We report the effects of nitrogen (N) plasma irradiation during the growth process after buffer layer growth on the characteristics of hexagonal (alpha) InN films grown on Si(1 1 1) substrates by electron-cyclotron resonance plasma-assisted molecular-beam epitaxy. (0 0 0 1)oriented single-crystalline alpha-InN films were dominantly grown independently of N plasma irradiation during the process. However, the crystalline qualities were greatly improved by irradiation in the high-growth-temperature region, effectively suppressing re-evaporation of N atoms from InN. Moreover, the n-type carrier concentrations of the N plasma-irradiated samples decreased to about 10(18)-10(19) cm(-3), one to two orders lower than those without irradiation. A strong infra-red PL emission of 0.75 eV was observed on the whole surface between 4 and 300 K, only from samples grown under weak N plasma conditions and irradiated by N plasma during the process. The origin of the residual donor impurities was determined to be related to N vacancy. (c) 2007 Elsevier B.V. All rights reserved.