Discrimination of individual atoms on Ge/Si(1 1 1)-(7 × 7) intermixed surface

Abstract

We have discriminated individual Ge atoms from the intermixed Ge/Si(1 1 1)-(7 × 7) surface using a non-contact atomic force microscope at a room temperature environment. In fact, Si–Ge (IV–IV) binary system is considered as one of the most difficult systems for atomic discrimination among atoms in the IV group because of the similarities in the electronic and chemical properties. However, in this study, we found one of the most attractive tools to discriminate a specific atom from the others even in the difficult Si–Ge system. Ge atoms are shown as dim spots in comparison to Si atoms with bright spots on the intermixed surface by a weak chemical bonding energy and/or a relaxation effect despite large atomic radius and high spatial position in both variable frequency shift and topographic images. The discrimination of individual atoms with respect to the chemical interaction variation will further provide a chance to manipulate different atomic species and assemble various nanostructures in near future.

Introduction

The realization of nanotechnology based on the bottom up process requires ultimate technologies such as identification (discrimination), manipulation, and assembly on atomic level. In particular, the nondestructive identification (discrimination) of individual atoms on the intermixed surface is the most elementary step of nanotechnology.

As a consequence of many efforts contributed for utilizing atomic force microscopy (AFM) [1] in the various application fields of nanotechnology for two decades [1], [2], [3], it is not hard any more to achieve the true atomic resolution images using a non-contact AFM (NC–AFM) [4], [5], [6], [7], [8], [9], [10]. The NC–AFM based on frequency modulation method (FM–AFM) [9], [10] nondestructively detects a tiny frequency shift in mechanical resonant oscillation of a cantilever by the weak attractive force acting between atoms of a tip apex and a sample surface. Thus, both the mechanism of the image acquisition and the information obtained by the NC–AFM are different from those of scanning tunneling microscope (STM) as well as of other scanning probe microscope (SPM) [11]. In the NC–AFM, the image is acquired by the formation of a short-range chemical bond between the atom of the AFM tip apex and the atoms of the sample surface [12], [13]. Lantz investigated experimentally the interaction force derived from the formation of a chemical bond that could reach the maximum value of 2.1 nN on Si(1 1 1)-(7 × 7) reconstructed surface at a cryogenic temperature of 7.2 K [2]. Moreover, Abe has realized measurement of the chemical bonding interaction force even at room temperature (RT) by the reproducible site-specific force spectroscopy with the compensation of the thermal drift using the atomic-tracking technique, and the value of interaction force between the tip and the sample was 2.2 nN on the Si(1 1 1)-(7 × 7) reconstructed surface [14].

Discriminating single Ge atoms on the Si(1 1 1) surface seems to be difficult because of the similarities in the electronic properties as well as the chemical properties. As Si–Ge binary system has only 4.2% difference in the radii of covalent bond (Si 1.17 Å, Ge 1.22 Å, the difference 0.05 Å) and 10% in covalent bonding energies (Si–Si 2.32 eV, Si–Ge 2.12 eV, the difference 0.20 eV), the clear discrimination of individual Ge atoms on the surface is difficult in comparison with other atoms in IV group such as Sn and Pb [16], [17]. The discrimination of individual atoms on Sn/Si(1 1 1)-(7 × 7) (the radii of covalent bond: Si 1.17 Å, Sn 1.40 Å, the difference 0.23 Å) and Pb/Si(1 1 1)-(7 × 7) (Si 1.17 Å, Pb 1.47 Å, the difference 0.30 Å) belong to IV group like Ge, has been investigated at RT by Sugimoto [15], [16]. It is reported that Sn or Pb atoms are imaged as the brighter spots than Si atoms on dilute (7 × 7) surfaces despite weak chemical bonding energies (Si–Si 2.32 eV, Si–Sn 1.95 eV, Si–Pb 1.67 eV). Accordingly, it is intriguing to investigate the discrimination mechanism of atom species in the same IV group such as Si, Ge, Sn and Pb atoms.

In this paper, we report the discrimination of Ge and Si atoms with respect to the chemical interaction variation on Ge/Si intermixed surface using a RT NC–AFM. In addition, we attempt to discuss a discrimination mechanism in Si–Ge binary system compared with Sn–Si system or Pb–Si system. We believe the results of this study give a possibility to discriminate and manipulate various atoms, thereby providing a way to assemble diverse nanostructures.