The formation of precipitates after an annealing process in low-temperature-grown GaAs0.6Sb0.4 is observed. We use high-resolution transmission electron microscopy for a detailed structure investigation and demonstrate the functionality of the material, which has a band gap of 0.86 eV, for ultrafast optical switches. The average diameter of the clusters is 5.7 nm in the bulk region, while larger clusters are observed near the interface to a 1.3% lattice mismatched Al0.77In0.23As buffer layer. After an annealing treatment, the sheet resistance is increased by five orders of magnitude to 1.7×108 Ω/sq, which is sufficiently high to enable Auston switch operation. We report terahertz (THz) emission from the material measured in a THz time-domain spectroscopy setup. THz generation is demonstrated by using a six interdigital finger metal-semiconductor-metal structure at the feed-point of a log-periodic antenna.