On STM imaging of GaAs(0 0 1)-(n×6) surface reconstructions: Does the (6×6) structure exist?

The ultrahigh vacuum scanning tunneling microscope (UHV-STM) was employed to investigate the evolution of GaAs(0 0 1)-(n×6) surface reconstructions. The (n×6) surface reconstructions have been referred to as As-rich transitional phases comprising of (1×6), (2×6), (3×6) and (6×6) reconstructions. During the evolution from (n×6) to the Ga-rich (4×2) reconstruction, our dual-bias (n×6) STM images reveal one alternating single As dimer chain with a periodicity giving n=2; and a pair of Ga dimer rows between the As chains with a periodicity of n=3, which has not been previously reported. A new (6×6) reconstruction model is proposed based on the atomic resolution dual bias STM images obtained; and we discuss whether such (6×6) reconstructions exist in the light of all STM data published to date.