Scanning Electron Microscopy – Cathodoluminescence (SEM-CL)

Scanning Electron Microscopy- Cathodoluminescence (SEM-CL)

SEM-Cathodoluminescence (SEM-CL) microscopy is based upon the characteristic Cathodoluminescence (CL) from a solid material, when excited by high-energy electron bombardment from a Scanning Electron Microscope (SEM). Cathodoluminescence can be explained by the band theory, in which electron populations in solids are confined to discrete energy bands, ranging from Valence to Conduction bands. In the case of solid Insulators and Semiconductors the band gap is sufficiently high, to prevent electrons from jumping easily from the Valence to Conduction bands. The electron bombardment of such a sample, during SEM-CL microscopy, transfers enough energy to Valence band electrons to jump up to the Conduction band. When the electrons fall back to the ground state, they may encounter impurities or structural defects that temporarily trap them (for microseconds). When the electrons vacate the trap to return to ground state, the energy lost may be of a wavelength that results in luminescence. This emission characterizes the impurity or structural defect. Actually, CL is a complex function of lattice structure, composition, structural defects, temperature and strain. In SEM-CL measurements, the emissions of interest are in the visible range of the electromagnetic spectrum (wavelengths of 400-700 nm), though some may occur in the UV and IR ranges. The intensity of emission is recorded by a CL detector and spectrum of intensity versus wavelength obtained. SEM-CL can be used either in focused mode to obtain spot spectra or the sample can be scanned, to obtain images that provide spatially averaged spectra.

SEM-CL is used in mineralogy, geology, ceramics, semiconductors and nanotechnology for detection of impurities, trace elements and microstructural defects.

Common Uses of SEM-CL

• Analysis of semiconductors
• Analysis of ceramics
• Study of surface plasmon resonances in metallic Nanoparticles.
• In mineralogy and geology to study rocks, minerals and fossils

Advantages of SEM-CL

• High sensitivity and spatial resolution
• Provides spot as well as spatially averaged images and spectra

Limitations of SEM-CL

• Interferences due to Fluorescence and Phosphorescence
• Conductive coating required on the sample

Industrial Applications of SEM-CL

• Ceramics
• Semiconductor industry
• Nanomaterials
• Mineralogy
• Geology
• Archaeology