Doping characteristics of group VT elements (Te and Se) on (100)-oriented GaAs are investigated at an extremely heavy doping level up to 5 x 10(20) cm(-3) on the basis of the surface stoichiometry control to improve the incorporation of impurities. By changing the gas injection sequences, the surface stoichiometry before the introduction of impurity precursors is controlled. The impurity concentration is measured by secondary ion mass spectroscopy (SIMS) analysis and the activation ratio is determined in conjunction with the results of Hall effect measurements. It is shown that the incorporation of Te and Se is extremely enhanced when DETe and DESe are exposed on the Ga-stabilized surface. From the electrical measurements and SIMS results, the segregation of defects due to doped impurity atoms is strongly suggested. The defect formation mechanism of heavily impurity doped GaAs is discussed in view of the formation of impurity-defect complex and the control of site-occupation of doped impurities.