Time-resolved optical and spectroscopic techniques

The microwave reflectivity of a semiconductor depends on the sample conductivity, i.e. on the density and mobility of free electrons and holes. Photoexcitation of the sample brings about an instantaneous increase of electron and hole densities and a corresponding change in microwave reflectivity. After excitation by a laser pulse, the decay of minority carriers by bulk and surface recombination, as well as by trapping, can be followed by the change in microwave reflectivity. For small changes in carrier density, the relationship between microwave reflectivity and carrier density is linear. 

The time-resolved microwave reflectivity (TRMR) techiuque is well established for contactless characterisation of minority-carrier lifetimes in semiconductors. It can be applied to map surface recombination in this case the sample is moved by an X-Y stage to allow spatially resolved measurement of the minority-carrier lifetime. 

Time-resolved photoinduced microwave conductivity measurements can be made as a function of applied potential. It has been shown that the measured minority-carrier lifetime r for moderately fast or slow interfacial charge transfer depends not only on the interfacial rate constant and surface recombination Ukc, but also on the energy band bending (AE) and the Debye length Ld (Tributsch, 1999). 

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