Photoluminescence

In the photoluminescence spectroscopy ( PL ) spectroscopy to be examined material is brought by absorption of light in the electronically excited energy states that reaches then under emission of light (spontaneous photon emission in the form of fluorescence or phosphorescence) again is energetically lower lying energy levels. The emitted light is detected and gives information about the electronic structure of the material.

Photoluminescence spectroscopy is a very sensitive method to investigate both intrinsic and defect -induced electronic transitions in semiconductors and insulators. It can optically active defects in concentrations of up to proven. While the first method mainly found application in basic research, it is used by the steadily increasing demand for high-purity or specifically doped materials becoming more common industrially for nondestructive, spatially resolved material characterization.

In the photoluminescence of semiconductors, electrons are raised from the valence to the conduction band by excitation with photons. These excited carriers recombine non-radiatively in the minimum of conduction and valence bands over discharge energy to the semiconductor crystal in the form of phonons. Depending on the crystal recombine these free charge carriers, electron-hole pairs or fall into deeper states and recombine from there. It can for example be free excitons bound to impurities, donors or acceptors excitons, donor - acceptor pairs and other conditions that result from higher-lying levels of the exciton (n = 1,2, ... ), or combinations of holes from different valence bands (A, B or C ) form detected. These states then recombine with characteristic lifetimes, which allows proof (eg via time-resolved photoluminescence spectroscopy ( TRPL, English:. Time-resolved PL) ). The released energy can be delivered in various forms, such as phonons through the crystal lattice, as photons or as Auger electrons. In the photoluminescence emitted by radiative recombination photons are detected. If several excited states exist, only transitions from the lowest state of can be observed by the very rapid thermalization in general. The measured radiation allowed important insights into the properties of the examined substance, for example in the defect inventory of the crystal and its band gap.

Most photoluminescence measurements at low temperatures ( the sample with liquid nitrogen, 77 K, or helium, 4 K, cooled) performed in order to prevent thermal ionization of the optical centers and the broadening of photoluminescence lines through lattice vibrations ( phonons) to avoid.

Experimental Setup

The luminescence of the material to be examined is obtained by the excitation light, for example by a laser of sufficient power. In general, a filter is provided in front of the laser, the non- lasing of the laser plasma lines that are in the relevant spectral region, is eliminated. The laser beam is focused by a lens onto the sample in the cryostat. The emitted light (and partly also by the laser light scattering ) is again focused by two lenses on the entrance slit of the monochromator. Ideally, a filter is mounted to the laser scattered light in front of the filter gap. In the monochromator, the radiation is (selectable depending on the desired wavelength range ) divided by variable grating spectrally and directed the dispersed light onto the detector. Where the optical signal is converted into a current or voltage signal depending on the type of detector. For amplification and phase-sensitive detection of the signal by means of a lock-in amplifier, the radiation must be (ideally equal before the radiation source ) is modulated by a chopper.