SEM Application in Material Sciences

The types of signals produced by an SEM include secondary electrons, back-scattered electrons (BSE), characteristic X-rays, light (cathodoluminescence or CL), elctron beam induced current (EBIC). These signals are widely used for materials analysis.

Secondary electron (SE) imaging of materials can be done in SUPRA and ULTRA SEMs using the most common Everhart-Thornley Detector (ETD) or In-Lens Detector

SE imaging using ETD allows producing high-resolution images of a sample surface, revealing details about 1 to 5 nm in size. Due to its position aside of the lens SEM micrographs yield a characteristic three-dimensional appearance useful for understanding the surface structure of a sample.

SE imaging  using In-Lens detector allows imaging at very small working distances (WD) obtaining high resolution images even at very low accelerating voltages. 

Fig. 2.  SE image of carbon nanotubes on carbon surface obtained at 3 kV accelerating voltage by In-Lens detector.

Fig. 3.  SE image of Tin balls on carbon surface obtained at 0.1 kV accelerating voltage by In-Lens detector.

Another special property of In-lens detector is its sensitivity to surface potential. This property allows in some cases reveal features that are not seen by common SE imaging using ETD.

Fig. 4. Potential Contrast of ferroelectric domain boundaries in CeO2 thin film imaged using In-Lens detector.

Back scattered electron imaging

Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by elastic scattering. BSE are often used in analytical SEM along with the spectra made from the characteristic X-rays. Because the intensity of the BSE signal is

strongly related to the atomic number (Z) of the specimen, BSE images can provide information about the distribution of different elements in the sample.
Back scattered electron imaging can be done in SUPRA using AsD (angle sensitive detector) and in ULTRA using ESB (energy selective backscattered detector)

Fig. 5. SE image (a) revealing topography contrast and  and ESB (b) image revealing atomic number contrast  of the same area of a sample presenting a mixture of Pt particles with ceramics.

Energy Dispersive Spectroscopy (EDS)

Characteristic X-rays are emitted when the electron beam removes an inner shell electron from the sample, causing a higher energy electron to fill the shell and release energy. These characteristic X-rays are used to identify the composition and measure the abundance of elements in the sample.

Cathodoluminescence (CL)

Cathodoluminiscemce is used to examine internal structures of semiconductors, rocks, ceramics, 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 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 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.
SEM Supra is equipped by retractable Panchromatic CL detector.

Fig. 6. SE (a) and CL (b) images of the same area of GaN semiconductor. Secondary electron image  does not reveal any defect. Panchromatic cathodoluminescence image shows high density of dislocations.

Electron Beam Induced Current (EBIC)

(EBIC) is a semiconductor analysis technique performed in a scanning electron microscope (SEM). It is used to identify buried junctions or defects in semiconductors, or to examine minority carrier properties. EBIC is similar to cathodoluminescence in that it depends on the creation of electron–hole pairs in the semiconductor sample by the microscope's electron beam. This technique is used in semiconductor failure analysis and solid-state physics.

Fig. 7. SE (a) and SE+EBIC (b) images of the gate area of GAN based transistor. SE image doesn't reveal any defects, SE+EBIC image reveals defect in the vicinity of the transistor's gate.

Environmental SEM  (ESEM)

Environmental SEM (ESEM) allows samples to be observed in low-pressure gaseous environments. This was made possible by the development of a secondary-electron detector capable of operating in the presence of water vapor and by the use of pressure-limiting apertures with differential pumping in the path of the electron beam to separate the vacuum region (around the gun and lenses) from the sample chamber.ESEM is especially useful for non-metallic and biological materials because coating with carbon or gold is unnecessary.