Although gallium oxide Ga2O3 is attracting much attention as a next-generation ultrawide bandgap semiconductor for various applications, it needs further optical characterization to support its use in higher-performance devices. In the present study, terahertz (THz) emission spectroscopy (TES) and laser THz emission microscopy (LTEM) are applied to Sn-doped, unintentionally doped, and Fe-doped β-Ga2O3 wafers. Femtosecond (fs) laser illumination generated THz waves based on the time derivative of the photocurrent. TES probes the motion of ultrafast photocarriers that are excited into a conduction band, and LTEM visualizes their local spatiotemporal movement at a spatial and temporal resolution of laser beam diameter and a few hundred fs. In contrast, one observes neither photoluminescence nor distinguishable optical absorption for a band-to-band transition for Ga2O3. TES/LTEM thus provides complementary information on, for example, the local mobility, surface potential, defects, band bending, and anisotropic photo-response in a noncontact, nondestructive manner. The results indicated that the band bends downward at the surface of an Fe-doped wafer, unlike with an n-type wafer, and the THz emission intensity is qualitatively proportional to the product of local electron mobility and diffusion potential, and is inversely proportional to penetration depth, all of which have a strong correlation with the quality of the materials and defects/impurities in them.
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 (ETHz) and gate voltage (Vg) curves taken from SiC MOS structures with different interface qualities and observed a steep ETHz–Vg 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 ETHz–Vg 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.
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