Abstract
Wide-gap semiconductors have received significant attention for their advantages over existing semiconductors in energy-efficient power devices. To realize stable and reliable wide-gap semiconductor devices, the basic physical properties, such as the electric properties on the surface and at the interface, should be revealed. Here, we report visualization of terahertz (THz) emission from the surface of GaN, which is excited by ultraviolet femtosecond laser pulses. We found that the THz emission is enhanced by defects related to yellow luminescence, and this phenomenon is explained through the modification of band structures in the surface depletion layer owing to trapped electrons at defect sites. Our results demonstrate that the surface potential in a GaN surface could be detected by laser-induced THz emission. Moreover, this method enables feasible evaluation of the distribution of non-radiative defects, which are undetectable with photoluminescence, and it contributes to the realization normally-off GaN devices.
Highlights
Because the band gap energy of gallium nitride (GaN) is 3.4 eV, the incident photon energy (4.8 eV) is sufficient to excite an electron from the valence band to the conduction band
The results suggest that an enhancement of the THz emission is induced by the defects in the GaN surface
The result that THz signals rapidly decrease under excitation with wavelengths above 360 nm, shown in Fig. 3, indicates that THz radiation is triggered by photoexcited carriers that are excited from the valence band to the conduction band
Summary
The PL measurement was performed at room temperature, and the excitation wavelength and photon density per pulse were 345 nm and 4.8 × 1013/cm[2], respectively These spectra contain the excitation source at 345 nm, the near-band-edge (NBE) emission peak at 365 nm, the luminescence peak by donor-acceptor pair (DAP) or conduction-band-to-acceptor (e -A) transition at 390 nm, and the broad luminescence around 500 nm, corresponding to photon energy of 2.2–2.4 eV. All THz signals rapidly decrease when the wavelengths are above 360 nm
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