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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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
≡
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Multicrystalline Si wafers: lifetime maps
After the POCl3 process stepAs-grown
Why?Lifetime maps carried out by ISC Konstanz (D)
PhotoConductance Decay (mW-PCD)
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
SEM: image magnification
Example of a series of increasing magnification: spherical lead particles
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Material – electron interaction
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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)
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
( )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]
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
-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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Multicrystalline Si wafers: lifetime mapsAs-grown (#162) after the POCl3 process step (# 145)
Internal gettering
Grains are free of active defects!
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
EBIC maps vs T: magnification 61xT=293K T=273K T=223K T=173K
T=120K T=100K T=90KSEM image
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Magnification 650xT=293K T=273K T=223K T=173K
T=120K T=100K T=90KSEM image
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
SE – EBIC comparisonBright spots
Bright spots: indicative of metal segregation at defects and low lifetime
Depleted impurity zones: less recombination
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
EBIC vs T
50 100 150 200 250 3000
10
20
30
40
50
Con
trast
(%)
Temperature (K)
Dislocation1 Dislocation 2 GB GB Twin
Dislocations active near room temperature: strong contamination
Impurities cm-1
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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)
Dislocation1 Dislocation 2 GB GB Twin
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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)
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
EBIC magnification
C~10%
Metal silicide precipitate1?
[1] T. Buonassisi et al. Appl. Phys. Lett. 87 (2005) 121918
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Iron contamination
As-grown Fe contamined
Ld= 140 μm Ld= 34 μm
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Iron content
3.0x1015
4.0x1014
4.0x1013
3.0x1012 cm−3
J. Chen et al. J. Apppl. Phys. 96 (2004) 5490
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Iron contaminationJ. Chen et al. J. Appl. Phys., Vol. 96, No. 10, 15 November 2004
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Relaxed SiGe buffer layers as virtual substrates (VS) for active layers
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Au
InGa
X=20%X=20%Front (Au) – Back (InGa);PC = 3 (spot 711 nm).
BeamBeam energyenergy ==15KeV15KeVTemperature =Temperature =300K300K
EBIC measurements
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Au
InGa
X=20%X=20%Front (Au) – Back (InGa);PC = 8 (spot 210 nm).
BeamBeam energyenergy ==25KeV25KeVTemperature =Temperature =300K300K
EBIC measurements
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
EBIC measurements
TD contrast = 4%
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Optical microscope: Normasky configuration
Etch pits are well defined at all Ge concentrations
20% 90%40%
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
EBIC vs chemical EPD counts
Concentration of impurities below 1012 cm3
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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.
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Isolation scheme
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
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
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
LBIC maps: edge problems
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
LBIC maps: IR vs UV laser
IR UV
Higher isolation for the UV laser treated sample
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
SEM images
IR UV
More redeposit (shunt) in IR laser treated sample
Università degli Studi di Milano”Bicocca”Dipartimento di Scienza dei Materiali
Thanks
Maurizio AcciarriDipartimento di Scienza dei Materiali
Università Milano BicoccaVia Cozzi 53 20125 Milano Italy
www.mater.unimib.itmaurizio.acciarri@unimib.it
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