|
Ultra-high vacuum (UHV) technologies and scanning tunneling microscope (STM) have made clean surfaces one of fundamental fields to study quantum mechanical phenomena of electrons. We have recently studied clean (111)A surface of an epitaxial InAs thin film. It was found that there remained In adatoms and they were donors responsible for electron accumulation at the surface. In this paper, using low-temperature STM, we investigate the effect of such In adatoms to the surface states especially in case of two In adatoms are placed closely. After the InAs thin film was grown on InAs(111)A substrate by molecular beam epitaxy, the sample was transferred to STM and characterized at 5 K in UHV. At the clean surface, each In adatom was found on the center of the In vacancy site of (2x2) reconstruction. At an isolated single In adatom, the closest six In atoms of (2x2) reconstruction to the adatom showed local density of states (LDOS) spectrum peak at roughly 1 eV above the Fermi level. This suggests In atoms on the top most In atomic surface plane form two-dimensional (2D) network of delocalized surface states. Such 2D delocalized states are confined by the ionized donor potential to show quantized resonance peak. When two In adatoms were placed closely, each adatom was still found on the center of the surface In vacancy site as the single In adatom. The closest distance of two In adatoms was equal to the distance between neighboring two In vacancies. In the LDOS spectrum, additional peaks appeared near the original spectrum peak of the single In adatom, when distance between two In adatoms was within a few nm. At these peak energies, the LDOS of the surface In atoms adjacent to the In adatoms are no longer the same symmetric patterns as those of the single In adatom. The density distribution is always mirror symmetric to the plane normal to <1 -1 0> orientation at the center between two In adatoms. Depending on the energy, LDOS enhancements at regions facing or facing away from each other were imaged. This indicates that the bound surface states at In adatoms were interacting coherently to form molecularly coupled states. |