The use of EBIC in solar cell characterization

The use of EBIC in solar cell characterization Maurizio Acciarri Mini PV Conference Trondheim 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 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

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

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

Physics and Chemistry of Semiconductors Defects, impurities and their interaction Solar energy (PV) conversion Facilities Oxygen, Carbon and Nitrogen Dislocations Precipitates Recombination processes (Si, SiC and SiGe) Optoelectronic Recombination processes at extended defects In-line characterization Thin films Solar cells 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

Impact of defects on solar cells efficiency C.B. Perfect crystal Radiative Radiative and emission non radiative emission Photoluminescence hυ B.V. Direct recombination lifetime: τ d Indirect recombination lifetime: τ i (Shockley-Read-Hall) τ SRH ( ) 2 Ei EF 2Ei Et EF Et EF τn0 h+ exp exp τ p0 1 h exp kt + + kt + + kt = 2( Ei EF) 1+ h + exp kt 1 τ n0 σ v N n th t

Impact of defects on solar cells efficiency For PV very high lifetime values are requested In PV is more meaningful the diffusion length (L): L = Dτ hυ C.B. B.V. 1 1 = τ τ d 1 + τ i 1 + τ s +.. D = kt q μ Defects may influence recombination and mobility

Multicrystalline Si wafers: lifetime maps PhotoConductance Decay (mw-pcd) As-grown After the POCl3 process step Why? Lifetime maps carried out by ISC Konstanz (D)

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-Gain Low-Noise Current Amplifier DLPCA-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 analysis and 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 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. Holder Sample Scanning e-beam 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

EBIC technique: lateral configuration I = I o e 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 30 Distance from the junction (μm) L=200 um L=50 um

EBIC technique: planar configuration E-beam Current (na) 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)

EBIC technique: how it works Multicrystalline Si wafer 1,0 110K 150K 230K 300K EBIC current (10-7 A) 0,8 0,6 0,4 100 150 200 250 300 Position (μm)

EBIC: theoretical profile around a grain boundary I( x, s, L) = I I 0 * αl = 1+ αl ( x, s, L) Donolato s formulation Diffusion problem in a semi-indefinite medium Boundary conditions: a) At the collecting surface s = b) At the grain boundary s = s 0 R where: I 0 I * ( x, s, L) + 2s k = dk dz sin( kz) dx exp( x ) h( x x 2 (2 + s) μ π μ μ 0 0 ( ) 2 2 μ = k + λ ; λ = 1/ L; s = v D s / L: diffusion length [μm] s: reduced recombination velocity [cm -1 ] D: diffusion coefficient [cm 2 s -1 ] 0, z)

EBIC: useful quantities I [u.arb] arb.] 1.2 1,1 1,0 1.0 0,9 0.8 0,8 0.6 0,7 0.4 0,6 0,5 0.2 0,4 0.0-80 0-60 50-40 -20 100 0 20 150 40 60 80 200 A σ x [μm] X [µm] Exp. data σ = 0.9 μm σ = 2.5 μm Contrast: I0 I C = I0 Area: A min Width: σ c 2 2 2 L = ( σ σ h ) 3 2 2 s = 3A( σ σ h ) 1

EBIC contrast vs T theory Segregated impurities at extended defects (dislocations, grain boundaries) increase their recombination activity. The analysis of the c EBIC (T ) allows the determination of the impurity concentration at defects. Increasing metal comtamination Type L Type mixed Type H Kveder et al. J. APPL. PHYS. 78, (1995), 4673

Multicrystalline Si wafers: lifetime maps As-grown (#162) after the POCl3 process step (# 145) Grains are free of active defects! Internal gettering

EBIC maps vs T: magnification 61x T=293K T=273K T=223K T=173K SEM image T=120K T=100K T=90K

Magnification 650x T=293K T=273K T=223K T=173K SEM image T=120K T=100K T=90K

SE EBIC comparison Bright spots Depleted impurity zones: less recombination Bright spots: indicative of metal segregation at defects and low lifetime

EBIC contrast vs T 1,0 110K 150K 2300K 300K D2 GB Twin GB EBIC current (10-7 A) 0,8 0,6 0,4 100 150 200 250 300 D1 50 Position (μm) Dislocation1 Dislocation 2 GB GB Twin 40 Contrast (%) 30 20 10 0 50 100 150 200 250 300 Temperature (K)

EBIC vs T 50 40 Dislocation1 Dislocation 2 GB GB Twin Impurities cm -1 Contrast (%) 30 20 10 0 50 100 150 200 250 300 Temperature (K) Dislocations active near room temperature: strong contamination

EBIC comparison between ingots 50 40 Dislocation1 Ingot 1 Ingot 2 Dislocation 2 GB GB Twin 15 14 Contrast (%) 30 20 10 Contrast (%) 13 12 11 10 9 8 7 Grain boundaries GB1 GB2 0 50 100 150 200 250 300 Temperature (K) 60 80 100 120 140 160 180 200 220 240 260 280 300 Temperature (K) 12 10 Dislocations Contrast (%) 8 6 4 D1 D2 D3 D4 2 0 80 100 120 140 Temperature (K) Less contamination in Ingot 2 Strong segregation at extended defects (GBs)

Impurity segregation during ingot growth Impurities Ingot bottom center ingot top C (%) increases with impurities segregation at BGs Università Mc Donald et degli al. J. Appl. Studi Phys. di 97, Milano Bicocca 033523 (2005)

EBIC: contrast evolution vs cell process P diffusion step 11 10 9 Contrast[%] 8 7 6 5 4 3 2 1 0 ContrastA ContrastB ContrastC ContrastD ContrastE As-grown #162 #144 #145 #147 #148 #Samples Contact anneal Contrast error ± 1%

EBIC magnification Metal silicide precipitate 1? [1] T. Buonassisi et al. Appl. Phys. Lett. 87 (2005) 121918 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 (3.0x10 12, 4.0 x10 13, 4.0x10 14, and 3.0x10 15 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 FeCl 3. Heat treatment at 950 C Cp4 etching Standard chemical cleaning procedure (RCA). Iron solubility in Si: S(T)=1.8 x10 26 e [-2.99/kbT] cm -3 S(900 C)=9.6 x 10 13 cm -3 [*] [*] A.A. Istratov et all Appl. Phys. A: Mateer. Sci. Process. 69 (1999) 13

Iron contamination Ld= 140 μm Ld= 34 μm As-grown Fe contamined

Iron content 3.0x10 15 4.0x10 14 4.0x10 13 3.0x10 12 cm 3 Dipartimento di Scienza J. Chen deiet Materiali al. J. Apppl. Phys. 96 (2004) 5490

Iron contamination J. 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 PECVD Common Characteristics: Doping: p(b) 1x10 16 cm -3 Grading rate: 7%/um Constant composition cap: 2 um Si cap: 10nm deposited at 550 C 2 μm 3 μm strained Si Au uniform SiGe 20% graded SiGe Si substrate

EBIC measurements Room temperature (300 K); At different acceleration voltages. E-beam depth penetration vs E-beam acceleration voltage (Kev) Au Interac. sphere 15keV R e ~0.6um Undoped strained Si Uniform SiGe n- doped 10n m 2 um Interac. sphere 25keV R e ~3um Graded SiGe n- doped p-type substrate Si subst. InGa ohmic contatct 3 um 15Ke 25KeV V

EBIC measurements Beam energy =15KeV Temperature =300K= X=20% Front (Au) Back (InGa); PC = 3 (spot 711 nm). Au InGa

EBIC measurements Beam energy =25KeV Temperature =300K= X=20% Front (Au) Back (InGa); PC = 8 (spot 210 nm). Au InGa

EBIC measurements TD contrast = 4%

Optical microscope: Normasky configuration 20% 40% 90% Etch pits are well defined at all Ge concentrations

EBIC vs chemical EPD counts Concentration of impurities below 10 12 cm 3

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.

Isolation scheme

LBIC ScanLab 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) mm 2 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 10 4..10 11 V/A Rise/fall time (10%-90%) 700ns at 10 4 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 Acciarri Università Milano Bicocca Via Cozzi 53 20125 Milano Italy www.mater.unimib.it maurizio.acciarri@unimib.it