Structure of Co-2 × 2 nanoislands grown on Ag/Ge(111)-√3 × √3 surface studied by scanning tunneling microscopy
© Huang et al; licensee Springer. 2012
Received: 29 November 2011
Accepted: 19 March 2012
Published: 19 March 2012
We have found that Co-2 × 2 islands grown on an Ag/Ge(111)-√3 × √3 surface have hcp structure with the (11-20) orientation. The island evolution involves transformation of the unit cell shape from parallelogram into rectangular, which is accompanied by the island shape transformation from hexagonal into stripe-like. Identified are two crystallographic directions for the island growth, the pseudo- and the pseudo-[1-100]. We have observed the occurrence of a lateral shift between the topmost and the underlying bilayers in the case of the island growth along the pseudo- direction. In contrast, the topmost and the underlying bilayers are unshifted for the growth along the pseudo-[1-100] direction.
Since its invention, scanning tunneling microscopy [STM] has served as a powerful means in providing an atomically resolved insight into the structure of solid surfaces . An interest in surface structure exists in a strong correlation between the morphology of a particular surface and its electronic, optical, and magnetic properties . Therefore, the need to characterize the surface morphology at the atomic level emerges in response to an increasing demand for materials with electronic and magnetic properties suitable for modern applications. Nowadays, in the face of tremendous progress in device miniaturization, the possibility to manipulate the properties of the surface seems to be particularly attracting. In principle, it may be achieved by depositing foreign atoms onto the surface, as it is known that numerous properties of the resultant epitaxial layers, such as lattice constant or electric conductance, are quite different from those of the substrate [3–6].
Among a palette of materials, the system Co/Ge represents a particularly promising case on account of a combination of the high-mobility substrate with a metal of an exceptionally high magnetization. However, a key concern is with Co-Ge intermixing, which results in the formation of non-ferromagnetic surface compounds.
An elegant solution which consists in Ag termination of the Ge(111) substrate surface and leads to the formation of a √3 × √3 reconstruction was proposed by Tsay et al.  for the growth of Co thin film. They have demonstrated that, in contrast to the non-ferromagnetic Co/Ge(111) surface, Co films grown on the Ag/Ge(111)-√3 × √3 substrate surface reveal magnetic properties and suggest that the intermediate Ag layer has buffering properties toward Co thin film growth by preventing the deposited Co atoms from a chemical reaction with the germanium surface.
The buffering properties of the Ag/Ge(111)-√3 × √3 surface may be accounted for in terms of its unique atomic arrangement, which is currently well established based on the experimental [8–14] and theoretical work . The approved structural models commonly propose that the √3 × √3 surface has a structure in which both the Ag atoms and the outermost Ge atoms are arranged in a triangular configuration. The formation of a Ge triangle saturates two of three surface dangling bonds, and the remaining bond is saturated with an Ag atom. Owing to such an arrangement, the Co atoms cannot combine with Ge(111) surface atoms very easily, and hence, the surface remains inert toward the deposits.
The unique properties of the Co/Ag/Ge(111)-√3 × √3 surface inspired the work in our laboratory where, in the last several years, the attention has been focused on the STM characterization of nanosized Co structures (nanoclusters and nanoislands) grown on the surface under discussion [15–17]. We have found that, depending on coverage and annealing temperature, the Co islands reveal either √13 × √13 or 2 × 2 reconstruction. Dual-polarity STM images of individual structures indicate that the islands differ in conducting properties. That is, the structures with the √13 × √13 periodicity, having the empty-state image significantly different from the filled-state image, exhibit behavior typical for semiconductors. In contrast, islands with the 2 × 2 reconstruction reveal a metallic character with the empty-state image almost identical to the filled-state ones. This observation has led us to conclude that the reported magnetic properties of the Co/Ag/Ge(111)-√3 × √3 films should be ascribed to the Co-2 × 2 phase rather than that of the Co-√13 × √13.
The application importance of the magnetic Co-2 × 2 phase provides a practical motivation to conduct more profound studies on the island growth on the Ag/Ge(111)-√3 × √3 surface. For example, a complete picture of the Co-2 × 2 island growth has been hampered by a lack of a model for the island surface structure. However, the situation for Co epitaxy is rather complicated as Co can grow in three different structures, face-centered cubic [fcc], hexagonal close-packed [hcp], and body-centered cubic [bcc] .
In this work, based on our experimental observations, we propose that the Co-2 × 2 islands grow on the Ag/Ge(111)-√3 × √3 surface in an hcp structure and have a (11-20) crystallographic orientation. We hope that our findings may be useful for controlling the magnetic nanoisland growth on the surface.
Experiment and methods
The Co-2 × 2 nanoislands were fabricated in situ with the use of an Omicron VT-STM (Omicron Taiwan R. O. C. Office Omega Scientific Taiwan Limited, Taipei, Taiwan, Republic of China) (base pressure approximately 2 × 10-10 mbar) operating in constant-current mode and equipped with well-collimated evaporators for Ag and Co deposition. The Ge(111)-c2 × 8 surface was achieved by cleaning p-type Ge(111) wafers (1 to 10 Ω cm resistivity, 500 μm thickness) by repeated cycles of Ar+ bombardment (1.0 keV, 10° to 90° incidence angle) followed by annealing at 920 K. The Ag/Ge(111)-√3 × √3 surface was prepared by exposing the substrate, kept at room temperature [RT], to an Ag beam from a K-cell dispenser for 90 min followed by annealing at 720 K. Then, the final surface was produced by Co deposition from an e-bombardment type evaporator for 30 min to obtain the coverage higher than 3 ML, which is suitable for fabrication of the desirable Co-2 × 2 phase. After deposition, the substrate was post-annealed at 670 K. All STM images presented in this paper were acquired at RT using KOH-etched W tips. The substrate temperatures were measured with a K-type thermocouple.
Results and discussion
We found previously that the structural properties of the Co islands grown on the Ag/Ge(111)-√3 × √3 surface are strongly influenced by the structure of the Ge(111) surface . This fact is caused by a very strong coupling between the Co atoms and the Ge(111) surface despite the presence of the Ag buffer layer. Therefore, in our further considerations, we neglect the presence of the Ag layer and propose a model for the Co-2 × 2 phase growth on the Ge(111) surface.
As seen in Figure 1a, the Co-2 × 2 islands adopt either hexagonal or stripe-like shapes. An insight into the inner structure of the individual islands is provided in Figure 1b, c. With respect to the shape, the unit cells of the hexagonal island (Figure 1b) at first glance seem to resemble parallelogram-shaped unit cells of the Ge(111)-1 × 1 surface, which indicates a strong influence of the substrate surface on the structure of the growing island. A closer inspection, however, reveals that the angle of the intersection between the island rows running in the directions shown in Figure 1b is larger as compared to that typical for the Ge(111)-1 × 1 surface (i.e., 70° vs. 60°). By analyzing a number of images, we have found that for the stripe-like islands, which are generally higher than their hexagonal counterparts, the above-mentioned discrepancy is even more distinct. For example, for the island shown in Figure 1c, the angle amounts to 77°. These observations have important implications for the model for the island growth. Namely, we can speculate that the island growth involves a distortion of the island rows with reference to the substrate surface rows. What is more, a degree of the distortion increases with the island height. As a consequence, the island unit cells undergo a shape transformation from initial parallelogram into rectangular. The alterations in the island inner structure are accompanied by noticeable changes in the island shape. That is, when the islands grow in height, the evolution proceeds to transform them from hexagonal-shaped into stripe-like-shaped. In view of that, in an attempt to construct the model for the Co-2 × 2 island growth, we focus on the stripe-like islands as a more evolved phase.
In order to ascribe a specific crystallographic structure to the Co-2 × 2 islands, we noted that the equilibrium phase for bulk Co is hcp at RT, and the Co layers generally adopt this structure when grown on metallic surfaces [19, 20]. However, it was demonstrated that, under special preparation conditions, the Co layer can grow epitaxially on the Au(111) surface with the fcc structure [21–23]. Tonner et al. added more confusion to the issue by demonstrating that the occurrence of a particular structure depends on the thickness of deposited films. That is, while the initial Co growth on the clean Cu(111) proceeds in the fcc phase, the transition from fcc into hcp takes place as the film thickness increases beyond two layers .
Geometrical characteristics of low-index Co crystallographic planes in bcc, fcc, and hcp structures.
Lattice parameters (pm)
Interlayer distance (pm)
251 × 251
251 × 355
251 × 251
251 × 251
434 × 407
251 × 407
284 × 284
284 × 402
402 × 402
400 × 400
We have observed that the Co-2 × 2 islands grown on the Ag/Ge(111)-√3 × √3 surface reveal hexagonal and stripe-like shapes. They commonly grow in the hcp structure and are oriented by the (11-20) face. However, in view of the island growth, the hexagonal islands may be regarded as a less evolved stage from which the stripe-like islands develop. The island shape evolution is accompanied by a transition of the unit cell shape from parallelogram-like into rectangular. In the majority of cases, the islands grow along the pseudo- crystallographic direction, giving the lattice mismatch of 2% between the growing phase and the substrate. Some 10% of islands grow along the pseudo-[1-100] direction for which the lattice mismatch amounts to 9%.
scanning tunneling microscopy.
The financial support of the National Science Council of the Republic of China (grant no. NSC 98-2112-M-003-006-MY3) is acknowledged.
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