Terahertz emission from semi-insulating GaAs with octadecanthiol-passivated surface

•THz surface emission spectroscopy is used as a new surface research tool for the study of semiconductor surfaces.•The surfaces under study are fresh, native oxidized, and octadecanthiol (ODT) passivated ones from (1 0 0) surface of GaAs.•Self-assembled ODT monolayer can stabilize GaAs (1 0 0) surface, and maintain a THz surface emission 1.4 times as efficient as the native oxidized surface.•Passivation of the surface can reduce the built-in electric field in the GaAs (1 0 0) surface.

Terahertz (THz) emission from octadecanthiol (ODT) passivated (1 0 0) surface of the semi-insulating GaAs was measured, and compared with those from the native oxidized and the fresh surfaces. It was shown that the self-assembled ODT monolayer can stabilize the GaAs (1 0 0) surface, and maintain a THz surface emission 1.4 times as efficient as the native oxidized surface under equal conditions. Surface passivation can reduce the built-in electric field in the depletion region of the GaAs (1 0 0), resulting in the suppression of the THz radiation to a different extent. Oxidation of GaAs surface reduces the THz amplitude mainly in the low-frequency region. These results indicate that GaAs can be made a more effective THz source by choosing molecular passivation technique. Conversely, the THz emission features such as polarity, amplitude, and phase from molecule-passivated surfaces may be used to characterize the attached molecules.