First-principles investigation of paramagnetic centers in ...giacomaz/Pres_Fismat2015.pdfGe(2)...

First-principles investigation of paramagneticcenters in v-SiO2, Ge-doped SiO2 and v-GeO2.

Luigi Giacomazzi1, L. Martin-Samos2, N. Richard3

CNR-IOM Democritos

Via Bonomea 265, Trieste, Italy2University of Nova Gorica

Vipavska 11c 5270-Ajdovscina, Slovenija.3CEA DAM, DIF

F-91297 Arpajon, France

[email protected]

October 1, 2015

FisMat2015, University of Palermo, Sept. 28 - Oct. 2, 2015 FP investigation of paramagnetic centers in v-SiO2, Ge-doped SiO

Motivation: Developing radiation tolerant optical fibers

ITER plasma diagnostic system

Space applications

monitoring systems for nuclear power plants and waste storage

S. Girard et al, IEEE Trans Nucl. Sci. 60, 2015 (2013).FisMat2015, University of Palermo, Sept. 28 - Oct. 2, 2015 FP investigation of paramagnetic centers in v-SiO2, Ge-doped SiO

E ′ centers in a-SiO2: E′γ, E

′α, and E ′

δ

Proposed models:

e−��

��

E’γ

e−��

��

E’α

?

��

��

E’δ

��

��

E’δ

?Griscom, Nucl. Inst. & Methods B1, 481 (1984).Buscarino et al. Phys. Rev. Lett 97, 135502 (2006).Jivanescu et al. Phys. Rev. B 83, 094118 (2011).


Ge paramagnetic centers in v -SiO2: Ge-E′, Ge(2)

Proposed models:

e−��

��

Ge−E’

e−

Ge(2): Griscom

e−

Ge(2): GLPC+

?Griscom, Opt. Mater. Express 1, 400 (2011).Fujimaki et al. Phys. Rev. B 57, 3920 (1998).


EPR parameters of E ′ centers: Methods & Models

GGA exchange-correlation functionals (80 ryd cutoff)

QE and QE-GIPAW (from www.qe-forge.org/):g-tensor GIPAW, Pickard and Mauri, PRL 88, 086403 (2002).

Positively charged oxygen vacancies in a-SiO2:

108 atoms model: remove a bridging oxygen froma chosen site and relax in theq = +1 charged state.

72 SiODC models are obtained(Si2 dimers)

relax in the q = +2 and againin the q = +1 (non-dimers)

N. Richard et al JNCS 351, 1825 (2005)FisMat2015, University of Palermo, Sept. 28 - Oct. 2, 2015 FP investigation of paramagnetic centers in v-SiO2, Ge-doped SiO

EPR parameters of E ′ centers: configurations

Configurations obtained after ab initio relaxation of SiODC models:

(72) dimer (15) puck 4x (33) puck 5x

(11) unpuckered (5) forward oriented

1.8 Ao

1.6 Ao

L. Giacomazzi, L. Martin-Samos, A. Boukenter, Y. Ouerdane, S. Girard, N.Richard, PRB 90, 014108 (2014).

Uchino et al PRB 74, 125203 (2006); PRL 84, 1475 (2000).FisMat2015, University of Palermo, Sept. 28 - Oct. 2, 2015 FP investigation of paramagnetic centers in v-SiO2, Ge-doped SiO

Results: Si DB puckered, unpuckered, and forward-oriented(FO) configurations

2.0 2.0005 2.001

-50

-40

-30

g2

Ais

o(29

Si)

(mT

) FO

E’α

E’γ

Expt.

1.999 1.9995 2.0 2.0005

-50

-40

-30

g3

Ais

o(29

Si)

(mT

) FO

E’α

E’γ

Expt.

-50 -40 -30

0.0

1.0

2.0

Aiso(29Si) (mT)

∆E (

eV)

forward−oriented Puckered

No way to find puckered configurations with A ∼ −49 mT andg2 ∼ 2.0010 as shown by the E ′

α.


Si DB ”interacting” model of the E ′α ?

2.6 2.8 3.0 3.2 3.4

-50

-40

-30

SiDB-OR (Å)

Ais

o(29

Si)

(mT

) A

3.0 3.4 3.8 4.2

-50

-40

-30

SiDB-OB (Å)

Ais

o(29

Si)

(mT

) B

e−

E’α

B A

A

(a)

A

(b)

3.05

2.78

trends could exist but local environment strongly affect Aiso(Si)

distance SiDB-OB,R ∼ 3 A does not seem a criterion supporting theexistence of an E ′ center at 49 mT.


EPR parameters of Ge centers: configurations

Configurations obtained after ab initio relaxation of GeODCmodels:

dimer puck 4x puck 5x

unpuckered

1.9 Ao

1.7 Ao

forward oriented


EPR parameters of Ge-E ′ and Ge-FO: distributions

1.99 1.995 2.0-30

-25

-20

-15

g2

Ais

o(73

Ge)

(m

T)

Ge-FO

Ge(2)

Ge-E’

Expt. Agnello et al EurPhysJB(2008) (a)

1.98 1.99 2.0-30

-25

-20

-15

g3

Ais

o(73

Ge)

(m

T)

Ge-FO

Ge(2)

Ge-E’

Expt. (b)

-5000 0 5000-10000

-5000

0

5000

Theory ∆g (ppm)

Exp

t. ∆

g (p

pm)

∆g3

∆g1

∆g2

(c)

Ge puckered → Ge-E ′

Ge-FO → Ge(2)

Expt.:∆gi = gi (Ge(2)) − gi (Ge− E′)Theory:∆gi = gi (GeFO)− gi (Ge− E′

c)


Ge(2) i.e. the Ge analougue of the E ′α

1.9925 1.995 1.9975 2.01.999

2.0

2.001

2.002

g2(Ge-E’ )

g2(

Si-E

’ )

FO

E’α/Ge(2)

E’γ /Ge-E’

Expt. Buscarino et al PRL2006 Agnello et al EurPhysJB2008 (a)

1.98 1.99 2.01.998

2.0

2.002

g3(Ge-E’ )

g3(

Si-E

’ )

FO

E’α/Ge(2)

E’γ /Ge-E’

Expt. (b)


The Ge(2) center in pure v -GeO2

0.0 0.0025 0.005 0.0075-30

-20

-10

0

g12

Ais

o(73

Ge)

(m

T)

GeFO

Ge(2)

Ge-Ge

E’-Ge

E’-Ge*

(a)

0.0 0.005 0.01 0.015-30

-20

-10

0

g13

Ais

o(73

Ge)

(m

T)

GeFO

Ge(2)E’-Ge

E’-Ge*

Ge-Ge

(b)

Giacomazzi, Martin-Samos, Richard, Microel Eng. 147, 130 (2015).


The Ge(2) is not a trapped elect. center as Ge(1)!

0.008 0.009 0.010.7

0.75

0.8

0.85

0.9

g13

δ V

(a)

150 160

O-Ge-O (deg)

(b)O−Ge−O

In Ge(2) g13 = 0.0140: for Griscom’s model to be true it implies a highly

deformed GeO−

4 tetrahedron with δV ∼ 0.5 as for O-Ge-O∼ 180◦.

L. Giacomazzi et al. Opt. Mater. Express 5, 1054 (2015).


Twofold Si and Ge (GLPC): in progress

Ge

1.75A

2.5 A

1.7A

Ge 1.9A

Ge−FO configuration GLPC configuration

+e

+e

−

−

By first principles relaxation of the neutral Ge-FO a two-fold Ge is

obtained. This is consistent with the observed Ge(2) generation from

GLPC [Agnello et al Eur. Phys. J. B 61, 25 (2008)].

0.0 0.5 1.00.0

0.5

1.0

1.5

reaction coordinate

ener

gy (

eV)

dimerTwofold Ge

A neutral Ge-Si (Si-Si) dimer can transform in to a twofold Ge (Si)

configuration by overcoming ∼ 1 eV (∼ 2) barrier.


Twofold Si and Ge (GLPC): in progress

Just in case you doubt it is really a GeODC(II): calculations of the opticalabsorption spectrum gives a peak at 5.1 eV !

Giacomazzi, Martin-Samos, Richard et al in progress (2015).


Conclusions

E ′α arises from ”forward-oriented” configurations, while no

clear evidence is found for the Si DB ”interacting model”.

Ge(2) originates from Ge-FO and thus is the Ge analogue ofthe E ′

α center.

As shown by this work, first-principles calculations of EPRparameters are a powerful tool for studying paramagneticdefects, not only for E ′ centers in vitreous silica but also indoped silica (e.g. Ge(2) and Ge-E ′ in Ge-doped SiO2).

Calculations of the optical absorption spectra are in progressfor the twofold Si and Ge configurations derived from theSi-FO and Ge-FO.

L. Giacomazzi, L. Martin-Samos, A. Boukenter, Y. Ouerdane, S. Girard, N.Richard, Opt. Mater Express 5, 1054 (2015).

L. Giacomazzi, Martin-Samos, N. Richard, Microel. Eng. 147, 130 (2015).L. Giacomazzi et al Phys. Rev. B 90, 014108 (2014).


Thank you for your attention


First-principles investigation of paramagnetic centers in ...giacomaz/Pres_Fismat2015.pdfGe(2)...

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