluminescence microscope which benefits from its ability to show actual visible color features immediately through the eyepiece.

In short, cathodoluminescence is a technique that can be implemented in an optical or electron microscope with the proper accessories, and allows the optical properties of non-metallic materials to be examined.

References

• B. G. Yacobi and D. B. Holt, Cathodoluminescence Microscopy of Inorganic Solids,New York, Plenum (1990)
• C. E. Norman, Microscopy and Analysis, March 2002, P.9-12
• S. A. Galloway et al., Physica Status Solidi (C), V0(3), P.1028-1032 (2003)
• C. M. Parish and P. E. Russell, Scanning Cathodoluminescence Microscopy, in Advances in Imaging and Electron Physics, V.147, ed. P. W. Hawkes, P. 1 (2007)