The surface structure of (NH4)2Sx-treated GaAs (100) was observed with a scanning tunneling microscope (STM) both in air and in vacuum. STM images obtained showed line shaped protrusions which run [110] direction and are 220 nm in width. For the sulfur treated n- and p-GaAs surfaces with 400°C-20 min heating, about 1 eV surface band bendings toward higher and lower binding energy side were observed, respectively. No band gap state appeared for either case.
