Surface effect of n-GaAs cap on the THz emission in LT-GaAs

The deposition of n-GaAs cap on low-temperature GaAs (LT-GaAs) improved the THz emission of LT-GaAs grown at a much lower temperature (< $$300\,^{\circ }$$ C), where the defect density is high, without compromising the spectral bandwidth and carrier lifetimes necessary for ultrafast THz detection. The LT-GaAs grown at $$220$$ and $$270\,^{\circ }$$ C showed a 192 and 10% enhancement THz emission peak-to-peak intensity, respectively, while the sample grown at $$310\,^{\circ }$$ C showed a 49% reduction. The n-GaAs cap reduced the As-related defects density in the LT-GaAs resulting to improved THz emission. The THz emission from the sample grown at $$310\,^{\circ }$$ C with already low defect density suffered possibly due to the free carrier absorption by the n-GaAs cap. The results are relevant in future material design of LT-GaAs based photoconductive antenna.