Non-Contact Atomic Force Microscopy and Scanning Tunneling Microscopy of Coexisting Reconstructions on Si(111)

The coexisting metastable reconstructions of the Si(111) surface have been investigated by non contact-atomic force microscopy (NC-AFM). True atomic resolution has been achieved in the NC-AFM imaging of the 7× 7, c(2× 8), 2× 2, c(2× 4), and √3× √3 coexisting reconstructions of the same quenched surface sample. A simple comparison with scanning tunneling microscopy (STM) results is given, and imaging of 2× 1 π-bonded chains island is also reported. [DOI: 10.1380/ejssnt.2005.258]

In the present study, we achieved atomic resolution in the NC-AFM topography of the coexisting reconstructions of the quenched Si(111) surface, namely, the 7 × 7, c(2 × 8), 2 × 2, c(2 × 4), and √ 3 × √ 3 reconstructions. We also observed on the very same quenched silicon sample 2 × 1 π-bonded chains islands. For the sake of comparison, two separated sets of observations were performed, one with a NC-AFM, the other with a STM.

II. EXPERIMENTAL
NC-AFM experiments were carried out by using a JSPM4500-A NC-AFM by JEOL, and STM measurements were taken with a JSPM-4500S STM by JEOL. Both equipments were operated at room temperature (RT) in an ultra-high vacuum (UHV) system with a base pressure of 8 × cantilever (Nano World NCH, n-rich silicon, resonance frequency of 330 kHz, spring constant of 42 N/m) was used to detect force gradients in the NC-AFM constant frequency shift topographic mode. To remove the oxide layer of the tip apex of the cantilever, the tip was intentionally crashed on Si (111) atomic steps. All STM topographs presented here have been performed with several home made metallic tips (polycrystalline W tips electrochemically prepared in NaOH). The STM constant current mode was used for topography.
Si(111) specimens cut from a CZ grown crystal wafer (1 mm × 7 mm × 0.3 mm, n-type, P doped, resistivity 0.1 Ω·cm, by JEOL) were cleaned in an ultrasonic bath of acetone for 30 min, then transferred to the UHV chamber to be degased at 600 • C for 15 h, and flashed several times at 1300 • C. During the flashing treatment, the vacuum pressure was kept lower than 3 × 10 −8 Pa and the temperature was monitored with an infrared pyrometer (IMPAC Infrared GmbH IGA-140). We used the rapid thermal radiation quenching [16] method to create 2 × 1 π-bonded chains islands that coexist with stable 7 × 7 reconstructed areas, and '1 × 1' areas containing metastable families of 2 × 2m (2 × 2, c(2 × 4), and c(2 × 8)), √ 3 × √ 3, and (2n + 1) × (2n + 1) (with n > 3) reconstructions (referred in the text as '1 × 1'). Reconstructed surfaces were prepared as follow: specimens were raised very rapidly from RT to 900 • C, flashed at 1300 • C for 5 s, quenched from 1300 • C to RT by cutting off the heating current (a cooling rate of 450 • C/s was measured), annealed at 200 • C for one hour, and thermalized at RT for another hour. 3b, 3c, and 4a) and the domain size for each metastable reconstruction is quite small, on the order of 10 × 10 nm 2 (see Fig. 1). These small metastable domains can be routinely imaged with NC-AFM, even when topological defects (disordered atomic arrangements, dislocations, and adatom vacancies) and Si clusters are present [17].

III. RESULTS AND DISCUSSION
The most stable of the all the metastable phases is the c(2×8) reconstruction, and the manifestation of its higher stability is that domains with relatively larger sizes, on the order of 50 × 50 nm 2 for the largest, can be observed. This is shown in the atomically resolved NC-AFM topographs of Figs. 2 and 3 (a), where c(2 × 8) double rows as long as 10 adatoms in size are measured. In Fig. 2, the c(2 × 8) adatoms double rows running in the three equivalent [110] directions reveal the threefold symmetry of the reconstruction. A domain boundary between the metastable c(2 × 8) and stable 7 × 7 phases is displayed in Fig. 3 (a), and it can be remarked here, that even if the Si(111)-7 × 7 reconstruction described by the dimeradatom-stacking-fault (DAS) model [18] is the most stable reconstruction of this surface, its topology is more complicated than the c(2 × 8) one [19]. In the case of Si(111)c(2 × 8), only the topmost layer reconstructs whereas four layers are involved in the 7 × 7 reconstruction. As seen in the c(2 × 8) model of Fig. 5(d) restatoms. The quality of the NC-AFM scans of Figs. 2 and 3 (a) is good since true atomic resolution of the reconstructed areas, dislocations, and Si clusters can be routinely performed. Observation of the restatoms of a c(2 × 8) structure has been reported in the case of NC-AFM imaging of Ge(111)-c(2 ×8) [10]. In the present case of Si(111)-c(2 × 8), we notice that atomic resolution of the restatoms could not be successfully achieved.
The domain boundary of Fig. 3a corresponds to the squared area in Fig 3b. Due to the large difference of surface contact potential (SCP) between the 7 × 7 and '1 × 1' triangles, these two phases are clearly resolved in the large scale NC-AFM images of Figs. 3 (b) and (c), taken with different sample voltages V s . We measured that with applied voltages V s ranging from 0 V (Fig. 4  (a)) to -0.6 V (Fig. 3 (b)), the 7 × 7 and '1 × 1' triangles appear as dark and bright features respectively. At V s = +0.6 V (Fig. 3 (c)) a contrast reversal occurs resulting in bright 7 × 7 and dark '1 × 1' triangles. These results are in good agreement with scanning Kelvin probe microscopy experiments that have already measured SCP differences between 7×7 and '1×1' domains [20]. Considerable discussion and detailed description related to SCP in the dynamic force microscopy of quenched silicon reconstructions will be reported in a future communication [21].
In the constant frequency shift scan of Fig. 4 (a), a large area of the quenched Si(111) surface is displayed, showing large dark and bright triangles, and three bright islands, corresponding to Si(111)-7 × 7 reconstructed regions, '1 × 1' areas, and 2 × 1 π-bonded chains islands respectively. The corners of the triangles formed by the 7×7 and '1×1' areas point in the [112] directions. Figure 4 bonded chains parallel to the [110] direction. The islands height is 1.6 nm, and their size distribution varies in a measured range of 2 × 10 2 to 1.8 × 10 4 nm 2 . Most of the islands have a rectangular shape, with a mean dimensions aspect ratio of L/l = 2.25, where L is the longest side (always parallel to the [110] chains direction), and l is the shortest side (in the [112] direction). As previously reported, a minority of islands exhibiting a complicated 'amoeba-like' shape [16] was also observed. We measured a surface coverage density of islands of 10 −4 nm −2 and noticed that their position on the surface is more likely to be at the boundary of 7 × 7 and '1 × 1' triangles.
Comparison between NC-AFM and STM topography experiments is given in Fig. 5 for the three main reconstructions of the Si(111) surface. NC-AFM and STM scans have been recorded with the same cantilever and STM tip respectively. From the imaging point of view, the quality and resolution of Fig. 4 (b) (NC-AFM) and (c) (STM) are in stark contrast. We experienced that obtaining atomic resolution of the Si(111)-2 × 1 together with the other coexisting metastable phases and the 7 × 7 reconstruction at the same time can be routinely achieved with STM, while it was almost impossible in the NC-AFM case. More precisely, rather than true atomic resolution of 2×1 π-bonded chains in the NC-AFM topography ( Fig.   5 (b)), we could only obtain straight features in the [110] direction having a corrugation height of 0.5Åwith a chain periodicity of 7.5Å, which strongly resemble, but do not completely match, the correct topography given by the STM measurements.

IV. PERSPECTIVES AND CONCLUSIONS
The models of the Si(111)-2 × 1, Si(111)-c(2 × 8), and Si(111)-7 × 7 reconstructions are well known and respectively described by the π-bonded chains model [22,23], a simple adatom restatom model [19], and the dimeradatom-stacking-fault (DAS) model [18]. Nevertheless, experimental results are still needed in order to unveil the band structure of the Si(111)-c(2 × 8) or the differences in the chemical reactivity of these reconstructions. In the case of group IV semiconductors, reconstruction, surface stabilization, electronic band structure, nature of the surface atoms and chemical reactivity are related to each other in a complex way. In order to study this complex interplay in the case of Si(111), it is thus an advantage to be able to probe directly on the same surface its three main reconstructions. In this report we have actually shown that it is possible to study concomitantly with the same surface science tool (NC-AFM or STM) the coexisting Si(111)-2 × 1, Si(111)-c(2 × 8), and Si(111)-7 × 7 reconstructions of the same quenched surface sample, opening the way for further direct comparison of the electronic spectroscopy or reactivity of these reconstructions [24]. In conclusion, we have broadened the scope of NC-AFM imaging on the Si(111) surface by achieving true atomic resolution on 7 × 7, c(2 × 8), 2 × 2, c(2 × 4), and, √ 3 × √ 3 coexisting reconstructions, and imaging 2 × 1 π-bonded chains island on the same quenched surface sample. We also confirmed that the SCP of 7 × 7 and c(2 × 8) are different.