For photoluminescence (PL), two complimentary methods need to be distinguished:

• Spectral PL (sPL), which is an indicator for the generally electronic quality of a semiconductor

• Time-resolved PL (TRPL), which gives direct access to the charge carrier lifetime

sPL records the spectral PL response of a semiconductor upon excitation with a laser of an energy exceeding the material’s band gap energy. The photoluminescence response results from the energy the electrons which were excited above the band gap by the laser will emit upon relaxation the valence band. Since within-band relaxation will usually occur thermally without any emission. Hence the spectral emission profile will solely reflect the bandgap of the material. Therefore, sPL is an excellent method for determining the band gap of a semiconductor. Potentially, the spectral signal can also be deconvoluted to identify certain defect levels within the bandgap. By evaluating the changes of the sPL signal with respect to peak widths, shape and intensity one can assess and compare the electronic quality since it reveals information about the amount of radiative and non-radiative charge carrier recombination as well as about the actual electronic levels.

Additionally, TRPL is another PL-based metrology method which has become increasingly important in recent years in the field of thin film photovoltaics. In contrast to sPL, the decay time of the PL signal is monitored (independently of the exact band gap energy) in TRPL as a metric for carrier lifetime. Thus, it is a valuable technique for characterizing the electronic quality of a semiconductor.

In combination, both techniques can yield a very comprehensive characterization of thin film semiconductors and thereby advance both quality control and process optimization. Both methods work even at room and moderately elevated temperatures and are hence suitable for in-line monitoring of layer properties in vacuum or ambient conditions.