Far-infrared characteristics of bulk and nanostructured wide-bandgap semiconductors

A review of far-infrared properties of popular bulk and nanostructured wide-bandgap semiconductors in the broadband terahertz region is presented. Such wide-bandgap semiconductor materials have shown promising applications in terahertz optoelectronics. The optical, dielectric or electric properties of bulk crystalline GaN, ZnO, and ZnS, and nanostructured ZnO and ZnS were characterized by terahertz time-domain spectroscopy measurements. Theoretical fitting based on dielectric models and effective medium models have shown good agreement with the measured results. The investigation reveals that the free-standing GaN exhibits a Drude-like behavior in the terahertz region, while the dielectric response of crystalline ZnO and ZnS is dominated by low-frequency transverse optical phonon modes. The ZnO tetrapod nanostructures exhibit very similar phonon resonances with that of single-crystal ZnO, whereas the phonon confinement in ZnS nanoparticles gave rise to a difference photon response compared to the bulk ZnS.