Ultrafast Terahertz Spectroscopy of Semiconducting Materials
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Abstract
Terahertz spectroscopy is a powerful non-contact technique to study the optical and electric properties of materials at sub-picosecond time scales. In this study, ultrafast time-resolved terahertz spectroscopy was performed on both silicon-on-sapphire (SOS) and iron pyrite (FeS2) nano-crystalline films that have potential application in photovoltaics. Transient conductivity after photoexcitation with ultrashort optical pulses was studied by extracting the complex conductivity from the terahertz spectra. The Drude model provides an excellent fit to the photoconductivity of SOS excited at moderate pump fluences. However, at higher excitation fluences, the SOS exhibits some carrier localization that is best described by a Drude-Smith model. The photoconductivity of the FeS2 nanocrystalline film exhibits Drude-Smith behavior with very strong carrier localization, and is around 20% of the photoconductivity of SOS. The relationship between the Drude scattering time and the photocarrier density in both materials can be described by the Caughey-Thomas model.