In this letter, scanning tunneling microscopy (STM) is utilized to investigate surface morphology and a local atomic structure of the GaAs(I 0 0) surfaces prepared by the HCl-isopropanol (HCl-iPA) treatment and annealing in ultrahigh vacuum (UHV). Low-energy electron diffraction (LEED) reveals that the (2 x 4) reconstruction appears on the HCl-iPA treated GaAs(1 0 0) surface after an UHV annealing at 300 degreesC. According to the STM images, this (2 x 4) structure is 7 phase. Both LEED and STM suggest improvement in surface quality of the HCl-iPA GaAs(1 0 0) with increasing the annealing temperature in the range of 300-400 degreesC. (C) 2002 Elsevier Science B.V. All rights reserved.