A Soft Incommensurate Reconstruction on PB/SI(111)
- PDF / 2,118,029 Bytes
- 4 Pages / 414.72 x 648 pts Page_size
- 52 Downloads / 145 Views
structures is important for understanding the SBH difference. In this work, we study the IC structure of 1 monolayer (ML) Pb on the Si(1 11) surface (1 ML = 7.84 x 1014 Pb atoms/cm 2). In contrast to the close-packed model proposed by Grey et al. [6], we find that Pb forms two types of trimers as a result of small lateral displacements from a lxi mesh. The trimers are arranged in alternating domains, whose sizes and shapes are strongly affected by local stress field and surface irregularities. We observe the transformation of the IC phase into an ordered I x 1 structure induced by a nearby reconstruction. Si(1 11) samples are etched using the Shiraki procedure [16] before being introduced into the ultra-high-vacuum chamber. The Si(l 1I )-7x7 surface is prepared by flashing the sample to 12500C, followed with annealing at 9200C for a few minutes. Two Pb deposition schemes are used in this study. In the conventional "one-stage deposition", Pb (99.999% in purity) is evaporated onto a clean Si(1 11) sample, which is then annealed at -3000C. A mixture of the lxl, 1/3 ML -vt3xV-/3 (in short, a(x/3), and Si(1 1 )-7x7 phases is observed for Pb coverages between 0.3 and 1ML. Samples having more than 1ML Pb show the IC phase and scattered 3D Pb islands. In the "two-stage deposition" scheme, we evaporated additional Pb at RT onto annealed samples with submonolayer initial Pb coverage. We observe coexistence of the Pb/Si-7x7 (the high coverage phase of RT deposition) and IC phases, which cannot be obtained in one-stage deposition. It allows us to determine the registration of Pb atoms in the IC phase, and observe interesting surface stress effects. Fig. 1 shows a tunneling image of a region of the IC surface, prepared by two-stage deposition. Surface Pb atoms are arranged in two types of trimers, differing in orientation by 600. Alternating domains of trimers having N x [-/R300 local periodicity can clearly be 153
Mat. Res. Soc. Symp. Proc. Vol. 355 @1995 Materials Research Society
IOA
I.A IW ! _ [1121 -
Fig. 1 65 A x 50 A tunneling image of a region of the Pb/Si(111) IC phase, taken at a sample bias of +2.0 V and a tunneling current of 50 pA. Parallel lines are drawn to indicate the structure of domain boundaries.
O@OPb adatom o IstlayerSiatom
Fig. 2 Atomic model for the IC structure in Fig. 1.
identified, but the IC structure does not have a long range order. Additionally, a quasi-lxl region with Pb atoms slightly displaced from the ideal lxWregistration sites can be seen. We believe the trimers and the quasi-Ix1 structure are the real atomic structure rather than merely an electronic artifact because they are consistently imaged on both polarities of the tunneling bias. Fig. 2 is the corresponding atomic model for the IC region outlined in Fig. 1. In the IC phase, each Pb atom is bonded to the Si atom underneath. The majority of Pb atoms are displaced laterally from their ideal TI site to form trimers centered on either T4 sites or H3 sites. Note that trimers centered on the T4 site are rotated 600 relative to those
Data Loading...
