Structural investigation of Ca/Si(111) surfaces

Abstract

The surface structures of different Ca/Si͑111͒ surfaces were studied by low-energy electron diffraction ͑LEED͒ and high-resolution core-level photoelectron spectroscopy. Five different phases were observed in LEED depending on the Ca coverage. The lowest coverage phase has both (3ϫ2) and c(6ϫ2) periodicities, and the highest coverage phase has a (2ϫ1) periodicity. The LEED patterns of the three intermediate phases were (5ϫ2), (7ϫ1), and (9ϫ1). In the Si 2p core-level spectra, three surface components were observed in both the lowest and highest coverage phases. These three surface components were also observed in each intermediate phase together with two other components. The presence of the two extra components indicates that the intermediate phases are not completely described by simple combinations of the two end phases as suggested in the literature. By considering the energy shift and intensity of each surface component, we conclude that the structure of the (3ϫ2) phase is basically the same as that of the honeycomb-chain-channel model with a Ca coverage of 1/6 ML, and the (2ϫ1) phase is formed by -bonded Seiwatz Si chains with a coverage of 0.5 ML. Moreover, taking the energy shifts and intensities of the extra surface components into account, we propose a structural model of the (5ϫ2) phase, whose Ca coverage is 0.3 ML.

Electronic structure of the Ca/Si(111)-(3×2)surface

“…The negligible correlation between the -bond state and the adsorbate is confirmed by the fact that this state is observed in the same binding energy range ͑1.5-2.5 eV͒ as similar dispersion in previous studies performed on monovalent 6,26 -29 and divalent 7,15,18,19 atom induced Si͑111͒-(3ϫ1) and (3ϫ2) HCC surfaces. Therefore, since ͑1͒ the basic structure of the Ca/Si͑111͒-(3ϫ2) surface is the same as the HCC structure of the Li/Si͑111͒-(3ϫ1) surface, [17][18][19] ͑2͒ the dispersion of S 3 shows good agreement with that of S 1 ϩ at k // larger than 0.9 Å Ϫ1 , and ͑3͒ the S 3 state is observed in a binding energy range of 1.5-2.5 eV, we conclude that the origin of S 3 is the -bond state between the C and D Si atoms displayed in Fig. 1͑b͒.…”

“…Among the other AEM induced (3ϫ2) surfaces, the Ca/Si͑111͒-(3ϫ2) surface was reported to have the same geometric structure as the Ba induced surface. [17][18][19] So far, three studies have reported the electronic structure of this surface, and a basic understanding is important in order to obtain a complete comprehension of the metal atom induced HCC structure. 8,18,19 However, these studies cover only a part of the surface Brillouin zone ͑SBZ͒, and the results are not consistent.…”

Structural investigation of the Ca/Si(111)-(3*2) surface using photoelectron diffraction

“…This kind of Si(111)-(3×1) surface has been reported to be formed by various adsorbates, e.g., alkali metals (AM's), alkaline-earth metals (AEM's), rare-earth metals (Eu, Sm and Yb), and Ag with a coverage of either 1/3 monolayer (ML) or 1/6 ML. Although some of the adsorbates actually show different diffraction patterns (Ag shows a (6×1) pattern [3][4][5][6], and Ca [7,8], Eu and Yb [9,10] show a (3×2) pattern) all reconstructed surfaces are widely believed to have a quite similar structure based on the similarity of the low-energy electron diffraction (LEED) I-V curves [11,12], scanning-tunnelingmicroscopy (STM) images [13][14][15][16][17], and surface core-level shift measurements [3,8,[17][18][19][20]. That is, the basic structures of all these surfaces follow the honeycomb-chainchannel (HCC) model [21][22][23], which was originally proposed for 1/3 ML monovalent atom induced Si(111)-(3×1) reconstructions, and the difference in diffraction pattern is proposed to originate from the different adsorption sites of the metal atoms.…”

“…Regarding the Ca induced Si(111)-(3×2) HCC surface, only weak ×2 streaks were observed in low-energy electron diffraction (LEED) at 300 K, while ×2 spots were observed at 100 K [7,8,25,26]. Taking into account that the surface energy for H 3 site adsorption ( Fig.…”

“…That is, Ca atoms are proposed to be adsorbed on only the T 4 site at 100 K, and to be adsorbed on both the T 4 and H 3 sites at 300 K. However, so far there is no detailed study about the atomic structure of the Ca/Si(111)-(3×2) surface at 300 K, and thus there is no evidence to support this proposition. Further, the Ca/Si(111) surface has been reported to form a series of quasi-1D (n×2) reconstructions (n =3, 5, 7, and 9) that culminates with a 1D (2×1) phase at 0.5 ML [7,8,25], and the intermediate phases are considered as combinations of the two end (3×2) and (2×1) phases. It is therefore essential to completely understand the atomic structure of the (3×2) phase to fully understand the physical properties of the Ca induced 1D and quasi-1D reconstructions on a Si(111) surface.…”