An indium-containing layer positioned underneath the InGaN/GaN quantum well (QW) active region is commonly used in high efficiency blue light-emitting diodes. Recent studies proposed that the role of this underlayer is to trap surface defects (SDs), which, otherwise, generate non-radiative recombination centers in the QWs. However, the origin and the nature of these defects remain unknown. Our previous study revealed that high-temperature growth of GaN promotes SD creation. In this work, we investigate the impact of the GaN-buffer growth temperature on the InGaN/GaN QW efficiency. We show that the 300 K photoluminescence decay time of a single QW deposited on 1-μm-thick GaN buffer dramatically decreases from few ns to less than 100 ps when the GaN buffer growth temperature is increased from 870 °C to 1045 °C. This internal quantum efficiency collapse is ascribed to the generation of SDs in the GaN buffer. A theoretical study of temperature-dependent defect formation energy in GaN suggests that these SDs are most likely nitrogen vacancies. Finally, we investigate the formation dynamics of SDs and show that they are mainly generated at the early stage of the GaN growth, i.e., within 50 nm, and then reach a steady state concentration mainly fixed by the GaN growth temperature.