GaAs-based photocathodes are the state-of-the-art in the production of highly spin-polarized electron beams for accelerator and microscopy applications. While various novel structures of GaAs have been shown to increase the degree of polarization and quantum efficiency, all GaAs-based photocathodes require activation to negative electron affinity (NEA) to operate at the photon energies where the highest spin polarization is achieved. In this work, we report on NEA activation of bulk GaAs performed using Sb-Cs-O. We show this activation layer to be more robust with respect to traditional Cs and O layer in terms of charge extraction lifetime. This new activation layer is shown to improve the dark lifetime up to one order of magnitude and the charge extraction lifetime up to a factor of about 60, when compared with the traditional Cs-O activating layer. Trade-offs with other relevant parameters like quantum efficiency and electron spin polarization are also discussed.