The surface reaction mechanism of GaAs MLE using a triethyl-gallium (TEG)-AsH3 system was studied by varying the sequence of gas injection of TEG and AsH3. The decomposition of TEG was found to be enhanced on the Ga-terminated (0 0 1)-oriented GaAs surface. The average lifetime of AsHx (x = 0, 1, 2) formed by the AsH3 supply was 8 s, and the lifetime of the ethyl group of the ethyl-gallium compound formed by the TEG supply was about 2 s. The gas supply sequence was chosen to be within the two lifetimes, and the self-limiting growth at the monolayer was achieved in the TEG pressure region of more than 5.3 x 10(-3) Pa at a temperature of 270 degreesC.