The change in the surface concentration of N ([N](s)) on a GaAs surface under N and As source injections is investigated using the N atomic layer doping (N-ALD) technique, and the key factor determining [N]s is discussed. The As and N precursors source gases are trisdimethylaminoarsenic (TDMAAs, [N(CH3)(2)](3)As) and monomethylhydrazine (MMHy, N2H3CH3), respectively. N-ALD layers are prepared by using two gas injection sequences (A: MMHy injection and B: MMHy and TDMAAs injections). [N](s) increases with decreasing growth temperature in both sequences. [N](s) in sequence A is higher than that of sequence B. In sequence B, Delta [N](s)/Delta t is proportional to exp(-t/tau(N)), where t and tau(N) are the gas injection time and the residence time of N, respectively. It is observed that the number of vacant sites, [V](s,N), remaining constant during gas injections. In sequence A, Delta[N](s)/Delta t cannot be fitted by a single exponential function, indicating that [V](s,N) is not constant. From these results, we suggest that the vacant sites at the surface are created not only by N desorption but also by As desorption. It has been found that As desorption is enhanced by MMHy injection. As desorption reaction has been confirmed by in situ auger electron spectroscopy measurements. These results indicate that [N](s) is determined by the competitive absorption between N and As, [V](s,N), and tau(N). (C) 2009 Elsevier B.V. All rights reserved.