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The carrier dynamics of photoexcited electrons in the vicinity of the surfaceof (NH4)2S-passivated GaAs were studied via terahertz (THz) emissionspectroscopy and optical-pump THz-probe spectroscopy. THz emission spectroscopymeasurements, coupled with Monte Carlo simulations of THz emission, revealedthat the surface electric field of GaAs reverses after passivation. Theconductivity of photoexcited electrons was determined via optical-pumpTHz-probe spectroscopy, and was found to double after passivation. Theseexperiments demonstrate that passivation significantly reduces the surfacestate density and surface recombination velocity of GaAs. Finally, we havedemonstrated that passivation leads to an enhancement in the power radiated byphotoconductive switch THz emitters, thereby showing the important influence ofsurface chemistry on the performance of ultrafast THz photonic devices.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter

Influence of surface passivation on ultrafast carrier dynamics and terahertz radiation generation in GaAs