Photo-excitation causes electrons within the material to move into permissible excited states. When these electrons return to their equilibrium states, the excess energy is released and may include the emission of light (a radiative process) or may not (a nonradiative process). The energy of the emitted light (photoluminescence) relates to the difference in energy levels between the two electron states involved in the transition between the excited state and the equilibrium state. The quantity of the emitted light is related to the relative contribution of the radiative process.
Band Gap DeterminationThe most common radiative transition in semiconductors is between states in the conduction and valence bands, with the energy difference being known as the band gap. Band gap determination is particularly useful when working with new compound semiconductors.
Impurity Levels and Defect DetectionRadiative transitions in semiconductors also involve localized defect levels. The photoluminescence energy associated with these levels can be used to identify specific defects, and the amount of photoluminescence can be used to determine their concentration.
Recombination MechanismsThe return to equilibrium, also known as "recombination," can involve both radiative and nonradiative processes. The amount of photoluminescence and its dependence on the level of photo-excitation and temperature are directly related to the dominant recombination process. Analysis of photoluminescence helps to understand the underlying physics of the recombination mechanism.
Material QualityIn general, nonradiative processes are associated with localized defect levels, whose presence is detrimental to material quality and subsequent device performance. Thus, material quality can be measured by quantifying the amount of radiative recombination.
Special Features of Photoluminescence spectroscopy
- Various excitation wavelengths allow for varying penetration depths into the material, and thus, varying levels of volume excitation.
- Detection of photoluminescence from 0.4 to 2.8 micrometers using diffraction and Fourier-transform-based systems.
- Mapping capabilities with 1-micrometer spatial resolution on the Fourier-transform-based system.
- Sample temperatures of 4 to 300 K.
- Sensitivity down to the level of parts per thousand, depending on impurity species and host.