The use of EBIC in solar cell
characterization
Maurizio Acciarri
Mini PV Conference
Trondheim 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 Science
1. Materials Science and Cultural Heritage. Luminescence Dating
2. Oxide Nanostructures and Silica-based Materials for Optical Technology
3. Energy storage materials. Chemical synthesis, crystal structure, theoretical models
4. Electrochemical activities
5. Chemistry of inorganic and organometallic materials
6. Surface chemical reactions: crystal growth and sorption processes
7. Physics and applications of lasers
8. Shape Memory Alloys
9. Organic materials for applications in photonics
10. Organic molecular systems for II order non-linear materials and low energy emitters
11. Nanostructured materials and magic angle spinning NMR
12. Chemical physics of semiconductors: defects, impurities and surfaces
13. Optical spectroscopy of semiconductors and semiconductor quantum structures
14. Organic Molecular Semiconductors
15. Photophysics of molecular semiconductors
16. Simulation and Modeling of the Epitaxial Growth of Semiconductor Nanostructures and
Films
17. Theoretical modeling and ab-initio simulation of material properties
18. Theory of Surface Science and Catalysis
19. 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 nonradiative 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) (20002003)
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
Physics and Chemistry of Semiconductors
Defects,
impurities and
their interaction
Oxygen, Carbon
and Nitrogen
Solar energy (PV)
conversion
Recombination
processes at
extended defects
Dislocations
In-line
characterization
Precipitates
Recombination
processes
(Si, SiC and SiGe)
Optoelectronic
Thin films
Solar cells
Facilities
•SEM – EBIC (77-300 K)
•LBIC e Fast-LBIC
•Hall effect
•SPV
•Solar simulator (5x5 cm2)
•FTIR (50-4000 cm-1)
•Photoluminescence (IRUV-Vis)
•XRD and Raman
spectroscopy (dept. facility)
•Optical microscope
•Chemistry laboratory for
etching and cleaning
•Furnaces (working
temperature up to 1500 °C)
•Evaporator
•Sputtering
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
Impact of defects on solar cells efficiency
Perfect crystal
C.B.
Radiative
Radiative
and emission
non radiative hυ
Photoluminescence
emission
B.V.
Direct recombination lifetime: τd
Indirect recombination lifetime: τi
(Shockley-Read-Hall)
⎡
τ SRH =
⎛ 2 ( Ei − EF ) ⎞
⎡
⎛ 2 Ei − Et − EF ⎞ ⎤
⎛ Et − E F ⎞ ⎤
⎟ ⎥ + τ p 0 ⎢1 + h + exp ⎜
⎟⎥
⎟ + exp ⎜
kT
kT
⎝
⎠⎦
⎝ kT ⎠ ⎦
⎣
⎝
⎠
⎛ 2 ( Ei − E F ) ⎞
1 + h + exp ⎜
⎟
kT
⎝
⎠
τ n 0 ⎢ h + exp ⎜
⎣
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
1
τ n0
≡ σ n vth N t
Impact of defects on solar cells efficiency
• For PV very high
C.B.
lifetime values are
requested
hυ
• In PV is more
meaningful the
B.V.
diffusion length (L):
L = Dτ
kT
μ
D=
q
1
τ
=
1
τd
+
1
τi
+
1
τs
+ ..
Defects may influence
recombination and mobility
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
Multicrystalline Si wafers: lifetime maps
PhotoConductance Decay (mW-PCD)
After the POCl3 process step
As-grown
Why?
Lifetime maps carried out by ISC Konstanz (D)
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-Gain
Low-Noise
Current Amplifier
DLPCA-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 analysis
and 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 electrontransparent 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.
Scanning
e-beam
Sample
Holder
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
Amplifier
EBIC technique: how it works
Different configuration can be used:
Lateral
Planar
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
L=200 um
L=50 um
I = I oe
x
−
L
EBIC Current (normalized)
1,00
0,95
0,90
0,85
0,80
0,75
0,70
0,65
0,60
0,55
0,50
0
5
10
15
20
25
Università degli Studi di Milano”Bicocca”
Distance from the junction (μm)
Dipartimento di Scienza dei Materiali
30
EBIC technique: planar configuration
Current (nA)
E-beam
Position (um)
I/V converter
amplifier
Gold contact
Space charge
region
Si wafer
Defect
Back contact
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
Multicrystalline Si wafer
110K
150K
230K
300K
-7
EBIC current (10 A)
1,0
0,8
0,6
0,4
100
150
200
Position (μm)
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
250
300
EBIC: theoretical profile around a grain boundary
Donolato’s formulation
Diffusion 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]
I ( x, s , L ) = I 0 − I * ( x, s , L )
s: reduced recombination velocity [cm-1]
αL
⎧
I
=
⎪ 0 1 + αL
D: diffusion coefficient [cm2s-1]
⎪
⎨
∞
∞
+∞
2
s
k
⎪ I * ( x, s , L ) =
dk 2
dz sin(kz ) ∫ dx exp(− μ x )h( x − x0 , z )
∫
∫
⎪⎩
π 0 μ (2μ + s) 0
−∞
(μ =
k 2 + λ2 ; λ = 1 / L; s = vs / D
)
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
EBIC: useful quantities
1.2
1,1
¾ Contrast:
I 0 − I min
C=
I0
[u.arb]
II[u.
arb.]
1,0
1.0
0,9
0.8
0,8
0.6
0,7
¾ Area: A
0.4
0,6
Exp. data
σ = 0.9 μm
σ = 2.5 μm
0.2
0,5
0,4
0.0
0
-80
-60
50
-40
⎧A
⎨
⎩σ
-20
100
0
[μm]
Xx[µm]
20
150
40
60
¾ Width: σc
200
80
⎧
2 2
2
(σ − σ h )
⎪L =
3
⎨
⎪s = 3 A(σ 2 − σ 2 ) −1
h
⎩
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
EBIC contrast vs T theory
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
Increasing metal comtamination
Type mixed
Kveder et al. J. APPL. PHYS. 78, (1995), 4673
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
Type H
Multicrystalline
Si
wafers:
lifetime
maps
As-grown (#162)
after the POCl3 process step (# 145)
Grains are free of active defects!
Internal gettering
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
EBIC maps vs T: magnification 61x
T=293K
SEM image
T=273K
T=223K
T=173K
T=120K
T=100K
T=90K
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
Magnification 650x
T=293K
SEM image
T=273K
T=223K
T=173K
T=120K
T=100K
T=90K
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
SE – EBIC comparison
Bright spots
Depleted impurity zones: less recombination
Bright spots: indicative of metal segregation at defects and low lifetime
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
EBIC contrast vs T
110K
150K
2300K
300K
-7
D2
EBIC current (10 A)
1,0
GB
0,8
0,6
0,4
100
150
200
250
Position (μm)
D1
300
Dislocation1
Dislocation 2
GB
GB Twin
50
40
Contrast (%)
GB Twin
30
20
10
0
50
100
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
150
200
Temperature (K)
250
300
EBIC vs T
Dislocation1
Dislocation 2
GB
GB Twin
50
Impurities cm-1
Contrast (%)
40
30
20
10
0
50
100
150
200
250
300
Temperature (K)
Dislocations active near room temperature: strong contamination
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
EBIC comparison between ingots
Ingot 1
50
Ingot 2
Dislocation1
Dislocation 2
GB
GB Twin
15
40
14
Contrast (%)
30
20
Grain boundaries
12
11
10
9
GB1
GB2
8
10
7
60
80
100
120
140
0
50
100
150
200
250
160
180
200
220
240
260
Temperature (K)
300
Temperature (K)
12
Dislocations
10
8
Contrast (%)
Contrast (%)
13
D1
D2
D3
D4
6
4
2
0
80
100
120
140
Temperature (K)
Less contamination
in Ingot 2
Università
degli Studi di Milano”Bicocca”
Dipartimento
di Scienza dei Materiali
Strong
segregation
at extended
defects (GBs)
280
300
Impurity segregation during ingot growth
Ingot bottom
Impurities
center
ingot top
C (%) increases
with impurities
segregation at BGs
Mc Donald etdegli
al. J. Appl.
Phys.di
97,Milano”Bicocca”
033523 (2005)
Università
Studi
Dipartimento di Scienza dei Materiali
EBIC: contrast evolution vs cell process
P diffusion step
11
10
9
8
ContrastA
ContrastB
ContrastC
ContrastD
ContrastE
Contrast[%]
7
6
5
4
3
2
1
0
#162
As-grown
#144
#145
#147
#Samples
Contrast error ± 1%
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
#148
Contact anneal
EBIC magnification
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
C~10%
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
Ld= 140 μm
Ld= 34 μm
As-grown
Fe contamined
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
Iron content
3.0x1015
4.0x1014
4.0x1013
3.0x1012 cm−3
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza
deietMateriali
J. Chen
al. J. Apppl. Phys. 96 (2004) 5490
Iron contamination
J. 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 PECVD
Common Characteristics:
Doping: p(B) 1x1016 cm-3
Grading rate: 7%/um
Constant composition cap: 2 um
Si cap: 10nm deposited at 550°C
Au
2 μm
strained Si
uniform SiGe 20%
3 μm
graded SiGe
Si substrate
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
EBIC measurements
Room temperature (300 K);
At different acceleration voltages.
E-beam depth penetration vs E-beam acceleration voltage
(Kev)
Au
Undoped strained Si
Interac.
sphere 15keV
Re~0.6um
Uniform SiGe ndoped
Graded SiGe ndoped
Interac.
sphere 25keV
Re~3um
p-type substrate
Si subst.
10n
m
2
um
3
um
25KeV
15Ke
V
InGa ohmic contatct
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
EBIC measurements
Beam energy =15KeV
Temperature =300K
Au
X=20%
Front (Au) – Back (InGa);
PC = 3 (spot 711 nm).
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
InGa
EBIC measurements
Beam energy =25KeV
Temperature =300K
Au
X=20%
Front (Au) – Back (InGa);
PC = 8 (spot 210 nm).
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
InGa
EBIC measurements
TD contrast = 4%
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali
Optical microscope: Normasky configuration
20%
40%
90%
Etch pits are well defined at all Ge concentrations
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 Laser
IR (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
LBIC
ScanLab Head:
•
Step response: (settling to 1/1000 full scale)
•
•
1% of full scale
10% of full scale
1.1 ms
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 Acciarri
Dipartimento di Scienza dei Materiali
Università Milano Bicocca
Via Cozzi 53 20125 Milano Italy
www.mater.unimib.it
[email protected]
Università degli Studi di Milano”Bicocca”
Dipartimento di Scienza dei Materiali