An atomically-controlled GaSb layer was fabricated on GaAs(001) substrates by using the surface exchange reaction between arsenic and antimony atoms. The exchange reaction process was controlled by molecular-beam irradiation of Sb 4 and was monitored by using reflection high-energy electron-beam diffraction (RHEED). One-cycle oscillation of RHEED specular-beam intensity during the Sb 4 irradiation was explained by the change of surface structure from GaAs to GaSb. From transmission electron microscopy and photoluminescence (PL) measurements, it was found that fabricated GaAs/GaSb quantum well (QW) structures have an abrupt heterointerface and high crystal quality. In scanning tunneling microscopy measurements, tunneling spectra obtained from the GaSb-terminated GaAs sample revealed suppression of density of surface states.