An Overview of MEIS Science – Past and

Future

Dr Tim Noakes

STFC Daresbury Laboratory, Daresbury Science and Innovation Campus, Keckwick Lane,

Daresbury, Warrington, Cheshire, WA4 4AD, UK

Overview

•The medium-energy ion scattering (MEIS) technique•Previous experiments using MEIS

• Surface structure• High resolution depth profiling• Thin film characterisation• Characterisation of nanostructures

•Future research areas•Summary

MEIS Technique

Medium energy light ions (50-250 keV H+ or He+) used to probe the surface and near surface of materials

– Energy losses during scattering• Elastic losses• Inelastic losses

– Angular variation in scattered ion intensity• Shadowing and blocking

Elastic scattering

• Simple ‘billiard ball’ collisions between ions and atoms• Conservation of energy and momentum relates ion energy loss to mass of target atom

Inelastic Energy Loss

• Inelastic energy losses arise from electronic excitations as ion passes through sample• Stopping powers well known (e.g. ‘SRIM 2011’)• Resolution degrades with depth as process is stochastic (energy loss straggling)

Angular Intensity Variation•Shadowing effects used to select number of layers illuminated•Blocking effects reveal relative positions of the atoms(i.e. the structure!)•Shifts in blocking dips related to layer spacings (surface relaxations, strain)•Amplitudes of dips indicate additional illumination (thermal vibrations, disorder)

LEIS, MEIS and RBSLEIS MEIS RBS

(1-5keV) (50-400keV) (0.5-4MeV)

Shadow cone > vibrational amplitude

Intrinsic surface specificity(1-3 atomic layers)

Shadow cone vibrational amplitude

Tunable surface specificity(1-100 atomic layers)

Shadow cone « vibrational amplitude

Low surface specificity(20-thousands atomic layers!

R.M.S. Vibrationa

l Amplitude

Medium Energy Ion Scattering

2D image

Angle (deg)

Ene

rgy

(keV

)

100

80

6085 110

Cou

nts

>

0

50

100

150

200

60 70 80 90 100

depth

depth

depthSi

Pd

Au

Scattered Ion Energy (keV)

Inte

nsity

(co

unts

)

700

900

1100

1300

1500

1700

85 95 105 115

Scattering Angle (deg)

Inte

nsity

(co

unts

)

Elastic scattering gives compositional information Inelastic scattering provides depth information (and morphology!)

Angular variation in the scattering intensity gives structure

Capabilities of MEIS• Depth selectivity, excellent structural sensitivity

• Surface structure (~2 pm resolution)

• Compositional sensitivity over the near surface• High resolution depth profiling (2 - 5 Å resolution)

• Ability to simultaneously determine composition and structure

• Full characterisation of thin film materials

• Path length sensitivity• Composition, structure and morphology of

nanoparticles

Surface StructureMetals and metal alloys

– Adsorbate induced reconstruction– Model catalysts– Complex metal alloys (e.g. quasicrystals)

Semiconductor materials– ‘Ideal’ Schottky Barriers – III-V growth surfaces

Oxides– Catalyst supports (e.g. TiO2)

TiO2(100)-(1x1) Surface Structure

Important support materials for catalystsNo agreement between previous SXRD and LEED studiesDifficult system to analyse since:

– Atoms have low Z– At least 12 parameters to

optimise

TiO2 data

Data taken in two azimuths probing different aspects of the structure and fitted using ‘VEGAS’ simulation software

TiO2(110) Surface Structure

MEIS structure similar to LEED data with surface bridging O atom relaxed outward

Parkinson et al, PRB 73 (2006) 245409

High Resolution Depth Profiling

Semiconductor device fabrication• Ion implants for semiconductor devices• High- gate dielectric materials

Structural materials• Oxide layers for Corrosion protection of light metal alloys

• Construction materials• Automotive, aerospace, rail and marine transport

applications

Corrosion Protection of Light Alloys

Typically dilute alloys of Aluminium used for improved corrosion resistance• Al-0.3at%Zn• Al-0.7at%W• Al-0.2at%Mn• Al-0.4at%Cu

What happens to minor alloying element during oxide film growth?

X-TEM image of anodized Al-0.4at%Cu sample

Enrichment in Al-0.4at%Cu Alloy

•Anodic oxidation leads to Cu enriched layer below the grown film•Film is stripped using chromic/phosphoric acid before analysis•Data reveals constant thickness of enriched layer with anodization time•Increase in Cu content attributed to increased cluster generation

Garcia-Vergara et al, App. Surf. Sci. 205 (2003) 121

Thin Film Characterisation

Systems which benefit from the simultaneous elucidation of composition and structure• Metal-on-metal growth (giant magneto-resistance films)• Quantum well systems (III-V materials, metals)• Spintronic materials (metal/semiconductor hybrids)

Cu on Co(0001) GrowthCu/Co multilayers commonly used for GMR layersMany studies of Co growth on Cu(111) but the reverse system less common because of the difficulty in preparing clean well ordered Co(0001) substratesSurfactant mediated epitaxy (SME) a possible way to improve growth• 1ML of Pb pre-deposited before Cu layer growth• Data taken before and after annealing to 300ºC

Cu Thin Film Structure•Cu data fitted using twinned fcc plus flat signal for disordered fraction• Less disorder (9±6%) for surfactant grown sample indicating improved crystal quality• Strain also required to fit data

3

4

5

6

DataSimulation28±4% disorder

No surfactant 300OC Anneal

Vis

ible

Cu

La

yers

(M

L)

3

4

5

6

80 90 100 110 120 130

19±4% disorder

1ML Pb Surfactant, 300OC Anneal

Scattering Angle (deg)

Pb Surfactant EffectFor 1ML Pb surfactant grown sample• 1-2 additional epitaxial layers• Reduced strain throughout the film

107

No Strain

111

Max. Strain

115No surfactant, post annealed

Ang

ular

Pos

ition

of f

cc<

110>

Blo

ckin

g

107

No Strain

111

Max. Strain

115

0 3 6 9

1ML Pb surfactant, post anealed

Depth Below Surface (ML)

2 6 10

Pb retained in grown layer

Surface Peak

Depth Below Surface (ML)

Inte

nsity

(co

unts

)

80 90 100 110 120 130

Scattering Angle (deg)

Inte

nsity

(co

unts

)

• Majority of Pb ‘floats’ on surface during growth• Some Pb retained in grown layer at a level of 3% ML• Sub-surface Pb mostly on lattice sites• Could strain relief contribute to the improved epitaxial growth?

Noakes et al, PRB 68 (2003) 155425

Nanoparticle Characterisation

Topographical information• Single element clusters

Compositional information• Bimetallic alloys (model catalysts)• III-V quantum dots

Structural Information• All the above!

‘Magic’ Height IslandsBi on i-Al63Cu24Fe13

4 layer islands form on top of pseudomorphic monolayer

Si(111) substrate

Bi monolayer

Bi clusters(bi-layers)

Ag on i-Al70Pd21Mn9

Ag on AlNiCo – QSE’s Moras et al, PRB 74 (2006) 121405(R)

Bi on Si(111) – bilayer formationNagao et al, PRL 93 (2004) 105501

Fournee et al, PRL 95 (2005) 155504

Structure of Bi Islands•Modelled using 25Å2 islands in 100Å2 box•Starting from bulk rhombohedral positions atoms allowed to move in Y and Z directions

a

b

c

0.8 Å

0.8 Å

a

b

c

a=cba=b=c

Rhombohedral ‘Black-Phosphorous’

3.5

3.6

3.7

3.8

3.9

4.0

90 100 110 120

Scattering Angle (deg)

Illu

min

atio

n (

laye

rs)

Y

Large downward movement of top atom leads to bi-layer formation

Energy Fits to Bi Data1ML data allows calibration, resolution, etc to be fitted1.5ML (nominal)•20% coverage of 4ML islands3 ML (nominal)•56% coverage of 4ML islands

0200400600800

10003 ML Bi

0200400600800

10001.5 ML Bi

Sca

ttere

d Io

n In

tens

ity (

coun

ts)

0200400600800

1000

95 96 97 98 99

1 ML Bi

Scattered Ion Energy (keV)

Fits improved (15% reduction in R-factor) by allowing 2 and 8 ML islands as well!

Noakes et al, PRB 82 (2010) 195418

Self-assembled InAs Quantum Dots on GaAs

Dot size and shape determined from AFM

Large 3D islandsQuantum Dots

Wetting Layer

InAs deposition on GaAs leads to:– InGaAs wetting layer– Regular well-defined quantum dots– Larger 3D islands

Quantum Dots Results

First independent measurement of the composition profile of materials of this type!

Wetting layer and large 3D islands included as well as quantum dots

In intensity fitted using linear profile from 20% to 100% at the top of the QD

P.Q. Quinn et al, App. Phys. Lett. 87 (2005) 153110

Future Research Using MEIS

Semiconductor device fabrication• Dielectric layers• Ion implantation• Metalisation

Catalysts• Oxide support materials• Bimetallic nanoparticles• Adsorbate induced segregation studies

Structural materials (light metal alloys)• Rail, automotive, marine and aerospace

applications

Future Research Using MEISBiomedical applications

• Joint replacements, dental implants

Photovoltaic materials• Multi-junction solar cells• II-VI quantum dot based solar cells• III-V quantum well LED’s

Magnetic materials• Magnetic tunnel junctions• Novel memory materials (MRAM, race track, etc)• Spintronic materials (metal-semiconductor

hybrids)

Future Research Using MEISThe ‘Hydrogen economy’

• Photo-catalysts• Hydrogen storage materials• Fuel cells

Photocathode materials• CaAs, tellurides, antimonides

Nanometrology• SIMS Calibration• Elipsometry and other optical techniques

Others???

SummaryMEIS is a fantastic technique for investigating the surface and near-surface region of materials

• Simultaneous measurement of composition and structure• High sensitivity to structural parameters (~2 pm)• Virtually monolayer depth resolution (2-5 Å)• Sensitivity to nanoparticle structure, morphology and

composition

MEIS can be used for a wide range of applications many of which fall into RCUK priority areas

• Sustainable energy• Environmental change• Life long health and well being• Nanoscience to nanoengineering

AcknowledgementsDaresbury - P. BaileyWarwick – G.S. Parkinson, M.A. Munoz-Marquez, P.D. Quinn, M.J. Gladys, R.E. Tanner and D.P. WoodruffManchester – S. Garcia-Vergara, P. Skeldon, G.E. Thompson, H. Habazaki and K. ShimizuLeeds – D.T Dekadjevi and M.A. HowsonLiverpool/Warwick – C.F. McConville, M. Draxler, M. Walker, M.G. Brown, A. Hentz, D.P. Woodruff, J. Smerdon, L. Leung and R. McGrathWarwick – P.D. Quinn, N.R. Wilson, S.A. Hatfield, C.F. McConville, G.R. Bell, S. Al-Harthi and F. Gard