Probing the surface potential of SiO₂/4H-SiC(0001) by terahertz emission spectroscopy

Terahertz (THz) emission spectroscopy (TES) was used to evaluate the properties of interfaces between thermally grown oxides and 4H-SiC(0001) substrates. Metal–oxide–semiconductor (MOS) structures with transparent electrodes were irradiated with a femtosecond laser pulse and the emitted THz signal was measured by changing the applied gate voltage. The amplitude of the THz pulse signal is dependent on the electric field, namely, band bending near the SiO₂/SiC interfaces, and thus contains information on the change in the surface potential of the SiC MOS structures. We compared the peak THz amplitude (E_THz) and gate voltage (V_g) curves taken from SiC MOS structures with different interface qualities and observed a steep E_THz–V_g curve for a high-quality SiO₂/SiC interface as compared with the curve for a structure with a higher interface state density. We also compared the E_THz–V_g and capacitance–voltage characteristics of SiC MOS capacitors and investigated the mechanism of THz emission from the SiC MOS structures to validate the ability of the TES technique for characterizing SiO₂/SiC interfaces.