In the paper the application of a complex UHV apparatus for in-situ investigation of low-temperature Ga-N ultra-thin film synthesis by thermal Ga atoms and hyperthermal nitrogen ions is presented. It was found by XPS that the highest content of Ga-N bonds was present in the layers grown on a 'nitrogen-rich' substrate at enhanced substrate temperature (400 degreesC). 'Nitrogen-rich' Si surfaces grew from hyperthermal nitrogen ions on substrates contaminated by oxygen ('native' and chemically etched Si). The 'gallium-rich' substrate possessed smaller number of Ga-N bonds. Higher content of Ga-O bonds was found on Si substrates covered by native oxide and chemically etched silicon. Due to low nitrogen ion currents the thickness of the films was small (<4 ML). (C) 2003 Elsevier B.V. All rights reserved.