We have investigated band-gap states in AlGaN/GaN hetero-structures with different growth conditions of GaN buffer layers in view of C impurity incorporation. The C incorporation was enhanced with decreasing the growth temperature of the GaN buffer layer between 1120 and 1170 degrees C. Acting in concert, three specific deep levels located at similar to 2.07, similar to 2.70, and similar to 3.23 eV below the conduction band were found to become dense significantly at the low growth temperature. Therefore, these levels are probably attributable to Ga vacancies and/or shallow C acceptors produced by the C impurity incorporation, and are likely in conjunction with each other. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.025202esl] All rights reserved.