an overview of meis science – past and future dr tim noakes stfc daresbury laboratory, daresbury...
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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