We have used in in situ scanning tunneling microscopy to study GaAs(0 0 1) surface treated by As-free high temperature surface cleaning method. The temperatures ranged from 575 to 655 degrees C and an optimum temperature of 605 degrees C was found. Its surface had x6 structure parallel to [1 1 0] direction and smooth morphology by congruent evaporation. However, many pit-like structures formed lower than 600 degrees C. On the other hand, with 610 degrees C and higher, the surface became rough, forming many grooves with {1 1 1}B and {1 1 1}A facets. At 655 degrees C, the roughness was reduced but the surface easily became milky because of rapid As desorption. (c) 2006 Elsevier B.V. All rights reserved.