Monatomic step structure on the Si(001) surface

Abstract

The atomic structure of the monatomic steps on the Si(001) surface is investigated by scanning tunneling microscopy. The kink at the intersection of the S A and S B steps induces a strong buckling along the rebonded dimer row in the lower terrace. On the upper terrace of the S B step, the kink induces a local p(2×2) structure directed toward the center of the step. The strains of second-layer atoms supporting the dimers are responsible for the reconstruction near the monatomic steps on the Si(001) surface.The step on the Si(001) surface is one of the most abundant intrinsic defects. It causes inhomogeneities on the clean Si(001) surface and affects the nucleation and growth in the epitaxy. Many theoretical and experimental studies have been devoted to resolve the atomic structure of the step [1-13]. The monatomic step on the Si(001) surface is classified into two types, S A and S B [3].For the S A step, two types of structure are expected, depending on the direction of the buckling. The theoretical calculations estimate those structures to have the same total energy within the limit of calculation accuracy [14], but only one of two structures has been observed in experiments [1]. The structure of the S B step classifies into three types depending on the structure of dimers at the step edge; rebonded, nonrebonded, and split. On a well-prepared clean Si(001) surface, the S B step edge is found to be of the rebonded type. When the surface is under a tensional stress near vacancy defects, the rebonded S B step edge is transformed into a split one. The nonrebonded S B step edge appears only in a special environment, namely one where free movement and/or supply of atoms is restricted [15]. These three types of step edge, in combination with the buckling of dimers, increase the structural varieties near the S B step [14].Since every step has a finite width, kinks are inevitably created at both ends. A kink is influenced by a strong asymmetrical environment that induces a local structure on neighboring dimer rows in the upper and lower terraces of the step [16]. Since the kink is located at the intersection of S A and S B steps, the S A and S B steps are strongly correlated through it.In this work, we investigate the atomic structure of the monatomic S A and S B steps by scanning tunneling microscopy (STM) and derive a simple geometric rule based on the bonding characteristics to explain the various structures near the steps. To avoid the complexity relating to the S B step, only the rebonded type (which is dominant on a well-prepared clean Si(001) surface) is taken into account. Experimental methodOur experiments were performed in an ultrahigh vacuum below 3 × 10 −10 Torr with a home-made STM. The Si(001) samples were cut from a P-doped Si(001) wafer (10 Ω cm) and wrapped with Ta foil at both ends to prevent direct contact with the Mo holder -a slight contact of the sample with the Mo holder causes contaminations and increases dimer vacancies on the sample surface after heat treatments. Then the samples were mounted on ...