Light absorption in Si or Ge nanostructures - GADEST2013

Synthesis and Light Absorption in Si or Ge Nanoclusters for Photovoltaics Applications

Salvo Mirabella

CNR-IMM Catania, ITALY

www.matis.imm.cnr.it Salvo Mirabella - [email protected] GADEST 2013, 23/09/2013

The Energy Problem

Average energy consumption increases with the standard of living [Source: A. Gasparella, Univ. Padova]

Average daily energy consumption [loe]

Energy for food Energy for house En. for agr.&ind. Energy for transp.


Terawatt Dilemma

TERAWATT DILEMMA Source: L. Kazmerski, NREL


Renewable sources

Sun is … enough Sun is … green Sun is … sustainable!


PV Module: Learning Curve

Every doubling of installed capacity a ~ 20% reduction in PV module price … A new Moore’s law-like lighthouse ?


Photovoltaic conversion

Single material solar cell

[F. Dimroth and S. Kurtz MRS Bull. 2007]

[Shockley-Queisser JAP 1961]

Lucky case: Sun spectrum & Abundant Si!!!


Multijunction solar cell

[F. Dimroth and S. Kurtz MRS Bull. 2007]


Best efficiencies

http://www.nrel.gov/ncpv/images/efficiency_chart.jpg


The quantum chance

All Si tandem solar cell UNSW group Green et al., 2005

Optical bandgap tunable in QD filling all the solar spectrum!

Light absorption in QDs ?

Si QDs

Ge QDs


OUTLINE

Light absorption and quantum effects:

• Ge nanostructures •Quantum wells •Quantum dots (size, matrix, proximity)

• Si nanostructures •Quantum dots (synthesis and phase effects) •Si:O alloy (material and preliminary device)

• Conclusions


OUTLINE




• Conclusions

Ideal study case Exciton Bohr radius Lower bandgap


Ge quantum well

Ar+

Magnetron Sputtering

deposition (@ RT)

2 ÷ 30 nm

TEM and RBS cross check: - Stoichiometric SiO2

- Ge density ̴̴ 4.3×1022 at/cm3

- QW free from holes ( 1D confinement!)

[S. Cosentino et al. NRL 8, 128 (2013)]


Ge quantum well

• 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

2opt

gTauc E

B

Tauc approach to derive: - optical bandgap (Eg) - absorption efficiency (BTauc)

Light absorption from single Ge QW


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 (

eV

×n

m)-1

]

(b)

(a)

En

erg

y g

ap

[e

V]

EG

Fit, Eg=E

g-Bulk+A/L

2

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)

Ge quantum well

hemA

2

22

strengthzii

Tauc Oee

mB 2

02||||

2~

Ideal case: - Eg follows QC rule - BTauc trend agrees with OStrenght trend for c-Ge QW

[S. Cosentino et al. NRL 8, 128 (2013)]

herr herr herr Close to the value of Barbagiovanni et al. JAP2012


OUTLINE


• Ge nanostructures •Quantum wells •Quantum dots (matrix, size, proximity)


• Conclusions


Ge quantum dots: matrix

Comparison between Si3N4 or SiO2 matrices

- Ge QDs size: <2 nm - Amorphous phase Si

3N

4

SiO

2 - Ge QDs size: 2-20 nm

- Crystalline phase

100 keV Ge (1-7x1016 cm-2 )

STEM HAADF (Z-contrast between Ge and Si3N4) cross section of Ge implanted matrices after annealing 850°C, 1h

Si3N4

SiO2


480 500 520 540 5600

500

1000

1500

2000

Si3N

4 matrix:

Ge as implanted

850 °C, 1h

RB

S Y

ield

channel

80 60 40 20

0.0

0.5

1.0

1.5

2.0

2.5(c)

Depth [nm]

Ge

co

nce

ntr

atio

n [×

10

22 /cm

3]


Ge diffusivity (850°C) in Si3N4 lower than : 7x10-17 cm2/s Yuan PRL209: in SiO2 D(Ge)=6.5x10-10 cm2/s

2.0 MeV He+

84°

Si3 N4

Ge diffusion in Si3N4 by RBS

[S. Mirabella et al. APL 101, 011911 (2012)] In Si3N4 Ge QD ripening limited by diffusivity



• larger for Ge QD in Si3N4 • Eg smaller in Si3N4, because of lower potential barrier

@700°C QD size 2-3 nm for both matrices

1 2 3 4 5 6 7 8 9 10

1

2

3

4

5

6

7

8

Energ

y g

ap [eV

]

Size [nm]

Si3N

4

SiO2

[S. Mirabella et al. APL 101, 011911 (2012)]


Ge quantum dots: size

Quartz/Si

Ar+

250 nm

UHV-Magnetron Sputtering Deposition

post thermal annealing at 600°C

SiGeO film SiO2 + Ge QDs

8 10 12 14 16 18 201

2

3

4

5

Mean Q

D s

ize [nm

]

Ge Concentration [%]

• Ge QD size variable with Ge at.%



2.0 2.5 3.0 3.5 4.0

1.4

1.6

1.8

2.0

Ge QDs in SiO2

Energy gap

Energ

y g

ap [eV

]Mean Diameter [nm]

• Large size-dependent shift of Eg

• Eg tunable in the 1.4 – 2.1 eV range • Eg in 2 nm QD larger than in 2 nm QW (1.8 eV)

Light absorption in Ge QD: size effect



2.0 2.5 3.0 3.5 4.0

3

6

9

12

BT

auc [

×1

01

8 e

V-1×

cm2]

Mean Diameter [nm]

Absorption Efficiency,

QD-QD distance <a>~2 nm

d d <a>

Absorption efficiency independent of the size: any role of QD-QD distance?

Absorption efficiency in Ge QD: size effect


Ge quantum dots: proximity

UHV-Magnetron Sputtering Deposition:

multilayer approach (SiGeO/SiO2)

d|| in-plane distance = 1 nm

d out-of-plane distance = 3 - 20 nm

Ge QD size 2-3 nm



Ge quantum dots: proximity

• Eg not affected by QD-QD distance • Absorption efficiency dependent on QD-QD distance • Long range QD-QD interaction (or between QD films)

Single layer d|| = d = 3 nm


d|| in-plane distance = 1 nm

d out-of-plane distance = 3 - 20 nm


Ge nanostructures

Preliminary application in light harvesting talk on Thursday 26th at 11:40

“High-efficiency photodetectors based on Ge quantum dots”

Optical bandgap can be tuned: • Eg increases with reducing the size • Eg increases in high potential matrix • Eg increases in 3D confined systems

Absorption efficiency can be tuned: • B increases by reducing thickness of the well • B increases in closer packaging of dots


OUTLINE




• Conclusions


Si quantum dots: synthesis

Quartz

SixO1-x

Deposition techniques: MS or PECVD 0.43<x<0.53 Post annealing (450-1250°C)

QD size ~ 4 nm Light emission ~ 1.3-1.5 eV Eg ~ 2.5 eV (Stokes shift) 600 700 800 900 1000 1100 1200

0.0

0.1

0.2

0.3

exc

= 488 nm

power = 10 mW

RT detection

after annealing 1100°C, 1h:

46-SL

43-S

46-C

PL

in

ten

sity

[a.

u.]

Wavelength [nm]

[S. Mirabella et al. JAP 106, 103505 (2009)]


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

EO

PT

g [e

V]

Temperature [°C]

Si QDs Eg: synthesis technique

Si-N, Si-H bonding (PECVD) Full Si-SiO2 separation (MS)

Increases with T (growing QD size!!!)

• Eg affected by deposition technique up to 900°C • unexpected Eg increase for T > 900°C


• Eg affected by synthesis (phase separation kinetics) • Eg affected by phase more than by size


[R. Guerra et al. PRB 79, 155320 (2009)]

Si QDs Eg: phase effect


OUTLINE


• Ge nanostructures •Quantum wells •Quantum dots (size, matrix,proximity)


• Conclusions


Si:O alloys: synopsys

1.8

2.1

2.4

2.7

3.0

40 50 60 70 80 90 100

EO

PT

G

[eV

]

MS grown

PECVD grown

40 50 60 70 80 90 10010

-1

100

101

102

103

104

(b)

(a)

[

×

cm

]

Si concentration [%]

Optical Absorption

600°C annealing

Electrical transport

900°C annealing

[B]peak

= 3×1020

B/cm3

MS grown

CVD grown

+ Forming gas


•EG of Si:O alloys matches the solar rainbow

•Charge transport affected by doping, Si content, defect saturation (forming gas)

First application in PV: preliminary solar cell


Si:O alloys: device

Device structure: Top TCO dep by sputter

p-i-n (20-200-20 nm) SiO alloy (70 Si at.%) by PECVD

Bottom TCO by sputter

PV effect measured (¼ sun illumination) -Good VOC

-Poor JSC (due to lower and carrier traps)

[G. Scapellato et al. JAP 114, 053507 (2013)]


Si:O alloys: spectral efficiency

I

L

P

I

e

hcQE

1

EG ≈ 2.2 eV EG ≈ 1.7 eV

Possible use in tandem cell

-Si Lower QE in Si:O (reduced ISC) QE peak shifted at higher photon energy (larger gap)

[G. Scapellato et al. JAP 114, 053507 (2013)]


CONCLUSIONS

Ge QDs

Ge QWs

Based on: -S. Cosentino et al. NRL 8, 128 (2013) -S. Mirabella et al. APL 101, 011911 (2012) -S. Mirabella et al. APL 102, 193105 (2013) -S. Mirabella et al. JAP 106, 103505 (2009) -S. Mirabella et al. JAP 108, 093507 (2010) -G. Scapellato et al. JAP 114, 053507 (2013)

Si QDs Si:O

Light absorption affected by: • Synthesis technique • Embedding matrix • 1D or 3D Confinement • NS density • Ns size


CONTRIBUTORS

S. Cosentino Ge NS

G. G. Scapellato Si:O PV cell

M. Miritello Sputter

I. Crupi Electr. meas.

A. Terrasi Si & Ge NS

F. Priolo Si QDs

F. Simone Opt. Meas.

G. Nicotra TEM Ge NS.

THANKS FOR YOUR ATTENTION!


chaired by H. Richter and W. Tumas

Organizers:

S. Mirabella (CNR, Catania) I. Gordon (IMEC, Leuven) J. Valenta (Ch. Univ., Prague) R. Turan (METU, Ankara)

E-MRS 2014 SPRING MEETING Technical sessions: May 26-30 Congress Center - Lille, France

Technology

Light absorption in Si or Ge nanostructures - GADEST2013