Measurement of electrical properties of GaN thin films using Terahertz-time domain spectroscopy

We demonstrate the noncontact and nondestructive evaluation of electrical properties of n-type GaN thin films on sapphire substrates using time domain spectroscopy in the THz frequency region (THz-TDS). DC resistivities of the GaN films with various free carrier densities are deduced by fitting the transmission spectra of the sample to the Drude model. The DC resistivities obtained by the THz-TDS show good agreement with those obtained by the conventional contact measurements. Mobilities of the free carriers in lightly doped GaN films are also determined by the Drude fit. It is found that the temperature dependence of the mobilities for the lightly doped films shows a peak at similar to 150K. The temperature dependences of the free carrier densities for the lightly doped films obtained by the THz measurements are compared with that predicted by a model with two kinds of donors reported previously.