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 × 10 16 cm-3 , 1.0 × 10 18 cm-3 and 1.5 × 10 18 cm-3 using 22.7 and 26.5 THz. The image contrast reflected the GaN crystals' carrier density.
THz imaging was performed in 2 s intervals with 1 mm resolution on a 3 in., 0.42 mm thick, as-cut n-type Silicon Carbide wafer. Carrier density, relaxation time, mobility, and resistivity obtained from imaging results are 0.91 × 1018 cm-3, 4.36 × 10-14 s, 218 cm2V-1s-1, and 3.14 × 10-2 Ωcm, respectively. Compared with the standard values provided by the manufacturers, the results suggest that THz imaging has reliable precision and accuracy.
A new nondestructive method using terahertz waves for determining the carrier density of GaAs is proposed. The reflectance around the longitudinal optical (LO) phonon frequency changes with carrier density, whereas the reflectance around the transverse optical (TO) phonon frequency is constant. The relative reflectance, which is evaluated from the reflectance at the two frequencies related to the TO and LO phonon, as a function of the carrier density of GaAs was calculated from the dielectric function. A broadband tunable terahertz source is convenient for producing two waves around these phonon frequencies. Imaging of n-type GaAs wafer using this new terahertz method is described.
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