We have investigated C-related deep-level defects in AlGaN/GaN hetero-structures with different MOCVD growth conditions of GaN buffer layers, employing a steady-state photo-capacitance spectroscopy technique on their Schottky barrier diodes. The C impurity incorporation was enhanced with decreasing the growth temperature of the GaN buffer layer between 1120 and 1170°C. Acting in concert, three specific deep levels located at ~2.07, ~2.75, and ~3.23 eV below the conduction band were found to become dense significantly at the low growth temperature, presumably attributable to Ga vacancies and shallow C acceptors produced by the C incorporation into the GaN buffer layer. Additionally, from photo-assisted turn-on current recovery measurements, by using 390 and 370 nm long-pass filters, the recovery time became significantly faster due to inactivation of their corresponding deep-level traps. Therefore, the ~2.75 and ~3.23 eV levels related to the residual C impurity on N sites are strongly responsible for the carrier-trapping phenomena in the bulk region of the hetero-structures.