The negative electron affinity GaAs photocathode forms when the Zn-doped GaAs beta 2(2 x 4) surface adsorbs atoms Cs and O. In this work, undoped and other four GaAs (1 0 0) beta 2(2 x 4) surface models with Zn doped in different positions are built. First-principles modeling techniques are used to investigate how the doped Zn impact on the GaAs beta 2(2 x 4) surface state and the best Zn-doping position is obtained. It is found that position III is a better position for Zn to dope than the others through the analysis of the atom structures, energy band structures, charge distributions, work functions and surface energy. Zn doped in position III can not only reduce the value of the work function which is interrelated with the photoelectrons escape but also be conducive to the adsorption of Cs to form the dipole layer which makes the GaAs (1 0 0) beta 2(2 x 4) surface the real negative electron affinity one. (C) 2013 Elsevier B.V. All rights reserved.