Terahertz Spectroscopy: Studying Carrier Dynamics in Semiconductor Nanostructures

Understanding the ultrafast dynamics of photoexcited carriers in semiconductor nanostructures and their dependence on sample morphology is crucial for their incorporation into photonic devices. Time-resolved terahertz (THz) spectroscopy (TRTS) is an all-optical, contact-free technique that directly measures the transient mobile carrier dynamics and terahertz conductivity in materials over picosecond time scales, and is uniquely suited as a probe of conductivity in nanomaterials. Using low temperature MBE-grown silicon films as an example, we show how TRTS can be used to probe microscopic photoconductivity as well as obtain crucial insights into sample morphology. The thin silicon films consist of a mixture of amorphous and crystalline phases, and their relative content changes drastically with growth temperature. Photoexcited carrier dynamics in these films are determined by film crystallinity: in the amorphous phase, carriers are trapped in bandtail states on sub-picosecond time scales, while the carriers excited in crystalline grains remain free for tens of picoseconds. The complex THz conductivity spectra obtained from the TRTS measurements show that the long range conductivity is significantly higher in films grown at higher temperatures that contain a larger fraction of crystalline material with larger crystal grain sizes.