Real-time imaging of reconstruction transitions during molecular-beam epitaxy on a GaAs surface has been performed by scanning electron microscopy in the secondary-electron mode, and reconstruction-transition-associated step bunching as well as debunching behaviors were investigated. On the $\mathrm{GaAs}\mathrm{}\left(111\right)B$ surface inclined 1° toward the [$\overline{1}\overline{1}2$] direction, uniform motion of atomic steps was observed at 620 °C with a growth rate of 0.1 ML/s under $\sqrt{19}\times \sqrt{19}$ reconstruction. When the growth temperature was raised to 625 °C, 1×1 high-temperature reconstruction [${(1\mathrm{}\times 1)}_{\mathrm{HT}}$] was found to appear from the step edges and step bunching occurred immediately. It was found that the average macrostep spacing increases in accordance with the domain sizes to a saturated value at a certain temperature. With an increase of the growth temperature, macrostep spacing was increased to cover hundreds of monatomic steps until the surface became a pure ${(1\mathrm{}\times 1)}_{\mathrm{HT}}$ reconstruction. However, as soon as the growth temperature was lowered down to 620 °C, step debunching occurred immediately in a few minutes with remarkable step motion.

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