Well-crystallized ZnO nanowire arrays were grown on GaN/sapphire by one-step chemical vapor deposition under control of the fabrication pressure of 1000– 2500 Pa and the best-aligned arrays were obtained at 1000 Pa. A photoluminescence study shows a red shift with nanowire diameter increase. Under 365-nm UV irradiation of 0.3 mW/cm2, the photoresponse study of the best ZnO ar- rays shows an ultra-fast tri-exponential rise with three constants of 0.148, 0.064 and 0.613 s, and a bi-exponential

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... cay behavior with two recovery constants of 30 and 270 ms. The ZnO/GaN heterojunction barriers could be responsible for the ultra-fast tri-exponential rise and bi-exponential de- cay behavior.