Controlling the effective mass of quantum well states in Pb/Si(111) by interface engineering

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Controlling the effective mass of quantum well states in Pb/Si(111) by interface engineering

Bartosz Slomski, Fabian Meier, Jürg Osterwalder, and J. Hugo Dil

Phys. Rev. B 83, 035409 – Published 14 January 2011

Abstract

The in-plane effective mass of quantum well states in thin Pb films on a Bi reconstructed Si(111) surface is studied by angle-resolved photoemission spectroscopy. It is found that this effective mass is a factor of 3 lower than the unusually high values reported for Pb films grown on a Pb reconstructed Si(111) surface. Through a quantitative low-energy electron diffraction analysis the change in effective mass as a function of coverage and for the different interfaces is linked to a change of about 2% in the in-plane lattice constant. To corroborate this correlation, density functional theory calculations are performed on freestanding Pb slabs with different in-plane lattice constants. These calculations show an anomalous dependence of the effective mass on the lattice constant including a change of sign for values close to the lattice constant of Si(111). This unexpected relation is due to a combination of reduced orbital overlap of the $6 p_{z}$ states and altered hybridization between the $6 p_{z}$ and the $6 p_{xy}$ derived quantum well states. Furthermore, it is shown by core-level spectroscopy that the Pb films are structurally and temporally stable at temperatures below 100 K.

Received 21 October 2010

DOI:https://doi.org/10.1103/PhysRevB.83.035409

Authors & Affiliations

Bartosz Slomski^1,2,*, Fabian Meier^1,2, Jürg Osterwalder¹, and J. Hugo Dil^1,2

¹Physik-Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
²Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland

^*bartosz.slomski@psi.ch

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Vol. 83, Iss. 3 — 1 January 2011

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Images

Figure 1
Band dispersion of (a) $n$ -Si(111)-( $7 \times 7$ ) and (b) $n$ -Si(111)- $(\sqrt{3} \times \sqrt{3})$ -Bi $(β)$ along the $\overset{̲}{Γ M}$ direction. (c) Energy spectrum of $n$ -Si(111)-( $7 \times 7$ ) at normal emission. (d–f) LEED patterns from the ( $7 \times 7$ ), Si(111)- $(\sqrt{3} \times \sqrt{3})$ -Bi $(β)$ and from the ( $1 \times 1$ ) surface of Pb/Bi/Si(111).Reuse & Permissions
Figure 2
(a) Photoemission of Pb and Bi 5 $d$ core levels from Bi- $\sqrt{3}$ /Si(111) (red/gray) and of 9-ML Pb/Bi/Si(111) (black). Both heavy metals $(Z_{Pb}$ $=$ 82, $Z_{Bi}$ $=$ 83) show strong spin-orbit split $d$ states. (b) Core levels of Bi before (red/gray circles) and after Pb deposition (black circles). The Pb overlayer induces a chemical shift by 350 meV toward a lower binding energy. Curves are shifted on the vertical scale for clarity. (c) Spectrum of a QWS shortly after preparation (triangles) and after 44 h (squares). (d) 5 $d$ core levels of Pb 24 h after preparation fitted with clean and oxidized Pb components.Reuse & Permissions
Figure 3
(a–c) Band dispersion of QWSs in 10-, 17-, and 19-ML Pb films on Bi/Si(111). (d) Band dispersion of Pb QWS on Pb/Si(111). (e) Extracted dispersion [(red) circles] from (a) as described in the text. For comparison the free-electron-like dispersion and dispersion of Pb/Pb/Si(111) are included. (f) Squares (red): effective masses of QWS for different thicknesses of Pb deposited on the Bi/Si(111) obtained from the fitting procedure as described in the text. Circles (blue): effective masses of Pb on Pb/Si(111) taken from Ref. 16.Reuse & Permissions
Figure 4
(a, b) Simplified picture of $p_{z}$ orbital overlap at both interfaces. (c) Calculated band dispersion along $\overset{̲}{M}$ - $\overset{̲}{Γ}$ - $\overset{̲}{M}$ with an in-plane lattice constant of (c) $a$ $=$ 3.5 Å and (d) $a$ $=$ 4.0 Å. (e) Top view of the Pb(111) surface and (f) effective masses for different in-plane lattice constants obtained from DFT including an exponential fit as described in the text.Reuse & Permissions
Figure 5
(a, b) (1,0) LEED spot of a Pb/Pb/Si(111) and Pb/Bi/Si(111) surface. (c) Horizontal cut through the maximum of intensity of the (1,0) spot for Bi [open (red) circles] and Pb [filled (black) circles]. (d) Relative lattice constant vs. coverage measured with LEED; (blue) circles represent Pb/Pb/Si(111); (red) square, Pb/Bi/Si(111).Reuse & Permissions
Figure 6
(a) Band dispersion of 12-ML Pb deposited on top of Bi/Si(111). (b) Normalized energy distribution curves for in-plane momenta $k_{∥} = 0 \pm 0.4$ Å $^{- 1}$ . Blue spectra represent quantum resonances; red (gray) spectra, fully confined QWSs. (c) Parabolic fit including the quantum resonances.Reuse & Permissions

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