Unreconstructed As atoms mixed with $(3\times 2)$ cells and $(6\times 6)$ supercells in low As pressure epitaxy on GaAs(001)

Since the early days of GaAs molecular beam epitaxy, it has been understood that on the As-stabilized growth surface, As lies in the form of ${\mathrm{As}}_2$ building blocks. At lower As pressure however, there is a transition to a $(3\times 1)$ growth surface observable by reflection high-energy electron diffraction. Using in situ scanning tunneling microscopy, we provide an atomically resolved observation of this still As rich surface that displays unreconstructed As atoms in $(1\times 1)$ cells as well as $(3\times 2)$ and $(6\times 6)$ reconstructed areas. Most surface As atoms do not display any dimer bonds. From the statistics of surface Ga atoms in $s{p}^2$ hybridization we propose that the background carbon acceptor impurities content is significantly reduced in the $(3\times 1)$ growth.

Figure 1

(Color) $200\mathrm{nm}\times 120\mathrm{nm}$ filled states STM image of the mixed reconstructed two terrace levels (upper yellowish, lower reddish) surface obtained after growth of GaAs at $620°\mathrm{C}$, $1\mathrm{\AA }∕\mathrm{s}$ and an As flux of $2.25\times {10}^{-6}\text{torr}$. The $\left[1\overline{1}0\right]$ rows close to the step edges are the signature of the $(n\times 6)$ area.

Figure 2

(Color) (a) Filled states STM image ($-2.4\mathrm{V}$, $270\mathrm{pA}$) of a terrace ledge: the As-dimer rows on the lower terrace are separated by $24\mathrm{\AA }$ corresponding to the $\times 6$ periodicity along [110]. (b) Filled states image ($-2.88\mathrm{V}$, $370\mathrm{pA}$) of an As-covered upper terrace: brighter As [110] rows are separated by $12\mathrm{\AA }$, corresponding to the $\times 3$ order visible on the RHEED pattern during growth.

Figure 3

(Color) (a) Filled states STM image showing the organization of the atomic As plane: 45% of As atoms are unreconstructed and 13% of As atoms are in faulted position leading to the formation of a local $(3\times 2)$ cell (a few are highlighted with pink frames). (b) Height profile along the white [110] line passing through As atoms in faulted position. (c) Atomic structure for the $(3\times 2)$ cell: the faulted As atoms are linked to two planar Ga $s{p}^2$ atoms.

Figure 4

(Color) (a) Filled states STM image of a $(6\times 6)$ reconstructed area: the underlying As atoms are zigzag patterned. (b) Atomic structure of the $(6\times 6)$ cell: the underlayer As plane is organized in $(3\times 2)$ subcells linked by one As dimer shifting from side to side at each connection between $(3\times 2)$ subcells.