subsurface structure of epitaxial silicides imaged by stm · 2007. 8. 28. · re-silicides...
TRANSCRIPT
Celia RogeroJose Angel Martín Gago Jorge Iribas Cerdá
ICMM-CSIC (Spain)
Subsurface Structure of Epitaxial Silicides imaged by STM
RE-Silicides epitaxially grown on Si(111)
Si(111)-7x7Si(111)-7x7
Si(111)400ºC
Si(111)
Y3Si5(0001)
RE-Silicides: General properties
Smallest Shottky Barrier HeightUsed for infrared detectors
Perfect lattice matchSi/Y3Si5 Multilayers with optical properties?
heavy-RE silicides all similar Same atomic & electronic structure, all metallicYttrium ok!
RE-Silicides: Fermi Surfaces
Y3Si5 Gd3Si5 Er3Si5
With <pA> matrix elements
Si(111)-p(1x1)+YSi21 ML 2D films
Si(111)bulk
Si bilayerRotated
BuriedY-plane
Characterized by LEED, XPD, DFT(C. Rogero, PhD Thesis)
Si(111)-p( 3 3)+Y3Si5> 1 ML 3D films
UpSi(111)
bulk
Rotated Si bilayer
Buried3D Y3Si5(Th3Pd5)
Si-vacancy ( 3 3) plane
Down
STM images: “old” works
p3m; Gago PRB(97) p6 ; Roge SS (97)
Up modelwith buckled top Si layer
Down modelwith lateral relaxations
Two different silicides?
Our STM images: 2 phases?
But images acquired on the same substrate=> Tip effect or 2 phases coexisting?
p3m p6
Theoretical Tools
Geometry/Energetics/Electronic StructureDFT-LDA: SIESTA
STM SimulationsDFT-LDA-Tersoff-Hamann: SIESTAGF-EHT+tip: green (www.icmm.csic.es/jcerda)
Semi-infinite leads
W(111)+4W W(111)+4Si
Atom resolved PDOS: EHT fits
Si-upSi-downSi-down-vY-subSi-bSi-aYSi-bSi-aYSi-bSi-aYSi-intbSi-intaY-intSi-intSi-b2Si-b1Si-surf
Si-up-vSi-upSi-downY-subSi-bSi-aYSi-bSi-aYSi-bSi-aYSi-intbSi-intaY-intSi-intSi-b2Si-b1Si-surf
Down Up
Si(111)-p( 3 3)+Y3Si5DFT Geometry
No Siup lateral shifts
More stable by 40 meV
DownUp
Top Siup layerNOT buckled
Si top bilayer+Y is a “p(1x1)” for both models=> Relaxations do not explain the STM images
STM Simulations
Experiment
p6/Up
p3m/Down
TH green
Si4apex
W4apex
2 phases coexist, despite tip effectsThe aspect of the images is dictated by the registry of theburied vacancies, NOT by relaxations at the top Si bilayer
Coexistence of two phases2 different vacancy registries
experimentgreen; Si4 apex
STM Simulations IIBuried Vacancy Domain Boundaries
The STM is probing 3rd layer buried vacancies
STM depth sensitivity
STM sensitivity to buried defects in semiconductorsis well established; Ph Ebert, SS Reports (99)
However, in metallic systems, and due to the efficient screening, it’s unexpected
STM depth sensitivity in metals
Ir atoms/chains in Cu(100)Heinze, PRL (99)
Si(111)-p(7x7) in Pb;Altfeder, PRL(98)
S, C, O in Pd(111)Rose, J Chem Phys (01)
Electronic Structure Covalent character of the Y-Si bonding
CDD p6/Up
p3m/Down
LDOS [Ef-0.2eV,Ef]
Despite system is metallic, screening not efficient due to the covalent character of the Y-Si bond
p3m/Down
CD
SiYSi
SiYSi
Conclusions
Solved the long standing controversy on the structure ofthe RE-silicide (0001) surfaces and the STM experiments
The symmetry in the STM images is NOT determined bysurface relaxations but it is dictated by the registry of thetop Si bilayer with respect to the buried vacancies
Unexpected STM depth-sensitivity (theory assisted):Buried (down to the 3rd layer) vacancies and domain boundaries can be resolved despite the system is metallic
It is the nature of the bond –covalent in this case, and not the metallicity of the system what determines the STMdepth sensitivity
Is there another technique to solve this?