Optimization of terahertz s-wave attenuated total reflection spectroscopy for characterizing a thin transport layer on an undoped semiconductor substrate.

Ultrathin conductive layers on wide-bandgap semiconductors are important in electronic devices, but their free-carrier transport properties are difficult to characterize nondestructively. Terahertz time-domain spectroscopic ellipsometry (THz-TDSE) is an attractive method for this purpose. However, conventional THz-TDSE has limited sensitivity to such layers in the low-terahertz range because the layer thickness is much smaller than the wavelength and the response is dominated by the dc conductivity. As a method to enhance the sensitivity, terahertz TDS in attenuated total reflection geometry has been demonstrated [Sun et al., J. Phys. Chem. C122, 17552 (2018)10.1021/acs.jpcc.8b05695 and Sun et al., Adv. Opt. Mater.8, 1900535 (2020)10.1002/adom.201900535]. However, details, including the condition giving the highest sensitivity as well as the reflection phase, have not been discussed. In this work, we investigate them systematically. Theoretical analysis indicates that the s-wave-based ATR-TDSE (sATR-TDSE) offers substantially higher sensitivity than conventional THz-TDSE and allows independent evaluation of carrier density and mobility even with a frequency below 1 THz. The refractive-index contrast between the prism and the substrate and the incidence angle giving the highest sensitivity are identified. We also experimentally validated the theoretical results using high-resistivity 4H-SiC substrates with and without a 225 nm thick doped epilayer. The resulting values of dc conductivity, carrier density, and mobility agree well with Hall measurements and empirical estimates. These results establish sATR-TDSE as a promising method for characterizing ultrathin conductive layers on wide-bandgap semiconductors and potentially two-dimensional materials such as graphene.