Creating and tuning charge trapping states by introducing atomic-scale defects are crucial for the optoelectronic synapses that parallelize sensing, processing, and memorizing of optical signals in a single device, which is essential for bioinspired neuromorphic computing. Herein, a mild Ar-plasma treatment approach to enable synaptic behavior in 2D semiconductor devices has been proposed and demonstrated in large-scale epitaxial GaSe/GaN heterostructures. The GaSe films were epitaxially grown on a GaN substrate by physical vapor deposition in an ultrahigh vacuum environment, while the devices were fabricated in situ using a shadow mask-assisted electrode deposition technique. A tailored mild Ar-plasma treatment on the GaSe films has been employed to create atomic-scale defects, which provide charge trapping states in the band gap without making morphological damage, as confirmed by the Raman spectra, scanning electron microscopy, and photoluminescence characterizations. Optoelectronic transport measurement under pulsed illumination of varying wavelengths reveals broadband photoresponse and significantly prolonged response time (×10