Cathodoluminescence electronic transitions

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

A possible candidate may be Tm ". For example, the doublets at 803 and 817 nm and at 796 and 813 nm are the strongest ones in cathodoluminescence spectra of fluorite and scheelite activated by Tm " (Blank et al. 2000). It is possible to suppose that the strong fines at 805 and 820 nm with a relatively short decay time of 60 ps in the titanite luminescence spectrum belong to Tm " ". They appear under 532 nm excitation and are evidently connected with the electron transition. Similar emission of Tm " was also detected in... 

These processes can be described by the concept of discrete energy levels in a crystal. In insulators and semiconductors, a forbidden band (band gap) exists sep-aratingthe valence and conduction bands (Figure 7.18). The presence of activators (lattice defects, impurity ions) that occupy discrete energy levels within the band gap are the important preconditions for cathodoluminescence (CL). Cathodoluminescence glow is thus caused by electronic transitions within the limits of the ion activator levels. 

Cathodoluminescence microscopy and spectroscopy techniques are powerful tools for analyzing the spatial uniformity of stresses in mismatched heterostructures, such as GaAs/Si and GaAs/InP. The stresses in such systems are due to the difference in thermal expansion coefficients between the epitaxial layer and the substrate. The presence of stress in the epitaxial layer leads to the modification of the band structure, and thus affects its electronic properties it also can cause the migration of dislocations, which may lead to the degradation of optoelectronic devices based on such mismatched heterostructures. This application employs low-temperature (preferably liquid-helium) CL microscopy and spectroscopy in conjunction with the known behavior of the optical transitions in the presence of stress to analyze the spatial uniformity of stress in GaAs epitaxial layers. This analysis can reveal,... 

ZnS, an SC with large of 3.68 eV at 25°C (Table 1), possesses a direct-band gap, which makes optical transitions very probable. Pure ZnS is characterized byn-type conductivity. ZnS is chemically more stable and thus technologically easier to manipulate than other compound SC materials [69]. ZnS is a luminescent material well known for its PL, EL, and cathodoluminescence. ZnS-based phosphors exhibit excellent conversion efficiencies for fast electrons into electron-hole pairs and are, therefore, among materials with the highest overall cathodoluminescence efficiency [70]. Because of its wide-band gap, ZnS has become one of the most applied active layers in TEEL devices [26]. 

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