Minority Carrier Lifetime Spectroscopy
NREL uses minority-carrier lifetime spectroscopy to study the recombination processes of materials optically or electronically.
Minority-carrier lifetime spectroscopy examines the return of photoexcited carriers back to equilibrium as a function of time and provides a measure of the "lifetime" of the excess carriers. This is accomplished with varying levels of volume excitation and sample temperatures of 4 K to 300 K.
One optical detection technique called time-correlated single-photon counting or time-resolved photoluminescence provides exceptionally fast system response times of 20 ps (optical detection from 0.4 to 1.0 µm) and 100 fs (optical detection from 0.5 to > 2 µm). Microwave-reflection photoconductive decay and resonant-coupled photoconductive decay are techniques to monitor the change in conductivity in both direct and indirect bandgap materials. The 5-ns time resolution for the microwave-reflection photoconductive decay systems (7 and 20 GHz) and ˜50-ns resolution for resonant-coupled photoconductive decay (˜450 MHz) are capable of 1- and 2-dimensional lifetime mapping with ˜1-mm spatial resolution.
Applications
- Minority-carrier lifetime
Measures material quality through a strong sensitivity to the presence of defects detrimental to device performance.
Maps of the polysilicon lifetime at various excitation wavelengths provide information on bulk, surface, and grain-boundary recombinations.
- Recombination processes
Determines the underlying physics of the recombination process by analyzing the dependence of lifetime on the level of photo-excitation and temperature. - Surface/interface recombination velocity
Distinguishes between recombination processes occurring in the bulk of the device and those occurring at interfaces or boundaries. This is accomplished by identifying the dependence of lifetime on geometry of the device being tested or excitation wavelength.
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