rare-earth silicides – a holistic study• a potentially very useful metal-silicon contact – low...

1CMD GroupDepartment of Physics

Rare-Earth Silicides –a Holistic Study

Matt Probert & Chris Eamesemail: [email protected]

web: http://www.cmt.york.ac.uk/cmd/


Outline

• Motivation

• Surface Physics– experimental and theoretical approaches

• Example 1 – Structure of Sm on Si surface– resolving STM and LEED conflicts

• Example 2 – Ho nanowire on Ge surface– structural and electronic model


Why study Rare-Earth Silicides?• A potentially very useful metal-silicon

contact– low Schottky barrier (~0.4 eV) on n-type Si– sensor applications

• Bulk rare-earth silicides have good lattice match to silicon

• Novel interface/surface structures• Novel electronic properties• Fundamental interest


Surface Physics• Much current interest -

$150 bn/year industry!• Understanding essential

for growth, catalysis, etc• Major impact on

electronic and atomic structure

• Surfaces may reconstruct in order to remove the effect of dangling bonds etc and hence attain lower energy state

Unreconstructed Si(111) Surface


Silicon Surface Reconstruction

Si (111)-7x7 Takayanagi Reconstruction

Brommer et alPRL 68, 1355 (1992)


Surfaces and Overlayers

• Overlayer periodicity related to bulk periodicity - in this case adatoms form 2x2 overlayer


Low Energy Electron Diffraction• Electrons with energies

~40-300eV diffracted from periodic surface mesh

• Collect elastically scattered electrons

• Surface sensitive• Complementary to other

experimental surface techniques such as STM

• Can be qualitative (e.g. indicating overlayerperiodicity and quality) or quantitative … Si (111)-7x7 70eV


Quantitative LEED• Intensity of each spot in the

LEED pattern depends on energy

• Intensity vs. Energy curves for each spot gives a unique fingerprint of the structure

• A difficult inverse problem to solve for the 3D structure which best fits the observations– Need phase shift of each

scattering event– Monte Carlo simulation with

many beams and look at yields, e.g. CAVLEED code

Kitayama et al Surf. Sci. V482-485, 1481, (2001)


Rare-Earth Silicide Preparation• Start with Si(111) or Ge(111) and clean under UHV to

give 7×7 or c(2×8) reconstruction• Deposit 1 ML of rare-earth onto surface and anneal to

about 550 °C• LEED shows formation of ordered silicide/germanide

with a 1×1 reconstruction• MEIS shows that trivalent rare-earth ions do not sit on

top of silicon/germanium– get an ordered, flat layer underneath a reverse-buckled

silicon/germanium overlayer– a 2D silicide/germanide

• 3D silicides prepared under different conditions.


LEED for Ho on Si(111)• Generate clean Si(111)-7x7

– Check quality with LEED

• Deposit 1ML of Ho• Anneal at 500oC for ~15mins• Check quality with LEED

• Acquire I(V) curves

• Try to find structure that best fits measured I(V)

40 eV

150eV


Generic 2D Structure

Trivalent Lanthanides e.g. Dy, Ho, Er, etc.

RE in flat layer on T4 sites under 1x1 reverse-buckled overlayer

Hydrogen passivationconverts reverse to normal buckling

Kitayama et al Surf. Sci. V482-485, 1481, (2001)


Materials Studied• York Surface Physics group has studied

– Heavy Rare Earth elements (Lanthanides) on Si and Ge• Divalent = Sm, Eu, Yb• Trivalent = La, Ce, Pr, Nd, Pm, Gd, Tb, Dy, Ho, Er, Tm• Done all except La, Ce, Pr, Pm• See “Trends and strain in the structures of 2D rare-earth silicides

studied using medium-energy ion scattering”, PRB 72, 165407 (2005)• Also Fe, Pb, Pd on Si etc

• With a variety of techniques– primarily STM and LEED in York– also MEIS at Daresbury– and now adding ab initio electronic structure calculations as a

complementary tool using CASTEP– a holistic approach


CASTEP for Surface Physics• Simple case: Si(100)-2x1• Small area surface so few

atoms required• Si atoms cheap to include

in calculation• Many experimental and

theoretical results to compare against

Unreconstructed Si(100) supercell


Si(100)-2x1 CASTEP convergence

=> 9KP, 360eV sufficient => 10Å Vacuum gap

Typical calculation time: 15 hours on 9 nodes of a Beowulf


Si(100)-2x1 Relaxed Structure

Asymmetric dimerisation


Si(100)-2x1 Calculated Properties

Electron density contours →

← Angular Momentum channel resolved density of states

← STM profile +2.0V


Si(100)-c4x2 Structure

40 atoms, 8 k-points, 260eV cutoff energy 16 days on 8 nodes of the White Rose Grid


Si(111)-1x1-Ho

Experiment Theory

Flatter top bilayer and more relaxed in theoretical result

CMD GroupDepartment of Physics

Example 1 –

Geometry Optimisation of Samarium on Silicon Surface


STM of Si(111)-3x2-Sm

STM shows 3x2 reconstruction and a 1D chain of Sm atoms …

Sm is divalent on Si …

Ab-Initio Calculation done by Palmino et al using VASP and 150eV cutoff to get STM structure


The Honeycomb Chain Channel (HCC)• 1/6 ML alkali earth metals (Ca, Mg, Ba) and divalent rare earth metals (Sm, Eu, Yb) form a 3x2 reconstruction and 1D chain of “metal” atoms 3x2 unit cell side view

3x2 unit cell from above

See PRL 81, 2296 (1998)and PRL 87, 56104 (2001)


The Honeycomb Chain Channel (HCC)• 1/3 ML alkali metals (Li, Na, K, Rb) form a 3x1 reconstruction

• Common silicon structure responsible



• 1/6 ML alkali earth metals (Ca, Mg, Ba) and divalent rare earth metals (Yb, Eu, Sm) form a 3x2 reconstruction


Problem with LEED of Si(111)-Sm

LEED shows 3x1 pattern not 3x2 like STM!Known problem for many metals on Si(111) eg Ba

Expected 3x2 Observed 3x1


Structural Models and Registry Shifts

●Si(111) 3x2-Ba structure suggested by Wigren et al, PRB 48, 11014 (1993).

● Qualitative calculation of effect of registry shift on LEED pattern by J. Schafer et al, PRB 67, 85411 (2003)

•interference of amplitudes from two registry shifted cells (1/2 unit cell shift) proposed to explain cancellation of x2 spots


Structural Model with a Registry Shift


Quantitative LEED Experiment

• Deposit 1ML Sm on clean Si(111)• Anneal 700 °C 15 mins• Clean, thermally resilient structure

3 x 1 LEED pattern 40eV 3 x 1 LEED pattern 80eV


LEED I(V) Curves• I(V) curves gathered from many

runs on freshly made surfaces– Compare curves using the Pendry

R-Factor (sensitive to peak positions)

– Reproducible: variation in Rp ~ 0.1

– Averaged to reduce noise

• Uppermost layer produces fractional and integer spots

• All surface layers produce integer spots – LEED beam typically penetrates ~ 5 layers

• Fractional spots are sensitive to Sm and Si in honeycomb chain


• Earlier ab initio calc structure to fit STM experiment– VASP DFT geometry

optimisation by Palmino et alPRB 67, 195413 (2003)

• R-Factor comparison– Good is <0.4– Acceptable is <0.5– Overall R-Factor is 0.78 – Poor R-Factors when

compared to LEED I(V)– Suggests HCC model and/or

Palmino is wrong• R-Factor calculated from

structure using CAVLEED• Where next?

VASP Ab Initio Structure vs Experiment


CASTEP: Basis Set Optimisation

• Convergence w.r.t. basis set size and Brillouin zone sampling• Check forces and total energy


Competing HCC Structures

• Two possible sites for Sm in HCC structure– H3 shown in (a) and (b)– T4 shown in (c) and (d)

• CASTEP geometry optimisation shows T4 is more stable than H3 by ~1eV

Silicon atoms are grey, samarium is black and the hydrogen atoms are white.


CASTEP Geometry Optimisation

• T4 structure relaxation performed on 32 processors of HPCx• Main relaxation found in interlayer spacings

Top view of T4

Side view of T4


CASTEP Structure vs Experiment• Pendry R-Factors from the CASTEP suggested structure

• Overall R-Factor is 0.48 (c.f. VASP Rp=0.78)

• Good is <0.4• Acceptable is <0.5 • Fractional spots better

than integer spots

• Suggests T4-HCC structure is valid but some further tweaking is still needed


Independent check – structure fitting

CASTEP result (white cross) found to be in Rp minimum (missing thermal expansion)

256 structure runs of CAVLEED on White Rose Grid parallel computer

• 1.5 hours per run• Step size 0.05Å

• Calculate Rp as function of interlayer spacing and so map R-factor surface


Sm on Si Summary• VASP ab initio result not refined enough to compare to quantitative surface science data: R-factor of 0.78

•CASTEP ab initio result agrees well with quantitative surface science data

• Careful optimisation of basis set parameters• New structure gives much better fit to experiment

•Independently verified by R-factor surface mapping• Need effects of thermal vibration/expansion• Further improved if include 60% T4 + 40% H3 mixture• Best overall R-factor now 0.42


Example 2 –

Holmium Nanowires on a Germanium surface


Ge(111)1x1-Ho Structure

• Structure common to trivalent RE of flat layer buried below reverse buckled Ge bilayer

• Details sensitive to Ho spin-state

Empty states STM 1.3V2nA experimentImage is ~5x5nm

Empty states STM 1.3VCASTEP with Tersoff-

Hamann schemeImage ~5x5nm


Ho Nanowires on Ge(111) : STM Observations

• Nanowires: self-assembled lines of atoms – HOT topic

• Never before seen on a (111) surface (due to 3-fold symmetry)

• Depending on coverage they can be parallel => 5x1 environment

•MEIS shows Ho is notsubsurface but no more info• What is the structure? Too dilute for LEED!0.25ML Ho on clean Ge(111)-c2x8

grown at 250°C not 500°C


Nanowires on Ge(111) – Structural Model

• Structure suggested by Chris Bonet based on STM dimensions –nanowire is 2 atoms wide!

• Is it stable?

• Does it agree with STM?


Nanowires on Ge(111) – CASTEP

CASTEP can get accurate surface structures:• Need high quality convergence• High cutoff energy, vacuum gap, 4 k-points• Use Ultra-Soft Pseudopotentials and GGA-PBE• Took 5 hours on HPCx (128 processors)

Side view electronic structure

Top view

CASTEP stable structure•Delicate balance of forces/bonding


Comparison with STM experiment

• Simulate STM from CASTEP calculated electronic structure using Tersoff-Hamann scheme

Empty states +1.5VTheory

Empty statesExperiment +1.5V, 2nA

Filled statesExperiment -2.0V, 2nA

Filled states -2.0VTheory

Image areas 4.2 x 3.3 nm

Atomic resolution

and agreement


Comparison with STS experiment

• Conducting properties of the nanowires• Theoretical DOS shows states at Fermi Level• Scanning Tunneling Spectroscopy shows nanowire has conducting states at Fermi Level (compare to Germanium with band gap)

Ab initio LDOS Experimental STS


Ho Nanowires on Ge Summary• Novel observation of nanowires on Ge(111)

• Model structure proposed on basis of STM measurements but could not be validated

• Structure confirmed by CASTEP geometry optimisation– and both filled and empty states STM simulation

• Conducting character of nanowire seen experimentally in STS measurement– and also in large LDOS at EF with CASTEP


Acknowledgements• York Surface Physics Group

– especially Steve Tear, Chris Bonet, Ed Perkins (now Nottingham)– http://www-users.york.ac.uk/~phys24/

• EPSRC funding

• “STM and ab initio study of holmium nanowires on a Ge(111) Surface”, C. Eames, C. Bonet, E.W. Perkins, M.I.J. Probert, S.P. Tear , PRB 74, 193318 (2006)

• “Quantitative LEED I-V and ab initio study of the Si(111)-3x2-Sm surface structure and the missing half order spots in the 3x1 diffraction pattern”, C. Eames, M.I.J. Probert, S.P. Tear, PRB (accepted ‘07)

rare-earth silicides – a holistic study• a potentially very useful metal-silicon contact – low...

Documents