Surface mapping of carrier density in a GaN wafer using a frequency-agile THz source

We developed a method for mapping the carrier density on a semiconductor substrate surface based on terahertz (THz)-reflective measurement. Reflectivity in the THz-frequency region away from the optical phonon frequency is sensitive to the carrier density in semiconductors. However, reflectivity in the optical phonon frequency regions is around 1.0, independent of the carrier density. We developed a THz-reflective spectral imaging system using a frequency-agile, ultra-widely tunable THz source (1-40 THz). Different reflective images were obtained from GaN samples of carrier density 2.5 × 10¹⁶ cm^-3, 1.0 × 10¹⁸ cm^-3 and 1.5 × 10¹⁸ cm^-3 using 22.7 and 26.5 THz. The image contrast reflected the GaN crystals' carrier density.