The use of EBIC in solar cell characterization - SINTEF · The use of EBIC in solar cell characterization Maurizio Acciarri ... belong to the Department of Material Science of the

Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali

The use of EBIC in solar cell characterization

Maurizio Acciarri

Mini PV ConferenceTrondheim No

9-10 January 2008


Department of Material Science

• The group of Physics and Chemistry of Semiconductors belong to the Department of Material Science of the University of Milano Bicocca, since 1998.

• To the Department are connected courses in:– Material Science– Optic and Optometry– Chemical Science and Technology– Goldsmith Science and Technology

• The Department is composed by:– 38 academic staff– 22 non academic staff– More than 76 PHD and post-doc students


Department of Material Science1. Materials Science and Cultural Heritage. Luminescence Dating2. Oxide Nanostructures and Silica-based Materials for Optical Technology3. Energy storage materials. Chemical synthesis, crystal structure, theoretical models4. Electrochemical activities5. Chemistry of inorganic and organometallic materials6. Surface chemical reactions: crystal growth and sorption processes7. Physics and applications of lasers8. Shape Memory Alloys9. Organic materials for applications in photonics10. Organic molecular systems for II order non-linear materials and low energy emitters11. Nanostructured materials and magic angle spinning NMR12. Chemical physics of semiconductors: defects, impurities and surfaces13. Optical spectroscopy of semiconductors and semiconductor quantum structures14. Organic Molecular Semiconductors15. Photophysics of molecular semiconductors16. Simulation and Modeling of the Epitaxial Growth of Semiconductor Nanostructures and

Films17. Theoretical modeling and ab-initio simulation of material properties18. Theory of Surface Science and Catalysis19. Theory of surfaces, interfaces, and bulk inorganic materials


Physics and Chemistry of Semiconductors

Research Focus on: the characterization of defect centers in semiconductors through the study of radiative and non-radiative recombination of carriers at impurity centers and point and extended defects (dislocations, grain boundaries).

Composition:• Dr. Maurizio Acciarri (assistant professor in Physics)• Dr. Simona Binetti (assistant professor in Physical

Chemistry)• 3 PHD • 3 final year students


Photovoltaic projects• Concepts for high efficiency multy-crystalline silicon solar cells

(Multi-Chess) (1990-1993)• Multi-Chess II (1993-1996)• Cost Effective Solar Silicon Tecnology (COSST) (1996-1999)• Fast in Line characterization tools for crystalline silicon material and

cell process quality control in the PV industry (FAST-IQ) (2000-2003)

• N-type Solar Grade Silicon for Efficient p+n Solar Cells (Nessi) (2002-2005)

• Nanocrystalline silicon film for photovoltaic and optoelectronic application (NanoPhoto) (2005-2008)

• Development of solar-grade silicon feedstock for wafers and cells, by purification and crystallisation (Foxy) (2006-2008)

• Cariplo national project (2002-2005) on SiGe thin film for optoelectronic and PV application


Solar energy (PV) conversion

Defects, impurities and

their interactionFacilities

Oxygen, Carbon and Nitrogen

Dislocations

Precipitates

Recombination processes

(Si, SiC and SiGe)

Recombination processes at

extended defects

In-line characterization

•SEM – EBIC (77-300 K)•LBIC e Fast-LBIC•Hall effect•SPV•Solar simulator (5x5 cm2)•FTIR (50-4000 cm-1)•Photoluminescence (IR-UV-Vis) •XRD and Raman spectroscopy (dept. facility)•Optical microscope•Chemistry laboratory for etching and cleaning•Furnaces (working temperature up to 1500 °C)•Evaporator•Sputtering

Optoelectronic Solar cells

Thin films

Physics and Chemistry of Semiconductors


Impact of defects on solar cells efficiency

( )

( )

0 02 2exp exp 1 exp

21 exp

i F i t F t Fn p

SRHi F

E E E E E E Eh hkT kT kT

E Eh

kT

τ ττ

⎡ ⎤−⎛ ⎞ − − −⎡ ⎤⎛ ⎞ ⎛ ⎞+ + + + +⎢ ⎥⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎢ ⎥⎝ ⎠ ⎝ ⎠⎣ ⎦⎝ ⎠⎣ ⎦=−⎛ ⎞

+ + ⎜ ⎟⎝ ⎠

B.V.

C.B.

hυRadiative emissionPhotoluminescence

Radiative and non radiativeemission

Direct recombination lifetime: τd

Indirect recombination lifetime: τi(Shockley-Read-Hall)

Perfect crystal

0

1n th t

n

v Nστ

≡


Impact of defects on solar cells efficiency

• For PV very high lifetime values are requested

• In PV is more meaningful the diffusion length (L):

..1111+++=

sid ττττ

B.V.

C.B.

hυ

τDL =

μqkTD = Defects may influence

recombination and mobility


Multicrystalline Si wafers: lifetime maps

After the POCl3 process stepAs-grown

Why?Lifetime maps carried out by ISC Konstanz (D)

PhotoConductance Decay (mW-PCD)


Scanning Electron Microscope

• The electron beam produced by an electron gun is focused to a point on the sample surface by two condenser lenses. The second condenser lens (sometimes also called as objective lens) focuses the beam to an extraordinarily small diameter of only 10-20 nm.

• Electrons, either SE or BSE, from the sample surface are detected by a detector and amplified to form images on the screen of a CRT.

Tescan VEGA TS 5136XM


Scanning Electron Microscopy – EDX - EBIC

• SEM– Tescan VEGA TS 5136XM

Variable Pressure (5x10-3- 500 Pa)

• EDX analysis – Genesis 4000 XMS Imaging 60

SEM• EBIC T= 80-300K.

– FEMTO Variable-GainLow-NoiseCurrent AmplifierDLPCA-200


SEM: image magnification

Example of a series of increasing magnification: spherical lead particles


Material – electron interaction


EBIC technique

• Electron beam induced current (EBIC) is a semiconductor analysis technique performed in a scanning electron microscope (SEM) or scanning transmission electron microscope (STEM). It is used to identify buried junctions or defects in semiconductors, or to examine minority carrier properties.

• EBIC depends on the creation of electron–hole pairs in the semiconductor sample by the microscope's electron beam.

• This technique is used in semiconductor failure analysisand solid-state physics.

• The spatial resolution is of the order of few µm (in SEM)– e-beam energy– Minority carrier lifetime


EBIC technique: how it works• EBIC employs a (SEM) on a

sample with a thin electron-transparent Schottky contact or a p-n junction.

• The short circuit current is amplified and displayed on a monitor synchronized with the electron beam scan.

• The electron beam induces carriers; the minority carriers either recombine at defects or are collected at the Schottky contact as current with the resulting signal being displayed on the monitor.

• The picture on the monitor thus shows a current sample map.

• Defects that are "electronically active" reduce the currents; they appear in dark contrasts.

Holder

Sample

Scanninge-beam

Amplifier


EBIC technique: how it works

Lateral Planar

Different configuration can be used:

p-n junction

Schottky diode

H.J. Leamy J. Appl. Phys. 53 (1982) R59


EBIC technique: lateral configuration

Lx

oeII−

=

0 5 10 15 20 25 300,50

0,55

0,60

0,65

0,70

0,75

0,80

0,85

0,90

0,95

1,00

EBIC

Cur

rent

(nor

mal

ized

)

Distance from the junction (μm)

L=200 um L=50 um


EBIC technique: planar configuration

Gold contactSpace charge region

Si wafer

DefectBack contact

Position (um)

Cur

rent

(nA

)E-beam

I/V converteramplifier

L determination also in planar configuration changing e-beam energy (penetration depth)


EBIC technique: how it works

100 150 200 250 3000,4

0,6

0,8

1,0

EBI

C c

urre

nt (1

0-7 A

)Position (μm)

110K 150K 230K 300K

Multicrystalline Si wafer


( )DvsLk

zxxhxdxkzdzs

kdksLsxI

LLI

LsxIILsxI

s / ;/1 ;

),()exp()sin()2(

2),,(

1

),,(),,(

22

0 002

*

0

*0

==+=

⎪⎪⎩

⎪⎪⎨

⎧

−−+

=

+=

−=

∫ ∫ ∫∞ ∞ ∞+

∞−

λλμ

μμμπ

αα

EBIC: theoretical profile around a grain boundary

Donolato’s formulationDiffusion problem in a semi-indefinite medium

Boundary conditions:

a) At the collecting surface s = ∞

b) At the grain boundary s = s0 ∈ ℜ where:L: diffusion length [μm]s: reduced recombination velocity [cm-1]

D: diffusion coefficient [cm2s-1]


-80 -60 -40 -20 0 20 40 60 800.0

0.2

0.4

0.6

0.8

1.0

1.2

I [u.

arb

.]

X [µm]

Contrast:

Area: A

⎪⎩

⎪⎨

⎧

−=

−=

−122

22

)(3

)(32

h

h

As

L

σσ

σσ

⎩⎨⎧σA

0

min0

IIIC −

=

EBIC: useful quantities

Width: σc

50 100 150 2000,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1

Exp. data

I [u.

arb]

x [μm]0

σ = 0.9 μm σ = 2.5 μm


EBIC contrast vs T theory

Kveder et al. J. APPL. PHYS. 78, (1995), 4673

Segregated impurities at extended defects (dislocations, grain boundaries) increase their recombination activity.

The analysis of the cEBIC(T°) allows the determination of the impurity concentration at defects.

Type L Type mixed Type HIncreasing metal comtamination


Multicrystalline Si wafers: lifetime mapsAs-grown (#162) after the POCl3 process step (# 145)

Internal gettering

Grains are free of active defects!


EBIC maps vs T: magnification 61xT=293K T=273K T=223K T=173K

T=120K T=100K T=90KSEM image


Magnification 650xT=293K T=273K T=223K T=173K

T=120K T=100K T=90KSEM image


SE – EBIC comparisonBright spots

Bright spots: indicative of metal segregation at defects and low lifetime

Depleted impurity zones: less recombination


EBIC contrast vs T

D2GB Twin GB

100 150 200 250 3000,4

0,6

0,8

1,0

EBI

C c

urre

nt (1

0-7 A

)

Position (μm)

110K 150K 2300K 300K

D1

50 100 150 200 250 3000

10

20

30

40

50

Con

trast

(%)

Temperature (K)

Dislocation1 Dislocation 2 GB GB Twin


EBIC vs T

50 100 150 200 250 3000

10

20

30

40

50

Con

trast

(%)

Temperature (K)


Dislocations active near room temperature: strong contamination

Impurities cm-1


EBIC comparison between ingots

60 80 100 120 140 160 180 200 220 240 260 280 300

7

8

9

10

11

12

13

14

15

GB1 GB2

Grain boundaries

Con

trast

(%)

Temperature (K)

80 100 120 1400

2

4

6

8

10

12

Dislocations

D1 D2 D3 D4

Con

trast

(%)

Temperature (K)

Ingot 1 Ingot 2

Less contamination in Ingot 2Strong segregation at extended defects (GBs)

50 100 150 200 250 3000

10

20

30

40

50

Con

trast

(%)

Temperature (K)



Impurity segregation during ingot growth

C (%) increases with impurities segregation at BGs

Ingot bottom center ingot topImpurities

Mc Donald et al. J. Appl. Phys. 97, 033523 (2005)


EBIC: contrast evolution vs cell process

#162 #144 #145 #147 #148

0

1

2

3

4

5

6

7

8

9

10

11

Con

trast

[%]

#Samples

ContrastA ContrastB ContrastC ContrastD ContrastE

Contrast error ± 1%

As-grown Contact anneal

P diffusion step


EBIC magnification

C~10%

Metal silicide precipitate1?

[1] T. Buonassisi et al. Appl. Phys. Lett. 87 (2005) 121918


Iron content determination

• Chen et al.[*] had demonstrated the possibility to correlate the EBIC contrast with the iron content.

• Samples were contaminated at different levels (3.0x1012, 4.0 x1013, 4.0x1014, and 3.0x1015 cm−3)

• [*] J. Chen et al. J. Apppl. Phys. 96 (2004) 5490


Iron contamination

• Samples: n-type Si• Iron deposition from an aqueous solution of FeCl3.• Heat treatment at 950 °C• Cp4 etching• Standard chemical cleaning procedure (RCA).

• Iron solubility in Si: – S(T)=1.8 x1026e[-2.99/kbT] cm-3

– S(900°C)=9.6 x 1013 cm-3 [*]

[*] A.A. Istratov et all Appl. Phys. A: Mateer. Sci. Process. 69 (1999) 13


Iron contamination

As-grown Fe contamined

Ld= 140 μm Ld= 34 μm


Iron content

3.0x1015

4.0x1014

4.0x1013

3.0x1012 cm−3

J. Chen et al. J. Apppl. Phys. 96 (2004) 5490


Iron contaminationJ. Chen et al. J. Appl. Phys., Vol. 96, No. 10, 15 November 2004


Relaxed SiGe buffer layers as virtual substrates (VS) for active layers


SiGe buffer layers growth by PECVDCommon Characteristics:Doping: p(B) 1x1016 cm-3

Grading rate: 7%/umConstant composition cap: 2 umSi cap: 10nm deposited at 550°C

Au

Si substrate

graded SiGe

uniform SiGe 20%

strained Si

2 μm

3 μm


15KeV

Au

p-type substrate

Graded SiGe n-doped

Uniform SiGe n-doped

Undoped strained Si 10nm2 um

3 um

InGa ohmic contatct

Si subst.

Interac. sphere 25keV Re~3um

Interac. sphere 15keV Re~0.6um

25KeV

E-beam depth penetration vs E-beam acceleration voltage(Kev)

Room temperature (300 K);At different acceleration voltages.

EBIC measurements


Au

InGa

X=20%X=20%Front (Au) – Back (InGa);PC = 3 (spot 711 nm).

BeamBeam energyenergy ==15KeV15KeVTemperature =Temperature =300K300K

EBIC measurements


Au

InGa

X=20%X=20%Front (Au) – Back (InGa);PC = 8 (spot 210 nm).

BeamBeam energyenergy ==25KeV25KeVTemperature =Temperature =300K300K

EBIC measurements


EBIC measurements

TD contrast = 4%


Optical microscope: Normasky configuration

Etch pits are well defined at all Ge concentrations

20% 90%40%


EBIC vs chemical EPD counts

Concentration of impurities below 1012 cm3


Edge Isolation of Solar Cells by Fiber LaserIR (1060 nm) and UV (355 nm)

• The removal of parasitic emitter diffusion flowing around Solar Cells Wafer Edges is mandatory in order to get high fill factors. In industry, the plasma etching of wafer stacks is very common, but this is an off-line process, and undesired chemicals have to be used.

• There is then a strong demand of other possibilities, to be performed in-line.

• One of the most appealing novel Edge Isolation process is Laser Scribing.


Isolation scheme


LBICScanLab Head:• Step response: (settling to 1/1000 full scale)

• 1% of full scale 1.1 ms• 10% of full scale 2.4 ms

• Typical image field: (170x170) mm2

• Resolution: 65536 pts on each axis• Spot size 65 μm ∅• Lasers: 633 nm, 780 nm and 830 nm• Variable neutral density filter (0.1, 0.2, 0.3, 1, 2, 3)Acquisition system:• Computer: Pentium III 550MHz• I/V converter:

– Transimpedance 104 ..1011 V/A– Rise/fall time (10%-90%) 700ns at 104

• Programming environment: LabView• GPIB 488II• NI-DAQ PCI MIO16E4 (250Ksample/s) via

Lock-in acquisition


LBIC maps: edge problems


LBIC maps: IR vs UV laser

IR UV

Higher isolation for the UV laser treated sample


SEM images

IR UV

More redeposit (shunt) in IR laser treated sample


Thanks

Maurizio AcciarriDipartimento di Scienza dei Materiali

Università Milano BicoccaVia Cozzi 53 20125 Milano Italy

[email protected]

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The use of EBIC in solar cell characterization - SINTEF · The use of EBIC in solar cell characterization Maurizio Acciarri ... belong to the Department of Material Science of the