CMP Seminar September 22, 2003 1 Growth and Structure of Thin Fe Films on the Ti-Al Interface C. V....

CMP Seminar September 22, 2CMP Seminar September 22, 2003003

11

Growth and Structure of Thin Fe Films on the Ti-Al Interface

Growth and Structure of Thin Fe Films on the Ti-Al Interface

C. V. RamanaC. V. Ramana

Montana State UniversityMontana State Universityhttp://www.physics.montana.edu/Ionbeams/ionbeams.htmlhttp://www.physics.montana.edu/Ionbeams/ionbeams.html


22

Stabilizing Metal-Metal Interfaces

• Metal thin film devices have layers ~ nm thicknessMetal thin film devices have layers ~ nm thickness• Diffusion occurs frequently: interface ~ nm thickDiffusion occurs frequently: interface ~ nm thick• Need to stabilize the interface, provide a template for Need to stabilize the interface, provide a template for

epitaxial growth, minimize interdiffusion epitaxial growth, minimize interdiffusion • Applications: magnetoresistive devices, spin electronicsApplications: magnetoresistive devices, spin electronics

• Surface energy (broken bonds)Surface energy (broken bonds)

• Chemical formation energyChemical formation energy

• Strain energyStrain energy A

Binterface

Smith et al, Appl. Surf. Sci. 219 (2003) 28


33

Experiment

• Substrate: Substrate: Al(100)Al(100)• Metal overlayers:Metal overlayers:

• FeFe • Ti Ti • Surface energy > Al surface energySurface energy > Al surface energy• Form Al compounds with Form Al compounds with HHformform < 0 < 0

• Use resistively heated wires ( ~ML/min)Use resistively heated wires ( ~ML/min)• Deposition at room temperatureDeposition at room temperature


44

Techniques and GoalsTechniques and Goals

• Rutherford backscattering and channeling (RBS/c)Rutherford backscattering and channeling (RBS/c)• Low-energy electron diffraction (LEED)Low-energy electron diffraction (LEED)• Low-energy ion scattering (LEIS)Low-energy ion scattering (LEIS)

Fe

Ti

Al


55

Overview of Rutherford Backscattering and Channeling

• MeV HeMeV He++ ions ions• Yield = Q Yield = Q (Nt) (Nt)• Fe (Ti) peak for coverageFe (Ti) peak for coverage• Al peak for structureAl peak for structure250 300 350 400 450

0

100

200

500 600 700 800 900

X 1/4

1 MeV He+ Ions Normal Incidence

Scattering Angle: 105o

Al(100) + 2 ML Ti + 3 ML Fe

Ti

Fe

Ion

Yie

ld (

Cou

nts)

Channel Number

Backscattered Ion Energy (KeV)


66

Fe Growth on Al(100) – Does Ti Interlayer Makes Any Difference?Fe Growth on Al(100) – Does Ti Interlayer Makes Any Difference?

-2 0 2 4 6 8 10 12 140

3

6

9

12

15

18

21

24

27

-2 0 2 4 6 8 10 12

0

2

4

6

8

10

12

14

16

18

20

22 Fe Growth on Al(100) + Ti

1 MeV He+, Normal Incidence, =105o

No Ti 1 ML Ti 2 ML Ti Simulation

Vis

ible

Al A

tom

s (1

015 a

tom

s/cm

2 )

Fe Coverage (1015 atoms/cm2)

Fe Coverage (Monolayers)

Vis

ible

Al A

tom

s (M

on

ola

yers

)

Ramana et al, Phys. Rev. Lett. 90 (2003) 66101


77

Fe-Fe ShadowingFe-Fe Shadowing

0 2 4 6 8 10 12

0

2

4

6

8

10

12

0 2 4 6 8 10

0

2

4

6

8

10Fe on 2 ML Ti + Al(100)

Fe

Co

vera

ge

(1

015 a

tom

s/cm

2 )

Fe Coverage (1015 atoms/cm2)

Random Aligned

Fe

Co

vera

ge

(M

on

ola

yers

)

Fe Coverage (Monolayers)


88

Angular Ion Yield Curves Angular Ion Yield Curves

0o

FCC Al

45o

FCC Fe

[010]

[100]

If it is fcc Fe


99

Angular Ion Yield Curves Cont…Angular Ion Yield Curves Cont…

FCC Al

45o

[010]

[100]

54.7o

BCC Fe

If it is bcc Fe


1010

Structure of Fe: Off-Normal Ion Channeling MeasurementsStructure of Fe: Off-Normal Ion Channeling Measurements

42 43 44 45 46 47 48

2.0x103

4.0x103

6.0x103

8.0x103

SimulationExperiment

Al Fe Polynomial Fit

Angle (Degrees)

Bac

ksca

ttere

d Io

n Yi

eld

(Cou

nts)

5

10

15

20

25

Al Fe Polynomial Fit

Angular Ion Yield H

itting Probability (M

L)


1111

Structure of Fe contd..Structure of Fe contd..

53 54 55 56 57

0.25

0.50

0.75

1.00

Al, Experiment Fe, Experiment Polynomial Fit Fe, Simulation Polynomial Fit

Angular Ion Yield

No

rma

lize

d Y

ield

Angle (Degrees)

θmin = 54.55o


1212

Lattice Parameters & CalculationsLattice Parameters & Calculations

c)TiAl: Tetragonala = 4.0155 Åc = 4.0625 Å

a) Fe: bcc2.8665 Å

b) Al: fcc (4.0496)2.8635 Å

Lattice Parameters

2.8560 Å


1313

Unrelaxed bcc FeUnrelaxed bcc Fe

54.74o

Volume is conservedVFe = CFe A = CFe X2

CFe = (2.8665)3/X2


1414

Fe (bcc) relaxed to Al substrateFe (bcc) relaxed to Al substrate

θ

54.74o

θ

54.65o

θ = tan-1(X / CFe )

Volume is conservedVFe = CFe A = CFe X2

CFe = (2.8665)3/X2


1515

Fe (bcc) relaxing to Ti-Al interfaceFe (bcc) relaxing to Ti-Al interface

54.74o

θ

θ

54.44o


1616

Calculations cont.…Calculations cont.…

XX CCFeFe θ

FeFe 2.86652.8665 -- 54.7454.74

AlAl 2.86352.8635 2.87252.8725 54.6554.65

TiAlTiAl 2.85602.8560 2.88762.8876 54.4454.44


1717

Why does it work?Why does it work?

• Bimetallic Formation Energies (kJ/mole-atom)Bimetallic Formation Energies (kJ/mole-atom)

Fe/Al: 25Fe/Al: 25 Ti/Al: 38Ti/Al: 38 Fe/Ti: 20Fe/Ti: 20

• Ti-Al bond stronger than Fe-Al or Fe-Ti so Ti prefers to stay Ti-Al bond stronger than Fe-Al or Fe-Ti so Ti prefers to stay near the Al interfacenear the Al interface

• Lattice matching: bcc Fe (100) unit cell has a = 2.86 Lattice matching: bcc Fe (100) unit cell has a = 2.86 ÅÅ

fcc Al (100) has Al-Al distance 2.86 fcc Al (100) has Al-Al distance 2.86 ÅÅ

hcp Ti (0001) has Ti-Ti distance 2.95 hcp Ti (0001) has Ti-Ti distance 2.95 ÅÅ

• Fe (small atom) more easily accommodated by Al(100) than Fe (small atom) more easily accommodated by Al(100) than Ti (larger atom). Formation of Ti-Al interface “stiffens” theTi (larger atom). Formation of Ti-Al interface “stiffens” the

surface, restrains movement of Fe into the substrate.surface, restrains movement of Fe into the substrate.


1818

Summary and ConclusionSummary and Conclusion

• Demonstrated the use of a metallic interlayer to Demonstrated the use of a metallic interlayer to stabilize a metal-metal interface and promote epitaxial stabilize a metal-metal interface and promote epitaxial growthgrowth

• Fe growth occurs in slightly distorted bcc structure on Fe growth occurs in slightly distorted bcc structure on the Al(100) surface with Ti interlayer at the interface. the Al(100) surface with Ti interlayer at the interface.


1919

AcknowledgementsAcknowledgements

• Prof. Richard J. SmithProf. Richard J. Smith

• Prof. Bum-Sik Choi (Korea) Prof. Bum-Sik Choi (Korea)

• Ion BeamersIon Beamers

• NSF (DMR-0077534)NSF (DMR-0077534)

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CMP Seminar September 22, 2003 1 Growth and Structure of Thin Fe Films on the Ti-Al Interface C. V....

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