Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 G. S. Oehrlein*, T. Schwarz-Selinger, K....

Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008

G. S. Oehrlein*, T. Schwarz-Selinger, K. Schmid,

M. Schlüter and W. Jacob

Interaction of Deuterium Atoms with Hard a-C:H Films:

Isotope Exchange, Soft-layer Formation and Steady-state Erosion

* work performed 2007 during sabbatical leave from: Department of Materials Science & Engineering and

Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland

20742

9th International Workshop on Hydrogen Isotopes in Fusion Reactor Materials, Salamanca, Spain, 3 June 2008


outline

motivation:motivation: fundamental understanding of H/C interaction(tritium removal from carbon surfaces with H0, D0)

strategy:strategy: exposure of a model system (a-C:H, a-C:D) to thermal H0, D0 beams at RT

surface modification and erosion: ellipsometry

isotope exchange: ion beam analysis

absolute values (cross sections, penetration depth…)


experimental

1. film preparation:

Usb = -300 V H/(H+C) = 0.33 nC = 9 ·10 22 cm-3, (1.8 g/cm3)

Si 001

rf plasma (CH4, 2Pa)

exposure of plasma deposited, hard a-C:H and a-C:D to thermal atomic H0 or D0 beams under UHV


experimental


W. Jacob et al., Review of Scientific Instruments 74, 5123-5136 (2003).

ellipsometry (633 nm)

H 2 or

D 2

T=210

0K

H0 , D

0

2. exposure:

Tsubstrate 320 K jH,D = 1.3 · 1015 cm-2s-1


experimental

3. ex-situ analysis: nuclear reaction analysis D(3He,p)4He

V.Kh. Alimov et al., Nucl. Instr. Meth. B234, 169 (2005).


3He

p@ 690 keV, 5 µC

p, 4He @ 135°

0.15 srmylar


erosion of a-C:H with H0

Vietzke and Philipps, Fusion Technol. 15, 108 (1989).Schlüter et al., J. Nuclear Mater. 136, 33 (2008).Horn et al., J. Chem. Phys. Lett. 231, 193 (1994).

present study


20 30 40 50

1.6

1.8

2.0

2.2

2.4 U

SB=-200 V

USB

=- 30 V

USB

=floating

USB

=floating, a-C:D

refr

activ

e in

dex

n p

H/(H+C) (at %)

model system a-C:H

Schwarz-Selinger et al., J. Appl. Phys. 86 (7), 3988 (1999).

presentstudy

’hard a-C:H’

growth from CH4, C2H2, C2H4 , C2H6, C3H8, C4H10 …

film properties like- hydrogen content- density- refractive index

are closely correlated


model system a-C:H

Schwarz-Selinger et al., J. Appl. Phys. 86(7), 3988 (1999).

film properties like- hydrogen content- density- refractive index

are closely correlated1.4 1.6 1.8 2.0 2.2 2.4 2.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

(g

/cm

3 )

refractive index n

density drop /thickness increase

by factor of 2

growth from CH4, C2H2, C2H4 , C2H6, C3H8, C4H10 …


ellipsometry

2 layer model

erosion

growth

a-C:H

SiSiO2

0 5 10 15 20 250

45

90

135

180

225

270

315

360

135 nm

n

(°)

(°)

rp : rs = tanei

SiO2

Si

d = 1 nm


ellipsometry

2 layer model

erosion

growth

a-C:H

SiSiO2

32.6 32.8 33.0 33.217

18

19

20

1

hard a-C:Hn=2.15, k=0.136

75.2 nm

(°)

(°

)


ellipsometry

3 layer model

erosion

growth

hard

soft

a-C:Ha-C:H

SiSiO2

32.6 32.8 33.0 33.217

18

19

20

1

2

hard a-C:Hn=2.15, k=0.136

75.2 nm+ soft a-C:Hn=1.6, k=0overlayer

(°)

(°

)


ellipsometry

3 layer model

erosion

growth

hard

soft

a-C:Ha-C:H

SiSiO2

32.6 32.8 33.0 33.217

18

19

20

3

convertinghard into soft

2

+ soft a-C:Hn=1.6, k=0overlayer

(°)

(°

)


ellipsometry

3 layer model

erosion

growth

hard

soft

a-C:Ha-C:H

SiSiO2

32.6 32.8 33.0 33.217

18

19

20

convertinghard into soft

3

2

1

hard a-C:Hn=2.15, k=0.136

75.2 nm+ soft a-C:Hn=1.6, k=0overlayer

(°)

(°

)


ellipsometry

3 layer model

erosion

growth

hard

soft

a-C:Ha-C:H

SiSiO2

32.6 32.8 33.0 33.217

18

19

20

conversionhard into soft

(°)

(°

)

D0 exposure


ellipsometry

3 layer model

erosion

growth

hard

soft

a-C:Ha-C:H

SiSiO2

32.6 32.8 33.0 33.217

18

19

20

lossof hard a-C:H conversion

hard into soft

(°)

(°

)

D0 exposure


ellipsometry

1 10 100 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.60.1 1 10 100

mo

dified

layer thickn

ess (nm

)

time (min)

fluence (1018 /cm2)

32.6 32.8 33.0 33.217

18

19

20


hard into soft

(°)

(°

)

D0 exposure


ellipsometry

32.6 32.8 33.0 33.217

18

19

20


hard into soft

(°)

(°

)

D0 exposure

250 500 7500.0

0.5

1.0

1.5

2.0

2.5

10 20 30 40 50

0.0

0.5

1.0

1.5

2.0

2.5 modified layer thickness (nm

)

time (min)

fluence (1018 /cm2)

er

oded

film

thi

ckne

ss (

nm)


NRA results: D0 (H0) areal density vs. exposure time

total uptake of D

1 10 100 10000

2

4

6

8

exposure time (min)

chna

ge o

f D

are

al d

ensi

ty (

10 1

5 at.

/cm

2 )

D0 on a-C:H

0.1 1 10 100

H0/D0 fluence (1018 at./cm2/s1)

initial thickness: 20 nm

expected D content of a14 Å thick soft layer:

6·1015 D0/cm2


1 10 100 10000

2

4

6

8

exposure time (min)

chna

ge o

f D

are

al d

ensi

ty (

10 1

5 at.

/cm

2 )

D0 on a-C:H

H0 on a-C:D0.1 1 10 100



loss of D (isotope exchange)

total uptake of D


initial D content of a14 Å thick hard layer:

5·1015 D0/cm2


1 10 100 10000

2

4

6

8

exposure time (min)

chna

ge o

f D

are

al d

ensi

ty (

10 1

5 at.

/cm

2 )

D0 on a-C:H

H0 on a-C:D

D0 on a-C:D

0.1 1 10 100



deuteration of extra sites

loss of D (isotope exchange)

total uptake of D



ellipsometry trajectories for D and H erosion

15 20 25 30 35

20

40

60

(a)

D

D

H

IONp20070314 d2 h2 comparison erosion

ELLIP9.OPJ, 2

Psi (deg.)

Del

ta (

deg.

)

H

switching between H and D: steady state erosion

no difference in optical response

erosion @ 650 K

ellipsometry does not see isotope exchange


change in D area density and modified layer thickness

1 10 100 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.60.1 1 10 100

mo

dified

layer thickn

ess (nm

)

time (min)

1 10 100 1000

0

2

4

6

8

chan

ge o

f D

are

al d

ensi

ty (

10 1

5 at.

/cm

2 ) fluence (1018 at. /cm2)

uptake of D without optical response isotope exchange


for a flux density j = 1.3·1015 D0/cm2 s

an initial hydrogen density nH = 0.33 · 12.1·1022 /cm3

an uptake of nD = 3·1015 D/cm2 in the first 5 minutes:

σ = 2·10-18 cm2 = 0.02 Å2

)()0()( tnntn HHD

jtntn HD exp1)0()(

jtndt

tdnH

H )()(

analysis

for simple isotope exchange we have:

Küppers et al.: abstraction of bonded H: 0.05 Å2 cm2

1 10 100 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.60.1 1 10 100

mo

dified

layer thickn

ess (nm

)

time (min)

1 10 100 1000

0

2

4

6

8

chan

ge o

f D

are

al d

ensi

ty (

10 1

5 at.

/cm

2 )

fluence (1018 at. /cm2)


interaction of H with a-C:H

known from literature * :

abstraction

addition

thermal activated erosion

* = 0.05 Å2

* = 1.3 Å2 (4.5 Å2 )

*J. Küppers, Surf. Science Reports 22, 249 (1995)


• 1.4 nm thickness, deuteration of extra sites– A) based on total number of C atoms in layer

– B) formation of C-D2 from C-D

i.e., comparable to D-exchange• Both steps must be going on.

analysis

σextra sites = 6·10-19 cm2 = 0.006 Å2

σextra sites = 1.3·10-18 cm2 = 0.013 Å2


3 stages of D interaction with a-C:H (at 50 °C)

1. isotope exchange• complete after ~ 30 min ( j = 2·1018 D0/cm2 )• comparison of the cross-section for this process with literature values

for H interaction with C:H shows that this corresponds to the cross-section of hydrogen abstraction from the C:H surface

2. creation of new C-D bonds• soft a-C:D layer formation • occurs up to ~ 200 min ( j = 2·1019 D0/cm2 )

3. erosion of a-C:H, • a soft a-C:D layer “remains” on the substrate, with roughly constant

thickness (1.4 nm)


interaction of H with a-C:H

known from literature * :

thermal activated erosion

k = f(T)

erosion ! H fluence !

*J. Küppers, Surf. Science Reports 22, 249 (1995)


sp2

Hydration and erosion circle:Horn et al., Chem. Phys. Lett. 231, 193

(1994)Zecho et alJ. Phys. Chem. B 105

(2001).

Chemical Erosion: microscopic model

1) chemisorption of H on sp2 site

H

= 1.3 Å2

H

spx

sp3

H

= 1.3 Å2

HCH32) chemisorption of H on spx site (hydration)

3) abstraction of H to form H2

H

H2

= 0.05 Å2

CH3Eact=1.7 eV

5) relaxation back to sp2 above 750 K

6) direct thermal decomposition to sp2

above 900 K with Eact=2.4 eV

4 a) thermal release of CH3 radicals from activated sites above 400 K

Eact=1.7 eV

CH3

4 b) chemisorption of H on spx site

= 1.3 Å2

H


Chemical erosion: structure dependence

E. Vietzke et al., Surf. Coat. Technol. 47 (1991) 156-161

Total Y of Ho on films of:

a-C:H (plasma-deposited), pre-irradiated graphite, graphite and diamond

disorder

order

1000 x reactivity of the surface depends critically on the surface structure


mechanistic picture

CHCH

C

HD

CHCH

C

DCDy

1. Isotope exchange 2. Deuteration ofextra sites – formationof highly deuteratedLayer;Thickness increase

3. Steady-state erosionwith modified layer ofconstant thicknesson top of substrate (x>>y)

CH CDCCD CD2

CD3

CDCDxHy Layer

CH

CHCH

C

DD

CHCH

C

HD

CHCH

C CHCH

C

DCDy

1. Isotope exchange 2. Deuteration ofextra sites – formationof highly deuteratedLayer;Thickness increase

3. Steady-state erosionwith modified layer ofconstant thicknesson top of substrate (x>>y)

CH CDCCD CD2

CD3

CDCDxHy Layer

CH

CHCH

C CHCH

C

DD


ellipsometry

32.6 32.8 33.0 33.217

18

19

20


hard into soft

(°)

(°

)

D0 exposure

1 10 1000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.60.1 1 10

mo

dified

layer thickn

ess (nm

)

time (min)

fluence (1018 /cm2)


ellipsometry

32.6 32.8 33.0 33.217

18

19

20


hard into soft

(°)

(°

)

D0 exposure

100 200 300 400 5000.0

0.5

1.0

1.5

2.0

5 10 15 20 25 30 35

0.0

0.5

1.0

1.5

2.0m

odified layer thickness (nm)

time (min)

fluence (1018 /cm2)

er

oded

film

thi

ckne

ss (

nm)


change in D area density and modified layer thickness

uptake of D without optical response isotope exchange

1 10 100 1000-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.60.1 1 10 100

mo

dified

layer thickn

ess (nm

)

time (min)

1 10 100 1000

0

2

4

6

8

chan

ge o

f D

are

al d

ensi

ty (

10 1

5 at.

/cm

2 ) fluence (1018 at. /cm2)


for a flux density j = 1.3·1015 D0/cm2 s

an initial hydrogen density nH = 0.33 · 12.1·1022 /cm3

an uptake of nD = 3·1015 D/cm2 in the first 5 minutes:

σ = 2·10-18 cm2 = 0.02 Å2

)()0()( tnntn HHD

jtntn HD exp1)0()(

jtndt

tdnH

H )()(

analysis

for simple isotope exchange we have:

Küppers et al.: abstraction of bonded H: 0.02 Å2 cm2

1 10 100 1000-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.60.1 1 10 100

mo

dified

layer thickn

ess (nm

)

time (min)

1 10 100 1000

0

2

4

6

8

chan

ge o

f D

are

al d

ensi

ty (

10 1

5 at.

/cm

2 )

fluence (1018 at. /cm2)

Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 G. S. Oehrlein*, T. Schwarz-Selinger, K....

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Transcript of Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 G. S. Oehrlein*, T. Schwarz-Selinger, K....