Phosphors are in myriad applications, including biofluorescent labels, display devices, plus LED and light bulb coatings. Phosphors emit very brightly relative to other materials and present a different apparent color depending on their composition. Cathodoluminescence (CL) in the electron microscope is an ideal technique for evaluating phosphor luminescence because interaction with the focused electron beam can generate luminescence signals from individual phosphor particles, even those as small as a few nanometers in size, to allow investigation of variations between phosphor particles.

CL spectroscopy is useful to analyze many different phosphors, including individual nanometer-sized crystals of Y_1.98Tb_0.02O₂S and Gd_1.98Tb_0.02O₂S. In those studies, photon emission maps demonstrate a high degree of uniformity. Spectra from the two materials also show an identical intensity ratio between the ⁵D₃ → ⁷F_J and ⁵D₄ → ⁷F_J transitions, important evidence that the concentration of (activated) Tb³⁺ cations is very similar in these different crystals. Other researchers are developing these nanometer-sized phosphors as biological labels that are stable under electron beam illumination.

Oxides and nitrides, such as sapphire (Al₂O₃) and gallium oxide (Ga₂O₃), are considered to be wide bandgap insulators. In these materials, the high energy electron beam of an electron microscope offers an ideal tool to study the electronic and optical properties without the need to resort to specialized deep ultraviolet lasers for photoluminescence. Cathodoluminescence (CL) is useful to determine the energy bandgap and to reveal the presence (and energy state) of electrically active defects. This makes it a preferred tool to study the basic optical properties of these materials as well as discriminate between different phases of compositionally and chemically identical materials, e.g., the anatase and rutile phases of TiO₂.

Recently, β-Ga₂O₃ receives increased attention due to its intrinsic ultra-wide bandgap and optical transparency in visible light, implying a natural application to solar-blind ultraviolet (UV) photodetection. There are limited techniques to determine material quality and characterize the defects that may be present. CL is one such a technique—CL spectroscopy has been used to reveal oxygen vacancy-related and (two) gallium vacancy-related energy levels within the β-Ga₂O₃ bandgap, including spectral changes associated with removing or creating these defects through materials processing. These defect levels compete with the near band edge luminescence and the relative ration of the two luminescence types provides insight into material quality.

Spectroscopic analysis of ultra-wide bandgap semiconductors

Dielectric materials are technologically important materials in the semiconductor industry used as gate oxides in metal oxide semiconductor field effect (MOSFET) transistors. Silicon dioxide and silicon nitride are studied extensively using cathodoluminescence (CL) to characterize the structure and electronic properties of point defect clusters.

In more recent years, CL continues to play an important role as high-κ dielectric materials replace SiO₂. For example, depth-resolved CL spectroscopy can detect and locate defects and interfacial states within ultra-thin (<4 nm) gate oxides. In hafnium oxide, CL techniques detect the presence of several oxygen vacancy defects, their evolution during thermal processing, as well as the formation of hafnium silicates (HfSiO₄) at the silicon-oxide interface. In LaLuO₃, defects produced by LaLuO₃–Si interdiffusion have been detected as well as suppression of these defects by monolayer thick Al₂O₃ interlayers.

Stress mapping in engineering ceramics

In many wide bandgap materials, the wavelength of the luminescence signal is useful to determine stress and strain fields. Luminescence in these materials results from energy transitions involving defects—point defect clusters or impurities—that place an energy level in the forbidden energy gap of the crystal's electronic structure. In many materials, the precise energy of this transition varies as a result of changes in the electronic structure of the crystal under applied stress. Spectrum imaging with high spectral and spatial resolution enables the stress field to be determined. The most noted application example of stress mapping is associated with alumina (using the R-line doublet associated with Cr³⁺); however, other examples including dielectric materials such as SiO₂ in semiconductor devices.