To study the correlation between defects and deep levels in a-plane GaN films grown on r-plane sapphire substrates, transmission electron microscopy (TEM) and deep level transient spectroscopy (DLTS) have been performed on three types of a-plane GaN samples grown using modified two-step growth (sample I), SiNx, interlayer (sample II), and patterned insulator on sapphire substrate (sample III). From the microstructure evolution in cross-sectional TEM images, it was shown that combination of growth techniques is highly efficient in the reduction of dislocation densities. Average dislocation densities of samples I, II, and III were about 2.2 x 10(9) cm(-2), 1.1 x 10(9) cm(-2), and 3.4 x 10(8) cm(-2), respectively. The trap a(t) E-c-E-t similar to 0.13 eV (E1) was observed in only sample I, and three electron traps at 0.28-0.33 eV (E2), 0.52-0.58 eV (E3), and 0.89-0.95 eV (E4) from the conduction band edge were measured common to all the samples. The analysis of trap properties indicated that E2 and E3 trap levels are strongly associated with the partial dislocations in a-plane GaN films. (c) 2011 Elsevier B.V. All rights reserved.