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The study reports peculiar light absorption in LT-GaAs due to the prominence of arsenic vacancies (V As), interstitials (As i), and neutral arsenic anti-sites (As Ga 0) after UHV annealing under no Arsenic vapour pressure. We compared unannealed and annealed samples' transient absorption behaviour and vibrational modes through ultrafast transient absorption spectroscopy (UFTS) (over 2.75 eV–0.775 eV spectral range) and RAMAN spectroscopy, respectively. We found that the UHV heating of LT-GaAs has augmented the tensile strain in the film by incorporating split As i. However, there is overall defect mitigation, as evident from the reduced TO phonon peak (RAMAN) and ∼59 meV reduction in Fermi level (UFTS). Even with reduced defects, the UFTS data for the annealed sample shows a higher switching speed and responsivity (1.75 ps faster) in the NIR range and a higher photoconductive gain in the visible region. [ABSTRACT FROM AUTHOR]