We have investigated the influence of growth temperature on N-polar GaN epitaxial layers deposited on sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The GaN films were grown at various temperatures (1050 degrees C, 1000 degrees C, 950 degrees C, 900 degrees C). The sheet resistivity of the GaN film was 2153 ohm/square at 950 degrees C, which is 6 times higher than that at 1050 degrees C (361 ohm/square). Secondary ion mass spectroscopy (SIMS) measurement confirmed that the increase of carbon impurity concentration was responsible for the above phenomena. High resolution X-ray diffraction (HRXRD), photoluminescence (PL) and Raman measurements showed that the GaN film quality did not deteriorate seriously at low growth temperature, implying that reducing the growth temperature would be a feasible method to obtain highly insulating N-polar GaN films. However, further reducing the growth temperature to 900 degrees C led to the sharp increase of oxygen impurity concentration and the decrease of sheet resistivity. This mechanism is explained in detail. (C) 2013 Published by Elsevier B.V.