The field emission (FE) characteristics of boron nitride (BN) nanofilms grown on flat and roughened GaN substrates are investigated. A significant improvement is achieved by the BN nanocoating sample in contrast to a BN thick coating sample. In order to obtain a large field enhancement factor, H-2 plasma treatment is employed to roughen the surface of GaN substrates. The roughened GaN substrates are demonstrated to be applicable to the BN nanofilms in further improving FE performance. An apparent enhancement is observed from the FE properties of BN nano/roughened GaN compared with those of BN nano/flat GaN and an optimum turn-on electric field of 3.2 V/mum is thus achieved. (C) 2004 American Vacuum Society.