- Cathodoluminescence
Cathodoluminescence is an optical and electrical
phenomenon whereby a beam ofelectron s is generated by anelectron gun (e.g.cathode ray tube ) and then impacts on a luminescent material such as aphosphor , causing the material to emit visible light. The most common example is the screen of atelevision . In geology, mineralogy and materials science ascanning electron microscope with specialized optical detectors, or an opticalcathodoluminescence 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.Cathodoluminescence occurs because the impingement of a high energy electron beam onto a
semiconductor will result in the promotion ofelectrons from thevalence band into theconduction band , leaving behind a hole. When an electron and a hole recombine, it is possible for aphoton to be emitted. The energy (color) of the photon, and the probability that a photon and not aphonon 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 ofband 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 ascanning 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 amonochromator and then detected with aphotomultiplier 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 entirespectrum at each point (hyperspectral imaging ) if the photomultiplier tube is replaced with aCCD 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 aquantum well s orquantum dots .Although
direct bandgap semiconductors such asGaAs orGaN are most easily examined by these techniques, indirect semiconductors such assilicon 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 inintegrated 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.
External links
* [http://plaza.snu.ac.kr/~lee2602/atlas/cath_intro.html An introduction to CL]
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)
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