Using the ammonia (NH3) plasma generated by a helical antenna surrounded by two magnetic coils, the transition of the discharge mode from low-density plasma to high-density one was observed. At the transition, the emission intensities from the H atoms and NH radicals especially increased in the optical emission spectroscopy, while the intensities of the other emission lines also increased abruptly. The nitridation of gallium arsenide (GaAs) surface was performed using the high-density NH3 plasma, and the properties of the nitrided surface layer were compared with those nitrided by high-density N-2 plasma using the same apparatus. From the spectroscopic ellipsometry measurements, the thickness of the nitrided layer was estimated to be 16-18 nm, while that by N-2 was 3-4 nm. From the Ga 3d spectra, the contamination with oxygen in the nitridation layer by NH3 plasma was less than that by N-2 plasma. (C) 2003 Elsevier Science B.V. All rights reserved.