We have systematically studied the atomistic growth processes of monolayer MoSe2 on GaAs(111)A and (111)B substrates. A combination of complementary techniques of reflection high-energy electron diffraction, scanning tunneling microscopy, and X-ray photoelectron spectroscopy allows us to study the evolution processes of surface and interface structures during the MoSe2 growth. Highly oriented MoSe2 films are epitaxially grown in two steps: the amorphous Mo islands are initially formed on Se-treated GaAs surfaces, which is followed by the crystallization into MoSe2 under the supply of a Se molecular beam. While the initial Mo deposition leads to the segregation of Se atoms from the Se-treated GaAs surface, the subsequent supply of the Se beam induces the reconstruction of the Se-terminated GaAs structure beneath the MoSe2 film.

Formation processes of Ga droplets on polar (111)A and (111)B surfaces of GaAs have been investigated. A single Ga atom forms a stable nucleus on the (111)A surface, so that the formation of extremely high-density of Ga droplets is achieved (2.8 × 1012 cm–2). On the (111)B surface, the initial Ga deposition on both As-rich (2 × 2) and Ga-rich (√19 × √19) reconstructions leads to the formation of a two-dimensional GaAs layer having a more Ga-rich (3 × 2) reconstruction. The Ga droplets are formed on the (3 × 2) surface with their densities being 4 orders of magnitude lower than those for the (111)A orientation.

Formation processes of Ga droplets on GaAs(001) have been systematically studied. We present the evidence that the surface atomic structures of the GaAs substrate dominate the surface diffusion of Ga atoms, which plays a key role in determining the size and density of Ga droplets. The Ga droplets are formed on the As-rich (2 × 4) and c(4 × 4)β surface after the modification of the initial surface reconstructions, while droplets are directly formed on the Ga-rich (4 × 6) surface. The density of Ga droplets on the (4 × 6) surface exceeds 1012 cm–2, which is significantly higher than that on the As-rich c(4 × 4)β surfaces.

The atomic structures and the formation processes of the Ga- and As-rich (2×2) reconstructions on GaAs(111)A have been studied. The Ga-rich (2×2) structure is formed by heating the As-rich (2×2) phase, but the reverse change hardly occurs by cooling the Ga-rich surface under the As2 flux. Only when the Ga-rich (2×2) surface covered with amorphous As layers was thermally annealed, the As-rich (2×2) surface is formed. The As-rich (2×2) surface consists of As trimers located at a fourfold atop site of the outermost Ga layer, in which the rest-site Ga atom is replaced by the As atom.Highlights► We studied the structure and formation processes of the As-rich (2×2) reconstruction of GaAs(111)A. ► The As-rich (2×2) surface consists of As trimers at a fourfold atop site with the rest-site As atom. ► The As-rich (2×2) surface is formed only when the Ga-rich (2×2) surface covered with amorphous As layers was annealed. ► The Ga-rich phase is formed by heating the As-rich phase, but the reverse change hardly occurs.

Reflection high-energy electron diffraction (RHEED), reflectance difference spectroscopy (RDS), and scanning tunneling microscopy (STM) have been used to study the anisotropic kinetics on the growing Ge(0 0 1) surface. While switching of dimer direction in alternate (2 × 1)/(1 × 2) layers causes the bilayer-period oscillations in RD response, RHEED oscillations are governed by variations in surface step densities. We show that the RHEED oscillations are strongly affected by the growth front morphology: when the growth front becomes distributed over several layers, the transition from bilayer- to monolayer-period occurs in RHEED oscillations.

Reflection high-energy electron diffraction (RHEED), reflectance difference spectroscopy (RDS), and scanning tunneling microscopy (STM) have been used to study the anisotropic kinetics on the growing Ge(0 0 1) surface. While switching of dimer direction in alternate (2 × 1)/(1 × 2) layers causes the bilayer-period oscillations in RD response, RHEED oscillations are governed by variations in surface step densities. We show that the RHEED oscillations are strongly affected by the growth front morphology: when the growth front becomes distributed over several layers, the transition from bilayer- to monolayer-period occurs in RHEED oscillations.