  Sb thin films have attracted wide interests due to their tunable band
structure, topological phases, and remarkable electronic properties. We
successfully grow epitaxial Sb thin films on a closely lattice-matched
GaSb(001) surface by molecular beam epitaxy. We find a novel anisotropic
directional dependence of their structural, morphological, and electronic
properties. The origin of the anisotropic features is elucidated using
first-principles density functional theory (DFT) calculations. The growth
regime of crystalline and amorphous Sb thin films was determined by mapping the
surface reconstruction phase diagram of the GaSb(001) surface under Sb$_2$
flux, with confirmation of structural characterizations. Crystalline Sb thin
films show a rhombohedral crystal structure along the rhombohedral (104)
surface orientation parallel to the cubic (001) surface orientation of the GaSb
substrate. At this coherent interface, Sb atoms are aligned with the GaSb
lattice along the [1-10] crystallographic direction but are not aligned well
along the [110] crystallographic direction, which results in anisotropic
features in reflection high-energy electron diffraction patterns, surface
morphology, and transport properties. Our DFT calculations show that the
anisotropic features originate from the GaSb surface, where Sb atoms align with
the Ga and Sb atoms on the reconstructed surface. The formation energy
calculations confirm that the stability of the experimentally observed
structures. Our results provide optimal film growth conditions for further
studies of novel properties of Bi$_{1-x}$Sb$_x$ thin films with similar lattice
parameters and an identical crystal structure as well as functional
heterostructures of them with III-V semiconductor layers along the (001)
surface orientation, supported by a theoretical understanding of the
anisotropic film orientation.
