Optical emission spectroscopy (OES) was used with the combinatorial growth method to investigate the behavior of excited nitrogen species in the growth of gallium nitride (GaN) using radio-frequency molecular beam epitaxy (RF-MBE). To determine the amounts of each excited nitrogen species in the nitrogen plasma, the integrated OES intensity (IOI) method was proposed. The IOI measurements revealed the following: more nitrogen ions (N-2(+)) were produced at the lower inlet pressure of nitrogen, the productivity of nitrogen atoms (N) and excited nitrogen atoms (N*) had an optimum value at some inlet pressure, whereas the productivity of excited nitrogen molecules (N-2*) saturated as the inlet pressure increased. The combination of the IOI measurement and the combinatorial growth of GaN showed that not only N and N*, but also the long-lifetime A(3)Sigma(+)(u) state of N* contributed to GaN growth using RF-MBE. (c) 2006 Elsevier B.V. All rights reserved.