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?