The influence of unintentionally doped carbon impurities of i-GaN layer on the performance of GaN-based p-i-n photodetectors is investigated.In this work three samples were prepared with different growth pressure(Sample A: 100 Torr,Sample B: 200 Torr,Sample C: 450 Torr)for the unintentionally doped GaN(i-GaN)layer growth.The results of GaN-based p-i-n PDs with different i-GaN growth pressure exhibited that the peak responsivity of Sample A is less than half of the values for Samples B and C and the dark current of Sample A is almost two orders of magnitude larger than those of Samples B and C.The results of secondary ion mass spectroscopy(SIMS)suggest that the residual carbon impurities in the i-layer has great effect on the generation of deep trap levels,and has a strong influence on the spectral responsivity and dark current of photodetectors[1,2].At the same time,the PL spectra exhibited that the carbon impurities are strongly involved in deep trap level-related yellow luminescence(YL)band [3-7].In conclusion,the spectral responsivity and dark current of GaN-based p-i-n PDs are closely related to the residual carbon impurities of i-GaN layer.Thus,the way to decrease the residual carbon impurity concentration of i-GaN layer,such as enlarging the growth pressure,can improve the performance of p-i-n photodetectors.