Photoluminescence is the emission of light which is caused by the irradiation of a substance with other light. The term embraces both fluorescence and phosphorescence, which differ in the time after irradiation over which the luminescence occurs. The radiated light is often visible, but can also be in the ultraviolet or infrared spectral region.
Photoluminescence in semiconductors and dielectrics can usually only occur for illumination of a substance with light which has a photon energy above the band gap energy. (Exceptions are cases with multiphoton absorption at high optical intensities.) The photoluminescence usually occurs for wavelengths around the band gap wavelength.
In many cases, photoluminescence occurs only with certain dopants or impurities in a material. For example, fluorescence can occur when rare-earth-doped laser gain media are pumped with light which can excite the rare earth ions. Here, the emission can occur in various wavelength bands corresponding to optical transitions of those ions. One may obtain more than one photon per absorbed photon, when there is a cascade of transitions.
Photoluminescence for the Characterization of Photonic Devices
Photoluminescence is sometimes used for the characterization of photonic devices or parts thereof. For example, one may irradiate a semiconductor wafer, on which additional layers for a semiconductor laser or a saturable absorber have been fabricated, with short-wavelength light for exciting photoluminescence. The optical spectrum of the photoluminescence (PL spectrum) and also the dependence of its intensity on the irradiation intensity and device temperature can deliver important information for device characterization. In particular, PL spectra and their intensity dependencies can allow one
- to determine the band gap energy
- to estimate the wavelength of maximum gain
- to determine the composition of ternary or quaternary layers
- to determine impurity levels (which can e.g. lead to the emission of light with longer wavelengths than otherwise possible)
- to investigate recombination mechanisms
Photoluminescence mappers (PL mappers) can spatially resolve the photoluminescence, e.g. from epitaxially grown wafers. PL maps can reveal, e.g., a radial dependency of layer thickness or material composition.
Note that the characterization of photoluminescence is possible before any electrodes if been applied to the semiconductor structure.
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