Light absorption enhancement in extremely confined Ge nanostructures

Light absorption enhancement in extremely confined Ge nanostructures

S. Mirabella R. Raciti, S. Cosentino, E. Barbagiovanni, M. Miritello, A. Terrasi

A.M. Mio, G. Nicotra, C. Spinella

R. Bahariqushchi, A. Aydinli

www.matis.imm.cnr.it [email protected] EMRS Spring meeting, 13/05/2015

Cullis et al. - J. Appl. Phys., 82, 909 (1997) Park et al., Phys. Rev. Lett. 86, 1355 (2001) E. Barbagiovanni et al. - J. Appl. Phys., 111, 034307 (2012)

5 10 150.5

1.0

1.5

2.0

2.5

3.0

Ener

gy g

ap [e

V]

Size [nm]

Si QD in SiO2

Si QW in SiO2

Ge QD in SiO2

The quantum chance

F. Priolo et al., Nature Nanotechnology 9, 19 (2014)

( ) 2*

22

2)(

LmbulkENSE gg

π+=

Si

SiO2


Ge versus Si QDs

1 2 3 4 5

103

104

105

106

Abso

rptio

n co

effic

ient

[cm

-1]

Energy [eV]

Silicon Germanium

Si QDs

Ge QDs

• Higher absorption coefficient • Larger size range for QCE • Bandgap tuning within solar spectrum


Quantum effect on Eg

E. Barbagiovanni et al. J. Appl. Phys., 111, 034307 (2012)

( ) 2*

22

2)(

LmbulkENSE gg

π+=Quantum confinement effect: does only size matter ?

Optical bandgap depends on several factors: how to model the QCE on Eg ? other effects on absorption ?

0 200 400 600 800 1000 1200

2,1

2,4

2,7

3,0

MS samples: 43-S 46-S

PECVD samples: 43-C 46-C 46-SL

EOPT

g [e

V]Temperature [°C]

Si QDs S. Mirabella et al. J. Appl. Phys., 106, 103505 (2009)

Si QDs - calculation P. Hapala et al., Phys. Rev. B, 87, 195420 (2013)


Quantum effect on absorption

SiGe/Ge/SiGe QW - calculation Kuo and Li,, Phys. Rev. B, 79, 245328 (2009)

herr −>herr −<herr −<<

Absorption efficiency depends on oscillator strength (Os) but … no clear experimental evidence!

Quantum confinement excitonic effect on absorption efficiency ?

( ) ( )∫∀

−−⋅⋅=k

BZvcs EEdkO

nce ωδ

πωρµπωσ 3

220

22

)2(24


OUTLINE

Light absorption in Ge quantum dots

• Extraction of optical properties (Eg and BTauc) • Interface effects on Eg

• confining potential

• Absorption enhancement • … towards extreme confinement

• Conclusions


OUTLINE





• Conclusions


Tauc model

8

Jan Tauc (1922-2010)

• Parabolic v.b. and c.b. approximation • Eg

opt energy difference (Ef-Ei) • BTauc ̴ absorption efficiency J.Tauc, Amorphous and Liquid Semiconductors, Plenum Press, London and New York Tauc plot

( ) ( )2optg

Tauc EB−⋅= ω

ωωα

( )optgTauc EB −= ωωα

S. Cosentino, S. Mirabella et al., Nanoscale Res. Lett. 8, 128 (2013)


Single Ge quantum well

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 5 10 15 20 25 30 1254

8

12

16

20

B [×

10-1 (e

V×nm

)-1]

(b)

(a)

Ener

gy g

ap [e

V] EG Fit, Eg=Eg-Bulk+A/L2

A = 4.35 [eV×nm2]

OS [×10

-4 nm-2]

Quantum well thickness [nm]

OS in Ge QW (theory, Kuo PRB2009)0.3

0.6

0.9

1.2

1.5

B (measured) Tauc approach to extract in Ge NS: - optical bandgap (Eg) - absorption efficiency (BTauc)

BTauc is proportional to the oscillator strength

S. Cosentino, S. Mirabella et al., Nanoscale Res. Lett. 8, 128 (2013)


OUTLINE





• Conclusions


Ge QDs synthesis

SiGeO film

PECVD or sputter (deposition 250°C: 8 - 20% Ge)

(600-800°C annealing in N2)

Ge QDs (2-8 nm) embedded in SiO2

5.0x1021 1.0x1022 1.5x10220

2

4

6

8

10

PECVD sputter

QD

size

[nm

]

Ge concentration [cm-3]


RBS

sigeo90_asdep_ran(x2y7).RBSSimulated

Channel600550500450400350300250200

Cou

nts

2.000

1.900

1.800

1.700

1.600

1.500

1.400

1.300

1.200

1.100

1.000

900

800

700

600

500

400

300

200

100

0

600 700 800 900 1000 1100 1200 1300 1400 1500 1600

Energy [keV]

Si

SiGeO

2 MeV He+

O

Si

Ge

• Ge QDs density (~ 1018 cm-3) and spacing (1-3 nm) • Light absorption analysis

5.0x1021 1.0x1022 1.5x10220

1

2

3

4

PECVD sputter

QD-

QD

dist

ance

[nm

]

Ge concentration [cm-3]


Light absorption cross section absorption (α) absorption cross section (σ): photon absorption probability per Ge dose

Dtασ =

• Red shift with decreasing QD size • Greater shift in PECVD w.r.t. sputter

1 2 3 4 510-19

10-18

10-17

PECVD QDs (3.5 nm) PECVD QDs (4.4 nm) Sputter QDs (3 nm) Sputter QDs (4 nm)

Abso

rptio

n cr

oss

sect

ion

[cm

2 ]

Energy [eV]

Relatore

Note di presentazione

200+200


Optical bandgap variation

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk

Opt

ical B

andg

ap [e

V]

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk EMA

Opt

ical B

andg

ap [e

V]

• Eg modulation is dependent on synthesis technique

( ) 2*

22

2)(

LmbulkENSE gg

π+=

QD

Is the interface playing a role ?

• Why different Eg modulation ?

• How to model the QCE ?

• Eg modulation is dependent on synthesis technique • EMA cannot account for none of the two


2 nm

Ge QD

TEM analysis

Z contrast profiling reveals systematically thinner interfaces in PECVD samples 2

1)(1)( 0 QDdiameterexf

xx≥Γ

+=

−Γ

−−

3.5 nm QD


EELS-STEM analysis

2 nm

• Chemical analysis of QD surrounding • Different oxide contribution

0.2

0.4

0.6

0.8

1.0

5 10 15 20 25 30 35 40 45 50 55 60

0.2

0.4

0.6

0.8

1.0

GeO

SiO2

PECVD

Inte

nsity

[a.u

.] Core Interface Matrix

Sputter

Ge

Inte

nsity

[a.u

.]

Energy [eV]

5 nm QD

5 nm QD

JEOL ARM200CF

www.beyondnano.it

Probe size: 0.2x0.2 nm

http://www.beyondnano.it/


EELS-STEM analysis

0.0

0.2

0.4

0.6

0.8

1.0

5 10 15 20 25 30 35 40 45 50 55 600.0

0.2

0.4

0.6

0.8

1.0

Sputter

Inte

nsity

[a.u

.]

PECVD

Inte

nsity

[a.u

.]

Energy [eV]

EELS core QD Fit

interband transition Ge Ge QD vol. plasmon SiO2 vol. plasmon Ge-Ge M4,5 band Ge-O M4,5 band

AGe-O

AGe-Ge

AGe-pl )( plGeGeGe

OGeOGe AA

AF−−

−− +

=

FGe-O ~ 16 % for sputter FGe-O ~ 8 % for PECVD

S. Cosentino, S. Mirabella et al., Nanoscale (2015) submitted

STEM: e-beam probe a cylinder of ~ 40 Ge atoms, 3 of which at surfaces

• Significant Ge-O surface contribution • Greater Ge-O contribution in sputter samples

e-beam


Ge/SiO2

V0,e 2.8 eV

V0,h 4.5 eV

Interface role

SiO2 SiO2 Ge QD

VB

CB

VB

CB

Potential well for e-

Potential well for h+

V0,e

V0,h

Ideal case Real case

SiO2 SiO2 Ge QD

GeO2

Ge/SiO2 Ge/GeO2

V0,e 2.8 eV 1.2 eV

V0,h 4.5 eV 3.6 eV


Interface effect on bandgap

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk

Opt

ical B

andg

ap [e

V]

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk EMA

Opt

ical B

andg

ap [e

V]

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk EMA SPDEM PECVD SPDEM Sputter

Opt

ical B

andg

ap [e

V]

E. G. Barbagiovanni, et al., J. Appl. Phys. (2012), 111, 034307 E. G. Barbagiovanni, et al. Physica E, (2014), 63, 14–20 E. G. Barbagiovanni, S. Mirabella et al., J. Appl. Phys. (2015) accepted S. Cosentino, S. Mirabella et al., Nanoscale (2015) submitted

Ge/SiO2 Ge/GeO2 PECVD Sputter

V0,e 2.8 eV 1.2 eV 1.1 eV 0.9 eV

V0,h 4.5 eV 3.6 eV 3.3 eV 2.8 eV

( )( )

+

⋅+= *

,

,*,

,

23

hc

hc

ec

ecbulkgg m

VmV

DDEDE

µ

SPDEM model well accounts for the Eg variation

www.matis.imm.cnr.it [email protected] EMRS Spring meeting, 13/05/2015 Paper of Eric on arxiv?

Interface effects

S. Cosentino, S. Mirabella et al., Nanoscale (2015) submitted

• GeO2 act as the confining potential • A thinner and GeO poor interface gives larger QCE

• … what about absorption efficiency ?

2 4 6 8 105.0x10-18

1.0x10-17

1.5x10-17

Absorption Efficiency

Ge QDs PECVD Ge QDs Sputter

B* Ta

uc [e

V-1×c

m2 ]

QD size [nm]

2X increase

Light absorption in Ge quantum dots in SiO2

Relatore

Note di presentazione

500+300 Mettere QCE classico (A=7.88) e poi le curve nuove dal JAP di Eric. La differenza del comportamento è da imputarsi al diverso confiment dovuto alla differente interfaccia fra i due campioni. In particolare l’effective mass è sensibile all’interfaccia, la quale poi determina un differente QCE.


OUTLINE





• Conclusions


Multilayer approach

Sample t

[nm] d

[nm] N

ML-2 2 20 15 ML-4 4.5 20 4

SL-330 330 - 1

Multilayer approach for: • narrowing size distribution (Zacharias APL2002) • increasing average distance among QDs Fixed SiO2 barrier thickness: 20 nm N SiGeO layers, from 4 to 15 Comparison with a single layer (330 nm)

d: SiO2 barrier

t: SiGeO layer


Absorption coefficient

1 2 3 4 5 6103

104

105

106

Ab

sorp

tion

coef

ficie

nt [c

m-1]

Energy [eV]

c-Ge bulk SL-330

1 2 3 4 5 6103

104

105

106

Ab

sorp

tion

coef

ficie

nt [c

m-1]

Energy [eV]

c-Ge bulk SL-330 ML-2 ML-4

• Multilayered samples show similar absorption onset to single layer

• … but much higher absorption efficiency!


RBS and TEM analysis

Sample t

[nm] d

[nm] N

Ge % in SiGeO

QD size

ML-2 2 20 15 10.6 2 ML-4 4.5 20 4 8.9 1.7

SL-330 330 - 1 10.0 2.9

450 500 5500

50

100

150

200

250

300

He+ backscattered from Ge atoms

RBS

yiel

d [c

ount

s]

Channel

ML-2 ML-4

2 MeV He+ beam165° backscattering angle


Light absorption Ge QD in MLs

10-17

10-16

σ [c

m2 ]

2 3 4 5 60

1

2

ML-2 ML-4 SL-330

(σ h

ν)1/

2 [10-9

cm x

eV1/

2 ]

Energy [eV]

• Similar optical bandgap • Strong increase of absorption efficiency • Independent modulation of Eg and B* • ML configuration allows for absorption increase

10 X increase!

Same Eg

R. Raciti et al. Poster CP2 #44 Today 14-16


Multiple screening effect

Si QDs - ε2 calculation in one Si QD R. Guerra et al., Phys. Rev. B, 84, 075342 (2011)

Local Field Effects Lower screening of e.m. radiation by induced polarization (local field effects)

2 4 6 8 100.0

5.0x10-17

1.0x10-16

1.5x10-16 PECVD Sputter ML-PECVD Ge bulk

B Ta

uc [e

V-1×c

m2 ]

QD size [nm]

15X increase


OUTLINE





• Conclusions


CONCLUSIONS

Based on: S. Mirabella et al. JAP 106, 103505 (2009) E. Barbagiovanni et al. JAP 111, 034307 (2012) S. Cosentino et al. NRL 8, 128 (2013) S. Mirabella et al. APL 102, 193105 (2013) E. Barbagiovanni et al. PE 63, 14 (2014) S. Cosentino et al. JAP 115, 043103 (2014) S. Cosentino et al. SOLMAT 135, 22 (2015) E. Barbagiovanni et al. JAP (2015) accepted S. Cosentino et al. Nanoscale (2015) submitted

SiO2 SiO2 Ge QD

GeO2

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

QD size [nm]

PECVD Sputter a-Ge bulk EMA SPDEM PECVD SPDEM Sputter

Opt

ical B

andg

ap [e

V]

2 3 4 5 60

1

2

(σ h

ν)1/

2 [10-9

cm x

eV1/

2 ]

Energy [eV]

10 X increase!

Optical bandgap in Ge QDs in SiO2 • variation with size • interface drives confinement • SPDEM model Absorption efficiency in Ge QDs in SiO2 • large increase in multilayer (reduced screening)

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Light absorption enhancement in extremely confined Ge nanostructures